title: Chronic nitrogen deposition alters tree allometric relationships: implications for biomass production and carbon storage.
abstract: As increasing levels of nitrogen (N) deposition impact many terrestrial ecosystems, understanding the potential effects of higher N availability is critical for forecasting tree carbon allocation patterns and thus future forest productivity. Most regional estimates of forest biomass apply allometric equations, with parameters estimated from a limited number of studies, to forest inventory data (i.e., tree diameter). However most of these allometric equations cannot account for potential effects of increased N availability on biomass allocation patterns. Using 18 yr of tree diameter, height, and mortality data collected for a dominant tree species (Acer saccharum) in an atmospheric N deposition experiment, we evaluated how greater N availability affects allometric relationships in this species. After taking into account site and individual variability, our results reveal significant differences in allometric parameters between ambient and experimental N deposition treatments. Large trees under experimental N deposition reached greater heights at a given diameter; moreover, their estimated maximum height (mean ± standard deviation: 33.7 ± 0.38 m) was significantly higher than that estimated under the ambient condition (31.3 ± 0.31 m). Within small tree sizes (5-10 cm diameter) there was greater mortality under experimental N deposition, whereas the relative growth rates of small trees were greater under experimental N deposition. Calculations of stemwood biomass using our parameter estimates for the diameter-height relationship indicated the potential for significant biases in these estimates (~2.5%), with under predictions of stemwood biomass averaging 4 Mg/ha lower if ambient parameters were to be used to estimate stem biomass of trees in the experimental N deposition treatment. As atmospheric N deposition continues to increase into the future, ignoring changes in tree allometry will contribute to the uncertainty associated with aboveground carbon storage estimates across a forest with a large geographic distribution in eastern North America.
authors.forename: Inés
authors.initials: I
authors.lastname: Ibáñez
authors.forename: Donald R
authors.initials: DR
authors.lastname: Zak
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Burton
authors.forename: Kurt S
authors.initials: KS
authors.lastname: Pregitzer
mesh_terms: D031002:Acer / Q000502:physiology*; D018533:Biomass; D002244:Carbon / Q000378:metabolism*; D009584:Nitrogen / Q000737:chemistry*; D013997:Time Factors; D014197:Trees / Q000378:metabolism*
chemical_list: D002244:Carbon; D009584:Nitrogen