Atmospheric nitrogen and sulfur pollution increased over much of the United States during the twentieth century from fossil fuel combustion and industrial agriculture. Despite recent declines, nitrogen and sulfur deposition continue to affect many plant communities in the United States, although which species are at risk remains uncertain. We used species composition data from >14,000 survey sites across the contiguous United States to evaluate the association between nitrogen and sulfur deposition and the probability of occurrence for 348 herbaceous species. We found that the probability of occurrence for 70% of species was negatively associated with nitrogen or sulfur deposition somewhere in the contiguous United States (56% for N, 51% for S). Of the species,15% and 51% potentially decreased at all nitrogen and sulfur deposition rates, respectively, suggesting thresholds below the minimum deposition they receive. Although more species potentially increased than decreased with nitrogen deposition, increasers tended to be introduced and decreasers tended to be higher-value native species. More vulnerable species tended to be shorter with lower tissue nitrogen and magnesium. These relationships constitute predictive equations to estimate critical loads. These results demonstrate that many herbaceous species may be at risk from atmospheric deposition and can inform improvements to air quality policies in the United States and globally.

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.

Significance Human activities have elevated nitrogen (N) deposition and there is evidence that deposition impacts species diversity, but spatially extensive and context-specific estimates of N loads at which species losses begin remain elusive. Across a wide range of climates, soil conditions, and vegetation types in the United States, we found that 24% of >15,000 sites were susceptible to N deposition-induced species loss. Grasslands, shrublands, and woodlands were susceptible to species losses at lower loads of N deposition than forests, and susceptibility to species losses increased in acidic soils. These findings are pertinent to the protection of biodiversity and human welfare and should be considered when establishing air quality standards.

University of Colorado, INSTAAR, Boulder, CO 80309; University of California, Riverside, Department of Botany and Plant Sciences and Center for Conservation Biology, Riverside, CA 92521; U.S. Environmental Protection Agency, National Center for Environmental Assessment, Washington, D.C. 20460; U.S. Geological Survey, Southwest Biological Science Center, Moab, UT 84532; U.S. Geological Survey, Western Ecological Research Center, Oakhurst, CA 93644; U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO 80226; University of New Mexico, Department of Biology, Albuquerque, NM 87131; USDA Forest Service, Pacific Northwest Region Air Resource Management Program, Corvallis, OR 97339; Marshall University, Department of Biological Sciences, Huntington, WV 25755; USDA Forest Service, Forest Inventory and Analysis Program, Portland, OR 97339; USDA Forest Service, Northern Research Station, Burlington, VT 05405; USDA Forest Service, Forest Inventory and Analysis Program, Anchorage, AK 99501; Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom; Arizona State University, School of Earth and Space Exploration & School of Life Sciences; and University of Wisconsin, Department of Botany, Madison, WI 53706

Plant diversity in groundwater-fed wetland ecosystems is typically extraordinarily high, yet the biogeochemical controls of this diversity are still incompletely understood. We hypothesized that fine-scale variation in sulfide would influence plant community composition via direct phytotoxicity and indirect mediation of phosphorus release from iron, coupled with gradients in other chemicals such as calcium. We measured porewater chemistry and plant species composition at 400 locations within a calcareous rich fen in central New York State. Groundwater-derived calcium (Ca2+) and sulfate and redox-sensitive sulfide and ferrous iron (Fe2+) showed high heterogeneity. Phosphorus availability was limited and not readily traceable to toxic sulfide, whereas nitrogen (TDN) was more abundant than expected. Using the corrected Akaike information criterion to select between competing models of toxic, nutrient, and mixed-chemistry influences on vegetation, we found that hydrogen sulfide explained decreases in total plant cover, cover of the three most frequently occurring species, dicot species density, plant height, and litter accumulation. Furthermore, sulfide coupled with calcium and phosphorus to explain plant species density and composition. Sulfide was more likely to explain decreased cover of dominant species than rare species. The presence and cover of uncommon species was often unexpectedly explained best by Fe2+ and sulfate, but all models of plant responses with environmental predictor variables were better than the “null” model of mean plant response that lacked environmental variables. An integrated geochemical assessment of coupled groundwater chemistry, redox-sensitive chemistry, and nutrient influences on plants demonstrated the importance of phytotoxic sulfide in explaining plant species density and composition.

Atmospheric deposition has long been recognized as an important source of pollutants and nutrients to ecosystems. The need for reliable, spatially explicit estimates of total atmospheric deposition (wet + dry + cloud) is central, not only to air pollution effects researchers, but also for calculation of input-output budgets, and to decision makers faced with the challenge of assessing the efficacy of policy initiatives related to deposition. Although atmospheric deposition continues to represent a critical environmental and scientific issue, current estimates of total deposition have large uncertainties, particularly across heterogeneous landscapes such as montane regions. We developed an empirical modeling approach that predicts total deposition as a function of landscape features. We measured indices of total deposition to the landscapes of Acadia (121 km2) and Great Smoky Mountains (2074 km2) National Parks (USA). Using approximately 300-400 point measurements and corresponding landscape variables at each park, we constructed a statistical (general linear) model relating the deposition index to landscape variables measured in the field. The deposition indices ranged over an order of magnitude, and in response to vegetation type and elevation, which together explained approximately 40% of the variation in deposition. Then, using the independent landscape variables available in GIS data layers, we created a GIS-relevant statistical nitrogen (N) and sulfur (S) deposition model (LandMod). We applied this model to create park-wide maps of total deposition that were scaled to wet and dry deposition data from the closest national network monitoring stations. The resultant deposition maps showed high spatial heterogeneity and a four- to sixfold variation in "hot spots" and "cold spots" of N and S deposition ranging from 3 to 31 kg N x ha(-1) x yr(-1) and from 5 to 42 kg S x ha(-1) x yr(-1) across these park landscapes. Area-weighted deposition was found to be up to 70% greater than NADP plus CASTNET monitoring-station estimates together. Model-validation results suggest that the model slightly overestimates deposition for deciduous and coniferous forests at low elevation and underestimates deposition for high-elevation coniferous forests. The spatially explicit deposition estimates derived from LandMod are an improvement over what is currently available. Future research should test LandMod in other mountainous environments and refine it to account for (currently) unexplained variation in deposition.

Pocket gophers (Geomys pinetis), Gopher Tortoises, armadillos, and fire ants were the primary soil disturbance agents in a longleaf pine ecosystem. Pocket gopher mounds were the most abundant soil disturbance and covered the greatest percentage of the study area. The most prominent feature of the pocket gopher soil disturbance regime was a strong peak in mound formation from November to January each year in the three-year study, with the location of mound clusters shifting from year to year. During the three-year study, the area disturbed in 0.25-ha plots ranged from 0.7-1.0% yr(-1). Pocket gopher mound formation rates were negatively correlated with air temperature and influenced by soil type in some cases, but mostly unaffected by prescribed fires.

Mounds formed by pocket gophers (Geomys pinetis) in a longleaf pine-wiregrass savanna had larger daily soil temperature fluctuations than surrounding matrix locations. Microsite temperature differences persisted for more than a year, but were largest during summer months. Mound soil also had lower total carbon, total nitrogen, available ammonium, available phosphate and slightly lower Soil moisture than matrix locations. These differences occurred in the context of an ecosystem where frequent fire consumes organic matter and removes herbaceous cover. The distinct microclimate and nutrient characteristics of gopher mounds did not significantly influence wiregrass germination, wiregrass seedling survival or longleaf pine Seedling survival. Wiregrass germination and seedling survival in the first growing season was low and did not differ between mound and matrix locations. Longleaf pine Seedling survival in the first growing season was 64-77% in the absence of fire, but no more than 15% in blocks that were burned. To see a relationship between mound microsite conditions and biotic response, it may be necessary to focus on microbes or Writ to other ecosystems.

A map of the vegetation of the Catskill Park, NY, was created using multi-temporal Landsat Thematic Mapper TM data and ancillary spatial data to support ecological studies in Catskill watersheds. The map emphasizes forest types defined by dominant tree species and depicts 24 vegetation classes. Mapping included a series Of Supervised classifications in a decision tree framework that allowed forest types to be distinguished using spectral characteristics and other environmental relationships (e.g., landscape position, elevation). Traditional contingency table analysis (based on limited ground sampling) suggests overall map accuracy ranging from 28% to 90%, depending on the level of aggregation of the original 24 map classes. Fuzzy accuracy assessment based on the same ground data suggests a 71% level of acceptable classification. The map indicates that maple-dominated forests are predominant in the Catskill region, but that beech and birch-dorninated forests become more important at higher elevations. Oak-dominated forests are very important along the eastern side of the Catskills, and conifer-dominated forests are largely restricted to mountaintops and stream bottoms.

Throughfall, the solution that falls from the forest canopy, is an important and commonly measured flux in forest ecosystem studies. Throughfall water and chemistry are highly variable spatially, requiring large numbers of collectors to quantify it. This and the fact that the solution can be chemically unstable make throughfall sampling very labor intensive, thus we have developed a method to reduce the field labor portion of this effort. Our throughfall collection method uses compact ion exchange resin columns that need only be collected every 1–2 months. The resin columns are subsequently extracted with 1.0 M potassium iodide (KI), releasing anions back into solution, with extraction efficiencies > 94% for sulfate, nitrate, and chloride. The extracts are analyzed by ion-chromatography (IC) to determine the total microequivalents of anions per unit area of collector surface collected over the period of resin column exposure. This ion exchange resin method was originally developed for a project in which we needed to deploy over 300 throughfall collectors to quantify throughfall variability across mountainous terrain with heterogeneous vegetation.

Soil disturbance by pocket gophers (Geomys pinetis) in a longleaf pine ecosystem had a limited impact on plant community composition, in contrast to significant changes documented in grassland ecosystems. No species or functional groups (grasses, forbs, or legumes) increased on pocket gopher mounds relative to undisturbed plots. Seedling recruitment was low, annuals were uncommon, and exotics were absent, with or without fire or soil disturbances. As a result of resprouting, however. plant stem density and species richness of plots measured before fire, one month after fire, 13 months after fire. and 25 months after fire were qualitatively similar. Simulated root destruction treatments kept bare soil microsites open longer, but still did not make them suitable refugia for colonizing species. The resprouting response shown here may be common in other ecosystems where climatic conditions are extreme. clonal perennial plants dominate the flora. and a history of frequent fire has exerted strong selective pressure for a generalized resprouting response to disturbance.