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Harvard Forest Symposium Abstract 2009

  • Title: 20 years of NEE measurements at the Harvard Forest Environmental Measurement Site
  • Primary Author: J. William Munger (Harvard University)
  • Additional Authors: Elizabeth Hammond Pyle (Harvard University); Josh McLaren (Harvard University); Leland Werden (Harvard University); Steven Wofsy (Harvard University)
  • Abstract:

    The Harvard Forest Environmental Measurements Site (HFEMS) flux tower is entering its twentieth year of operation, making this an auspicious point to reflect on the observed trends. A 30m tower and adjacent instrument building were constructed in the summer of 1989 and first measurements started in October. Initial instrumentation and procedures were not consistent with later operation and the record of continuous CO2 exchange data starts at the end of the 1991 growing season. In 1993 a set of 40 ecological sampling plots were established along 8 500m transects radiating to the northwest and southwest of the flux tower to quantify above-ground biomass and species composition. Three plots were inundated when beavers dammed a nearby stream and 3 were located in an area that was selectively harvested. Biomass data are derived from the 34 remaining undisturbed plots. All trees larger than 10cm dbh were fitted with band dendrometers in 1998 and diameter increment has been measured annually since then. Additional plot measurements include annual litter input, Leaf Area Index (LAI), surveys of coarse woody debris (CWD) and recruitment and mortality of trees in each plot.

    The forest surrounding the HFEMS tower has been accumulating carbon for the duration of the measurement period. Cumulative carbon uptake (Figure 1) shows clearly the annual cycle of CO2 release by respiration in the dormant season and CO2 uptake by photosynthesis during the growing season. In each year uptake exceeds the emission. Before 2001 the annual mean carbon uptake was about 2 Mg-C ha-1y-1, and has doubled in recent years with increased interannual variability (Figure 2). Above-ground biomass at HFEMS has increased from 102 to 121 Mg-C ha-1 from 1993 to 2008, with red oak accounting for ~66% of the total increase (Figure 2). Annual litter inputs have grown from 1.27 in 1998 to 1.72 in 2007; leaf area in the canopy has increased, for all measured years except 2006, as well (Figure 3). The capacity of a more than 100 yr old forest to accelerate its carbon uptake rate is somewhat surprising. We attribute it in part to increased dominance of red oak, which has a higher photosynthetic efficiency compared to red maple. Additionally, the number of days per year with net CO2 uptake has been increasing (Figure 4). A linear fit yields a trend of 1.5 day yr-1. Besides extending the duration of carbon uptake, warmer early spring periods may support development of denser canopy and increased nutrient uptake, contributing to an increase in maximum photosynthetic rates in mid summer. These observations highlight the critical contribution from long-term measurements that can observe ecological change on the time scale of successional dynamics and climate variability.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • Munger_HF_symp2008_Fig1.pdf