Site-level importance of broadleaf deciduous trees outweighs the legacy of high nitrogen (N) deposition on ecosystem N status of Central Appalachian red spruce forests
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Background and aims
Atmospheric nitrogen (N) deposition can influence forest ecosystem N status, and the resilience of forests to the effects of N deposition depends on a number of co-occurring environmental factors that regulate N retention or loss. In this study, we test the idea that N deposition may have important and long-lasting impacts on patterns of N cycling by using field and laboratory techniques to assess N status in seven high-elevation Central Appalachian red spruce (Picea rubens Sarg.) forests located at sites that historically received moderate to high inputs of N atmospheric deposition.
During 2011 and 2012, we measured multiple indices of N availability (e.g. foliar/soil C:N and δ15N, resin ion-exchange, and N transformation rates) that integrate N cycling over seasonal to decadal time scales. Using a model selection approach, we compared the strength of the association between various environmental factors and temporally-integrated indices of N status in a series of regression models.
Site-level differences in the relative importance value of broadleaf deciduous (BD) trees consistently explained most of the observed variation in N status. Soil C:N was significantly lower for sites with greater BD importance (R 2 = 0.67–0.77), and there was a strong positive relationship between BD importance and soil δ15N content (R 2 = 0.64–0.85). Despite a four-fold difference in historic deposition across the seven forest sites, we did not observe any significant relationships between site N status and N deposition.
These findings suggest that potential legacy effects of N deposition were obscured by the influence of BD importance on N status at these sites. Our results add strong support to the idea that predicting the resilience of forests to the effects of N deposition requires detailed knowledge on the contribution of tree species composition to soil N cycling and retention.
KeywordsNitrogen N deposition N availability Broadleaf deciduous trees Red spruce
We would like to thank Amy Hessl, Bradley Breslow, and Benjamin Hedin for assistance with site selection and fieldwork. In addition, we thank the US Forest Service and, in particular, Stephanie Connolly and Kent Karriker for granting us access to these sites to perform fieldwork. Finally we would like to thank Edward Brzostek for his helpful comments and critiques on an earlier draft of this manuscript. This work was supported by the WVU Office of the Dean’s Awards for Research Team Scholarship (ARTS), the WVU Research Corporation’s Program to Stimulate Competitive Research (PSCoR), and in part by the National Science Foundation Research Experience for Undergraduates (NSF-REU) program.
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