Repeated measures from FIA data facilitates analysis across spatial scales of tree growth responses to nitrogen deposition from individual trees to whole ecoregions

The abundance of temporally and spatially consistent Forest Inventory and Analysis data facilitates hierarchical/multilevel analysis to investigate factors affecting tree growth, scaling from plot-level to continental scales. Herein we use FIA tree and soil inventories in conjunction with various spatial climate and soils data to estimate species-specific responses of tree growth to nitrogen (N) deposition across the contiguous United States. Plot-level analyses have shown that N deposition affects tree growth but not uniformly. Increases in bio-available N can stimulate tree growth rates but also impair soil fertility, increase plant susceptibility to pathogen infection, and alter competition between plant species. How these effects scale to regional landscapes will in part determine the trajectory of forest composition and health. We use the repeated measures in FIA data to calculate growth rates of thousands of individual trees nationwide and then compare them to other available spatial data for climate and soil and deposition chemistry. Specifically, we address the following questions: 1) What are the species-specific growth responses to N deposition? and 2) What are the variances of these responses with respect to scales of individual tree, FIA plot, and ecoregion? Tree growth responses were nonuniform across the more than 100 species examined. Growth rates varied across the range of N deposition to include continuous increases in growth, continuous decreases in growth and threshold responses among the different species. Important covariates affecting tree growth in addition to N deposition from FIA data include canopy position and various soil characteristics. We hypothesize that large variances across individual tree, FIA plot, and ecoregion scales will indicate scale-dependent covariates and regional sensitivity of forests to N deposition.