On the tracks of Nitrogen deposition effects on temperate forests at their southern European range - an observational study from Italy

Temperature and leaf form drive contrasting sensitivity to nitrogen deposition across European forests

Raised emissions of biologically reactive nitrogen (N) have intensified N deposition, enhancing tree productivity globally. Nonetheless, the drivers of forest sensitivity to N deposition remain unknown. We used stem growth data from 62,000 trees across Europe combined with N deposition data to track the effects of air temperature and precipitation on tree growth's sensitivity to N deposition and how it varied depending on leaf form over the past 30 years. Overall, N deposition enhanced conifer growth (until 30 kg N ha-1 yr-1) while decreasing growth for broadleaved angiosperms. Lower temperatures led to higher growth sensitivity to N deposition in conifers potentially exacerbated by N limitation. In contrast, higher temperatures stimulated growth sensitivity to N deposition for broadleaves. Higher precipitation equally increased N deposition sensitivity in all leaf forms. We conclude that air temperature and leaf form are decisive in disentangling the effect of N deposition in European forests, which provides crucial information to better predict the contribution of N deposition to land carbon sink enhancement.

Atmospheric deposition patterns in bulk open field precipitation and throughfall in Aleppo pine forest and black pine forest on the eastern Adriatic coast

The Mediterranean region, with its unique ecological characteristics, is particularly sensitive to global environmental changes, including climate change and impact of air pollution. Although Aleppo pine and black pine forests are the most abundant on the eastern Adriatic coast, atmospheric deposition in these forests is poorly studied. Changes in the chemical composition of precipitation as it passes through the tree canopy can lead to soil and groundwater eutrophication, and soil acidification, which affects plant vitality. In this study, the dynamics of ion deposition in Aleppo pine forest (Pinus halepensis Mill.) and black pine forest (Pinus nigra Arnold) on the eastern Adriatic coast are investigated, focusing on throughfall and bulk open field depositions. The aim of our research was to fill the gaps in understanding the influence of tree canopies on deposition fluxes in two different Mediterranean pine stands and to compare total inorganic nitrogen loads with critical loads. Over a period of two years, bulk open field precipitation and throughfall were sampled, measured and analysed using the ICP Forest methodology. The results indicate significant differences in ion deposition between bulk open field and throughfall, with throughfall showing higher values for almost all ions. The highest enrichment ratio was determined for K+. The comparison of the actual inorganic nitrogen load with the critical nitrogen load for Mediterranean pine forests revealed that the inorganic nitrogen load exceeded the critical load in the Aleppo pine forest. Ion deposition increased in the throughfall compared to bulk precipitation, which can be attributed to the seasonality of precipitation, including leaching and long dry periods. These findings enhance our understanding of ion deposition fluxes in vulnerable Mediterranean pine ecosystems and emphasize the need for long-term research on this topic in the actual changing environmental conditions.

Air pollution in Białowieża forest: Analysis of short-term trends from 2014 to 2021

Air pollution caused by sulphur dioxide (SO2) and nitrogen oxides (NOx) has negative impacts on forest health and can initiate forest dieback. Long-term monitoring and analysis of these pollution are carried out in Białowieża Forest in NE Poland due to the threats from abiotic, biotic and anthropogenic factors. The main objective of our study was to monitor the levels and trends of air pollutant deposition in Białowieża Forest. During a short-term monitoring period over six years (2014-2021), the concentration of SO2 in the air decreased significantly (from 2.03 μg m-3 in December 2015 to 0.20 μg m-3 in July 2016), while the concentration of NO2 in the air showed a non-significant decrease (from 8.24 μg m-3 in December 2015 to 1.61 μg m-3 May 2016). There was no significant linear trend in the wet deposition of S-SO4 anions. Mean monthly S-SO4 deposition varies between 4.54 and 94.14 mg m-2month-1. Wet nitrogen deposition, including oxidized nitrogen (N-NO3) and reduced nitrogen (N-NH4), showed a non-significant increase. Mean monthly precipitation of N-NO3 and N-N H4 ranged from 1.91 to 451.73 mg m-2month-1. Neither did total sulphur deposition nor total nitrogen deposition exceed the mean deposition values for forests in Europe (below 6 ha-1yr-1 and 3-15 ha-1yr-1, respectively). Our results indicate that air pollutants originate from local sources (households), especially from the village of Białowieża, as demonstrated by the level and spatial distribution of air pollutant deposition. This indicates that air pollutants from the village of Białowieża could spread to other parts of Białowieża Forest in the future and may have a negative impact on forest health and can initiate forest dieback. It is therefore important to continue monitoring air pollution to assess the threats to this valuable forest ecosystem.

OsEIL1 is involved in the response to heterogeneous high ammonium in rice: A split-root analysis

Ammonium (NH4+) concentrations in rice fields show heterogeneous spatial distribution under the combined influences of nitrogen fertilizer application and modern agronomic practices. However, the characteristics and mechanisms of rice roots in response to heterogeneous NH4+ supply are not well understood. Here, we found a systemic response of rice roots to heterogeneous and high (10 mM) NH4+ supply using a split-root experiment, and show root growth on the NH4+-free (NO3-) side was also inhibited by localized high-NH4+ supply. Moreover, OsEIL1 (encoding a core transcription factor in the ethylene signaling pathway) was found to be involved in the response of rice roots to heterogeneous NH4+. OsEIL1 mutation significantly increased the inhibitory effect of localized high-NH4+ on root growth of the NO3- side, as well as significantly increased NH4+ efflux there. Furthermore, our results indicate that the mitigating effect of OsEIL1 on NH4+ efflux is related to the regulated expression of OsVTC1-3 (encoding a GDP-mannose pyrophosphorylase). These findings provide insight into the mechanisms by which OsEIL1 responds to heterogeneous high NH4+ and contribute to our understanding of rice adaptation to heterogeneous NH4+ supply.

Effects of simulated nitrogen deposition on the nutritional and physiological status of beech forests at two climatic contrasting sites in Italy

Anthropogenic activities have resulted in a significant increase of reactive nitrogen (N) compounds in the atmosphere and a rise in N deposition on forest ecosystems. Increasing N loads impact forest productivity and health, altering tree physiological status and nutrient balance with possible beneficial and detrimental consequences. The impact of N deposition has received considerable attention by scientific research, covering medium and high latitudes, while experimental studies in the Mediterranean area are almost absent. The present study used a manipulative approach, through replicated N additions (background deposition, 30, 60 kg N ha^-1yr^-1) to simulate the cumulative effects of N deposition in two beech (Fagus sylvaticaL.) forests located in contrasting climatic regions of Italy. Leaf nutrients and photosynthetic pigments were tested as monitoring indicators after four years of N fertilization. Foliar N and pigment concentrations indicated not limiting N conditions at both forest sites, although changes in chlorophylls and carotenoids showed an early response of the canopy to N additions. N-to-phosphorus (P) and sulfur (S) ratios increased under elevated N fertilization, which could be partly related to the relative enhancement of foliar N concentration, and partly associated with the reduction of foliar P and S. The two eutrophic beech forests monitored were not severely affected by chronic N addition, not showing critical nutritional and physiological impairments over the short to medium period. However, the modifications in leaf nutrient and pigment compositions showed an incipient stress response and accentuated the differences induced by climatic and soil characteristics at the two sites. The potential use of nutrients and photosynthetic pigments in monitoring forest N deposition under contrasting climatic conditions and the eventual limits of manipulative experiments are discussed.

OsEIL1 protects rice growth under NH4+ nutrition by regulating OsVTC1-3-dependent N-glycosylation and root NH4+ efflux

Rice is known for its superior adaptation to ammonium (NH₄⁺ ) as a nitrogen source. Compared to many other cereals, it displays lower NH₄⁺ efflux in roots and higher nitrogen-use efficiency on NH₄⁺ . A critical role for GDP-mannose pyrophosphorylase (VTC1) in controlling root NH₄⁺ fluxes was previously documented in Arabidopsis, but the molecular pathways involved in regulating VTC1-dependent NH₄⁺ efflux remain unclear. Here, we report that ETHYLENE-INSENSITIVE3-LIKE1 (OsEIL1) acts as a key transcription factor regulating OsVTC1-3-dependent NH₄⁺ efflux and protein N-glycosylation in rice grown under NH₄⁺ nutrition. We show that OsEIL1 in rice plays a contrasting role to Arabidopsis-homologous ETHYLENE-INSENSITIVE3 (AtEIN3) and maintains rice growth under NH₄⁺ by stabilizing protein N-glycosylation and reducing root NH₄⁺ efflux. OsEIL1 constrains NH₄⁺ efflux by activation of OsVTC1-3, but not OsVTC1-1 or OsVTC1-8. OsEIL1 binds directly to the promoter EIN3-binding site (EBS) of OsVTC1-3 in vitro and in vivo and acts to increase the transcription of OsVTC1-3. Our work demonstrates an important link between excessive root NH₄⁺ efflux and OsVTC1-3-mediated protein N-glycosylation in rice grown under NH₄⁺ nutrition and identifies OsEIL1 as a direct genetic regulator of OsVTC1-3 expression.

Legacy effects of drought on nitrate leaching in a temperate mixed forest on karst

With climate change the occurrence of summer droughts is expected to increase in Central Europe. This could lead to increased nitrate (NO₃^-) leaching when water scarcity affects the N-uptake capacity of trees and increases soil N availability due to early leaf senescence and higher litter input. In the present study, we used 16 years of ecological monitoring data from the LTER research site "Zöbelboden" in Austria. The monitoring site is a mixed Spruce-Sycamore-Ash-Beech forest on karst, which is representative for many watersheds that supply drinking water in Austria. We found that in the year after a summer drought, NO₃^- leaching via soil water seepage was significantly elevated compared to the long-term mean. While in normal years, NO₃^- leaching was primarily affected by soil water seepage volume, after a summer drought these controls changed and NO₃^- leaching was controlled by NO₃^- input via precipitation, tree N uptake, and vapor-pressure deficit. Furthermore, higher aboveground litter input during dry years was correlated with increased NO₃^- leaching in the following year. Our findings show that NO₃^- leaching from temperate mountain forests on karst is susceptible to summer drought, which could affect drinking water quality in the Central European Alps in the future, especially in combination with forest disturbances like bark beetle outbreaks, which are often a direct consequence of drought damage to trees.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

The availability of carbon (C) from high levels of atmospheric carbon dioxide (CO2 ) and anthropogenic release of nitrogen (N) is increasing, but these increases are not paralleled by increases in levels of phosphorus (P). The current unstoppable changes in the stoichiometries of C and N relative to P have no historical precedent. We describe changes in P and N fluxes over the last five decades that have led to asymmetrical increases in P and N inputs to the biosphere. We identified widespread and rapid changes in N:P ratios in air, soil, water, and organisms and important consequences to the structure, function, and biodiversity of ecosystems. A mass-balance approach found that the combined limited availability of P and N was likely to reduce C storage by natural ecosystems during the remainder of the 21st Century, and projected crop yields of the Millennium Ecosystem Assessment indicated an increase in nutrient deficiency in developing regions if access to P fertilizer is limited. Imbalances of the N:P ratio would likely negatively affect human health, food security, and global economic and geopolitical stability, with feedbacks and synergistic effects on drivers of global environmental change, such as increasing levels of CO2 , climatic warming, and increasing pollution. We summarize potential solutions for avoiding the negative impacts of global imbalances of N:P ratios on the environment, biodiversity, climate change, food security, and human health.

Assessment of Atmospheric Deposition and Vitality Indicators in Mediterranean Forest Ecosystems

Considering the fragility of the Mediterranean environment, there is an increasing need to improve the knowledge of this forest environment. The aim of this study was to examine the effects of air pollution on the forest ecosystem’s condition by analyzing tree vitality. The study area was chosen to represent the most important and the most common species in Mediterranean forest ecosystems of the Eastern Adriatic coast. Quercus pubescens, Quercus ilex, Pinus halepensis, and Pinus nigra plots were equipped with rain collectors and dendrometer bands. Sampling, measurements, and analyses of atmospheric deposition, foliar nutrient, defoliation, and growth were all carried out. Results showed that actual N deposition loads were the lowest in Aleppo pine forest and the highest in holm oak forests. This, however, did not have an effect on the concentrations of N in foliage. Most elements’ concentrations were in the plausible range. No relevant differences in mean defoliation between the plots were observed. The plots with a lower percentage of basal area increment (BAI%) were found to have lower defoliation. The research was conducted to bridge the gap in the knowledge of air pollutants and vitality indicators in different forest types. These findings are a valuable contribution to the sustainable forest management of Mediterranean forest.

Processing Tomato–Durum Wheat Rotation under Integrated, Organic and Mulch-Based No-Tillage Organic Systems: Yield, N Balance and N Loss

In a 4-year study, the biannual crop rotation processing tomato–durum wheat was applied to three cropping systems: (i) an innovative organic coupled with no-tillage (ORG+) where an autumn-sown cover crop was terminated by roller-crimping and then followed by the direct transplantation of processing tomato onto the death-mulch cover; (ii) a traditional organic (ORG) with autumn-sown cover crop that was green manured and followed by processing tomato; and (iii) a conventional integrated low-input (INT) with bare soil during the fall–winter period prior to the processing tomato. N balance, yield and N leaching losses were determined. Innovative cropping techniques such as wheat–faba bean temporary intercropping and the direct transplantation of processing tomato into roll-crimped cover crop biomass were implemented in ORG+; the experiment was aimed at: (i) quantifying the N leaching losses; (ii) assessing the effect of N management on the yield and N utilization; and (iii) comparing the cropping system outputs (yield) in relation to extra-farm N sources (i.e., N coming from organic or synthetic fertilizers acquired from the market) and N losses. The effects of such innovations on important agroecological services such as yield and N recycling were assessed compared to those supplied by the other cropping systems. Independently from the soil management strategy (no till or inversion tillage), cover crops were found to be the key factor for increasing the internal N recycling of the agroecosystems and ORG+ needs a substantial improvement in terms of provisioning services (i.e., yield).

Can nutrient fertilization mitigate the effects of ozone exposure on an ozone-sensitive poplar clone?

We tested the independent and interactive effects of nitrogen (N; 0 and 80 kg ha^-1), phosphorus (P; 0, 40 and 80 kg ha^-1), and ozone (O₃) application/exposure [ambient concentration (AA), 1.5 × AA and 2.0 × AA] for five consecutive months on biochemical traits of the O₃-sensitive Oxford poplar clone. Plants exposed to O₃ showed visible injury and an alteration of membrane integrity, as confirmed by the malondialdehyde by-product accumulation (+3 and +17% under 1.5 × AA and 2.0 × AA conditions, in comparison to AA). This was probably due to O₃-induced oxidative damage, as documented by the production of superoxide anion radical (O₂^-, +27 and +63%, respectively). Ozone per se, independently from the concentrations, induced multiple signals (e.g., alteration of cellular redox state, increase of abscisic acid/indole-3-acetic acid ratio and reduction of proline content) that might be part of premature leaf senescence processes. By contrast, nutrient fertilization (both N and P) reduced reactive oxygen species accumulation (as confirmed by the decreased O₂^- and hydrogen peroxide content), resulting in enhanced membrane stability. This was probably due to the simultaneous involvement of antioxidant compounds (e.g., carotenoids, ascorbate and glutathione) and osmoprotectants (e.g., proline) that regulate the detoxification processes of coping with oxidative stress by reducing the O₃ sensitivity of Oxford clone. These mitigation effects were effective only under AA and 1.5 × AA conditions. Nitrogen and P supply activated a free radical scavenging system that was not able to delay leaf senescence and mitigate the adverse effects of a general peroxidation due to the highest O₃ concentrations.

A New Wetness Index to Evaluate the Soil Water Availability Influence on Gross Primary Production of European Forests

Rising temperature, drought and more-frequent extreme climatic events have been predicted for the next decades in many regions around the globe. In this framework, soil water availability plays a pivotal role in affecting vegetation productivity, especially in arid or semi-arid environments. However, direct measurements of soil moisture are scarce, and modeling estimations are still subject to biases. Further investigation on the effect of soil moisture on plant productivity is required. This study aims at analyzing spatio-temporal variations of a modified temperature vegetation wetness index (mTVWI), a proxy of soil moisture, and evaluating its effect on gross primary production (GPP) in forests. The study was carried out in Europe on 19 representative tree species during the 2000–2010 time period. Results outline a north–south gradient of mTVWI with minimum values (low soil water availability) in Southern Europe and maximum values (high soil water availability) in Northeastern Europe. A low soil water availability negatively affected GPP from 20 to 80%, as a function of site location, tree species, and weather conditions. Such a wetness index improves our understanding of water stress impacts, which is crucial for predicting the response of forest carbon cycling to drought and aridity.

Excess Nitrogen in Temperate Forest Ecosystems Decreases Herbaceous Layer Diversity and Shifts Control from Soil to Canopy Structure

Research Highlights: Excess N from atmospheric deposition has been shown to decrease plant biodiversity of impacted forests, especially in its effects on herbaceous layer communities. This work demonstrates that one of the mechanisms of such response is in N-mediated changes in the response of herb communities to soil resources and light availability. Background and Objectives: Numerous studies in a variety of forest types have shown that excess N can cause loss of biodiversity of herb layer communities, which are typically responsive to spatial patterns of soil resource and light availability. The objectives of this study were to examine (1) gradients of temporal change in herb composition over a quarter century, and (2) spatial patterns of herb cover and diversity and how they are influenced by soil resources and canopy structure. Materials and Methods: This study used two watersheds (WS) at the Fernow Experimental Forest, West Virginia, USA: WS4 as an untreated reference and WS3 as treatment, receiving 35 kg N/ha/yr via aerial application. Herb cover and composition was measured in seven permanent plots/WS from 1991 to 2014. In 2011, soil moisture and several metrics of soil N availability were measured in each plot, along with measurement of several canopy structural variables. Backwards stepwise regression was used to determine relationships between herb cover/diversity and soil/canopy measurements. Results: Herb diversity and composition varied only slightly over time on reference WS4, in contrast to substantial change on N-treated WS3. Herb layer diversity appeared to respond to neither soil nor canopy variables on either watershed. Herb cover varied spatially with soil resources on WS4, whereas cover varied spatially with canopy structure on WS3. Conclusions: Results support work in many forest types that excess N can decrease plant diversity in impacted stands. Much of this response is likely related to N-mediated changes in the response of the herb layer to soil N and light availability.

Nitrogen deposition affects both net and gross soil nitrogen transformations in forest ecosystems: A review

Nitrogen (N) deposition has rapidly increased and is influencing forest ecosystem processes and functions on a global scale. Understanding process-specific N transformations, i.e., gross N transformations, in forest soils in response to N deposition is of great significance to gain mechanistic insights on the linkages between global N deposition and N availability or loss in forest soils. In this paper, we review factors controlling N mineralization, nitrification and N immobilization, particularly in relation to N deposition, discuss the limitations of net N transformation studies, and synthesize the literature on the effect of N deposition on gross N transformations in forest ecosystems. We found that more than 97% of published papers evaluating the effect of N deposition (including N addition experiments that simulate N deposition) on soil N cycle determined net rates of mineralization and nitrification, showing that N deposition significantly increased those rates by 24.9 and 153.9%, respectively. However, studies on net N transformation do not provide a mechanistic understanding of the effect of N deposition on N cycling. To date, a small number of studies (<20 published papers) have directly quantified the effect of N deposition on gross N transformation rates, limiting our understanding of the response of soil N cycling to N deposition. The responses to N deposition of specific N transformation processes such as autotrophic nitrification, heterotrophic nitrification, dissimilatory nitrate reduction to ammonium, N mineralization, and N immobilization are poorly studied. Future research needs to use more holistic approaches to study the impact of N deposition on gross N transformation rates, N loss and retention, and their microbial-driven mechanisms to provide a better understanding of the processes involved in N transformations, and to understand the differential responses between forest and other ecosystems.

Modelling study of soil C, N and pH response to air pollution and climate change using European LTER site observations

Current climate warming is expected to continue in coming decades, whereas high N deposition may stabilize, in contrast to the clear decrease in S deposition. These pressures have distinctive regional patterns and their resulting impact on soil conditions is modified by local site characteristics. We have applied the VSD+ soil dynamic model to study impacts of deposition and climate change on soil properties, using MetHyd and GrowUp as pre-processors to provide input to VSD+. The single-layer soil model VSD+ accounts for processes of organic C and N turnover, as well as charge and mass balances of elements, cation exchange and base cation weathering. We calibrated VSD+ at 26 ecosystem study sites throughout Europe using observed conditions, and simulated key soil properties: soil solution pH (pH), soil base saturation (BS) and soil organic carbon and nitrogen ratio (C:N) under projected deposition of N and S, and climate warming until 2100. The sites are forested, located in the Mediterranean, forested alpine, Atlantic, continental and boreal regions. They represent the long-term ecological research (LTER) Europe network, including sites of the ICP Forests and ICP Integrated Monitoring (IM) programmes under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP), providing high quality long-term data on ecosystem response. Simulated future soil conditions improved under projected decrease in deposition and current climate conditions: higher pH, BS and C:N at 21, 16 and 12 of the sites, respectively. When climate change was included in the scenario analysis, the variability of the results increased. Climate warming resulted in higher simulated pH in most cases, and higher BS and C:N in roughly half of the cases. Especially the increase in C:N was more marked with climate warming. The study illustrates the value of LTER sites for applying models to predict soil responses to multiple environmental changes.

The sap flow-based assessment of atmospheric trace gas uptake by three forest types in subtropical China on different timescales

Assessing the uptake of trace gases by forests contributes to understanding the mechanisms of gas exchange between vegetation and the atmosphere and to evaluating the potential risk of these pollutant gases to forests. In this study, the multi-timescale characteristics of the stomatal uptake of NO, NO₂, SO₂ and O₃ by Schima superba, Eucalyptus citriodora and Acacia auriculiformis were investigated by continuous sap flow measurements for a 3-year period. The peak canopy stomatal conductance (G_C) for these three species appeared between 9:00 and 12:00, which was jointly regulated by the vapour pressure deficit (VPD) and photosynthetically active radiation (PAR). Additionally, annual and seasonal variations in the stomatal uptake of trace gases for these three tree species suggested that there was a combination effect between canopy stomatal conductance and ambient concentration on the uptake of trace gases. Furthermore, the result demonstrated that the trace gas absorption capacities among these three forest types followed the order of S. superba > E. citriodora > A. auriculiformis. The findings of this study have theoretical significance and application value in assessing air purification and the risk of harm to forests in Southern China.

Sustainable Land Management, Adaptive Silviculture, and New Forest Challenges: Evidence from a Latitudinal Gradient in Italy

Aimed at reducing structural homogeneity and symmetrical competition in even-aged forest stands and enhancing stand structure diversity, the present study contributes to the design and implementation of adaptive silvicultural practices with two objectives: (1) preserving high wood production rates under changing environmental conditions and (2) ensuring key ecological services including carbon sequestration and forest health and vitality over extended stand life-spans. Based on a quantitative analysis of selected stand structure indicators, the experimental design was aimed at comparing customary practices of thinning from below over the full standing crop and innovative practices of crown thinning or selective thinning releasing a pre-fixed number of best phenotypes and removing direct crown competitors. Experimental trials were established at four beech forests along a latitudinal gradient in Italy: Cansiglio, Veneto; Vallombrosa, Tuscany; Chiarano, Abruzzo; and Marchesale, Calabria). Empirical results indicate a higher harvesting rate is associated with innovative practices compared with traditional thinning. A multivariate discriminant analysis outlined significant differences in post-treatment stand structure, highlighting the differential role of structural and functional variables across the study sites. These findings clarify the impact of former forest structure in shaping post-treatment stand attributes. Monitoring standing crop variables before and after thinning provides a basic understanding to verify intensity and direction of the applied manipulation, the progress toward the economic and ecological goals, as well as possible failures or need for adjustments within a comprehensive strategy of adaptive forest management.

Nitrogen deposition outweighs climatic variability in driving annual growth rate of canopy beech trees: Evidence from long-term growth reconstruction across a geographic gradient

In this study, we investigated the role of climatic variability and atmospheric nitrogen deposition in driving long-term tree growth in canopy beech trees along a geographic gradient in the montane belt of the Italian peninsula, from the Alps to the southern Apennines. We sampled dominant trees at different developmental stages (from young to mature tree cohorts, with tree ages spanning from 35 to 160 years) and used stem analysis to infer historic reconstruction of tree volume and dominant height. Annual growth volume (G_V ) and height (G_H ) variability were related to annual variability in model simulated atmospheric nitrogen deposition and site-specific climatic variables, (i.e. mean annual temperature, total annual precipitation, mean growing period temperature, total growing period precipitation, and standard precipitation evapotranspiration index) and atmospheric CO₂ concentration, including tree cambial age among growth predictors. Generalized additive models (GAM), linear mixed-effects models (LMM), and Bayesian regression models (BRM) were independently employed to assess explanatory variables. The main results from our study were as follows: (i) tree age was the main explanatory variable for long-term growth variability; (ii) GAM, LMM, and BRM results consistently indicated climatic variables and CO₂ effects on G_V and G_H were weak, therefore evidence of recent climatic variability influence on beech annual growth rates was limited in the montane belt of the Italian peninsula; (iii) instead, significant positive nitrogen deposition (N_dep ) effects were repeatedly observed in G_V and G_H ; the positive effects of N_dep on canopy height growth rates, which tended to level off at N_dep values greater than approximately 1.0 g m^-2  y^-1 , were interpreted as positive impacts on forest stand above-ground net productivity at the selected study sites.

Assessment of climate change effects on mountain ecosystems through a cross-site analysis in the Alps and Apennines

Mountain ecosystems are sensitive and reliable indicators of climate change. Long-term studies may be extremely useful in assessing the responses of high-elevation ecosystems to climate change and other anthropogenic drivers from a broad ecological perspective. Mountain research sites within the LTER (Long-Term Ecological Research) network are representative of various types of ecosystems and span a wide bioclimatic and elevational range. Here, we present a synthesis and a review of the main results from ecological studies in mountain ecosystems at 20 LTER sites in Italy, Switzerland and Austria covering in most cases more than two decades of observations. We analyzed a set of key climate parameters, such as temperature and snow cover duration, in relation to vascular plant species composition, plant traits, abundance patterns, pedoclimate, nutrient dynamics in soils and water, phenology and composition of freshwater biota. The overall results highlight the rapid response of mountain ecosystems to climate change, with site-specific characteristics and rates. As temperatures increased, vegetation cover in alpine and subalpine summits increased as well. Years with limited snow cover duration caused an increase in soil temperature and microbial biomass during the growing season. Effects on freshwater ecosystems were also observed, in terms of increases in solutes, decreases in nitrates and changes in plankton phenology and benthos communities. This work highlights the importance of comparing and integrating long-term ecological data collected in different ecosystems for a more comprehensive overview of the ecological effects of climate change. Nevertheless, there is a need for (i) adopting co-located monitoring site networks to improve our ability to obtain sound results from cross-site analysis, (ii) carrying out further studies, in particular short-term analyses with fine spatial and temporal resolutions to improve our understanding of responses to extreme events, and (iii) increasing comparability and standardizing protocols across networks to distinguish local patterns from global patterns.

Scarce evidence of ozone effect on recent health and productivity of alpine forests-a case study in Trentino, N. Italy

We investigated the significance of tropospheric ozone as a factor explaining recent tree health (in terms of defoliation) and productivity (in terms of basal area increment, BAI) in 15 ICP Forests level I and one level II plots in alpine forests in Trentino (N. Italy). Mean daily ozone summer concentrations varied between 30 and 72 parts per billion (ppb) leading to large exceedance of concentration-based critical levels set to protect forest trees. Phytoxic ozone dose (POD₀) estimated at the level II plot over the period 1996-2009 was 31-61 mmol m^-2 projected leaf area (PLA). The role of ozone was investigated taking into account other site and environmental factors. Simple linear regression, multiple linear regression (MLR, to study mean periodical defoliation and mean periodical BAI), and linear mixed models (LMM, to study annual defoliation data) were used. Our findings suggest that-regardless of the metric adopted-tropospheric ozone is not a significant factor in explaining recent status and trends of defoliation and BAI in the alpine region examined. Both defoliation and BAI are in turn driven by biotic/abiotic damage, nutritional status, DBH (assumed as a proxy for age), and site characteristics. These results contrast with available ozone-growth dose response relationships (DRRs) and other observational studies. This may be due to a variety of concurrent reasons: (i) DRRs developed for individual saplings under controlled condition are not necessarily valid for population of mature trees into real forest ecosystems; (ii) some observational studies may have suffered from biased design; and (iii) since alpine forests have been exposed to high ozone levels (and other oxidative stress) over decades, possible acclimation mechanisms cannot be excluded.

In search for evidence: combining ad hoc survey, monitoring, and modeling to estimate the potential and actual impact of ground level ozone on forests in Trentino (Northern Italy)

A 5-year project was carried out over the period 2007-2011 to estimate the potential and actual ozone effect on forests in Trentino, Northern Italy (6207 km²) (Ozone EFFORT). The objective was to provide explicit answers to three main questions: (i) is there a potential risk placed by ozone to vegetation? (ii) are there specific ozone symptoms on vegetation, and are they related to ozone levels? (iii) are there ozone-related effects on forest health and growth? Different methods and techniques were adopted as follows: monitoring ozone levels, ad hoc field survey for symptoms on vegetation and chlorophyll-related measurements, modeling to upscale ozone measurements, ozone flux estimation, statistical analysis, and modeling to detect whether a significant effect attributable to ozone exists. Ozone effects were assessed on an ad hoc-introduced bioindicator, on spontaneous woody species, and on forest trees. As for question (i), the different ozone-risk critical levels for both exposure and stomatal flux were largely exceeded in Trentino, evidencing a potentially critical situation for vegetation. As for question (ii), specific ozone foliar symptoms related to ozone exposure levels were observed on the introduced supersensitive Nicotiana tabacum L. cv Bel-W3 and on the spontaneous, ozone-sensitive Viburnum lantana L., but not on other 33 species surveyed in the field studies. Regarding question (iii), statistical analyses on forest health (in terms of defoliation) and growth (in terms of basal area increment) measured at 15 forest monitoring plots and tree rings (at one site) revealed no significant relationship with ozone exposure and flux. Instead, a set of factors related to biotic and abiotic causes, foliar nutrients, age, and site were identified as the main drivers of forest health and growth. In conclusion, while ozone levels and fluxes in the investigated region were much higher than current critical levels, evidence of impact on vegetation-and on forest trees in particular-was limited.

Contrasting effects of nitrogen addition on soil respiration in two Mediterranean ecosystems

Increased atmospheric nitrogen (N) deposition is known to alter ecosystem carbon source-sink dynamics through changes in soil CO₂ fluxes. However, a limited number of experiments have been conducted to assess the effects of realistic N deposition in the Mediterranean Basin, and none of them have explored the effects of N addition on soil respiration (R _s ). To fill this gap, we assessed the effects of N supply on R _s dynamics in the following two Mediterranean sites: Capo Caccia (Italy), where 30 kg ha^-1 year^-1 was supplied for 3 years, and El Regajal (Spain), where plots were treated with 10, 20, or 50 kg N ha^-1 year^-1 for 8 years. Results show a complex, non-linear response of soil respiration (R _s ) to N additions with R _s overall increasing at Capo Caccia and decreasing at El Regajal. This suggests that the response of R _s to N addition depends on dose and duration of N supply, and the existence of a threshold above which the N introduced in the ecosystem can affect the ecosystem's functioning. Soil cover and seasonality of precipitations also play a key role in determining the effects of N on R _s as shown by the different responses observed across seasons and in bare soil vs. the soil under canopy of the dominant species. These results show how increasing rates of N addition may influence soil C dynamics in semiarid ecosystems in the Mediterranean Basin and represent a valuable contribution for the understanding and the protection of Mediterranean ecosystems.

Functional indicators of response mechanisms to nitrogen deposition, ozone, and their interaction in two Mediterranean tree species

The effects of nitrogen (N) deposition, tropospheric ozone (O3) and their interaction were investigated in two Mediterranean tree species, Fraxinus ornus L. (deciduous) and Quercus ilex L. (evergreen), having different leaf habits and resource use strategies. An experiment was conducted under controlled condition to analyse how nitrogen deposition affects the ecophysiological and biochemical traits, and to explore how the nitrogen-induced changes influence the response to O3. For both factors we selected realistic exposures (20 kg N ha-1 yr-1 and 80 ppb h for nitrogen and O3, respectively), in order to elucidate the mechanisms implemented by the plants. Nitrogen addition resulted in higher nitrogen concentration at the leaf level in F. ornus, whereas a slight increase was detected in Q. ilex. Nitrogen enhanced the maximum rate of assimilation and ribulose 1,5-bisphosphate regeneration in both species, whereas it influenced the light harvesting complex only in the deciduous F. ornus that was also affected by O3 (reduced assimilation rate and accelerated senescence-related processes). Conversely, Q. ilex developed an avoidance mechanism to cope with O3, confirming a substantial O3 tolerance of this species. Nitrogen seemed to ameliorate the harmful effects of O3 in F. ornus: the hypothesized mechanism of action involved the production of nitrogen oxide as the first antioxidant barrier, followed by enzymatic antioxidant response. In Q. ilex, the interaction was not detected on gas exchange and photosystem functionality; however, in this species, nitrogen might stimulate an alternative antioxidant response such as the emission of volatile organic compounds. Antioxidant enzyme activity was lower in plants treated with both O3 and nitrogen even though reactive oxygen species production did not differ between the treatments.

Ecological impacts of atmospheric pollution and interactions with climate change in terrestrial ecosystems of the Mediterranean Basin: Current research and future directions

Mediterranean Basin ecosystems, their unique biodiversity, and the key services they provide are currently at risk due to air pollution and climate change, yet only a limited number of isolated and geographically-restricted studies have addressed this topic, often with contrasting results. Particularities of air pollution in this region include high O₃ levels due to high air temperatures and solar radiation, the stability of air masses, and dominance of dry over wet nitrogen deposition. Moreover, the unique abiotic and biotic factors (e.g., climate, vegetation type, relevance of Saharan dust inputs) modulating the response of Mediterranean ecosystems at various spatiotemporal scales make it difficult to understand, and thus predict, the consequences of human activities that cause air pollution in the Mediterranean Basin. Therefore, there is an urgent need to implement coordinated research and experimental platforms along with wider environmental monitoring networks in the region. In particular, a robust deposition monitoring network in conjunction with modelling estimates is crucial, possibly including a set of common biomonitors (ideally cryptogams, an important component of the Mediterranean vegetation), to help refine pollutant deposition maps. Additionally, increased attention must be paid to functional diversity measures in future air pollution and climate change studies to establish the necessary link between biodiversity and the provision of ecosystem services in Mediterranean ecosystems. Through a coordinated effort, the Mediterranean scientific community can fill the above-mentioned gaps and reach a greater understanding of the mechanisms underlying the combined effects of air pollution and climate change in the Mediterranean Basin.

Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest

Nitrogen (N) deposition (NDEP ) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A 15 N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy NDEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of NDEP on the relative change in C (∆C/∆N) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used 15 N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this 15 N to an equivalent amount in simulated NDEP treatments. By the end of the experiment, three times as much 15 N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more 15 N tracer from litter decomposition than from simulated mineral NDEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more 15 N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of NDEP on forest ∆C/∆N may not be apparent from mineral 15 N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of NDEP is its influence on the rate of N release from litter.

Forest ecosystems of temperate climatic regions: from ancient use to climate change

871 I. 871 II. 874 III. 875 IV. 878 V. 882 884 References 884 SUMMARY: Humans have long utilized resources from all forest biomes, but the most indelible anthropogenic signature has been the expanse of human populations in temperate forests. The purpose of this review is to bring into focus the diverse forests of the temperate region of the biosphere, including those of hardwood, conifer and mixed dominance, with a particular emphasis on crucial challenges for the future of these forested areas. Implicit in the term 'temperate' is that the predominant climate of these forest regions has distinct cyclic, seasonal changes involving periods of growth and dormancy. The specific temporal patterns of seasonal change, however, display an impressive variability among temperate forest regions. In addition to the more apparent current anthropogenic disturbances of temperate forests, such as forest management and conversion to agriculture, human alteration of temperate forests is actually an ancient phenomenon, going as far back as 7000 yr before present (bp). As deep-seated as these past legacies are for temperate forests, all current and future perturbations, including timber harvesting, excess nitrogen deposition, altered species' phenologies, and increasing frequency of drought and fire, must be viewed through the lens of climate change.

Response of Quercus velutina growth and water use efficiency to climate variability and nitrogen fertilization in a temperate deciduous forest in the northeastern USA

Nitrogen (N) deposition and changing climate patterns in the northeastern USA can influence forest productivity through effects on plant nutrient relations and water use. This study evaluates the combined effects of N fertilization, climate and rising atmospheric CO2on tree growth and ecophysiology in a temperate deciduous forest. Tree ring widths and stable carbon (δ(13)C) and oxygen (δ(18)O) isotopes were used to assess tree growth (basal area increment, BAI) and intrinsic water use efficiency (iWUE) ofQuercus velutinaLamb., the dominant tree species in a 20+ year N fertilization experiment at Harvard Forest (MA, USA). We found that fertilized trees exhibited a pronounced and sustained growth enhancement relative to control trees, with the low- and high-N treatments responding similarly. All treatments exhibited improved iWUE over the study period (1984-2011). Intrinsic water use efficiency trends in the control trees were primarily driven by changes in stomatal conductance, while a stimulation in photosynthesis, supported by an increase in foliar %N, contributed to enhancing iWUE in fertilized trees. All treatments were predominantly influenced by growing season vapor pressure deficit (VPD), with BAI responding most strongly to early season VPD and iWUE responding most strongly to late season VPD. Nitrogen fertilization increasedQ. velutinasensitivity to July temperature and precipitation. Combined, these results suggest that ambient N deposition in N-limited northeastern US forests has enhanced tree growth over the past 30 years, while rising ambient CO2has improved iWUE, with N fertilization and CO2having synergistic effects on iWUE.

Does canopy nitrogen uptake enhance carbon sequestration by trees?

Temperate forest (15) N isotope trace experiments find nitrogen (N) addition-driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply (15) N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional (15) N additions typically trace mineral (15) N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/(15) N ha(-1)  yr(-1) to Sitka spruce saplings. We compared tree and soil (15) N recovery among treatments where enrichment was due to either (1) a (15) N-enriched litter layer, or mineral (15) N additions to (2) the soil or (3) the canopy. We found that 60% of (15) N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of (15) N applied to the soil was found in these pools. (15) N recovery from litter was low and highly variable. (15) N partitioning among biomass pools and age classes also differed among treatments, with twice as much (15) N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from (15) N applied to the soil, scaled to real-world conditions, was 43 kg C kg N(-1) , similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N(-1) . Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Changes in composition, ecology and structure of high-mountain vegetation: a re-visitation study over 42 years

High-mountain ecosystems are increasingly threatened by climate change, causing biodiversity loss, habitat degradation and landscape modifications. However, very few detailed studies have focussed on plant biodiversity in the high mountains of the Mediterranean. In this study, we investigated the long-term changes that have occurred in the composition, structure and ecology of high-mountain vegetation in the central Apennines (Majella) over the last 42 years. We performed a re-visitation study, using historical and newly collected vegetation data to explore which ecological and structural features have been the most successful in coping with climatic changes. Vegetation changes were analysed by comparing geo-referenced phytosociological relevés collected in high-mountain habitats (dolines, gentle slopes and ridges) on the Majella massif in 1972 and in 2014. Composition analysis was performed by detrended correspondence analysis, followed by an analysis of similarities for statistical significance assessment and by similarity percentage procedure (SIMPER) for identifying which species indicate temporal changes. Changes in ecological and structural indicators were analysed by a permutational multivariate analysis of variance, followed by a post hoc comparison. Over the last 42 years, clear floristic changes and significant ecological and structural variations occurred. We observed a significant increase in the thermophilic and mesonitrophilic plant species and an increment in the frequencies of hemicryptophytes. This re-visitation study in the Apennines agrees with observations in other alpine ecosystems, providing new insights for a better understanding of the effects of global change on Mediterranean high-mountain biodiversity. The observed changes in floristic composition, the thermophilization process and the shift towards a more nutrient-demanding vegetation are likely attributable to the combined effect of higher temperatures and the increase in soil nutrients triggered by global change. The re-visitation approach adopted herein represents a powerful tool for studying climate-related changes in sensitive high-mountain habitats.

Critical loads and nitrogen availability under deposition and harvest scenarios for conifer forests in Ireland

In this study we calculated the critical load of nutrient nitrogen (N) for Irish forest plots (n=380) under two harvesting scenarios: conventional stem-only harvest (SOH) and stem plus branch harvest (SBH) and two deposition scenarios: current and with a 10% increase in reduced-N. In addition, current N status was assessed using the following data from forest monitoring plots: forest floor C:N, foliar N and plant root simulation (PRS™) probe N supply rate. Average critical loads were 15.3 kg N ha(-1)year(-1) under SOH and 19.5 kg N ha(-1)year(-1) under SBH. Average total (wet+dry) N deposition was 18 kg N ha(-1)year(-1), ranging from 8.6 to 26 kg Nha(-1)year(-1). As a result, critical loads were exceeded at 67% of sites under SOH and 40% of sites under SBH. However, there was little evidence of exceedance at monitored plots. Foliar and forest floor C:N data indicated that most of these sites had low to intermediate N status. There were considerable differences in N cycling between soil types. Plant root simulation (PRS™) probe data indicated that this was likely due to differences in net N-mineralization and nitrification. Our results indicate that many sites are currently N limited but critical load exceedance suggests that these systems will accumulate N over time. The findings have implications for forest management, allowing for the assessment of nutrient management under different harvest scenarios.

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep ) for four forests in the United Kingdom subjected to different Ndep : Scots pine and beech stands under high Ndep (HN, 13-19 kg N ha(-1)  yr(-1) ), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N ha(-1)  yr(-1) ). Changes of NO3 -N and NH4 -N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ(18) O, Δ(17) O and δ(15) N in NO3 (-) and δ(15) N in NH4 (+) , were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4 -N and NO3 -N concentrations in RF compared to the LN sites. Similar values of δ(15) N-NO3 (-) and δ(18) O in RF suggested similar source of atmospheric NO3 (-) (i.e. local traffic), while more positive values for δ(15) N-NH4 (+) at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, (15) N-enriched NH4 (+) in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ(18) O and Δ(17) O, we quantified for the first time the proportion of NO3 (-) in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ(17) O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ(17) O.