Chronic nitrogen deposition alters tree allometric relationships and growth resilience to drought: Implications for biomass production and carbon storage under global change
As increasing levels of atmospheric nitrogen (N) deposition impact many terrestrial ecosystems, understanding the potential effects of higher N availability is critical for forecasting future forest productivity and ecosystem carbon (C) storage. Most regional estimates of forest biomass use allometric equations, with parameters estimated from a limited number of individuals that are based only in tree diameter. Presently, allometric equations ignore potential effects of increased N availability on biomass allocation patterns. Using 19 years of tree diameter, height, and mortality collected for a dominant tree species (Acer saccharum) in an atmospheric N deposition experiment, we evaluated how changes in N availability affect allometric relationships. We also investigated how growing under N deposition affected the capacity of this species to recover after a drought period.
Our results reveal significant differences in allometric parameters between ambient and experimental N deposition treatments. Trees under the experimental N deposition reach greater heights at a given diameter; moreover, their estimated maximum height (mean ± SD: 33.7 m ± 0.38 m) was significantly greater than that estimated under the ambient condition (31.4 m ± 0.31 m). Simulations of stem biomass production using our parameter estimates for the diameter-height relationship indicated the potential for significant biases in these estimates (4-2%, with under predictions of stem biomass ranging from 2.44 to 5.96 Mg/ha), if ambient parameters were to be used to estimate stem biomass of trees in the experimental N deposition treatment. When evaluating radial growth rates we found that trees in N deposition had higher growth rates than ambient trees only under wetter than average conditions, under average or drier conditions trees in the ambient treatment grew more and also recovered faster from previous drier conditions.
As atmospheric N deposition continues to increase into the future, ignoring changes in tree growth and patterns of biomass allocation due to this agent of global change will underestimate aboveground C storage across a forest with a large geographic distribution in eastern North America. In addition, as environmental conditions shift the effects of N deposition in carbon storage, positive or negative, will depend on the climatic trajectory of the site.