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Direct measurements of forest-atmosphere exchanges of carbon dioxide, water, energy, and other trace gases allow us to observe whole-ecosystem metabolism as well as quantify the role of forests as sources of trace gases, the input of gaseous pollutants and nutrients, and the role of forests in cleansing the atmosphere. The eddy-covariance flux tower at the Harvard Forest Environmental Measurements Site (HFEMS) was installed in 1989 and provides the world's longest continuous record of net ecosystem CO2 exchange, evaporation, and energy flux between the atmosphere and a forest, at hourly time resolution. The CO2 exchange is partitioned into respiration and photosynthetic components. Additional towers in a hemlock stand and a younger deciduous stand on Little Prospect Hill were installed to examine the differences in carbon flux due to species composition and stand age. At all three flux-tower sites, the forest is accumulating carbon and contributing to the terrestrial "sink" for carbon that currently is reducing the growth atmospheric CO2. The atmospheric carbon fluxes provide a top-down look at forest growth and are complemented by ground measurements of vegetation and litter at large and small plots where we can identify which components in the forest are gaining and losing carbon. The combination of short-term carbon and water exchange fluxes observed at the towers with long-term changes in biomass and species composition observed at permanent plots provides a very powerful constraint to develop and evaluate ecosystem process models.
Because of its rural location downwind of major east-coast urban areas, but away from nearby pollution sources, the Harvard Forest site is an important observatory for tracking regional air quality. In addition to the carbon flux measurements at HFEMS, we measure ozone and nitrogen oxide to quantify potential air pollutant impacts on the forest and assess the role of the forest in removing these pollutants from the atmosphere. Several measurement campaigns have focused on emissions of volatile hydrocarbons such as isoprene from the vegetation. These compounds are an important precursor for atmospheric chemical reactions.
Key facilities and measurement programs
- 4 eddy flux and walk-up towers
- Various soil-respiration chamber campaigns
- Micrometerological - advection studies