Document Details
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Title |
The effects of salvage logging on the net ecosystem productivity of MPB-attacked lodgepole pine forests of the northern-BC interior | ||||||||||
Author |
Brown, Mathew | ||||||||||
Date |
2008 | ||||||||||
Abstract |
Background: The current BC outbreak of mountain pine beetle (MPB) (Dendroctonus ponderosae Hopk.), which began in 2000, had killed a total of 280 million m3 of pine through the end of 2004 (Eng et al. 2005), and is predicted to kill no less than 80% of the pine volume in the province by 2020 (Eng et al. 2005). Pine accounts for almost 30% of the timber volume in the timber harvesting land base of BC. The pine stands located in the Sub-Boreal Spruce biogeoclimatic zone (SBS) account for about 70% of BC?s timber production (Meidinger and Pojar 1991). The MPB outbreak is predicted to peak in 2006 with the annual volume of 90 million m3 merchantable pine killed on the timber harvesting landbase. This compares with the pre-MPB annual allowable cut (AAC) from public lands in BC of about 81.5 million m3 (The State of Canada?s Forests, 2003-4, NRCan). The AAC was increased by 4.9 million m3 in the fall of 2004 to allow for salvage logging in the timber supply areas (TSAs) of Lakes, Quesnel and Prince George (Snetsinger, 2005). Surveys of three pine-leading (>50% pine overstory), MPB-affected stands in north central BC indicated that 20-25% of pine-leading stands had such poor secondary structure that total salvage logging and planting was the only way to mitigate merchantable timber losses (Coates et al., 2006). Monitoring of the regeneration of pine in salvage-logged (SL) and not-salvage-logged (NSL) areas has been recommended (Eng, 2004). Measurements of the annual net ecosystem productivity of salvage-logged clearcuts are a key diagnostic tool for assessing the effects of forest management practices on the ecosystem carbon (C) balance, and hence regenerative potential, of MPB-affected stands. Net ecosystem productivity (NEP) is a direct measure of whether an ecosystem is a source of (NEP < 0), or sink for (NEP > 0) atmospheric C over a time period of interest and is calculated as P ? R: the difference between gross ecosystem photosynthesis (P) and respiration (R). Clearcutting removes the canopy component of the aboveground biomass of the stand, thus dramatically reducing P. The removal of the mature trees also removes a significant component of C release from the metabolic activity in roots, boles and leaves (autotrophic component of R, or Ra). Extremes in microclimate (temperature, atmospheric humidity and soil water content) caused by the shift of radiant energy exchange from up in the canopy to near the ground surface (Spittlehouse and Stathers, 1990) bring about changes in the survival and growth rates of planted seedlings and understory (Fleming et al., 1998). Also affected are soil biological and chemical processes that control the decomposition rates (heterotrophic component of R, or Rh) of belowground biomass and slash created by the logging process (Prescott et al., 1997). C balance research in harvested forest stands: Growing season measurements of NEP have been made in early successional forests following fire (Amiro, 2001), and clearcut harvesting (Kowalski et al. 2003; Pypker and Fredeen, 2002a,b, 2003; Rannik et al, 2002). Stands varying in age from 1 to 10 years following clearcut harvest in a variety of biogeoclimatic zones have been observed to be annual CO2 sources. Humphreys et al. (2005) used year-round eddy covariance measurements to determine that a harvested coastal Douglas fir stand lost between 520 and 620 g C m-2 yr-1 of CO2 to the atmosphere over a 3-year period ? nearly twice the amount of CO2 removed from the atmosphere per year by a nearby 50-year-old Douglas fir stand (Morgenstern et al., 2004). Notably, it was found that following harvesting, increases in annual P were more than offset by increases in annual R over the study period and that the influence of weather on annual NEP was at least as important as stand regeneration. Pypker and Fredeen (2002b) measured NEP in a sub-boreal clearcut 5 and 6 years following harvest. The clearcut was observed to be a C source in both years when ann ... | ||||||||||
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