Linking plants and soils to understand ecosystem thresholds in response to N deposition at a continental level
Tree mortality in response to elevated nitrogen deposition has been observed in forested ecosystems in the United States, but the mechanisms underlying these declines, and the reasons why some forests appear resilient, are unknown. As part of a USGS Powell Center Working group, we assembled data from plot-based studies across the United States and Canada that included both soil chemistry and plant responses. Our goal was to determine whether soil chemistry could explain heterogeneity in forest responses and to determine how responses varied as a function of tree species, ecosystem, N load, and soil type. We hypothesized that soil base saturation and soil acidity influence tree vulnerability to N deposition. Moreover, we expected that tree species have unique responses to soil chemistry and N load, and that individual tree species can mediate these effects due to internal regulation of carbon balance.
Our review suggests that there are at least three trajectories of tree species' response to nitrogen deposition that can lead to one of several outcomes: enhanced productivity; no change in productivity – potentially mediated through species’ shifts; or, reductions in productivity that can lead to forest decline. Overall, our work enhances understanding of ecosystem threshold dynamics by evaluating the role of initial conditions (e.g., base saturation), individual tree species’ responses, and potential tissue-level, physiological mechanisms (e.g., oxidative stress). This complexity hindered our ability to generalize responses across ecosystems at a continental scale due to a lack of consistent data across studies. However, by generating families of response curves informed by empirical studies, our work contributes to a deeper understanding of the conditions that promote ecosystem resilience.