Forests in Europe face significant changes in climate, which in interaction with air quality changes, may significantly affect forest productivity, stand composition and carbon sequestration in both vegetation and soils. Identified knowledge gaps and research needs include: (i) interaction between changes in air quality (trace gas concentrations), climate and other site factors on forest ecosystem response, (ii) significance of biotic processes in system response, (iii) tools for mechanistic and diagnostic understanding and upscaling, and (iv) the need for unifying modelling and empirical research for synthesis. This position paper highlights the above focuses, including the global dimension of air pollution as part of climate change and the need for knowledge transfer to enable reliable risk assessment. A new type of research site in forest ecosystems ("supersites") will be conducive to addressing these gaps by enabling integration of experimentation and modelling within the soil-plant-atmosphere interface, as well as further model development. (C) 2011 Elsevier Ltd. All rights reserved.

Critical loads are thresholds of atmospheric deposition below which harmful ecological effects do not occur. Because lichens are sensitive to atmospheric deposition, lichen-based critical loads can foreshadow changes of other forest processes. Here, we derive critical loads of nitrogen (N) and sulfur (S) deposition for continental US and coastal Alaskan forests, based on nationally consistent lichen community surveys at 8855 sites. Across the eastern and western US ranges of 459 lichen species, each species' realized optimum was the N or S atmospheric deposition value at which it most frequently occurred. The mean of optima for all species at a site, weighted by their abundances, was defined as a community “airscore” indicative of species’ collective responses to atmospheric deposition. To determine critical loads for adverse community compositional shifts, we then modeled changes in airscores as a function of deposition, climate and forest habitat predictors in nonparametric multiplicative regression. Critical loads, indicative of initial shifts from pollution-sensitive toward pollution-tolerant species, occurred at 1.5 kg N ha⁻¹ y⁻¹ and 2.7 kg S ha⁻¹ y⁻¹. Importantly, these critical loads remain constant under any climate regime nationwide, suggesting both simplicity and nationwide applicability. Our models predict that preventing excess N deposition of just 0.2–2.0 kg ha⁻¹ y⁻¹ in the next century could offset the detrimental effects of predicted climate warming on lichen communities. Because excess deposition and climate warming both harm the most ecologically influential species, keeping conditions below critical loads would sustain both forest ecosystem functioning and climate resilience.

Soil nitrogen (N) leaching is recognized to have negative effects on the environment. There is a lack of studies on different simultaneously occurring drivers of environmental change, including changing rainfall and N deposition, on soil N leaching. In this study, a two factorial field experiment was conducted in a Korean pine forest with the following four treatments: 30% of throughfall reduction (TR), 50 kg N ha⁻¹ yr⁻¹ of N addition (N+), throughfall reduction plus N addition (TRN+) and natural forest (CK). The zero-tension pan lysimeter method was used to assess the response of soil N leaching loss to manipulated N addition and throughfall reduction. The results showed that the soil N leaching loss in natural forest was 5.0 ± 0.4 kg N ha⁻¹yr⁻¹, of which dissolved organic nitrogen (DON) accounted for 48%. Compared to natural forest, six years of N addition (NH₄NO₃, 50 kg N ha⁻¹ year⁻¹) significantly (P < 0.05) increased soil N leaching losses by 122%, especially in the form of NO₃⁻; a 30% reduction in throughfall slightly decreased N leaching losses by 23%; in combination, N addition and throughfall reduction increased N leaching losses by 48%. There was a strong interaction between N addition and throughfall reduction, which decreased N leaching loss by approximately 2.5 kg N ha⁻¹ yr⁻¹. Our results indicated that drought would diminish the enhancing effect of N deposition on soil N leaching. These findings highlight the importance of incorporating both N deposition and precipitation and their impacts on soil N leaching into future N budget assessments of forest ecosystems under global environmental change.

The effect of long-term nitrogen (N) and sulfur (S) deposition on litter mass loss and changes in carbon (C), N, and S composition and enzyme activities during litter decomposition was investigated in a boreal forest. This study included four N × S treatments: control (CK), N application (30 kg N ha−1 yr−1), S application (30 kg S ha−1 yr−1), and N plus S application (both at 30 kg ha−1 yr−1). Two experiments were conducted for 22 months: 1) a common litter decomposition experiment with litter bags containing a common litter (same litter chemistry) and 2) an in-situ litter decomposition experiment with litter from each treatment plot (and thus having different litter chemistry). Litterbags were placed onto the four treatment plots to investigate the direct effect of N and S addition and the combined effect of N and/or S addition and litter chemistry on litter decomposition, respectively. Regardless of the source of litter, N and/or S addition affected C, N and S composition at a certain period of the experiment but did not affect litter mass loss and enzyme activity throughout the experiment, indicating that the N and S addition rates were below the critical level required to affect C and N cycling in the studied ecosystem. However, the greater change in N composition per unit of litter mass loss in the N addition treatment than in the other treatments in the common litter but not in the in-situ litter experiment, suggests that the effect of N addition on N loss and retention depends on the initial litter chemistry. We conclude that the studied N and S addition rates did not affect litter decomposition and elemental cycling in the studied forest ecosystem even though the N and S addition rates were much greater than their ambient deposition rates.

Forest ecosystem has long been suggested as a vital component in the global mercury (Hg) biogeochemical cycling. However, there remains large uncertainties in understanding total Hg (THg) and methylmercury (MeHg) variations and their controlling factors during the whole hydrological processes in forest ecosystems. Here, we quantified Hg mass flow along hydrological processes of wet deposition, throughfall, stemflow, litter leachate, soil leachate, surface runoff, and stream, and litterfall Hg deposition, and air-forest floor elemental Hg (Hg⁰) exchange flux to set up a Hg mass balance in a subtropical forest of China. Results showed that THg concentration in stream was lower than that in wet deposition, while an opposite characteristic for MeHg concentration, and both THg and MeHg fluxes of stream were lower than those of wet deposition. Variations of THg and MeHg in throughfall and litter leachate had strong direct and indirect effects on controlling variations of THg and MeHg in surface runoff, soil leachate and stream, respectively. Especially, the net Hg methylation was suggested in the forest canopy and forest floor layers, and significant particulate bound Hg (PBM) filtration was observed in soil layers. The Hg mass balance showed that the litterfall Hg deposition was the main Hg input for forest floor Hg, and the elemental Hg vapor (Hg⁰) re-emission from forest floor was the dominant Hg output. Overall, we estimated the net THg input flux of 13.8 μg m⁻² yr⁻¹ and net MeHg input flux of 0.6 μg m⁻² yr⁻¹ within the forest ecosystem. Our results highlighted the important roles of forest canopy and forest floor to shape Hg in output flow, and the forest floor is a distinct sink of MeHg.

Average nitrogen (N) deposition across Europe has declined since the 1990s. This resulted in decreased N inputs to forest ecosystems especially in Central and Western Europe where deposition levels are highest. While the impact of atmospheric N deposition on forests has been receiving much attention for decades, ecosystem responses to the decline in N inputs received less attention. Here, we review observational studies reporting on trends in a number of indicators: soil acidification and eutrophication, understory vegetation, tree nutrition (foliar element concentrations) as well as tree vitality and growth in response to decreasing N deposition across Europe. Ecosystem responses varied with limited decrease in soil solution nitrate concentrations and potentially also foliar N concentrations. There was no large-scale response in understory vegetation, tree growth, or vitality. Experimental studies support the observation of a more distinct reaction of soil solution and foliar element concentrations to changes in N supply compared to the three other parameters. According to the most likely scenarios, further decrease of N deposition will be limited. We hypothesize that this expected decline will not cause major responses of the parameters analysed in this study. Instead, future changes might be more strongly controlled by the development of N pools accumulated within forest soils, affected by climate change and forest management.

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.

Understorey communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understoreys often respond sensitively but inconsistently to drivers of ecological change, including nitrogen (N) deposition. Nitrogen deposition effects, reflected in the concept of critical loads, vary greatly not only among species and guilds, but also among forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstoreys, and available plant species pools. Nitrogen effects on understorey trajectories can also vary due to differences in surrounding landscape conditions; ambient browsing pressure; soils and geology; other environmental factors controlling plant growth; and, historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understoreys. We review the literature to examine evidence for context dependency in N deposition effects on forest understoreys. We also use data from 1814 European temperate forest plots to test the ability of multi-level models to characterize context-dependent understorey responses across sites that differ in levels of N deposition, community composition, local conditions and management history. This analysis demonstrated that historical management, and plot location on light and pH-fertility gradients, significantly affect how understorey communities respond to N deposition. We conclude that species' and communities' responses to N deposition, and thus the determination of critical loads, vary greatly depending on environmental contexts. This complicates our efforts to predict how N deposition will affect forest understoreys and thus how best to conserve and restore understorey biodiversity. To reduce uncertainty and incorporate context dependency in critical load setting, we should assemble data on underlying environmental conditions, conduct globally distributed field experiments, and analyse a wider range of habitat types.

In Mediterranean areas, dry deposition is a major component of the total atmospheric N input to natural habitats, particularly to forest ecosystems. An innovative approach, combining the empirical inferential method (EIM) for surface deposition of NO₃⁻ and NH₄⁺ with stomatal uptake of NH₃, HNO₃ and NO₂ derived from the DO₃SE (Deposition of Ozone and Stomatal Exchange) model, was used to estimate total dry deposition of inorganic N air pollutants in four holm oak forests under Mediterranean conditions in Spain. The estimated total deposition varied among the sites and matched the geographical patterns previously found in model estimates: higher deposition was determined at the northern site (28.9 kg N ha⁻¹ year⁻¹) and at the northeastern sites (17.8 and 12.5 kg N ha⁻¹ year⁻¹) than at the central-Spain site (9.4 kg N ha⁻¹ year⁻¹). On average, the estimated dry deposition of atmospheric N represented 77% ± 2% of the total deposition of N, of which surface deposition of gaseous and particulate atmospheric N averaged 10.0 ± 2.9 kg N ha⁻¹ year⁻¹ for the four sites (58% of the total deposition), and stomatal deposition of N gases averaged 3.3 ± 0.8 kg N ha⁻¹ year⁻¹ (19% of the total deposition). Deposition of atmospheric inorganic N was dominated by the surface deposition of oxidized N in all the forests (means of 54% and 42% of the dry and total deposition, respectively). The relative contribution of NO₂ to dry deposition averaged from 19% in the peri-urban forests to 11% in the most natural site. During the monitoring period, the empirical critical loads provisionally proposed for ecosystem protection (10–20 kg N ha⁻¹ year⁻¹) was exceeded in three of the four studied forests.

Forests are considered a pool of mercury in the global mercury cycle. However, few studies have investigated the distribution of mercury in the forested systems in China. Tieshanping forest catchment in southwest China was impacted by mercury emissions from industrial activities and coal combustions. Our work studied mercury content in atmosphere, soil, vegetation and insect with a view to estimating the potential for mercury release during forest fires. Results of the present study showed that total gaseous mercury (TGM) was highly elevated and the annual mean concentration was 3.51 ± 1.39 ng m−2. Of the vegetation tissues, the mercury concentration follows the order of leaf/needle > root > bark > branch > bole wood for each species. Total ecosystem mercury pool was 103.5 mg m−2 and about 99.4% of the mercury resides in soil layers (0–40 cm). The remaining 0.6% (0.50 mg m−2) of mercury was stored in biomass. The large mercury stocks in the forest ecosystem pose a serious threat for large pluses to the atmospheric mercury during potential wildfires and additional ecological stress to forest insect: dung beetles, cicada and longicorn, with mercury concentration of 1983 ± 446, 49 ± 38 and 7 ± 5 ng g−1, respectively. Hence, the results obtained in the present study has implications for global estimates of mercury storage in forests, risks to forest insect and potential release to the atmosphere during wildfires.