Controls on ecosystem nitrogen (N) status and carbon (C) balance in red spruce forests located in a region of historically high atmospheric N deposition.

Atmospheric deposition of reactive nitrogen (N) can have a strong influence on patterns of carbon (C) and N cycling in forest ecosystems. High-elevation red spruce (Picea rubens Sarg.) forests in the central Appalachian Mountains have historically been among the most sensitive ecosystems to the negative effects of N deposition. I examined the primary environmental factors that influence 1) N status, 2) belowground C balance, and 3) red spruce aboveground growth in seven high-elevation study sites that are located in an area where N deposition has been historically moderate to high. I established the seven sites in 2011 along a transect of N deposition within the central Appalachian Mountains with the goal of determining whether historically elevated rates of N deposition were continuing to have long-term effects on ecosystem function. With respect to N-induced changes to forest N status, I found no evidence that patterns of N availability were associated with N deposition; instead, I found that site-level differences in the abundance of broadleaf deciduous tree species were a consistently better predictor of N status at these sites. Likewise, annual rates of soil respiration and total belowground C allocation (TBCA) were positively correlated with a higher abundance of broadleaf species, but were unrelated to N deposition. These results suggest that the abundance of broadleaf tree species had a strong influence on N status and soil C balance via differences in the quality and decomposability of aboveground litterfall inputs. I found that the variation in red spruce aboveground growth along this transect was primarily related to differences in mean annual temperature between sites. I observed a weak negative trend relating red spruce growth to imbalances in foliar gas exchange (i.e. ratio of foliar dark respiration to net photosynthesis), suggesting that tree growth was reduced in locations where photosynthetic C uptake was more significantly offset by C losses via respiration. Overall, these red spruce growth patterns indicate that cooler temperatures were a stronger limiting factor to aboveground production than N availability or N deposition. This research suggests that environmental factors apart from N deposition control C and N cycling dynamics in these forests, despite the fact that red spruce ecosystems throughout the Northeast have been negatively affected by acidic deposition in the past. However, while rates of N deposition have significantly declined since their peak in the mid 1980s, it is possible that current rates of N deposition may still have some impacts on ecosystem function, but these effects were undetectable given the overwhelming influence of tree species. Overall, these results are encouraging and could indicate that red spruce forests in this region are in a period of recovery from pollution