Harvard Forest LTER

Determining the Mechanisms for Carbon Sequestration in Temperate Forests

Northern mid-latitude forests appear to be important sinks for as much as global 25% of the CO2 added to the atmosphere-globally-by fossil fuel burning. But the net sink varies interannually, correlating with global-scale climate variations.When the HFR-LTER was initiated in 1989, the group set out to determine whether typical New England forests were a net sink for CO2, how large the sink was, and which processes controlled it. Rates for Net Ecosystem Exchange (NEE) of CO2 at Harvard Forest have been measured each hour since 1990, using the eddy covariance technique.

Very different processes control carbon uptake on long and short time scales. Ecosystem responses to weather and climate (e.g., variations in growing season length or cloudiness) regulated seasonal and annual fluctuations. Land-use legacies, especially those affecting stand age and composition, dominate carbon balances in longer time scales. Thus short-term variations of NEE at Harvard Forest reflect prompt forest response to environmental influences, while inter-annual variations reflect interactions of climate variations with ecosystem properties on annual time scales, affecting tree mortality, respiration rates, length of the growing season, and available light. Seasonal and annual climatic anomalies are often coherent over large spatial scales, and the processes described here are significant in mediating inter-annual variations of the rate of increase of atmospheric CO2. The long-term carbon budget at Harvard Forest reflects the slowest-changing ecosystem properties: stand age and composition, soil fertility, coarse woody debris. The enormous areas occupied by mid-succession forests (30-100 yrs old) in the U.S. have been cited as the major factor in present terrestrial uptake of carbon, for which these studies provide strong quantitative support and mechanistic understanding.

The long-term record of NEE (Fig. 1a) shows average net uptake of 2.0(0.4 Mg C ha-1yr-1, with interannual variations exceeding 50%.

Long-term Investigations of Nitrogen Saturation in Temperate Forest Ecosystems

Human activity has augmented the cycling of nitrogen through the biosphere more than any other element.Nitrogen is a key element in the acidification of soils and streams due to acid deposition, and has profound effects on ecosystem function. In 1989, the HFR-LTER group published a series of hypotheses concerning the expected responses of N limited forest ecosystems to chronically elevated N deposition. The findings show that it is indeed possible to induce nitrification and nitrate leaching with chronic N additions, although this took much longer in the very N-poor hardwood stand than expected (Fig. 1). Mechanisms by which large amounts of added N are retained in these systems are now a key area of study. In addition, the application of nitrogen did induce significant reductions in growth rates in the pine stand (Fig. 2). By the year 2001, the pines receiving the highest level of N deposition were dead. To-date there has not been any growth declines in deciduous, broad-leaved stands. This result may assist us in interpreting historical episodes of forest decline and anticipating future trends in forest health in areas of high N deposition

Figure 2. Cumulative uptake compared well with biometric data that indicated storage of 1.6(0.4 Mg C ha-1 yr-1) over 8 years, 60% in live biomass and the balance in coarse woody debris and soils