Canopy mortality has doubled in Europe's temperate forests over the last three decades

Long-term annual estimation of forest above ground biomass, canopy cover, and height from airborne and spaceborne sensors synergies in the Iberian Peninsula

The Mediterranean Basin has experienced substantial land use changes as traditional agriculture decreased and population migrated from rural to urban areas, which have resulted in a large forest cover increase. The combination of Landsat time series, providing spectral information, with lidar, offering three-dimensional insights, has emerged as a viable option for the large-scale cartography of forest structural attributes across large time spans. Here we develop and test a comprehensive framework to map forest above ground biomass, canopy cover and forest height in two regions spanning the most representative biomes in the peninsular Spain, Mediterranean (Madrid region) and temperate (Basque Country). As reference, we used lidar-based direct estimates of stand height and forest canopy cover. The reference biomass and volume were predicted from lidar metrics. Landsat time series predictors included annual temporal profiles of band reflectance and vegetation indices for the 1985-2023 period. Additional predictor variables including synthetic aperture radar, disturbance history, topography and forest type were also evaluated to optimize forest structural attributes retrieval. The estimates were independently validated at two temporal scales, i) the year of model calibration and ii) the year of the second lidar survey. The final models used as predictor variables only Landsat based metrics and topographic information, as the available SAR time-series were relatively short (1991-2011) and disturbance information did not decrease the estimation error. Model accuracies were higher in the Mediterranean forests when compared to the temperate forests (R2 = 0.6-0.8 vs. 0.4-0.5). Between the first (1985-1989) and the last (2020-2023) decades of the monitoring period the average forest cover increased from 21 ± 2% to 32 ± 1%, mean height increased from 6.6 ± 0.43 m to 7.9 ± 0.18 m and the mean biomass from 31.9 ± 3.6 t ha-1 to 50.4 ± 1 t ha-1 for the Mediterranean forests. In temperate forests, the average canopy cover increased from 55 ± 4% to 59 ± 3%, mean height increased from 15.8 ± 0.77 m to 17.3 ± 0.21m, while the growing stock volume increased from 137.8 ± 8.2 to 151.5 ± 3.8 m3 ha-1. Our results suggest that multispectral data can be successfully linked with lidar to provide continuous information on forest height, cover, and biomass trends.

Forest disturbance regimes and trends in continental Spain (1985-2023) using dense landsat time series

Forest disturbance regimes across biomes are being altered by interactive effects of global change. Establishing baselines for assessing change requires detailed quantitative data on past disturbance events, but such data are scarce and difficult to obtain over large spatial and temporal scales. The integration of remote sensing with dense time series analysis and cloud computing platforms is enhancing the ability to monitor historical disturbances, and especially non-stand replacing events along climatic gradients. Since the integration of such tools is still scarce in Mediterranean regions, here, we combine dense Landsat time series and the Continuous Change Detection and Classification - Spectral Mixture Analysis (CCDC-SMA) method to monitor forest disturbance in continental Spain from 1985 to 2023. We adapted the CCDC-SMA method for improved disturbance detection creating new spectral libraries representative of the study region, and quantified the year, month, severity, return interval, and type of disturbance (stand replacing, non-stand replacing) at a 30 m resolution. In addition, we characterised forest disturbance regimes and trends (patch size and severity, and frequency of events) of events larger than 0.5 ha at the national scale by biome (Mediterranean and temperate) and forest type (broadleaf, needleleaf and mixed). We quantified more than 2.9 million patches of disturbed forest, covering 4.6 Mha over the region and period studied. Forest disturbances were on average larger but less severe in the Mediterranean than in the temperate biome, and significantly larger and more severe in needleleaf than in mixed and broadleaf forests. Since the late 1980s, forest disturbances have decreased in size and severity while increasing in frequency across all biomes and forest types. These results have important implications as they confirm that disturbance regimes in continental Spain are changing and should therefore be considered in forest strategic planning for policy development and implementation.

Using climate envelopes and earth system model simulations for assessing climate change induced forest vulnerability

Changing climatic conditions threaten forest ecosystems. Drought, disease and infestation, are leading to forest die-offs which cause substantial economic and ecological losses. In central Europe, this is especially relevant for commercially important coniferous tree species. This study uses climate envelope exceedance (CEE) to approximate species risk under different future climate scenarios. To achieve this, we used current species presence-absence and historical climate data, coupled with future climate scenarios from various Earth System Models. Climate scenarios tended towards drier and warmer conditions, causing strong CEEs especially for spruce. However, we show that annual averages of temperature and precipitation obscure climate extremes. Including climate extremes reveals a broader increase in CEEs across all tree species. Our study shows that the consideration of climate extremes, which cannot be adequately reflected in annual averages, leads to a different assessment of the risk of forests and thus the options for adapting to climate change.

Effects of experimental canopy openness on wood-inhabiting fungal fruiting diversity across succession

While the succession of terrestrial plant communities is well studied, less is known about succession on dead wood, especially how it is affected by environmental factors. While temperate forests face increasing canopy mortality, which causes considerable changes in microclimates, it remains unclear how canopy openness affects fungal succession. Here, we used a large real-world experiment to study the effect of closed and opened canopy on treatment-based alpha and beta fungal fruiting diversity. We found increasing diversity in early and decreasing diversity at later stages of succession under both canopies, with a stronger decrease under open canopies. However, the slopes of the diversity versus time relationships did not differ significantly between canopy treatments. The community dissimilarity remained mainly stable between canopies at ca. 25% of species exclusively associated with either canopy treatment. Species exclusive in either canopy treatment showed very low number of occupied objects compared to species occurring in both treatments. Our study showed that canopy loss subtly affected fungal fruiting succession on dead wood, suggesting that most species in the local species pool are specialized or can tolerate variable conditions. Our study indicates that the fruiting of the fungal community on dead wood is resilient against the predicted increase in canopy loss in temperate forests.

Does long-term drought or repeated defoliation affect seasonal leaf N cycling in young beech trees?

Forest trees adopt effective strategies to optimize nitrogen (N) use through internal N recycling. In the context of more recurrent environmental stresses due to climate change, the question remains of whether increased frequency of drought or defoliation threatens this internal N recycling strategy. We submitted 8-year-old beech trees to 2 years of either severe drought (Dro) or manual defoliation (Def) to create a state of N starvation. At the end of the second year before leaf senescence, we labeled the foliage of the Dro and Def trees, as well as that of control (Co) trees, with 15N-urea. Leaf N resorption, winter tree N storage (total N, 15N, amino acids, soluble proteins) and N remobilization in spring were evaluated for the three treatments. Defoliation and drought did not significantly impact foliar N resorption or N concentrations in organs in winter. Total N amounts in Def tree remained close to those in Co tree, but winter N was stored more in the branches than in the trunk and roots. Total N amount in Dro trees was drastically reduced (-55%), especially at the trunk level, but soluble protein concentrations increased in the trunk and fine roots compared with Co trees. During spring, 15N was mobilized from the trunk, branches and twigs of both Co and Def trees to support leaf growth. It was only provided through twig 15N remobilization in the Dro trees, thus resulting in extremely reduced Dro leaf N amounts. Our results suggest that stress-induced changes occur in N metabolism but with varying severity depending on the constraints: within-tree 15N transport and storage strategy changed in response to defoliation, whereas a soil water deficit induced a drastic reduction of the N amounts in all the tree organs. Consequently, N dysfunction could be involved in drought-induced beech tree mortality under the future climate.

Growth data of outlying plantations allows benchmarking the tolerance to climate extremes and drought stress in the European larch

Introduction

Plantations located outside the species distribution area represent natural experiments to assess tree tolerance to climate variability. Climate change amplifies warming-related drought stress but also leads to more climate extremes.

Methods

We studied plantations of the European larch (Larix decidua), a conifer native to central and eastern Europe, in northern Spain. We used climate, drought and tree-ring data from four larch plantations including wet (Valgañón, site V; Santurde, site S), intermediate (Ribavellosa, site R) and dry (Santa Marina, site M) sites. We aimed to benchmark the larch tolerance to climate and drought stress by analysing the relationships between radial growth increment (hereafter growth), climate data (temperature, precipitation, radiation) and a drought index.

Results

Basal area increment (BAI) was the lowest in the driest site M (5.2 cm2 yr-1; period 1988-2022), followed by site R (7.5 cm2 yr-1), with the youngest and oldest and trees being planted in M (35 years) and R (150 years) sites. BAI peaked in the wettest sites (V; 10.4 cm2 yr-1; S, 10.8 cm2 yr-1). We detected a sharp BAI reduction (30% of the regional mean) in 2001 when springto-summer conditions were very dry. In the wettest V and S sites, larch growth positively responded to current March and June-July radiation, but negatively to March precipitation. In the R site, high April precipitation enhanced growth. In the driest M site, warm conditions in the late prior winter and current spring improved growth, but warm-sunny conditions in July and dry-sunny conditions in August reduced it. Larch growth positively responded to spring-summer wet conditions considering short (1-6 months) and long (9-24 months) time scales in dry (site M) and wet-intermediate (sites S and R) sites, respectively.

Discussion

Larch growth is vulnerable to drought stress in dry slow-growing plantations, but also to extreme spring wet-cloudy events followed by dry-hot conditions in wet fast-growing plantations.

Growth of tree (Pinus sylvestris) and shrub (Amelanchier ovalis) species is constrained by drought with higher shrub sensitivity in dry sites

We lack understanding of how variable is radial growth of coexisting tree and shrub species, and how growth is constrained by drought depending on site aridity. Here, we compared the radial growth of two widespread and coexisting species, a winter deciduous shrub (Amelanchier ovalis Medik.) and an evergreen conifer tree (Pinus sylvestris L.). We sampled four sites in Northeastern Spain subjected to different aridity levels and used dendrochronological methods to quantify growth patterns and responses to climate variables. The growth of the two species varied between regions, being lower in the driest sites. The first-order autocorrelation (growth persistence) was higher in more mesic sites but without clear differences between species. Tree and shrub growth negatively responded to elevated summer temperatures and positively to spring-summer precipitation and wet conditions. However, negative growth responses of the shrub to drought were only observed in the two driest sites in contrast to widespread responses of the tree. Abrupt growth reductions were common in the drier sites, but resilience indices show that the two species rapidly recovered pre-drought growth levels. The lower growth synchrony of the shrub as compared to the tree can be due to the multistemmed architecture, fast growth and low stature of the shrub. Besides, the high dependency of the shrub growth on summer rainfall can explain why drought limitations were only apparent in the two driest sites. In any case, results point out to the dendrochronological potential of shrubs, which is particularly relevant giving its ability to inhabit woodlands and treeless regions under harsh climatic conditions. Nevertheless, further research is required to elucidate the capacity of shrub species to tolerate drought, as well as to understand how shrubs thrive in water- and cold-limited environments.

Novel light regimes in European forests

Tree canopies are one of the most recognizable features of forests, providing shelter from external influences to a myriad of species that live within and below the tree foliage. Canopy disturbances are now increasing across European forests, and climate-change-induced drought is a key driver, together with pests and pathogens, storms and fire. These disturbances are opening the canopy and exposing below-canopy biodiversity and functioning to novel light regimes-spatial and temporal characteristics of light distribution at forest floors not found previously. The majority of forest biodiversity occurs in the shade within and below tree canopies, and numerous ecosystem processes are regulated at the forest floor. Altered light regimes, in interaction with other global change drivers, can thus strongly impact forest biodiversity and functioning. As recent European droughts are unprecedented in the past two millennia, and this has initiated probably the largest pulse of forest disturbances in almost two centuries, we urgently need to quantify, understand and predict the impacts of novel light regimes on below-canopy forest biodiversity and functions. This will be a crucial element in delivering much-needed information for policymakers and managers to adapt European forests to future no-analogue conditions.

"Mortality, or not mortality, that is the question …": How to Treat Removals in Tree Survival Analysis of Central European Managed Forests

Tree mortality is an objective forest health criterion and is particularly suitable for long-term and large-scale studies of forest condition. However, it is impossible to determine actual tree mortality in Central European managed forests where trees are removed for various reasons. In this case, the only way to approximate tree mortality is to define the range in which it occurs. This can be carried out by including in the mortality calculations either dead trees that remain in the stand at the end of the assessment period or additionally trees that have been removed from the stand. We used data from the annual forest monitoring surveys in Poland from 2009 to 2022 for pine, spruce, oak and birch to perform a survival analysis in which we included all removals or sanitary cuttings either as censored or complete observations. The differences between the calculated mortality rates were significant, indicating the importance of how removals are treated in the analysis. To assess which method used for mortality calculation was more appropriate, we compared values for last recorded defoliation and severity of damage from live, dead and thinned or salvaged trees. For all species studied, significant differences were found between dead trees or trees removed by sanitation cuts and living trees or trees removed by thinning, suggesting that not only dead trees remaining in the forest, but also trees removed by sanitation cuts, should be considered when calculating mortality in managed stands. We also recommend the use of survival analysis in forest monitoring as a routine method for assessing the health of stands.

Significant increase in natural disturbance impacts on European forests since 1950

Over the last decades, the natural disturbance is increasingly putting pressure on European forests. Shifts in disturbance regimes may compromise forest functioning and the continuous provisioning of ecosystem services to society, including their climate change mitigation potential. Although forests are central to many European policies, we lack the long-term empirical data needed for thoroughly understanding disturbance dynamics, modeling them, and developing adaptive management strategies. Here, we present a unique database of >170,000 records of ground-based natural disturbance observations in European forests from 1950 to 2019. Reported data confirm a significant increase in forest disturbance in 34 European countries, causing on an average of 43.8 million m³ of disturbed timber volume per year over the 70-year study period. This value is likely a conservative estimate due to under-reporting, especially of small-scale disturbances. We used machine learning techniques for assessing the magnitude of unreported disturbances, which are estimated to be between 8.6 and 18.3 million m³ /year. In the last 20 years, disturbances on average accounted for 16% of the mean annual harvest in Europe. Wind was the most important disturbance agent over the study period (46% of total damage), followed by fire (24%) and bark beetles (17%). Bark beetle disturbance doubled its share of the total damage in the last 20 years. Forest disturbances can profoundly impact ecosystem services (e.g., climate change mitigation), affect regional forest resource provisioning and consequently disrupt long-term management planning objectives and timber markets. We conclude that adaptation to changing disturbance regimes must be placed at the core of the European forest management and policy debate. Furthermore, a coherent and homogeneous monitoring system of natural disturbances is urgently needed in Europe, to better observe and respond to the ongoing changes in forest disturbance regimes.

Interannual adjustments in stomatal and leaf morphological traits of European beech (Fagus sylvatica L.) demonstrate its climate change acclimation potential

The current projections of climate change might exceed the ability of European forest trees to adapt to upcoming environmental conditions. However, stomatal and leaf morphological traits could greatly influence the acclimation potential of forest tree species subjected to global warming, including the single most important forestry species in Europe, European beech. We analysed stomatal (guard cell length, stomatal density and potential conductance index) and leaf (leaf area, leaf dry weight and leaf mass per area) morphological traits of ten provenances from two provenance trials with contrasting climates between 2016 and 2020. The impact of meteorological conditions of the current and preceding year on stomatal and leaf traits was tested by linear and quadratic regressions. Ecodistance was used to capture the impact of adaptation after the transfer of provenances to new environments. Interactions of trial-provenance and trial-year factors were significant for all measured traits. Guard cell length was lowest and stomatal density was highest across beech provenances in the driest year, 2018. Adaptation was also reflected in a significant relationship between aridity ecodistance and measured traits. Moreover, the meteorological conditions of the preceding year affected the interannual variability of stomatal and leaf traits more than the meteorological conditions of the spring of the current year, suggesting the existence of plant stress memory. High intraspecific variability of stomatal and leaf traits controlled by the interaction of adaptation, acclimation and plant memory suggests a high acclimation potential of European beech provenances under future conditions of global climate change.

Summer temperatures reach the thermal tolerance threshold of photosynthetic decline in temperate conifers

Climate change-related environmental stress has been recognized as a driving force in accelerating forest mortality over the last decades in Central Europe. Here, we aim to elucidate the thermal sensitivity of three native conifer species, namely Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver fir (Abies alba), and three non-native species, namely Austrian pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii) and Atlas cedar (Cedrus atlantica). Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (F_v /F_m ) with increasing temperature, was measured under varying levels of heat stress and compared with the turgor loss point (π_tlp ) as a drought resistance trait. We calculated three different leaf thermotolerance traits: the temperature at the onset (5%) of the F_v /F_m decline (T5), the temperature at which F_v /F_m was half the maximum value (T50) and the temperature at which only 5% F_v /F_m remained (T95). T5 ranged from 38.5 ± 0.8 °C to 43.1 ± 0.6 °C across all species, while T50 values were at least 9 to 11 degrees above the maximum air temperatures on record for all species. Only Austrian pine had a notably higher T5 value than recorded maximum air temperatures. Species with higher T5 values were characterized by a less negative π_tlp compared to species with lower T5. The six species could be divided into 'drought-tolerant heat-sensitive' and 'drought-sensitive heat-tolerant' groups. Exposure to short-term high temperatures thus exhibits a considerable threat to conifer species in Central European forest production systems.

Multiscale spatial analysis of headwater vulnerability in South-Central Chile reveals a high threat due to deforestation and climate change

Headwaters represent an essential component of hydrological, ecological, and socioeconomical systems, by providing constant water streams to the complete basin. However, despite the high importance of headwaters, there is a lack of vulnerability assessments worldwide. Identifying headwaters and their vulnerability in a spatially explicit manner can enable restauration and conservation programs. In this study, we assess the vulnerability of headwaters in South-Central Chile (38.4 to 43.2°S) considering multiple degradation factors related to climate change and land cover change. We analyzed 2292 headwaters, characterizing multiple factors at five spatial scales by using remote sensing data related to Land Use and Cover Change (LUCC), human disturbances, vegetation cover, climate change, potential water demand, and physiography. We then generated an index of vulnerability by integrating all the analyzed variables, which allowed us to map the spatial distribution of headwater vulnerability. Finally, to estimate the main drivers of degradation, we performed a Principal Components Analysis with an Agglomerative Hierarchical Clustering, that allowed us to group headwaters according to the analyzed factors. The largest proportion of most vulnerable headwaters are located in the north of our study area with 48.1 %, 62.1 %, and 28.1 % of headwaters classified as highly vulnerable at 0, 10, and 30 m scale, respectively. The largest proportion of headwaters are affected by Climate Change (63.66 %) and LUCC (23.02 %) on average across all scales. However, we identified three clusters, in which the northern cluster is mainly affected by LUCC, while the Andean and Coastal clusters are mainly affected by climate change. Our results and methods present an informative picture of the current state of headwater vulnerability, identifying spatial patterns and drivers at multiple scales. We believe that the approach developed in this study could be useful for new studies in other zones of the world and can also promote Chilean headwater conservation.

Tree crown damage and its effects on forest carbon cycling in a tropical forest

Crown damage can account for over 23% of canopy biomass turnover in tropical forests and is a strong predictor of tree mortality; yet, it is not typically represented in vegetation models. We incorporate crown damage into the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), to evaluate how lags between damage and tree recovery or death alter demographic rates and patterns of carbon turnover. We represent crown damage as a reduction in a tree's crown area and leaf and branch biomass, and allow associated variation in the ratio of aboveground to belowground plant tissue. We compare simulations with crown damage to simulations with equivalent instant increases in mortality and benchmark results against data from Barro Colorado Island (BCI), Panama. In FATES, crown damage causes decreases in growth rates that match observations from BCI. Crown damage leads to increases in carbon starvation mortality in FATES, but only in configurations with high root respiration and decreases in carbon storage following damage. Crown damage also alters competitive dynamics, as plant functional types that can recover from crown damage outcompete those that cannot. This is a first exploration of the trade-off between the additional complexity of the novel crown damage module and improved predictive capabilities. At BCI, a tropical forest that does not experience high levels of disturbance, both the crown damage simulations and simulations with equivalent increases in mortality does a reasonable job of capturing observations. The crown damage module provides functionality for exploring dynamics in forests with more extreme disturbances such as cyclones and for capturing the synergistic effects of disturbances that overlap in space and time.

Climate change-associated multifactorial stress combination: A present challenge for our ecosystems

Humans negatively influence Earth ecosystems and biodiversity causing global warming, climate change as well as man-made pollution. Recently, the number of different stress factors have increased, and when impacting simultaneously, the multiple stress conditions cause dramatic declines in plant and ecosystem health. Although much is known about how plants and ecosystems are affected by each individual stress, recent research efforts have diverted into how these biological systems respond to several of these stress conditions applied together. Studies of such "multifactorial stress combination" concept have reported a severe decrease in plant survival and microbiome biodiversity along the increasing number of factors in a consistent directional trend. In addition, these results are in concert with studies about how ecosystems and microbiota are affected by natural conditions imposed by climate change. Therefore, all this evidence should serve as an important warning in order to decrease pollutants, create strategies to deal with global warming, and increase the tolerance of plants to multiple stressful factors in combination. Here we review recent studies focused on the impact of abiotic stresses on plants, agrosystems and different ecosystems including forests and microecosystems. In addition, different strategies to mitigate the impact of climate change in ecosystems are discussed.

Forest tree species distribution for Europe 2000-2020: mapping potential and realized distributions using spatiotemporal machine learning

This article describes a data-driven framework based on spatiotemporal machine learning to produce distribution maps for 16 tree species (Abies alba Mill., Castanea sativa Mill., Corylus avellana L., Fagus sylvatica L., Olea europaea L., Picea abies L. H. Karst., Pinus halepensis Mill., Pinus nigra J. F. Arnold, Pinus pinea L., Pinus sylvestris L., Prunus avium L., Quercus cerris L., Quercus ilex L., Quercus robur L., Quercus suber L. and Salix caprea L.) at high spatial resolution (30 m). Tree occurrence data for a total of three million of points was used to train different algorithms: random forest, gradient-boosted trees, generalized linear models, k-nearest neighbors, CART and an artificial neural network. A stack of 305 coarse and high resolution covariates representing spectral reflectance, different biophysical conditions and biotic competition was used as predictors for realized distributions, while potential distribution was modelled with environmental predictors only. Logloss and computing time were used to select the three best algorithms to tune and train an ensemble model based on stacking with a logistic regressor as a meta-learner. An ensemble model was trained for each species: probability and model uncertainty maps of realized distribution were produced for each species using a time window of 4 years for a total of six distribution maps per species, while for potential distributions only one map per species was produced. Results of spatial cross validation show that the ensemble model consistently outperformed or performed as good as the best individual model in both potential and realized distribution tasks, with potential distribution models achieving higher predictive performances (TSS = 0.898, R2 logloss = 0.857) than realized distribution ones on average (TSS = 0.874, R2 logloss = 0.839). Ensemble models for Q. suber achieved the best performances in both potential (TSS = 0.968, R2 logloss = 0.952) and realized (TSS = 0.959, R2 logloss = 0.949) distribution, while P. sylvestris (TSS = 0.731, 0.785, R2 logloss = 0.585, 0.670, respectively, for potential and realized distribution) and P. nigra (TSS = 0.658, 0.686, R2 logloss = 0.623, 0.664) achieved the worst. Importance of predictor variables differed across species and models, with the green band for summer and the Normalized Difference Vegetation Index (NDVI) for fall for realized distribution and the diffuse irradiation and precipitation of the driest quarter (BIO17) being the most frequent and important for potential distribution. On average, fine-resolution models outperformed coarse resolution models (250 m) for realized distribution (TSS = +6.5%, R2 logloss = +7.5%). The framework shows how combining continuous and consistent Earth Observation time series data with state of the art machine learning can be used to derive dynamic distribution maps. The produced predictions can be used to quantify temporal trends of potential forest degradation and species composition change.

Determining the role of climate change in India's past forest loss

Tropical forests in India have declined at an alarming rate over the past century, with extensive literature focusing on the high contributions of agricultural expansions to deforestation, while the effects of climate change have largely been overlooked. Climate change effects, such as increasing temperatures, drought and flooding, have already occurred, and are projected to worsen. Climate velocity, a metric that accounts for spatial heterogeneity in climate, can help identify contiguous areas under greater climate stress and potential climate refuges in addition to traditional temporal trends. Here, we examined the relative contribution of climate changes to forest loss in India during the period 2001-2018, at two spatial (regional and national) and two temporal (seasonal and annual) scales. This includes, for the first time, a characterization of climate velocity in the country. Our findings show that annual forest loss increased substantially over the 17-year period examined (2001-2018), with the majority of forest loss occurring in the Northeast region. Decreases in temporal trends of temperature and precipitation were most associated with forest losses, but there was large spatial and seasonal variation in the relationship. In every region except the Northeast, forest losses were correlated with faster velocities of at least one climate variable but overlapping areas of high velocities were rare. Our findings indicate that climate changes have played an important role in India's past forest loss, but likely remain secondary to other factors at present. We stress concern for climates velocities recorded in the country, reaching 97 km year-1 , and highlight that understanding the different regional and seasonal relationships between climatic conditions and forest distributions will be key to effective protection of the country's remaining forests as climate change accelerates.

Distinguishing Abrupt and Gradual Forest Disturbances With MODIS-Based Phenological Anomaly Series

Capturing forest disturbances over time is increasingly important to determine the ecosystem's capacity to recover as well as aiding a timely response of foresters. With changes due to climate change increasing the frequencies, a better understanding of forest disturbances and their role in historical development is needed to, on the one hand, develop forest management approaches promoting ecosystem resilience and, on the other hand, provide quick and spatially explicit information to foresters. A large, publicly available satellite imagery spanning more than two decades for large areas of the Earth's surface at varying spatial and temporal resolutions represents a vast, free data source for this. The challenge is 2-fold: (1) obtaining reliable information on forest condition and development from satellite data requires not only quantification of forest loss but rather a differentiated assessment of the extent and severity of forest degradation; (2) standardized and efficient processing routines both are needed to bridge the gap between remote-sensing signals and conventional forest condition parameters to enable forest managers for the operational use of the data. Here, we investigated abiotic and biotic disturbances based on a set of ground validated occurrences in various forest areas across Germany to build disturbance response chronologies and examine event-specific patterns. The proposed workflow is based on the R-package "npphen" for non-parametric vegetation phenology reconstruction and anomaly detection using MODIS EVI time series data. Results show the potential to detect distinct disturbance responses in forest ecosystems and reveal event-specific characteristics. Difficulties still exist for the determination of, e.g., scattered wind throw, due to its subpixel resolution, especially in highly fragmented landscapes and small forest patches. However, the demonstrated method shows potential for operational use as a semi-automatic system to augment terrestrial monitoring in the forestry sector. Combining the more robust EVI and the assessment of the phenological series at a pixel-by-pixel level allows for a changing species cover without false classification as forest loss.

Impact of an Extremely Dry Period on Tree Defoliation and Tree Mortality in Serbia

This paper presents research results on forest decline in Serbia. The results were obtained through monitoring defoliation of 34 tree species at 130 sample plots during the period from 2004 to 2018. This research aimed to determine whether the occurrence of defoliation and tree mortality were caused by drought. Defoliation was assessed in 5% steps according to the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) methodology. All the trees recorded as dead were singled out, and annual mortality rates were calculated. To determine changes in air temperature and precipitation regimes during the study period, we processed and analysed climatic data related to air temperature and precipitation throughout the year and in the growing season at 28 main weather stations in Serbia. Tree mortality patterns were established by classifying trees into three groups. The first group of trees exhibited a gradual increase in defoliation during the last few years of monitoring, with dying as the final outcome. The second group was characterised by sudden death of trees. The third group of trees reached a higher degree of defoliation immediately after the first monitoring year, and the trees died after several years. Tree mortality rates were compared between years using the Standardised Precipitation Evaporation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI), the most common methods used to monitor drought. The most intensive forest decline was recorded during the period from 2013 to 2016, when the largest percentage of the total number of all trees died. According to the annual mortality rates calculated for the three observation periods (2004-2008, 2009-2013, and 2014-2018) the highest forest decline rate was recorded in the period from 2014 to 2018, with no statistically significant difference between broadleaved and coniferous tree species. As the sample of coniferous species was small, the number of sample plots should be increased in order to achieve better systematic forest condition monitoring in Serbia. The analysis of the relationship between defoliation and climatic parameters proved the correlation between them. It was noted that the forest decline in Serbia was preceded by an extremely dry period with high temperatures from 2011 to 2013, supporting the hypothesis that it was caused by drought. We therefore conclude that these unfavourable climatic conditions had serious and long-term consequences on forest ecosystems in Serbia.

Relationships between the fecundity of bark beetles and the presence of antagonists

Although previous research has documented the occurrence of antagonists of bark beetles, the studies have only evaluated individual antagonists and have not assessed the overall effect of all antagonists on adult beetles. In this study, we determined which body-cavity antagonists were associated with a reduction in the fecundity and maternal gallery lengths of two important species of bark beetles: Ips typographus on Norway spruce and I. cembrae on European larch. We evaluated these relationships under natural conditions by collecting maternal females in galleries and examining their internal organs. The antagonists in the I. typographus hemolymph had significant negative associations with fecundity and gallery length. These antagonists were mainly nematodes and parasitoids in the hemocoel. In contrast, a positive association between gregarine presence and I. typographus fecundity was found. No antagonist that was likely to significantly alter I. cembrae fecundity or maternal gallery length was proven. Our study provides the first comprehensive assessment of antagonists that may have the potential impact on reduction the fecundity and thereby mass occurrence of these bark beetles.

MASTREE+: Time-series of plant reproductive effort from six continents

Significant gaps remain in understanding the response of plant reproduction to environmental change. This is partly because measuring reproduction in long-lived plants requires direct observation over many years and such datasets have rarely been made publicly available. Here we introduce MASTREE+, a data set that collates reproductive time-series data from across the globe and makes these data freely available to the community. MASTREE+ includes 73,828 georeferenced observations of annual reproduction (e.g. seed and fruit counts) in perennial plant populations worldwide. These observations consist of 5971 population-level time-series from 974 species in 66 countries. The mean and median time-series length is 12.4 and 10 years respectively, and the data set includes 1122 series that extend over at least two decades (≥20 years of observations). For a subset of well-studied species, MASTREE+ includes extensive replication of time-series across geographical and climatic gradients. Here we describe the open-access data set, available as a.csv file, and we introduce an associated web-based app for data exploration. MASTREE+ will provide the basis for improved understanding of the response of long-lived plant reproduction to environmental change. Additionally, MASTREE+ will enable investigation of the ecology and evolution of reproductive strategies in perennial plants, and the role of plant reproduction as a driver of ecosystem dynamics.

Mutually inclusive mechanisms of drought-induced tree mortality

Unprecedented tree dieback across Central Europe caused by recent global change-type drought events highlights the need for a better mechanistic understanding of drought-induced tree mortality. Although numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk. We studied 9435 young trees of 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighborhood competition influence individual mortality risk. Following the most extreme global change-type drought since record in 2018, one third of these trees died. Across species, hydraulic safety margins (HSMs) were negatively and a shift towards a higher sugar fraction in the non-structural carbohydrate (NSC) pool positively associated with mortality risk. Moreover, trees infested by bark beetles had a higher mortality risk, and taller trees a lower mortality risk. Most neighborhood interactions were beneficial, although neighborhood effects were highly species-specific. Species that suffered more from drought, especially Larix spp. and Betula spp., tended to increase the survival probability of their neighbors and vice versa. While severe tissue dehydration marks the final stage of drought-induced tree mortality, we show that hydraulic failure is interrelated with a series of other, mutually inclusive processes. These include shifts in NSC pools driven by osmotic adjustment and/or starch depletion as well as pest infestation and are modulated by the size and species identity of a tree and its neighbors. A more holistic view that accounts for multiple causes of drought-induced tree mortality is required to improve predictions of trends in global forest dynamics and to identify mutually beneficial species combinations.

Compound climate events increase tree drought mortality across European forests

Climate change can lead to the simultaneous occurrence of extreme droughts and heat waves increasing the frequency of compound events with unknown impacts on forests. Here we use two independent datasets, a compiled database of tree drought mortality events and the ICP-Forest level I plots, to study the impacts of the simultaneous occurrence of hot summers, with elevated vapour pressure deficit (VPD), and dry years on forest defoliation and mortality across Europe. We focused on tree drought mortality and background mortality rates, and we studied their co-occurrence with compound events of hot summers and dry years. In total, 143 out of 310 mortality events across Europe, i.e. 46% of cases, corresponded with rare compound events characterized by hot summers and dry years. Over the past decades, summer temperature increased in most sites and severe droughts resulted in compound events not observed before the 1980s. From the ICP-Forest plots we identified 291 (1718 trees) and 61 plots (128 trees) where severe defoliation and mortality, respectively, were caused by drought. The analyses of these events showed that 34% and 27% of the defoliation and mortality cases corresponded with rare compound climate events, respectively. Background mortality rates across Europe in the period 1993-2013 presented higher values in regions where summer temperature and VPD more steeply rose, where drought frequency increased. The steady increase in summer temperatures and VPD in Southern and Eastern Europe may favor the occurrence of compound events of hot summers and dry conditions. Giving that both, local and intense tree drought mortality events and background forest mortality rates, are linked to such compound events we can expect an increase in forest drought mortality in these European regions over the next decades.

Using historical spy satellite photographs and recent remote sensing data to identify high-conservation-value forests

High-conservation-value forests (HCVFs) are critically important for biodiversity and ecosystem service provisioning, but they face many threats. Where systematic HCVF inventories are missing, such as in parts of Eastern Europe, these forests remain largely unacknowledged and therefore often unprotected. We devised a novel, transferable approach for detecting HCVFs based on integrating historical spy satellite images, contemporary remote sensing data (Landsat), and information on current potential anthropogenic pressures (e.g., road infrastructure, population density, demand for fire wood, terrain). We applied the method to the Romanian Carpathians, for which we mapped forest continuity (1955-2019), canopy structural complexity, and anthropogenic pressures. We identified 738,000 ha of HCVF. More than half of this area was identified as susceptible to current anthropogenic pressures and lacked formal protection. By providing a framework for broad-scale HCVF monitoring, our approach facilitates integration of HCVF into forest conservation and management. This is urgently needed to achieve the goals of the European Union's Biodiversity Strategy to maintain valuable forest ecosystems.

Beetle diversity is higher in sunny forests due to higher microclimatic heterogeneity in deadwood

Microclimate is a crucial driver of saproxylic beetle assemblages, with more species often found in sunny forests than in shady ones. Whether this pattern is caused by a higher detectability due to increased beetle activity under sunny conditions or a greater diversity of beetles emerging from sun-exposed deadwood remains unclear. This study examined whether sun exposure leads to higher microclimatic heterogeneity in deadwood and whether this drives beetle diversity in deadwood logs and at forest stand scale. Saproxylic beetles were sampled at the stand scale using flight-interception traps and at object scale using stem-emergence traps on deadwood logs at the same site. The variability in wood surface temperature was measured on single logs and between logs as a proxy for microclimatic heterogeneity in deadwood. Abundance in sunny forests was higher at the stand scale, and in shady forests at the object scale. The estimated number of species was higher in sunny forests at both scales and correlated positively with temperature variability on single logs and between logs at the stand scale and, albeit weakly, with temperature variability on single logs at the object scale. Gamma-diversity, and thus beta-diversity, across logs at the object scale was higher in sunny forests. These findings indicate that sun exposure promotes saproxylic beetle diversity due to higher microclimatic heterogeneity within and between deadwood logs. Our study therefore corroborates previous research demonstrating the importance of canopy cover and microclimate for forest biodiversity.

Climate-induced tree senescence leads to a transient increase in reproductive success of a large woodpecker species

Climate change-induced mortality of trees is a concerning phenomenon for global forest ecosystems. The rapid decay and death of long-lived trees can significantly impact forest dynamics, with effects that transmit through ecological networks, becoming more evident in organisms occupying high trophic levels, such as large and specialized woodpecker species. However, understanding how populations of high trophic level species respond to climate change is still a challenge. In this study it was analyzed 32-year data of social groups of the Magellanic Woodpecker (Campephilus magellanicus) in North Patagonia, a region facing increasingly frequent droughts and increased temperatures. A positive trend in the size of woodpecker social groups as a response to climate-induced tree senescence was tested. A causal structural equation model examining climate- tree senescence- woodpecker relationships was used. Increasing nonlinear trends and positive interannual growth rates (>10%) for tree senescence and group size were found. Lowland forest sites had higher levels of tree senescence and more numerous social groups. The causal model supported the positive effect of mean temperature on tree senescence and the positive association of woodpeckers with tree senescence. These results provide evidence of a climate-induced increase in tree senescence that causes an increase in the size of woodpecker social groups. It is suggested that accelerated decay and mortality of trees in the northern Patagonian forests will decrease the stocks of deadwood in the long term, threatening the persistence of this large woodpecker species.

Fungal fruit body assemblages are tougher in harsh microclimates

Forest species are affected by macroclimate, however, the microclimatic variability can be more extreme and change through climate change. Fungal fruiting community composition was affected by microclimatic differences. Here we ask whether differences in the fruiting community can be explained by morphological traits of the fruit body, which may help endure harsh conditions. We used a dead wood experiment and macrofungal fruit body size, color, and toughness. We exposed logs of two host tree species under closed and experimentally opened forest canopies in a random-block design for four years and identified all visible fruit bodies of two fungal lineages (Basidio- and Ascomycota). We found a consistently higher proportion of tough-fleshed species in harsher microclimates under open canopies. Although significant, responses of community fruit body size and color lightness were inconsistent across lineages. We suggest the toughness-protection hypothesis, stating that tough-fleshed fruit bodies protect from microclimatic extremes by reducing dehydration. Our study suggests that the predicted increase of microclimatic harshness with climate change will likely decrease the presence of soft-fleshed fruit bodies. Whether harsh microclimates also affect the mycelium of macrofungi with different fruit body morphology would complement our findings and increase predictability under climate change.

A First Assessment of Canopy Cover Loss in Germany’s Forests after the 2018–2020 Drought Years

Central Europe was hit by several unusually strong periods of drought and heat between 2018 and 2020. These droughts affected forest ecosystems. Cascading effects with bark beetle infestations in spruce stands were fatal to vast forest areas in Germany. We present the first assessment of canopy cover loss in Germany for the period of January 2018–April 2021. Our approach makes use of dense Sentinel-2 and Landsat-8 time-series data. We computed the disturbance index (DI) from the tasseled cap components brightness, greenness, and wetness. Using quantiles, we generated monthly DI composites and calculated anomalies in a reference period (2017). From the resulting map, we calculated the canopy cover loss statistics for administrative entities. Our results show a canopy cover loss of 501,000 ha for Germany, with large regional differences. The losses were largest in central Germany and reached up to two-thirds of coniferous forest loss in some districts. Our map has high spatial (10 m) and temporal (monthly) resolution and can be updated at any time.

An Improved Forest Structure Data Set for Europe

Today, European forests face many challenges but also offer opportunities, such as climate change mitigation, provision of renewable resources, energy and other ecosystem services. Large-scale analyses to assess these opportunities are hindered by the lack of a consistent, spatial and accessible forest structure data. This study presents a freely available pan-European forest structure data set. Building on our previous work, we used data from six additional countries and consider now ten key forest stand variables. Harmonized inventory data from 16 European countries were used in combination with remote sensing data and a gap-filling algorithm to produce this consistent and comparable forest structure data set across European forests. We showed how land cover data can be used to scale inventory data to a higher resolution which in turn ensures a consistent data structure across sub-regional, country and European forest assessments. Cross validation and comparison with published country statistics of the Food and Agriculture Organization (FAO) indicate that the chosen methodology is able to produce robust and accurate forest structure data across Europe, even for areas where no inventory data were available.

Detection of Bark Beetle Disturbance at Tree Level Using UAS Multispectral Imagery and Deep Learning

This study aimed to examine the potential of convolutional neural networks (CNNs) for the detection of individual trees infested by bark beetles in a multispectral high-resolution dataset acquired by an unmanned aerial system (UAS). We compared the performance of three CNN architectures and the random forest (RF) model to classify the trees into four categories: pines, sbbd (longer infested trees when needles turn yellow), sbbg (trees under green attack) and non-infested trees (sh). The best performance was achieved by the Nez4c3b CNN (kappa 0.80) and Safaugu4c3b CNN (kappa 0.76) using only RGB bands. The main misclassifications were between sbbd and sbbg because of the similar spectral responses. Merging sbbd and sbbg into a more general class of infested trees made the selection of model type less important. All tested model types, including RF, were able to detect infested trees with an F-score of the class over 0.90. Nevertheless, the best overall metrics were achieved again by the Safaugu3c3b model (kappa 0.92) and Nez3cb model (kappa 0.87) using only RGB bands. The performance of both models is comparable, but the Nez model has a higher learning rate for this task. Based on our findings, we conclude that the Nez and Safaugu CNN models are superior to the RF models and transfer learning models for the identification of infested trees and for distinguishing between different infestation stages. Therefore, these models can be used not only for basic identification of infested trees but also for monitoring the development of bark beetle disturbance.

Medium- (MR) and Very-High-Resolution (VHR) Image Integration through Collect Earth for Monitoring Forests and Land-Use Changes: Global Forest Survey (GFS) in the Temperate FAO Ecozone in Europe (2000–2015)

Monitoring of land use, land-use changes, and forestry (LULUCF) plays a crucial role in biodiversity and global environmental challenges. In 2015, the Food and Agriculture Organization of the United Nations (FAO) launched the Global Forest Survey (GFS) integrating medium- (MR) and very-high-resolution (VHR) images through the FAO’s Collect Earth platform. More than 11,150 plots were inventoried in the Temperate FAO ecozone in Europe to monitor LULUCF from 2000 to 2015. As a result, 2.19% (VHR) to 2.77% (MR/VHR) of the study area underwent LULUCF, including a 0.37% (VHR) to 0.43% (MR/VHR) net increase in forest lands. Collect Earth and VHR images have also (i) allowed for shaping a preliminary structure of the land-use network, showing that cropland was the land type that changed most and that cropland and grassland were the more frequent land uses that generated new forest land, (ii) shown that, in 2015, mixed and monospecific forests represented 44.3% and 46.5% of the forest land, respectively, unlike other forest sources, and (iii) shown that 14.9% of the area had been affected by disturbances, particularly wood harvesting (67.47% of the disturbed forests). According to other authors, the area showed a strong correlation between canopy mortality and reported wood removals due to the transition from past clear-cut systems to “close-to-nature” silviculture.

Altered growth conditions more than reforestation counteracted forest biomass carbon emissions 1990-2020

Understanding the carbon (C) balance in global forest is key for climate-change mitigation. However, land use and environmental drivers affecting global forest C fluxes remain poorly quantified. Here we show, following a counterfactual modelling approach based on global Forest Resource Assessments, that in 1990-2020 deforestation is the main driver of forest C emissions, partly counteracted by increased forest growth rates under altered conditions: In the hypothetical absence of changes in forest (i) area, (ii) harvest or (iii) burnt area, global forest biomass would reverse from an actual cumulative net C source of c. 0.74 GtC to a net C sink of 26.9, 4.9 and 0.63 GtC, respectively. In contrast, (iv) without growth rate changes, cumulative emissions would be 7.4 GtC, i.e., 10 times higher. Because this sink function may be discontinued in the future due to climate-change, ending deforestation and lowering wood harvest emerge here as key climate-change mitigation strategies.

The Role of Remote Sensing for the Assessment and Monitoring of Forest Health: A Systematic Evidence Synthesis

Forests are increasingly subject to a number of disturbances that can adversely influence their health. Remote sensing offers an efficient alternative for assessing and monitoring forest health. A myriad of methods based upon remotely sensed data have been developed, tailored to the different definitions of forest health considered, and covering a broad range of spatial and temporal scales. The purpose of this review paper is to identify and analyse studies that addressed forest health issues applying remote sensing techniques, in addition to studying the methodological wealth present in these papers. For this matter, we applied the PRISMA protocol to seek and select studies of our interest and subsequently analyse the information contained within them. A final set of 107 journal papers published between 2015 and 2020 was selected for evaluation according to our filter criteria and 20 selected variables. Subsequently, we pair-wise exhaustively read the journal articles and extracted and analysed the information on the variables. We found that (1) the number of papers addressing this issue have consistently increased, (2) that most of the studies placed their study area in North America and Europe and (3) that satellite-borne multispectral sensors are the most commonly used technology, especially from Landsat mission. Finally, most of the studies focused on evaluating the impact of a specific stress or disturbance factor, whereas only a small number of studies approached forest health from an early warning perspective.

Storm and fire disturbances in Europe: Distribution and trends

Abiotic forest disturbances are an important driver of ecosystem dynamics. In Europe, storms and fires have been identified as the most important abiotic disturbances in the recent past. Yet, how strongly these agents drive local disturbance regimes compared to other agents (e.g., biotic, human) remains unresolved. Furthermore, whether storms and fires are responsible for the observed increase in forest disturbances in Europe is debated. Here, we provide quantitative evidence for the prevalence of storm and fire disturbances in Europe 1986-2016. For 27 million disturbance patches mapped from satellite data, we determined whether they were caused by storm or fire, using a random forest classifier and a large reference dataset of true disturbance occurrences. We subsequently analyzed patterns of disturbance prevalence (i.e., the share of an agent on the overall area disturbed) in space and time. Storm- and fire-related disturbances each accounted for approximately 7% of all disturbances recorded in Europe in the period 1986-2016. Storm-related disturbances were most prevalent in western and central Europe, where they locally accounted for >50% of all disturbances, but we also identified storm-related disturbances in south-eastern and eastern Europe. Fire-related disturbances were a major disturbance agent in southern and south-eastern Europe, but fires also occurred in eastern and northern Europe. The prevalence and absolute area of storm-related disturbances increased over time, whereas no trend was detected for fire-related disturbances. Overall, we estimate an average of 127,716 (97,680-162,725) ha of storm-related disturbances per year and an average of 141,436 (107,353-181,022) ha of fire-related disturbances per year. We conclude that abiotic disturbances caused by storm and fire are important drivers of forest dynamics in Europe, but that their influence varies substantially by region. Our analysis further suggests that increasing storm-related disturbances are an important driver of Europe's changing forest disturbance regimes.

Transcriptional response of mushrooms to artificial sun exposure

Climate change causes increased tree mortality leading to canopy loss and thus sun-exposed forest floors. Sun exposure creates extreme temperatures and radiation, with potentially more drastic effects on forest organisms than the current increase in mean temperature. Such conditions might potentially negatively affect the maturation of mushrooms of forest fungi. A failure of reaching maturation would mean no sexual spore release and, thus, entail a loss of genetic diversity. However, we currently have a limited understanding of the quality and quantity of mushroom-specific molecular responses caused by sun exposure. Thus, to understand the short-term responses toward enhanced sun exposure, we exposed mushrooms of the wood-inhabiting forest species Lentinula edodes, while still attached to their mycelium and substrate, to artificial solar light (ca. 30°C and 100,000 lux) for 5, 30, and 60 min. We found significant differentially expressed genes at 30 and 60 min. Eukaryotic Orthologous Groups (KOG) class enrichment pointed to defense mechanisms. The 20 most significant differentially expressed genes showed the expression of heat-shock proteins, an important family of proteins under heat stress. Although preliminary, our results suggest mushroom-specific molecular responses to tolerate enhanced sun exposure as expected under climate change. Whether mushroom-specific molecular responses are able to maintain fungal fitness under opening forest canopies remains to be tested.

Application of Low-Cost MEMS Spectrometers for Forest Topsoil Properties Prediction

Increasing temperatures and drought occurrences recently led to soil moisture depletion and increasing tree mortality. In the interest of sustainable forest management, the monitoring of forest soil properties will be of increasing importance in the future. Vis-NIR spectroscopy can be used as fast, non-destructive and cost-efficient method for soil parameter estimations. Microelectromechanical system devices (MEMS) have become available that are suitable for many application fields due to their low cost as well as their small size and weight. We investigated the performance of MEMS spectrometers in the visual and NIR range to estimate forest soil samples total C and N content of Ah and Oh horizons at the lab. The results were compared to a full-range device using PLSR and Cubist regression models at local (2.3 ha, n: Ah = 60, Oh = 50) and regional scale (State of Saxony, Germany, 184,000 km2, n: Ah = 186 and Oh = 176). For each sample, spectral reflectance was collected using MEMS spectrometer in the visual (Hamamatsu C12880MA) and NIR (NeoSpetrac SWS62231) range and using a conventional full range device (Veris Spectrophotometer). Both data sets were split into a calibration (70%) and a validation set (30%) to evaluate prediction power. Models were calibrated for Oh and Ah horizon separately for both data sets. Using the regional data, we also used a combination of both horizons. Our results show that MEMS devices are suitable for C and N prediction of forest topsoil on regional scale. On local scale, only models for the Ah horizon yielded sufficient results. We found moderate and good model results using MEMS devices for Ah horizons at local scale (R2≥ 0.71, RPIQ ≥ 2.41) using Cubist regression. At regional scale, we achieved moderate results for C and N content using data from MEMS devices in Oh (R2≥ 0.57, RPIQ ≥ 2.42) and Ah horizon (R2≥ 0.54, RPIQ ≥2.15). When combining Oh and Ah horizons, we achieved good prediction results using the MEMS sensors and Cubist (R2≥ 0.85, RPIQ ≥ 4.69). For the regional data, models using data derived by the Hamamatsu device in the visual range only were least precise. Combining visual and NIR data derived from MEMS spectrometers did in most cases improve the prediction accuracy. We directly compared our results to models based on data from a conventional full range device. Our results showed that the combination of both MEMS devices can compete with models based on full range spectrometers. MEMS approaches reached between 68% and 105% of the corresponding full ranges devices R2 values. Local models tended to be more accurate than regional approaches for the Ah horizon. Our results suggest that MEMS spectrometers are suitable for forest soil C and N content estimation. They can contribute to improved monitoring in the future as their small size and weight could make in situ measurements feasible.

Stand-scale climate change impacts on forests over large areas: transient responses and projection uncertainties

The increasing impacts of climate change on forest ecosystems have triggered multiple model-based impact assessments for the future, which typically focused either on a small number of stand-scale case studies or on large scale analyses (i.e., continental to global). Therefore, substantial uncertainty remains regarding the local impacts over large areas (i.e., regions to countries), which is particularly problematic for forest management. We provide a comprehensive, high-resolution assessment of the climate change sensitivity of managed Swiss forests (~10,000 km2 ), which cover a wide range of environmental conditions. We used a dynamic vegetation model to project the development of typical forest stands derived from a stratification of the Third National Forest Inventory until the end of the 22nd century. Two types of simulations were conducted: one limited to using the extant local species, the other enabling immigration of potentially more climate-adapted species. Moreover, to assess the robustness of our projections, we quantified and decomposed the uncertainty in model projections resulting from the following sources: (1) climate change scenarios, (2) local site conditions, and (3) the dynamic vegetation model itself (i.e., represented by a set of model versions), an aspect hitherto rarely taken into account. The simulations showed substantial changes in basal area and species composition, with dissimilar sensitivity to climate change across and within elevation zones. Higher-elevation stands generally profited from increased temperature, but soil conditions strongly modulated this response. Low-elevation stands were increasingly subject to drought, with strong negative impacts on forest growth. Furthermore, current stand structure had a strong effect on the simulated response. The admixture of drought-tolerant species was found advisable across all elevations to mitigate future adverse climate-induced effects. The largest uncertainty in model projections was associated with climate change scenarios. Uncertainty induced by the model version was generally largest where overall simulated climate change impacts were small, thus corroborating the utility of the model for making projections into the future. Yet, the large influence of both site conditions and the model version on some of the projections indicates that uncertainty sources other than climate change scenarios need to be considered in climate change impact assessments.

Global meta-analysis of tree decline impacts on fauna

Significant portions of the world's forests have been impacted by severe and large-scale tree declines characterised by gradual but widespread loss of vigour and subsequent death of either single or several tree species. Tree deaths represent a threat for fauna that are dependent on forest habitats for their survival. Although tree declines have received considerable scientific attention, surprisingly, little is known about their impacts on fauna. In total, we calculated 631 effect sizes across 59 studies that quantified the impact of tree declines on animal abundance. Data representing 186 bird species indicated an overall increase in bird abundance in response to tree declines (meta-analysis mean ± estimation g = 0.172 ± 0.053 [CI 0.069 to 0.275], P = 0.001); however, there was substantial variability in responses (significant heterogeneity P < 0.001) with a strong influence of diet as well as nesting guild on bird responses. Granivores (especially ground-foraging species, e.g. Passerellidae species), bark-foraging insectivores (e.g. woodpeckers), as well as ground- and cavity-nesting species apparently benefitted from tree declines, while nectarivorous birds [and, although not significant, aerially foraging insectivores (e.g. flycatchers) and leaf-gleaning insectivores (canopy-feeding)] were less common in the presence of tree declines. Data representing 33 mammal species indicate a tendency for detrimental effects of tree declines on mammals that use trees as refuges, while aerial foragers (i.e. bats) may benefit from opening up the canopy. Overall the average effect for mammals was neutral (meta-analysis mean estimation g = -0.150 ± 0.145 [-0.433 to 0.134], P = 0.302). Data representing 20 reptile species showed an insufficient range of responses to determine any diet or foraging effect on their responses. Data for 28 arthropod taxa should be considered with caution, as we could not adequately separate taxa according to their specialisations and reliance on key habitat. The data broadly suggest a detrimental effect of tree declines (meta-analysis mean estimation g = -0.171 ± 0.072 [-0.311 to -0.031], P = 0.017) with ground-foraging arthropods (e.g. detritivores and predators such as spiders and centipedes) more likely to be detrimentally impacted by tree declines. The range of responses to tree declines signifies substantially altered animal communities. In many instances, altered ecosystem function due to loss of key animal services will represent a significant threat to forest health.

Satellite-based habitat monitoring reveals long-term dynamics of deer habitat in response to forest disturbances

Disturbances play a key role in driving forest ecosystem dynamics, but how disturbances shape wildlife habitat across space and time often remains unclear. A major reason for this is a lack of information about changes in habitat suitability across large areas and longer time periods. Here, we use a novel approach based on Landsat satellite image time series to map seasonal habitat suitability annually from 1986 to 2017. Our approach involves characterizing forest disturbance dynamics using Landsat-based metrics, harmonizing these metrics through a temporal segmentation algorithm, and then using them together with GPS telemetry data in habitat models. We apply this framework to assess how natural forest disturbances and post-disturbance salvage logging affect habitat suitability for two ungulates, roe deer (Capreolus capreolus) and red deer (Cervus elaphus), over 32 yr in a Central European forest landscape. We found that red and roe deer differed in their response to forest disturbances. Habitat suitability for red deer consistently improved after disturbances, whereas the suitability of disturbed sites was more variable for roe deer depending on season (lower during winter than summer) and disturbance agent (lower in windthrow vs. bark-beetle-affected stands). Salvage logging altered the suitability of bark beetle-affected stands for deer, having negative effects on red deer and mixed effects on roe deer, but generally did not have clear effects on habitat suitability in windthrows. Our results highlight long-lasting legacy effects of forest disturbances on deer habitat. For example, bark beetle disturbances improved red deer habitat suitability for at least 25 yr. The duration of disturbance impacts generally increased with elevation. Methodologically, our approach proved effective for improving the robustness of habitat reconstructions from Landsat time series: integrating multiyear telemetry data into single, multi-temporal habitat models improved model transferability in time. Likewise, temporally segmenting the Landsat-based metrics increased the temporal consistency of our habitat suitability maps. As the frequency of natural forest disturbances is increasing across the globe, their impacts on wildlife habitat should be considered in wildlife and forest management. Our approach offers a widely applicable method for monitoring habitat suitability changes caused by landscape dynamics such as forest disturbance.

Emergent vulnerability to climate-driven disturbances in European forests

Forest disturbance regimes are expected to intensify as Earth’s climate changes. Quantifying forest vulnerability to disturbances and understanding the underlying mechanisms is crucial to develop mitigation and adaptation strategies. However, observational evidence is largely missing at regional to continental scales. Here, we quantify the vulnerability of European forests to fires, windthrows and insect outbreaks during the period 1979–2018 by integrating machine learning with disturbance data and satellite products. We show that about 33.4 billion tonnes of forest biomass could be seriously affected by these disturbances, with higher relative losses when exposed to windthrows (40%) and fires (34%) compared to insect outbreaks (26%). The spatial pattern in vulnerability is strongly controlled by the interplay between forest characteristics and background climate. Hotspot regions for vulnerability are located at the borders of the climate envelope, in both southern and northern Europe. There is a clear trend in overall forest vulnerability that is driven by a warming-induced reduction in plant defence mechanisms to insect outbreaks, especially at high latitudes.

Natural disturbances imperil healthy and productive forests, but quantifying their effects at large scales is challenging. Here the authors apply machine learning to disturbance records and satellite data to quantify and map European forest vulnerability to fires, windthrows, and insect outbreaks through 1979-2018.

Do bark beetle outbreaks amplify or dampen future bark beetle disturbances in Central Europe?

Bark beetle outbreaks have intensified in many forests around the globe in recent years. Yet, the legacy of these disturbances for future forest development remains unclear. Bark beetle disturbances are expected to increase further because of climate change. Consequently, feedbacks within the disturbance regime are of growing interest, for example, whether bark beetle outbreaks are amplifying future bark beetle activity (through the initiation of an even-aged cohort of trees) or dampening it (through increased structural and compositional diversity).We studied bark beetle-vegetation-climate interactions in the Bavarian Forest National Park (Germany), an area characterised by unprecedented bark beetle activity in the recent past. We simulated the effect of future bark beetle outbreaks on forest structure and composition and analysed how disturbance-mediated forest dynamics influence future bark beetle activity under different scenarios of climate change. We used process-based simulation modelling in combination with machine learning to disentangle the long-term interactions between vegetation, climate and bark beetles at the landscape scale.Disturbances by the European spruce bark beetle were strongly amplified by climate change, increasing between 59% and 221% compared to reference climate. Bark beetle outbreaks reduced the dominance of Norway spruce (Picea abies (L.) Karst.) on the landscape, increasing compositional diversity. Disturbances decreased structural diversity within stands (α diversity) and increased structural diversity between stands (β diversity). Overall, disturbance-mediated changes in forest structure and composition dampened future disturbance activity (a reduction of up to -67%), but were not able to fully compensate for the amplifying effect of climate change. Synthesis. Our findings indicate that the recent disturbance episode at the Bavarian Forest National Park was caused by a convergence of highly susceptible forest structures with climatic conditions favourable for bark beetle outbreaks. While future climate is increasingly conducive to massive outbreaks, the emerging landscape structure is less and less likely to support them. This study improves our understanding of the long-term legacies of ongoing bark beetle disturbances in Central Europe. It indicates that increased diversity provides an important dampening feedback, and suggests that preventing disturbances or homogenizing post-disturbance forests could elevate the future susceptibility to large-scale bark beetle outbreaks.

Over half of western United States' most abundant tree species in decline

Changing forest disturbance regimes and climate are driving accelerated tree mortality across temperate forests. However, it remains unknown if elevated mortality has induced decline of tree populations and the ecological, economic, and social benefits they provide. Here, we develop a standardized forest demographic index and use it to quantify trends in tree population dynamics over the last two decades in the western United States. The rate and pattern of change we observe across species and tree size-distributions is alarming and often undesirable. We observe significant population decline in a majority of species examined, show decline was particularly severe, albeit size-dependent, among subalpine tree species, and provide evidence of widespread shifts in the size-structure of montane forests. Our findings offer a stark warning of changing forest composition and structure across the western US, and suggest that sustained anthropogenic and natural stress will likely result in broad-scale transformation of temperate forests globally.

Excess forest mortality is consistently linked to drought across Europe

Pulses of tree mortality caused by drought have been reported recently in forests around the globe, but large-scale quantitative evidence is lacking for Europe. Analyzing high-resolution annual satellite-based canopy mortality maps from 1987 to 2016 we here show that excess forest mortality (i.e., canopy mortality exceeding the long-term mortality trend) is significantly related to drought across continental Europe. The relationship between water availability and mortality showed threshold behavior, with excess mortality increasing steeply when the integrated climatic water balance from March to July fell below -1.6 standard deviations of its long-term average. For -3.0 standard deviations the probability of excess canopy mortality was 91.6% (83.8-97.5%). Overall, drought caused approximately 500,000 ha of excess forest mortality between 1987 and 2016 in Europe. We here provide evidence that drought is an important driver of tree mortality at the continental scale, and suggest that a future increase in drought could trigger widespread tree mortality in Europe.

The influence of climate change and canopy disturbances on landslide susceptibility in headwater catchments

Forests have an important regulating function on water runoff and the occurrence of shallow landslides. Their structure and composition directly influence the risk of hydrogeomorphic processes, like floods with high sediment transport or debris flows. Climate change is substantially altering forest ecosystems, and for Central Europe an increase in natural disturbances from wind and insect outbreaks is expected for the future. How such changes impact the regulating function of forest ecosystems remains unclear. By combining methods from forestry, hydrology and geotechnical engineering we investigated possible effects of changing climate and disturbance regimes on shallow landslides. We simulated forest landscapes in two headwater catchments in the Eastern Alps of Austria under four different future climate scenarios over 200 years. Our results indicate that climate-mediated changes in forest dynamics can substantially alter the protective function of forest ecosystems. Climate change generally increased landslide risk in our simulations. Only when future warming coincided with drying landslide risk decreased relative to historic conditions. In depth analyses showed that an important driver of future landslide risk was the simulated vegetation composition. Trajectories away from flat rooting Norway spruce (Picea abies (L.) Karst.) forests currently dominating the system towards an increasing proportion of tree species with heart and taproot systems, increased root cohesion and reduced the soil volume mobilized in landslides. Natural disturbances generally reduced landslide risk in our simulations, with the positive effect of accelerated tree species change and increasing root cohesion outweighing a potential negative effect of disturbances on the water cycle. We conclude that while the efficacy of green infrastructure such as protective forests could be substantially reduced by climate change, such systems also have a strong inherent ability to adapt to changing conditions. Forest management should foster this adaptive capacity to strengthen the protective function of forests also under changing environmental conditions.

Contrasting vulnerability of monospecific and species-diverse forests to wind and bark beetle disturbance: The role of management

Wind and bark beetle disturbances have increased in recent decades, affecting Europe's coniferous forests with particular severity. Management fostering forest diversity and resilience is deemed to effectively mitigate disturbance impacts, yet its efficiency and interaction with other disturbance management measures remain unclear.We focused on Central Europe, which has become one of the hotspots of recent disturbance changes. We used the iLand ecosystem model to understand the interplay between species composition of the forest, forest disturbance dynamics affected by climate change, and disturbance management. The tested measures included (a) active transformation of tree species composition toward site-matching species; (b) intensive removal of windfelled trees, which can support the buildup of bark beetle populations; and (c) reduction of mature and vulnerable trees on the landscape via modified harvesting regimes.We found that management systems aiming to sustain the dominance of Norway spruce in the forest are failing under climate change, and none of the measures applied could mitigate the disturbance impacts. Conversely, management systems fostering forest diversity substantially reduced the level of disturbance. Significant disturbance reduction has been achieved even without salvaging and rotation length reduction, which is beneficial for ecosystem recovery, carbon, and biodiversity. Synthesis and applications: We conclude that climate change amplifies the contrast in vulnerability of monospecific and species-diverse forests to wind and bark beetle disturbance. Whereas forests dominated by Norway spruce are not likely to be sustained in Central Europe under climate change, different management strategies can be applied in species-diverse forests to reach the desired control over the disturbance dynamic. Our findings justify some unrealistic expectations about the options to control disturbance dynamics under climate change and highlight the importance of management that fosters forest diversity.

Reducing rotation age to address increasing disturbances in Central Europe: Potential and limitations

Forest disturbance regimes are intensifying in many parts of the globe. In order to mitigate disturbance impacts a number of management responses have been proposed, yet their effectiveness in addressing changing disturbance regimes remains largely unknown. The strong positive relationship between forest age and the vulnerability to disturbances such as windthrows and bark beetle infestations suggests that a reduced rotation length can be a potent means for mitigating the impacts of natural disturbances. However, disturbance mitigation measures such as shortened rotation lengths (SRL) can also have undesired consequences on ecosystem services and biodiversity, which need to be considered in their application. Here, we used the process-based landscape and disturbance model iLand to investigate the effects of SRL on the vulnerability of a 16,000 ha forest landscape in Central Europe to wind and bark beetle disturbances. We experimentally reduced the current rotation length (between 100 and 115 years) by up to -40% in 10% increments, and studied effects on disturbance dynamics under current and future climate conditions over a 200-year simulation period. Simultaneously, we quantified the collateral effects of SRL on forest carbon stocks and indicators of biodiversity. Shortening the rotation length by 40% decreased disturbances by 14%. This effect was strongly diminished under future climate change, reducing the mitigating effect of shortened rotation to < 6%. Collateral effects were severe in the initial decades after implementation: Reducing the rotation length by 40% caused a spike in harvested timber volume (+ 92%), decreased total forest carbon storage by 6% and reduced the number of large trees on the landscape by 20%. The long-term effects of SRL were less pronounced. At the same time, SRL caused an increase in tree species diversity. Shortening rotation length can reduce the impact of wind and bark beetle disturbances, but the overall efficiency of the measure is limited and decreases under climate change. Given the potential for undesired collateral effects we conclude that a reduction of the rotation length is no panacea for managing increasing disturbances, and should be applied in combination with other management measures reducing risks and fostering resilience.

Long- and Short-Term Inorganic Nitrogen Runoff from a Karst Catchment in Austria

Excess nitrogen (N) deposition and gaseous N emissions from industrial, domestic, and agricultural sources have led to increased nitrate leaching, the loss of biological diversity, and has affected carbon (C) sequestration in forest ecosystems. Nitrate leaching affects the purity of karst water resources, which contribute around 50% to Austria’s drinking water supply. Here we present an evaluation of the drivers of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment in the Austrian Alps (LTER Zöbelboden) from 27 years of records. In addition, a hydrological model was used together with climatic scenario data to predict expected future runoff dynamics. The study area was exposed to increasing N deposition during the 20th century (up to 30 to 35 kg N ha−1 y−1), which are still at levels of 25.5 ± 3.6 and 19.9 ± 4.2 kg N ha−1 y−1 in the spruce and the mixed deciduous forests, respectively. Albeit N deposition was close to or exceeded critical loads for several decades, 70–83% of the inorganic N retained in the catchment from 2000 to 2018, and NO3- concentrations in the runoff stayed <10 mg L−1 unless high-flow events occurred or forest stand-replacing disturbances. We identified tree growth as the main sink for inorganic N, which might together with lower runoff, increase retention of only weakly decreasing N deposition in the future. However, since recurring forest stand-replacement is predicted in the future as a result of a combination of climatically driven disturbance agents, pulses of elevated nitrate concentrations in the catchment runoff will likely add to groundwater pollution.

Effects of disturbance patterns and deadwood on the microclimate in European beech forests

More frequent and severe disturbances increasingly open the forest canopy and initiate tree regeneration. Simultaneously, increasing weather extremes, such as drought and heat, are threatening species adapted to cool and moist climate. The magnitude of the microclimatic buffering capacity of forest canopies to mitigate hot and dry weather conditions and its disturbance-induced reduction remains poorly quantified. Also, the influence of disturbance legacies (e.g., deadwood) on forest microclimate is unresolved. In a unique manipulation experiment we investigated (i) the microclimatic buffering capacity of forest canopies in years with different climatic conditions; (ii) the impacts of spatial disturbance patterns on surface light and microclimate; and (iii) the effect of deadwood presence and type on microclimate. Treatments included two disturbance patterns (i.e., aggregated and distributed), four deadwood types (i.e., standing, downed, standing and downed, removed), and one untreated control (i.e., nine treatments in total), replicated at five sites dominated by European beech (Fagus sylvatica L.) in southeastern Germany. We measured forest floor light conditions and derived diurnal extremes and variation in temperature (T) and vapor pressure deficit (VPD) during four consecutive summer seasons (2016 - 2019). The buffering capacity of intact forest canopies was higher in warm and dry years. Surface light was significantly higher in spatially aggregated disturbance gaps compared to distributed disturbances of similar severity. An increase in surface light by 10 % relative to closed canopies elevated T_max and VPD_max by 0.42°C and 0.04 kPa, respectively. Deadwood presence and type did not affect the forest microclimate significantly. Microclimatic buffering under forest canopies can dampen the effects of climate change. However, increasing canopy disturbances result in more light penetrating the canopy, reducing the microclimatic buffering capacity of forests. We conclude that forest management should foster microclimatic buffering in forests as one element of a multi-pronged strategy to counter climate change.

Evidence of non-stationary relationships between climate and forest responses: Increased sensitivity to climate change in Iberian forests

Climate and forest structure are considered major drivers of forest demography and productivity. However, recent evidence suggests that the relationships between climate and tree growth are generally non-stationary (i.e. non-time stable), and it remains uncertain whether the relationships between climate, forest structure, demography and productivity are stationary or are being altered by recent climatic and structural changes. Here we analysed three surveys from the Spanish Forest Inventory covering c. 30 years of information and we applied mixed and structural equation models to assess temporal trends in forest structure (stand density, basal area, tree size and tree size inequality), forest demography (ingrowth, growth and mortality) and above-ground forest productivity. We also quantified whether the interactive effects of climate and forest structure on forest demography and above-ground forest productivity were stationary over two consecutive time periods. Since the 1980s, density, basal area and tree size increased in Iberian forests, and tree size inequality decreased. In addition, we observed reductions in ingrowth and growth, and increases in mortality. Initial forest structure and water availability mainly modulated the temporal trends in forest structure and demography. The magnitude and direction of the interactive effects of climate and forest structure on forest demography changed over the two time periods analysed indicating non-stationary relationships between climate, forest structure and demography. Above-ground forest productivity increased due to a positive balance between ingrowth, growth and mortality. Despite increasing productivity over time, we observed an aggravation of the negative effects of climate change and increased competition on forest demography, reducing ingrowth and growth, and increasing mortality. Interestingly, our results suggest that the negative effects of climate change on forest demography could be ameliorated through forest management, which has profound implications for forest adaptation to climate change.