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Harvard Forest Symposium Abstract 2014

• Title: Long term trends and variability in carbon exchange at Harvard Forest EMS
• Primary Author: J. William Munger (Harvard University)
• Additional Authors: Evan Goldman (Harvard University)
• Abstract:

  The Harvard Forest Environmental Measurements Site (HFEMS) tower was established in 1989. Continuous CO2 flux measurements that can be integrated to determine annual carbon exchange between forest and atmosphere have been made since 1992. In 1995 we established a set of 10m diameter plots along eight 500m transects radiating from the tower in the dominant upwind direction to sample the vegetation in the tower footprint. These have been resurveyed annually to quantify above ground woody increments. In addition, leaf area index (LAI) has been measured on each plot, with about biweekly measurements throughout the growing season since 2005. These LAI measurements identify variation in phenology as well as the maximum values for LAI each year (Figure 1). Litter is collected from the plots each year and sorted by species to determine species-specific litter inputs.
  
  Throughout the last 2 decades the forest surrounding HFEMS has been accumulating carbon. Annual NEP increased from a mean value of about 2 Mg-C ha-1y-1 during the 1990’s up to a maximum of 6 Mg-C ha-1y-1 in 2008. NEP declined over the next two years, following an ice storm in 2008, and a particularly wet and cloudy spring in 2009 (Figure 2). Measurements of fine and coarse woody debris in 2009 documented a significant pulse of woody litter from the ice storm. Resurveys in the summer of 2013 suggest that the pulse of debris from that storm has decayed significantly and the pool of dead wood near EMS is consistent with the growth trend that was apparent before 2008. Annual NEP recovered in 2011 and 2012. The status of the forest canopy as indicated by maximum LAI during the growing season, and the onset of canopy development influences annual NEP. Annual carbon balances were reduced in 1998 and 2009, when maximum LAI was reduced. Above-ground wood growth generally follows annual NEP, though there is some evidence for a lag. AGWI continued to increase from 2005 onward until it dropped in 2011, while NEP showed a similar reduction the year before.
  
  A synthesis of several temperate forest sites including the Harvard Forest EMS shows a general pattern of increasing annual NEP. The data are consistent with a trend of increasing water use efficiency enabled by rising ambient CO2 levels [Keenan et al.,2013].
  
  
  
  
  
  Keenan, T. F., D. Y. Hollinger, G. Bohrer, D. Dragoni, J. W. Munger, H. P. Schmid, and A. D. Richardson (2013), Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise, Nature, 499(7458), 324-+, doi:10.1038/nature12291.
  
  

• Research Category: Forest-Atmosphere Exchange
  Large Experiments and Permanent Plot Studies


• Figures:
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