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Harvard Forest Symposium Abstract 2012

• Title: Ecosystem physiology in disturbed environments of New England: Understanding processes and assessing climate implications from plots to landscapes
• Primary Author: Christopher Williams (Clark University)
• Additional Authors: Julian Hadley (Harvard Forest); J. William Munger (Harvard University); Melanie Vanderhoof (Clark University)
• Abstract:

  Temperate forests store a large amount of carbon vulnerable to release from climate change, disturbance and land conversion processes. Assessing such vulnerability is one of the grand challenges facing terrestrial ecosystem ecologists and earth system scientists today. Temperate forests also play an important role in regulating Earth’s climate system via biogeophysical effects of shortwave and longwave radiative fluxes and evapotranspiration. This raises another grand challenge we face, namely, to quantify the full climate forcings of land change, including not just biogeochemical effects but biogeophysical effects as well. Earth system models have provided important clues, suggesting that these carbon and energy balance (esp. albedo) effects from deforestation and land conversion lead to cooling in the boreal zone and warming in the tropics, but are offsetting in the temperate zone. However their precise balances are poorly understood in part because of a lack of field measurements. And while land conversions have received significant attention, cycles of forest disturbance and recovery also perturb carbon, water, and energy balances in significant ways. This process of land conversion within-class, i.e. “forest land remaining forest land”, has yet to be incorporated into such models and is also poorly understood because of limited observations.
  
  
  
  This project is quantifying such variation and indentifying underlying mechanisms with detailed observations across the local network of flux towers including the recent and ongoing measurements in hardwood deciduous broadleaf forest (LPH, EMS), the hemlock stand (HEM), and the ClearCut site (CC) During the group’s first three years of research experience at Harvard Forest we have begun to document the early trajectory of carbon, water, and energy in the ClearCut and have started to develop new synthesis from comparisons among sites within the local network. Preliminary findings from the Harvard Forest CC site indicate that gross ecosystem productivity (GEP) was initially low in the clearcut during the first year of regrowth. However, after just one year of recovery GEP at the clearcut already recovered to a level similar to that in the mature deciduous forest (EMS site), in part due to the clearcut’s earlier green-up and productivity in spring. Despite this rapid return of productivity, the ClearCut site continued to emit a large amount of CO2 with massive ecosystem respiration (Reco) partly owing to vigorous decomposition of the abundant coarse woody debris (17% based on ancillary measurements), litter, and dead roots left onsite after harvest. It remains to be seen how long this site will act as a net source of CO2 to the atmosphere. Clearcutting has also influences water fluxes. Direct measurement of evapotranspiration (ET) in the Harvard Forest ClearCut site has shown initial reduction in the first year after clearing but greater annual ET than the neighboring mature forests by the second year. This rapid recovery of ET is a somewhat surprising result, and emerges partly from elevated water fluxes recorded in winter, spring and fall potentially linked to evaporation of snowmelt on the sun-exposed snowpack and/or earlier leaf-out and later senescence. Albedo and surface energy balance results have yet to be compared across the network.
  
  
  
  Figure 1. Total climatic season (DJF, MAM, etc.) and annual (ANN) carbon dioxide and water fluxes for four flux tower sites within the Harvard Forest LTER network. Abbreviations are: Gross Ecosystem Productivity (GEP), Net Ecosystem Productivity (NEP), Ecosystem Respiration (Reco), clearcut site in 2009 (CC09) and 2010 (CC10), the mature hardwood site (EMS), the intermediate aged hardwood site (LPH), and the hemlock stand (HEM).

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


• Figures:
WilliamsCA_HFSymposium2012_Fig1.png