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Tower-based eddy covariance measurements of forest-atmosphere carbon dioxide (CO2) exchange from many sites around the world indicate that there is considerable year-to-year variation in net ecosystem exchange (NEE). Here, we use a statistical modeling approach to partition the interannual variability in NEE (and its component fluxes, ecosystem respiration, Reco, and gross photosynthesis, Pgross) into two main effects: variation in environmental drivers (air and soil temperature, solar radiation, vapor pressure deficit, and soil water content) and variation in the biotic response to this environmental forcing (as characterized by the model parameters). The model is applied to a 9-year data set from the Howland AmeriFlux site, a spruce-dominated forest in Maine, USA. Gap-filled flux measurements at this site indicate that the forest has been sequestering, on average, 190 g C m−2 yr−1, with a range from 130 to 270 g C m−2 yr−1. Our fitted model predicts somewhat more uptake (mean 270 g C m−2 yr−1), but interannual variation is similar, and wavelet variance analyses indicate good agreement between tower measurements and model predictions across a wide range of timescales (hours to years). Associated with the interannual variation in NEE are clear differences among years in model parameters for both Reco and Pgross. Analysis of model predictions suggests that, at the annual time step, about 40% of the variance in modeled NEE can be attributed to variation in environmental drivers, and 55% to variation in the biotic response to this forcing. As model predictions are aggregated at longer timescales (from individual days to months to calendar year), variation in environmental drivers becomes progressively less important, and variation in the biotic response becomes progressively more important, in determining the modeled flux. There is a strong negative correlation between modeled annual Pgross and Reco ( r=−0.93, P≤0.001); two possible explanations for this correlation are discussed. The correlation promotes homeostasis of NEE: the interannual variation in modeled NEE is substantially less than that for either Pgross or Reco [ABSTRACT FROM AUTHOR]

The great challenge now facing forest ecosystem scientists and managers is to address the need for multiple ecosystem services over relatively large spatial and temporal scales (e.g., whole national forests over 50- to 100-year time frames). Here we present a new conceptual model for the study of forest ecosystems that aids in the analysis of factors that influence ecosystem structure, function, and services. We then go on to show how this model has been applied to the long-term Hubbard Brook Ecosystem Study. Our new model has three main components: (1) controllers, (2) ecosystem pattern and process, and (3) ecosystem functions and services. The controllers are the factors that drive ecosystem pattern and process; we split them into two groups, state factors and variable-stochastic factors. This new model will help to ensure a comprehensive approach to forest ecosystem analysis and will facilitate interactions of research with policy and management at many locations. [ABSTRACT FROM AUTHOR]

Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition and mobility in the environment. They also provide case studies of coastal eutrophication and its effects on terrestrial ecosystems in the northeastern and western United States. Surface water data suggest a significant increase in nitrate losses with N deposition. N deposition is altering N status in northeastern forests.

This article focuses on the multiple linkages among the ecological and human health effects of reactive nitrogen (Nr) molecules as they move from one environmental system to another. Human production of food and energy is the dominant continental process that breaks the triple bond in molecular nitrogen (N) and creates Nr species. Circulation of anthropogenic Nr in Earth's atmosphere, hydrosphere, and biosphere has a wide variety of consequences, which are magnified with time as Nr moves along its biogeochemical pathway. The same atom of Nr can cause multiple effects in the atmosphere, in terrestrial ecosystems, in freshwater and marine systems, and on human health. Breaking the triple bond holding the two N atoms together requires a significant amount of energy--energy that can be mustered only in high-temperature processes or by a small number of specialized N-fixing microbes.

Presents a series of hypotheses regarding the stages of forest ecosystem response to chronic nitrogen deposition from fossil fuel combustion. Discussion of the human contribution to pollutants in the atmosphere and deposition rates on the surface; Effects of nitrogen emissions on air quality, ozone, water quality and forest health; Definition of nitrogen saturation; Stages in the development of nitrogen saturation; Interactions with other forms of air pollution; Ongoing research.

Examines the excess of nitrogen in the ecosystems of North America. Summarization of data collected from various case studies; In-depth look at the nitrogen-saturated ecosystems; Discussion on management strategies used to reduce nitrogen losses from watersheds.

Presents a study which combined leaf-level ozone response data from independent ozone fumigation studies with a forest ecosystem model to stimulate the effects of ambient ozone on mature hardwood forests. Effects of ozone pollution on forest vegetation; Reduction in leaf carbon gain; Detailed information on the results from the study; Information on the sensitivity analysis carried out on forest ecosystem.

Discusses environmental impoverishment and management. Impact of human activity; View that nature exists only where human activity does not; Restoration ecology; Reconstruction of communities and ecosystems.