Harvard Forest >

Harvard Forest Symposium Abstract 2013

• Title: Carbon dynamics and responses to disturbance in two New England forest stands
• Primary Author: Allison Dunn (Worcester State University)
• Abstract:

  In this study, we investigate the carbon balance of two differently-aged stands in Harvard Forest, and their differential response to the 2008 ice storm. This study began in 2008 and consists of twelve plots across two separate study areas. One study area ("control") is the unharvested portion of plantation 25-H in the Prospect Hill Tract, which was planted in 1925 with red pine. The other study area ("harvest") is a nearby former red pine plantation that was harvested in 1990. Six 10-m radius circular plots were established at random locations in each of the study areas. All trees ≥ 5 cm diameter breast height (DBH; 1.3 m) were identified, measured, and tagged, totaling nearly 800 trees across the 18 plots. Coarse and fine woody debris ≥ 2.5 cm diameter was surveyed using the line-intercept method along randomly oriented 10 m transects. Annual DBH surveys were conducted from 2008 through 2012; woody debris surveys in 2008 and 2010.
  
  
  
  Aboveground woody biomass was calculated using species-specific allometric equations. Both the control site and the harvest site had significant changes in aboveground carbon storage over the 2008-2011 study period. Aboveground carbon storage at the control site declined from 137.2 ± 7.8 Mg C ha-1 in 2008 to 114.1 ± 19.9 Mg C ha-1 in 2011 (Figure 1). The harvest site, in contrast, increased its aboveground carbon storage over the same time period (35.2 ± 9.9 Mg C ha-1 to 41.6 ± 10.6 Mg C ha-1).
  
  
  
  Aboveground biomass represents the residual between annual growth and mortality. Growth at the 1990 harvest site averaged 3.5 Mg C ha-1 yr-1 (Figure 2), significantly higher than that observed at the nearby EMS tower (1 – 2.5 Mg C ha-1 yr-1, Urbanski et al. 2007). The annual fluxes at the control site present a more complex picture (Figure 3). Mortality rates in the control site between September 2008 and September 2009 averaged 11.4 Mg C ha-1 yr-1, significantly above the mortality rates of 0.6 Mg C ha-1 yr-1 measured near the EMS tower (Barford et al. 2001.) This mortality pulse is attributable to the ice storm of December 2008, which had a significant impact on the overstory of red pine. Red pine mortality from 2008-2010 was 21.8 Mg C ha-1, representing a 16% decline in total species biomass. In contrast, the understory species increased in biomass during the same time period, adding 1 Mg C ha-1 yr-1.
  
  
  
  Woody debris biomass was calculated using the methods of Van Wagner (1968) and the decay-class- specific bulk densities of Liu et al. (2007.) The 2008 ice storm had a significant impact on woody debris stocks at the red pine plantation, which increased from 6.8 Mg C ha-1 to 29.9 Mg C ha-1 between 2008 and 2010. The harvest site, in contrast, experienced no ice storm-related enhancements to woody debris stocks, which slightly decreased from 5.8 Mg C ha-1 to 4.1 Mg C ha-1 between 2008 and 2010. This pulse of woody debris into the control site by the ice storm will affect ecosystem carbon exchange for decades as it decomposes. Respiration from the 2010 woody debris pool was estimated using the model of Liu et al. (2007) using decay-class specific decay constants. Carbon loss to the atmosphere from the ice-storm enhanced woody debris pool at the red pine plantation is significant, and will remain above 0.5 Mg C ha-1 through 2030.
  
  
  
  Carbon exchange at these sites evidently depends on both stand age and disturbance history. The harvest site is clearly in a period of vigorous growth, sequestering an average of 3.5 Mg C ha-1 y-1. In contrast, the control site is in a period of decline, vulnerable to carbon loss via disturbance (11.4 Mg C ha-1 yr-1 following the ice storm) and via slow shrinkage (0.5 Mg C ha-1 y-1 for red pines). Our results indicate that the control site has shifted from a carbon sink to a carbon source, and that disturbance events such as ice storms serve as an accelerant to the process. The long-term legacy of this ice storm is significant; the carbon efflux, especially through 2020, is of a similar magnitude to that sequestered by a nearby 75-100-year old stand at Harvest Forest (1.0 – 4.7 Mg C ha-1 yr-1, Urbanski et al. 2007). These results also underscore the resilience of younger forest stands like the control site, which is vigorously sequestering carbon, and will likely do so for many decades to come.
  
  

• Research Category:


• Figures:
C:UsersadunnDocumentsProfessionalHarvard Forestsymposium2013Figure1.pdf
C:UsersadunnDocumentsProfessionalHarvard Forestsymposium2013Figure2.pdf
C:UsersadunnDocumentsProfessionalHarvard Forestsymposium2013Figure3.pdf