Nitrogen deposition influences tree sapling diversity in forests of the southeastern United States

Background/Question/Methods:  A number of ecosystems types have experienced shifts in species diversity as anthropogenic sources increase nitrogen’s presence in the environment. Many studies have analyzed the connection between nitrogen (N) deposition and diversity, although these studies tend to focus on grasslands or small-scale forest patches. Additionally, few studies have focused on the relationship between N deposition and tree diversity. Because of the challenges inherent in studying responses of long-lived species, like trees, sapling trees provide an opportunity to examine tree response to N deposition. We thus pose the following question: How has increased N deposition impacted sapling diversity on a regional scale? To examine this question, we chose to use the southeastern U.S. due to extensive soil and plant community composition data available from the Carolina Vegetation Survey (CVS) database. Using N deposition data from the National Atmospheric Deposition Program (NADP), we created linear mixed models to quantify N deposition’s impact on sapling diversity, and whether there are interactions between N deposition and other factors that can influence sapling survival: light availability, soil pH, and herbaceous plant cover.

Results/Conclusions:  In our study, soil pH does not appear to influence sapling diversity with increasing levels of N deposition. However, we did find two important interactions with N deposition: that of herbaceous plant cover and light availability. We found that N deposition increases sapling diversity in forests with low herbaceous cover, but not in forests with high herbaceous cover. We suggest that the increased N availability encourages tree recruitment for a wider range of species, regardless of competitive ability; but as herbaceous cover increases, only species with higher competitive abilities can survive, and N deposition does not appear to have an effect on sapling diversity in those forests. Forests with higher light availability showed a decrease in sapling diversity with increasing N deposition. The mechanism of this interaction is unclear, but may be related to time since last disturbance, which impacts light availability and species composition. Sapling diversity appears to respond to N deposition, and may do so in unexpected ways. It is important to continue this path of questioning to better understand how the lag effects of N deposition on long-lived species may influence the tree diversity in future forests.