Forest health is affected by multiple stress agents (e.g. pests and pathogens, climate extremes and deposition) further influenced by site and stand characteristics. The critical load - the level of deposition below which no harmful ecological effects are expected to occur – can be used to quantify risk to forest health. In this study, Nitrogen Critical Loads Assessment by Site (N-CLAS), an online GIS analysis and visualization tool, was used to examine spatial patterns of critical loads and exceedance of N deposition for 23 tree species of management concern across 12 level III ecoregions in the northeastern United States.
This novel, high resolution (30m) landscape-scale assessment maps critical loads, target loads, and exceedances for each species present through a series of calculations that incorporate the influence of climate, topographic and soil factors that modify tree response to N deposition. For every pixel across the landscape, N-CLAS calculates an adjusted critical load for individual species and an aggregate critical load for all species present.
Results/Conclusions
Our analysis indicates that eighty-six percent of the forested area in the region (98M acres) is in exceedance of the most protective critical load. The magnitude of this exceedance is highest (6-8 kg N ha-1 yr-1) in the southwestern part of the study region where N deposition is highest. The magnitude of exceedance is lowest (1-2 kg N ha-1 yr-1) in the northeastern and northwestern part of the study area where N deposition is lowest.
For forests in the northeastern U.S., N-CLAS indicates that the most N-sensitive species - those with the lowest critical loads across large portions of the study area– include yellow birch (Betula alleghaniensis), butternut (Juglans cinerea), eastern white pine (Pinus strobus), Bigtooth aspen (Populus grandidentata), Quaking aspen (Populus tremuloides), northern red oak (Quercus rubra), and American elm (Ulmus americana). While the extent and magnitude of the risk varies spatially, these species consistently determine the most protective critical load thresholds in the mixed stands they occupy.
This analysis indicates that N deposition continues to be a risk factor for forests across the northeast, with a high degree of spatial variability that should be used to inform management plans. Because temperature and precipitation values outside of the optimum range were common drivers of low adjusted critical loads for sensitive species, such analyses will become more important as the effects of climate change continue to impact the ability of the region's forests to respond to stressors.