Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA
Introduction
Our introductory paper for this special section (Aber and Magill, 2004) presents a brief review of the literature on global patterns of N deposition, and the long-term effects of this deposition on forests through nitrogen saturation. This paper describes the Chronic N Amendment Study at the Harvard Forest (HF) (Petersham, MA, USA) in detail and presents the long-term baseline measurements established at the beginning of treatments in 1988. Long-term measurements trace the retention and loss of added N, changes in pool size or concentration in different parts of the system, and the effects of N accumulation on plant and microbial processes, all in the context of the nitrogen saturation hypothesis (Aber et al., 1989, Aber et al., 1998). These basic measurements present the context for the detailed studies reported in other papers in the chronic N special section of this issue.
Section snippets
Study site and experimental design
The chronic nitrogen addition experiment is a core experiment of the Harvard Forest Long-term Ecological Research (LTER), and is located at the Harvard Forest in central Massachusetts (42°30′N, 72°10′W). Two contrasting forest stands were selected for this study, each with different land-use histories that have been well documented as part of the Harvard Forest silvicultural and land management records. The red pine (Pinus resinosa Ait.) plantation was established in 1926 and is located on the
Nitrogen inputs and losses
Background nitrogen deposition at the Harvard Forest (0.66–0.8 g N m−2 per year) is moderate for the northeastern United States (Munger et al., 1996, Ollinger et al., 1993, Lovett and Lindberg, 1993) and substantially lower than many experimental sites in Europe (Dise and Wright, 1995, Dise et al., 1998, Gundersen et al., 1998). We assume that deposition has remained relatively constant over the course of the experiment, as total US emissions of NOx have not changed during the experimental period (
Comparisons with previous work and other studies from the chronic N plots
Many of the findings reported in this paper continue or accelerate trends seen in earlier data summaries of the chronic N study. Losses of inorganic N remain high in the high N plots (higher in pines than hardwoods) and low N plots in the pine stand also have measurable DIN losses. Foliar and fine root N concentrations are elevated significantly. Trends in tree decline in the pine high N plot have accelerated and complete mortality in that stand is likely in the near future. Canopy condition
Acknowledgements
This research was funded by the National Science Foundation Long-Term Ecological Research Program and the USDA National Research Initiative Competitive Grants Program. Many people have contributed to the success and longevity of this project, including Gloria Quigley, Steve Newman, Richard Bowden, Joe Hendricks, Matt Kizlinski, Shannon Cromley, Jeremy Fontenault and Colin Pinney. Special thanks goes to the root sorting crew of 1999: Alex Sherman, Dan Vorosmarty, Cheryl Parker and Tracey
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