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Harvard Forest Symposium Abstract 2017

  • Title: Twenty Five Years of Flux Observations at the Harvard Forest; Healthy, mature Northeastern Forests are a Consistent Carbon Sink
  • Primary Author: J. William Munger (Harvard University)
  • Additional Authors: David Orwig (Harvard Forest); Timothy Whitby (Harvard University); Steven Wofsy (Harvard University)
  • Abstract:

    Because trees grow slowly, long-term observations are essential for detecting responses to climatic shifts and vegetation dynamics in forests. At the Harvard Forest in central Massachusetts, eddy-covariance fluxes have been measured since 1991 in a hardwood-dominated stand that was established in the early 1900's. Flux measurements commenced in 2004 for a hemlock-dominated stand that has been present for at least 200 years. Plot-based observations surrounding the towers provide complementary data on species composition, woody biomass, and litter production. Above-ground biomass has increased more than 30%. Mean annual temperatures have been rising on average 0.3C per decade. The hardwood stand is a net carbon sink, with periods of increasing carbon uptake punctuated by downturns in response to combinations of unfavorable conditions. The hemlock stand has lower peak carbon uptake rates than the oak dominated hardwood, but because it is evergreen, carbon uptake starts early in the spring and continues until late fall while temperatures are above freezing. The resulting rates of woody biomass accumulation in the two stands are remarkably similar despite differences in age. Over the past 5 years an infestation by hemlock wooly adelgids (HWA) has led to a noticeable decline in CO2 uptake by the Hemlock stand, and death of a few individual trees (Figure 1). HWA will ultimately kill all the hemlocks; hardwoods (mostly black birch) will sprout in the gaps as they form. Because hemlock stands have different seasonality and provide a distinct soil and sub-canopy light environment, their mortality and replacement by hardwood species will have significant impacts on forest dynamics and carbon balance.
    A key observation from this work is that in the absence of severe disturbance or management activity, mature northeastern forests are consistent carbon sinks. Moderate disturbances by ice-storms, cold-cloudy springs, and summer droughts cause at most a reduction in carbon uptake for at most 1-2 years, but do not release accumulated carbon. The outcome of HWA infestation in this intensively studied forest will allow quantification of ecosystem carbon metabolism in response to a stand-replacing disturbance.

    Figure 1. Average carbon fluxes observed the deciduous-dominated EMS stand, the Hemlock-dominated Hemlock stand, and the mixed forest stand outside the Hemlock sector are computed as 8-day means for the pre-adelgid period prior to 2012 (solid lines) and the post-adelgid period after 2012 (dashed lines). Negative values are CO2 flux from atmosphere to forest. The annual cycle of carbon flux has not changed noticeably in the EMS stand. The Hemlock dominated stand, where adelgid damage is observed, shows a large decrease in the magnitude of net carbon uptake in the peak summer growing season. Fluxes at the Hemlock tower influenced by mixed forest outside the predominantly hemlock sector show some reduction in carbon uptake only in the shoulder season when deciduous trees are still dormant. Numbers in the legend indicate the sum of CO2 flux during the growing season in kg m-2.

  • Research Category: Large Experiments and Permanent Plot Studies; Invasive Plants, Pests & Pathogens; Forest-Atmosphere Exchange

  • Figures:
  • Annual_FCO2.pdf