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
Smith, K. R., J. M. Mathias, B. E. McNeil, W. T. Peterjohn and R. B. Thomas . 2016.
Abstract
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.
Methods During 2011 and 2012, we measured multiple
indices of N availability (e.g. foliar/soil C:N and δ15
N,
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 actors and temporally-integrated indices of N status in a
series of regression models.
Results 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 (R2 = 0.67-0.77), and there was a strong
positive relationship between BD importance and soil δ15
N content (R2 = 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.
Conclusions 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.
Key Words
Nitrogen . N deposition . N availability . Broadleaf deciduous trees . Red spruce
