Title: Nitrogen fixing trees in the United States
Description: Patterns and controls of net primary production (NPP) remain a critical question in ecology especially as climate modeling efforts expand. Nutrients, particularly nitrogen (N), can regulate NPP, which couples the N and C cycles. Biological nitrogen fixation (BNF) is the primary natural pathway by which new N enters ecosystems. The magnitude of the natural BNF flux is still not well constrained and the effect of this new N on forest demography and C storage is not well understood. In chapter 1 we use tree census data and two approaches of estimating BNF to make an estimate of the total N fixed by trees across the U.S.: 0.30-0.88 Tg N yr-1 (1.4-3.4 kg N ha-1 yr-1), smaller than previously expected and on par with N inputs from understory or asymbiotic BNF and less than inputs from N deposition. The tree BNF input is dominated by two tree genera: Robinia and Alnus. In chapter 2 we use mixed effect models of forest census data to show that N-fixing trees have no net effect on forest biomass accumulate rate, indicating that though they can fertilize forests on long timescales, during the course of their lives the competitive influences they exert on neighbors balance any fertilization effect they may have.

However, the net effect of N-fixing trees on forest development and carbon storage depends on local factors and can be significantly facilitative in contexts where N-fixers are less competitive or when neighbors occupy different forest niches. In chapter 3 we develop a theoretical model which shows lateral leaf litter is a plausible mechanism for observed N-fixer effects, wherein the percent of litter nutrients shared with neighbors can range from almost 0% for small trees to >90% for large isolated trees in low wind, fast decomposition environments. Litter nutrients spread more in windy environments or from trees whose leaf litter falls farther from trees and diffuses more quickly. In sum, N-fixing trees play an important role in temperate forests representing an important N input, however, the flux is smaller than previously expected and the fertilization effect of N-fixing trees is not observed during the census interval.