Effects of nitrogen saturation on tree growth and death in a mixed-oak forest

Ecologists have long treated temperate forests as systems limited by nitrogen (N) availability. Widespread concern about the effects of anthropogenic N on forested ecosystems has inspired experimental simulation of the effects of chronic N addition. This study investigates the effects of 8 years (1996-2003) of simulated chronic N deposition on tree growth and mortality in an upland mixed-oak forest at the Institute of Ecosystem Studies (IES), Millbrook, NY, USA. Nitrogen was added at a rate of 50-100 kg N ha(-1) y(-1), with atmospheric deposition (wet + dry, inorganic N) at the site averaging 10 kg N ha(-1) y(-1). The experiment used a paired design, with six fertilized plots and six non-fertilized control plots. We measured the diameter of all of the trees in the plots, recorded mortality, and extracted increment cores and measured their annual growth rings. We found that fertilization caused both increased mortality (N-saturation effect) and increased relative growth rate of surviving trees (fertilization effect). The combined effect was a net loss of living basal area in most of the fertilized stands. Growth and mortality effects varied among sites and species. Within sites, N fertilization caused increased growth in some trees and mortality of neighboring trees in the same site, indicating that N saturation is manifest as a mosaic phenomenon mediated by sensitivity of individual trees and soil microsites. No interactions were observed between site, treatment, and species for basal area growth, but oak species suffered greater mortality. Decreased Ca:Al ratio in organic soil is a possible explanation for increased mortality, though the specific mechanism is unknown. The growth and mortality responses at this deciduous forest may be more similar to those previously observed in coniferous forests, calling into question the hypothesis that deciduous forests are less sensitive to the effects of N-saturation. (c) 2007 Elsevier B.V. All rights reserved.