Harvard Forest Symposium Abstract 2010

Title: Quantifying the Impact of an Ice Storm on the Carbon Budget of a Mixed Deciduous Forest in Central Massachusetts
Primary Author: J. William Munger (Harvard University)
Additional Authors: Leland Werden (Harvard University); Steven Wofsy (Harvard University)
Abstract:
L. K. Werden1, T. L. Connell2, L. K. Sanchez3, S. C. Wofsy1, and J. W. Munger1

1Harvard Forest, SEAS,

2Harvard Forest REU program,

3DOE GCEP undergrad program

Measurements of long term carbon flux (NEE) and biomass inventories at the Harvard Forest Environmental Measurements Site (HFEMS) show increasing carbon sequestration over the past 20 years. The mechanisms behind this increase in carbon storage are not entirely understood, but natural forest succession or recovery from previous disturbances are prime candidates. In December 2008 large areas of central Massachusetts including Harvard Forest were affected by a severe ice storm. Ice storms affect the forest immediately by creating gaps and creating a pulse of woody debris input to the forest floor. The forest may be affected in the following growing season by reduction in leaf area. The magnitude and duration of shifts in the forest carbon balance are difficult to predict. Long-term observations of carbon fluxes at the EMS tower and forest biomass at the array of adjacent ecological plots provide a unique opportunity to assess the impact from this storm. During the summer of 2009 we performed a complete fine and coarse woody debris (FWD, CWD) survey and measured leaf area indices (LAI) on 33 established biometry plots. Although breakage of very large branches and tipping of whole trees was observed around the Harvard Forest, on the 33 plots that were sampled only one had a whole downed tree. Overall statistics given below exclude this plot. If it is included the inputs are larger, but the standard errors are much larger as well. Averaged over the plots, CWD input from the ice storm event alone (0.57 Mg-C ha-1) was comparable to the non-ice-storm CWD that had accumulated over the three preceding years (0.64 Mg-C ha-1) (Figure 1). FWD inputs (0.47 Mg-C ha-1) were comparable to CWD (Figure 2). LAI was reduced on average by 0.86 m2 m 2 compared to summer of 2008 (Figure 3), interrupting a trend of increasing LAI that began in 1998. The magnitude of LAI reduction on a plot was correlated with the quantity of ice-storm-derived FWD on the plot, implicating canopy damage as the main cause of the observed LAI reduction, rather than unusually wet and cloudy weather that characterized the spring and early summer of 2009 (Figure 4). LAI reduction did not correlate to downed CWD, indicating that damage to small twigs and branches, which make up FWD and hold the majority of leaves on each tree, had the most impact on leaf area in the first summer following the storm. Preliminary light curves, relating carbon uptake, determined by eddy covariance at the HFEMS tower, to the photosynthetically active photon flux (PPFD), do not show a significant decline in carbon uptake rates per unit PAR for the post storm year (2009) compared to the pre-disturbance years (2007, 2008).(Figure 5). Though we might have expected carbon uptake to be decreased by the reduction in leaf area it appears that reduction in leaf area may have mostly been in the Red maples (Acer rubrum), which are generally subcanopy species and have lower leaf-level photosynthetic rates. Overall, red maples make an insignificant contribution to annual biomass increase, though they are a major part of the standing biomass, second only to the Red oak (Quercus rubra). 52% of the FWD down due to the ice storm was identified as Red maple (Figure 6). Because the ice storm preferentially removed less productive leaf area it has not had a large impact on the carbon uptake rate in the forest surrounding the EMS tower.
Research Category: Forest-Atmosphere Exchange