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.    

Our analysis indicates that most forests in the northeastern United States are at risk of detrimental effects from N deposition, although the extent and magnitude of the risk varies spatially Eighty-six percent of the forested area in the region (98M acres) is in exceedance of the 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 (e.g. North Central Hardwood Forests, Erie Drift Plain, Driftless Area, and Southern Michigan/Northern Indiana Drift Plains ecoregions).  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 (e.g. Acadian Plains and Hills and the Northern Minnesota Wetland ecoregions). White pine (Pinus strobus), quaking aspen (Populus tremuloides), yellow birch (Betula alleghaniensis), northern red oak (Quercus rubra) and bigtooth aspen (Populus grandidentata) are in exceedance of the critical load across large portions of the study area–between 31 to 56 million acres are in exceedance for each species. Red pine (Pinus resinosa), pitch pine (Pinus rigida), and chestnut oak (Quercus prinus), which occur over a small geographic extent, are in exceedance of the critical load over 89 to 100 percent of their range.