Norway spruce at the trailing edge: the effect of landscape configuration and composition on climate resilience

Resilience and vulnerability: distinct concepts to address global change in forests

Resilience and vulnerability are important concepts to understand, anticipate, and manage global change impacts on forest ecosystems. However, they are often used confusingly and inconsistently, hampering a synthetic understanding of global change, and impeding communication with managers and policy-makers. Both concepts are powerful and have complementary strengths, reflecting their different history, methodological approach, components, and spatiotemporal focus. Resilience assessments address the temporal response to disturbance and the mechanisms driving it. Vulnerability assessments focus on spatial patterns of exposure and susceptibility, and explicitly address adaptive capacity and stakeholder preferences. We suggest applying the distinct concepts of resilience and vulnerability where they provide particular leverage, and deduce a number of lessons learned to facilitate the next generation of global change assessments.

A balancing act: Principles, criteria and indicator framework to operationalize social-ecological resilience of forests

Against a background of intensifying climate-induced disturbances, the need to enhance the resilience of forests and forest management is gaining urgency. In forest management, multiple trade-offs exist between different demands as well as across and within temporal and spatial scales. However, methods to assess resilience that consider these trade-offs are presently lacking. Here we propose a hierarchical framework of principles, criteria, and indicators to assess the resilience of a social-ecological system by focusing on the mechanisms behind resilience. This hierarchical framework balances trade-offs between mechanisms, different parts of the social-ecological system, ecosystem services, and spatial as well as temporal scales. The framework was developed to be used in a participatory manner in forest management planning. It accounts for the major parts of the forest-related social-ecological system and considers the multiple trade-offs involved. We demonstrate the utility of the framework by applying it to a landscape dominated by Norway spruce (Picea abies (L.) Karst.) in Central Europe, managed for three different management goals. The framework highlights how forest resilience varies with the pursued management goals and related management strategies. The framework is flexible and can be applied to various forest management contexts as part of a participatory process with stakeholders. It thus is an important step towards operationalizing social-ecological resilience in forest management systems.

Assembly and Annotation of Red Spruce (Picea rubens) Chloroplast Genome, Identification of Simple Sequence Repeats, and Phylogenetic Analysis in Picea

We have sequenced the chloroplast genome of red spruce (Picea rubens) for the first time using the single-end, short-reads (44 bp) Illumina sequences, assembled and functionally annotated it, and identified simple sequence repeats (SSRs). The contigs were assembled using SOAPdenovo2 following the retrieval of chloroplast genome sequences using the black spruce (Picea mariana) chloroplast genome as the reference. The assembled genome length was 122,115 bp (gaps included). Comparatively, the P. rubens chloroplast genome reported here may be considered a near-complete draft. Global genome alignment and phylogenetic analysis based on the whole chloroplast genome sequences of Picea rubens and 10 other Picea species revealed high sequence synteny and conservation among 11 Picea species and phylogenetic relationships consistent with their known classical interrelationships and published molecular phylogeny. The P. rubens chloroplast genome sequence showed the highest similarity with that of P. mariana and the lowest with that of P. sitchensis. We have annotated 107 genes including 69 protein-coding genes, 28 tRNAs, 4 rRNAs, few pseudogenes, identified 42 SSRs, and successfully designed primers for 26 SSRs. Mononucleotide A/T repeats were the most common followed by dinucleotide AT repeats. A similar pattern of microsatellite repeats occurrence was found in the chloroplast genomes of 11 Picea species.

The State of Dark Coniferous Forests on the East European Plain Due to Climate Change

As a result of global climate changes, negative processes have been recorded in the coniferous forests of the Northern Hemisphere. Similar processes are observed in the Urals, including in Udmurtia. In the course of this research, archival analysis methods were used, as well as field research methods. In the process of analyzing archival materials in the Urals, a reduction of spruce forests was observed. If in the 20th century the share of spruce forests in the region was 50%, then in the 21th century it decreased to 35%. As a result of this research, it was revealed that the most unfavorable sanitary condition was recorded in the boreal-subboreal zone of Udmurtia, with a sanitary condition index of 3.2 (from 2.62 to 3.73). The main reason for the unfavorable sanitary condition of spruce forests was the vital activity of Ips typographus L. According to our research, in 11 sample plots out of 18, a high score for sanitary condition was associated with the vital activity of bark beetles. The correlation coefficient of the index of the sanitary condition of plantings and the number of individuals of Picea obovata Ledeb. affected by Ips typographus L. was0.93. Bark beetle activity has increased in the 21th century, which is associated with changing climatic factors. Unstable precipitation over recent years (differences of more than 100 mm) and an average temperature increase of 1.2 °C were observed in the region. The most significant increase in temperature over the past 10 years was observed in winter, which in turn affected the high survival rate of insect pests.

Tree species admixture increases ecosystem service provision in simulated spruce- and beech-dominated stands

Climate-adaptive forest management aims to sustain the provision of multiple forest ecosystem services and biodiversity (ESB). However, it remains largely unknown how changes in adaptive silvicultural interventions affect trade-offs and synergies among ESB in the long term. We used a simulation-based sensitivity analysis to evaluate popular adaptive forest management interventions in representative Swiss low- to mid-elevation beech- and spruce-dominated forest stands. We predicted stand development across the twenty-first century using a novel empirical and temperature-sensitive single-tree forest stand simulator in a fully crossed experimental design to analyse the effects of (1) planting mixtures of Douglas-fir, oak and silver fir, (2) thinning intensity, and (3) harvesting intensity on timber production, carbon storage and biodiversity under three climate scenarios. Simulation results were evaluated in terms of multiple ESB provision, trade-offs and synergies, and individual effects of the adaptive interventions. Timber production increased on average by 45% in scenarios that included tree planting. Tree planting led to pronounced synergies among all ESBs towards the end of the twenty-first century. Increasing the thinning and harvesting intensity affected ESB provision negatively. Our simulations indicated a temperature-driven increase in growth in beech- (+ 12.5%) and spruce-dominated stands (+ 3.7%), but could not account for drought effects on forest dynamics. Our study demonstrates the advantages of multi-scenario sensitivity analysis that enables quantifying effect sizes and directions of management impacts. We showed that admixing new tree species is promising to enhance future ESB provision and synergies among them. These results support strategic decision making in forestry.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10342-022-01474-4.

Will forest dynamics continue to accelerate throughout the 21st century in the Northern Alps?

Observational evidence suggests that forests in the Northern Alps are changing at an increasing rate as a consequence of climate change. Yet, it remains unclear whether the acceleration of forest change will continue in the future, or whether downregulating feedbacks will eventually decouple forest dynamics from climate change. Here we studied future forest dynamics at Berchtesgaden National Park, Germany by means of a process-based forest landscape model, simulating an ensemble of 22 climate projections until the end of the 21st century. Our objectives were (i) to assess whether the observed acceleration of forest dynamics will continue in the future, (ii) to analyze how uncertainty in future climate translates to variation in future forest disturbance, structure, and composition, and (iii) to determine the main drivers of future forest dynamics. We found that forest dynamics continue to accelerate in the coming decades, with a trend towards denser, structurally more complex and more species rich forests. However, changes in forest structure leveled off in the second half of the 21st century regardless of climate scenario. In contrast, climate scenarios caused trajectories of tree species change to diverge in the second half of the 21st century, with stabilization under RCP 2.6 and RCP 4.5 scenarios and accelerated loss of conifers under RCP 8.5. Disturbance projections were 3 to 20 times more variable than future climate, whereas projected future forest structure and composition varied considerably less than climate. Indirect effects of climate change via alterations of the disturbance regime had a stronger impact on future forest dynamics than direct effects. Our findings suggest that dampening feedbacks within forest dynamics will decelerate forest change in the second half of the 21st century. However, warming beyond the levels projected under RCP 4.5 might profoundly alter future forest disturbance and composition, challenging conservation efforts and ecosystem service supply.

Forest Management under Climate Change: A Decision Analysis of Thinning Interventions for Water Services and Biomass in a Norway Spruce Stand in South Germany

Climate change is producing threats to forests’ capacity of regulating water regimes. Therefore, thinning strategies can be applied to mitigate climate change impacts more efficiently by providing more spaces for trees to utilize resources e.g., water and nutrients. This study examined the effects of different thinning intensities and intervals on water characteristics and biomass growth of a 75-year-old Norway spruce (Picea abies) stand in the Black Forest, Germany. Here we used a water and management sensitive update of the process-based forest growth model 3PG, 3PG-Hydro. We applied light (10%), moderate (30%), and heavy thinning (50% intensity) in the interval of 10, 25, and 50 years of the management period. We simulated growth with climate change scenario RCP 8.5 data from 1995 to 2065. We analyzed the effects of the different thinning regimens on biomass, evapotranspiration as well as water yield. Thinning intensity and interval as well as their interaction have significant influence on production of stand biomass and water yield for all thinning regimes applied (p < 0.05). However, there is no significant difference (p > 0.05) in accumulated biomass (thinned biomass added to the stand biomass) between the applied thinning regimes. Light thinning in a long interval (50 years) produced highest stand biomass among the applied thinning regimes. Furthermore, the prediction showed that accumulated water yield increased with increasing thinning intensity. Our study concludes that repeated moderate thinning at intermediate intervals results in a high water yield without losing biomass production.

Arthropod dark taxa provide new insights into diversity responses to bark beetle infestations

Natural disturbances are increasing around the globe, also impacting protected areas. Although previous studies have indicated that natural disturbances result in mainly positive effects on biodiversity, these analyses mostly focused on a few well established taxonomic groups, and thus uncertainty remains regarding the comprehensive impact of natural disturbances on biodiversity. Using Malaise traps and meta-barcoding, we studied a broad range of arthropod taxa, including dark and cryptic taxa, along a gradient of bark beetle disturbance severities in five European national parks. We identified order-level community thresholds of disturbance severity and classified barcode index numbers (BINs; a cluster system for DNA sequences, where each cluster corresponds to a species) as negative or positive disturbance indicators. Negative indicator BINs decreased above thresholds of low to medium disturbance severity (20%-30% of trees killed), whereas positive indicator BINs benefited from high disturbance severity (76%-98%). BINs allocated to a species name contained nearly as many positive as negative disturbance indicators, but dark and cryptic taxa, particularly Diptera and Hymenoptera in our data, contained higher numbers of negative disturbance indicator BINs. Analyses of changes in the richness of BINs showed variable responses of arthropods to disturbance severity at lower taxonomic levels, whereas no significant signal was detected at the order level due to the compensatory responses of the underlying taxa. We conclude that the analyses of dark taxa can offer new insights into biodiversity responses to disturbances. Our results suggest considerable potential for forest management to foster arthropod diversity, for example by maintaining both closed-canopy forests (>70% cover) and open forests (<30% cover) on the landscape.

Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California

Quantifying the responses of forest disturbances to climate warming is critical to our understanding of carbon cycles and energy balances of the Earth system. The impact of warming on bark beetle outbreaks is complex as multiple drivers of these events may respond differently to warming. Using a novel model of bark beetle biology and host tree interactions, we assessed how contemporary warming affected western pine beetle (Dendroctonus brevicomis) populations and mortality of its host, ponderosa pine (Pinus ponderosa), during an extreme drought in the Sierra Nevada, California, United States. When compared with the field data, our model captured the western pine beetle flight timing and rates of ponderosa pine mortality observed during the drought. In assessing the influence of temperature on western pine beetles, we found that contemporary warming increased the development rate of the western pine beetle and decreased the overwinter mortality rate of western pine beetle larvae leading to increased population growth during periods of lowered tree defense. We attribute a 29.9% (95% CI: 29.4%-30.2%) increase in ponderosa pine mortality during drought directly to increases in western pine beetle voltinism (i.e., associated with increased development rates of western pine beetle) and, to a much lesser extent, reductions in overwintering mortality. These findings, along with other studies, suggest each degree (°C) increase in temperature may have increased the number of ponderosa pine killed by upwards of 35%-40% °C^-1 if the effects of compromised tree defenses (15%-20%) and increased western pine beetle populations (20%) are additive. Due to the warming ability to considerably increase mortality through the mechanism of bark beetle populations, models need to consider climate's influence on both host tree stress and the bark beetle population dynamics when determining future levels of tree mortality.

Time matters: Resilience of a post-disturbance forest landscape

Present-day disturbances are transforming European forest landscapes, and their legacies determine the vulnerability and resilience of the emergent forest generation. To understand these legacy effects, we investigated the resilience of the aboveground forest biomass (B_abg) to a sequence of disturbances affecting the forest in different recovery phases from the initial large-scale impact. We used the model iLand to simulate windthrows that affected 13-24% of the B_abg in a Central European forest landscape. An additional wind event was simulated 20, 40, 60, or 80 years after the initial impact (i.e., sequences of two windthrows were defined). Each windthrow triggered an outbreak of bark beetles that interacted with the recovery processes. We evaluated the resistance of the B_abg to and recovery after the impact. Random Forest models were used to identify factors influencing resilience. We found that B_abg resistance was the lowest 20 years after the initial impact when the increased proportion of emergent wind-exposed forest edges prevailed the disturbance-dampening effect of reduced biomass levels and increased landscape heterogeneity. This forest had a remarkably high recovery rate and reached the pre-disturbance B_abg within 28 years. The forest exhibited a higher resistance and a slower recovery rate in the more advanced recovery phases, reaching the pre-disturbance B_abg within 60-80 years. The recovery was enhanced by higher levels of alpha and beta diversity. Under elevated air temperature, the bark beetle outbreak triggered by windthrow delayed the recovery. However, the positive effect of increased temperature on forest productivity caused the recovery rate to be higher under the warming scenario than under the reference climate. We conclude that resilience is not a static property, but its magnitude and drivers vary in time, depending on vegetation feedbacks, interactions between disturbances, and climate. Understanding these mechanisms is an essential step towards the operationalization of resilience-oriented stewardship.

Understanding and Modeling Forest Disturbance Interactions at the Landscape Level

Disturbances, both natural and anthropogenic, affect the configuration, composition, and function of forested ecosystems. Complex system behaviors emerge from the interactions between disturbance regimes, the vegetation response to those disturbances, and their interplay with multiple drivers (climate, topography, land use, etc.) across spatial and temporal scales. Here, we summarize conceptual advances and empirical approaches to disturbance interaction investigation, and used those insights to evaluate and categorize 146 landscape modeling studies emerging from a systematic review of the literature published since 2010. Recent conceptual advances include formal disaggregation of disturbances into their constituent components, embedding disturbance processes into system dynamics, and clarifying terminology for interaction factors, types, and ecosystem responses. Empirical studies investigating disturbance interactions now span a wide range of approaches, including (most recently) advanced statistical methods applied to an expanding set of spatial and temporal datasets. Concurrent development in spatially-explicit landscape models, informed by these empirical insights, integrate the interactions among natural and anthropogenic disturbances by coupling these processes to account for disturbance stochasticity, disturbance within and across scales, and non-linear landscape responses to climate change. Still, trade-offs between model elegance and complexity remain. We developed an index for the degree of process integration (i.e., balance of static vs. dynamic components) within a given disturbance agent and applied it to the studies from our systematic review. Contemporary model applications in this line of research have applied a wide range process integration, depending on the specific question, but also limited in part by data and knowledge. Non-linear “threshold” behavior and cross-scaled interactions remain a frontier in temperate, boreal, and alpine regions of North America and Europe, while even simplistic studies are lacking from other regions of the globe (e.g., subtropical and tropical biomes). Understanding and planning for uncertainty in system behavior—including disturbance interactions—is paramount at a time of accelerated anthropogenic change. While progress in landscape modeling studies in this area is evident, work remains to increase model transparency and confidence, especially for understudied regions and processes. Moving forward, a multi-dimensional approach is recommended to address the uncertainties of complex human-ecological dynamics.

Widespread regeneration failure in forests of Greater Yellowstone under scenarios of future climate and fire

Changing climate and disturbance regimes are increasingly challenging the resilience of forest ecosystems around the globe. A powerful indicator for the loss of resilience is regeneration failure, that is, the inability of the prevailing tree species to regenerate after disturbance. Regeneration failure can result from the interplay among disturbance changes (e.g., larger and more frequent fires), altered climate conditions (e.g., increased drought), and functional traits (e.g., method of seed dispersal). This complexity makes projections of regeneration failure challenging. Here we applied a novel simulation approach assimilating data-driven fire projections with vegetation responses from process modeling by means of deep neural networks. We (i) quantified the future probability of regeneration failure; (ii) identified spatial hotspots of regeneration failure; and (iii) assessed how current forest types differ in their ability to regenerate under future climate and fire. We focused on the Greater Yellowstone Ecosystem (2.9 × 10⁶  ha of forest) in the Rocky Mountains of the USA, which has experienced large wildfires in the past and is expected to undergo drastic changes in climate and fire in the future. We simulated four climate scenarios until 2100 at a fine spatial grain (100 m). Both wildfire activity and unstocked forest area increased substantially throughout the 21st century in all simulated scenarios. By 2100, between 28% and 59% of the forested area failed to regenerate, indicating considerable loss of resilience. Areas disproportionally at risk occurred where fires are not constrained by topography and in valleys aligned with predominant winds. High-elevation forest types not adapted to fire (i.e., Picea engelmannii-Abies lasiocarpa as well as non-serotinous Pinus contorta var. latifolia forests) were especially vulnerable to regeneration failure. We conclude that changing climate and fire could exceed the resilience of forests in a substantial portion of Greater Yellowstone, with profound implications for carbon, biodiversity, and recreation.

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.

Ash dieback, soil and deer browsing influence natural regeneration of European ash (Fraxinus excelsior L.)

European ash (Fraxinus excelsior L.) dieback affects both overstory trees and natural regeneration. The decline of ash caused by severe crown defoliation and branch mortality has a high impact on ash natural regeneration. The site factors affecting the disease symptoms vary significantly and are not fully understood. Hence, we aimed to assess the joined effects of soil fertility and moisture (expressed by soil pH, CaCO₃ content, and summer groundwater table level), herbivory, and health conditions connected with Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, and Hosoya infestation on natural regeneration of F. excelsior. We examined 32 ash stands in Western Poland across soil fertility gradient. We established randomly selected circular plots (400 m²) in forests with ≥60% of ash in overstory species composition. We assessed natural regeneration density, the proportion of browsed trees, and trees damaged by ash dieback using generalized mixed-effects models. We found a higher proportion of damaged trees in neutral and base soils than in acid soils. Moreover, we found a low proportion of damaged trees in sites with low groundwater table levels. High CaCO₃ content decreased the proportion of browsed trees, similarly as high shrub cover. The density of F. excelsior natural regeneration depended on groundwater table level, canopy cover, and proportion of damaged trees. We also found a positive relationship between density and deer browsing. The factor responsible for the higher infestation of saplings (low groundwater table level) also influenced natural regeneration density. This way, our study revealed how soil properties influence ash natural regeneration directly and indirectly. We showed that ash dieback will have a more severe impact on ash regeneration in ash typical sites. This is essential for predicting forest recovery and the ability to resilience after disturbances caused by H. fraxineus. CAPSULE: Joined effects of ash dieback, soil pH, and CaCO₃ content, together with soil moisture, as well as deer browsing, affect ash regeneration.

The Effect of Crown Social Class on Bark Thickness and Sapwood Moisture Content in Norway Spruce

The research study examined the effect of tree properties (crown social class, diameter at breast height (DBH), and tree height) on bark thickness (BT) and sapwood moisture content (SMC) in Norway spruce (Picea abies (L.) H. Karst.). Both examined variables were shown to be positively affected by DBH and tree height. The relationship between DBH and SMC varied among crown social classes, while the relationship between DBH and BT was relatively constant across crown social classes. Crown social class had a relatively small effect on BT and SMC, having a more pronounced effect on SMC than on BT. The relationship between tree height and BT did not vary across crown social classes, while the relationship between SMC and tree height was found to change slightly across crown social classes. Measurements of BT and SMC in the field are affordable, fast, and easy to use. Both variables could potentially be used to improve predictions of bark beetle attacks, as they reflect the physiological state of an individual tree.

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.