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Net ecosystem production of three coastal Douglas-fir stands at different stages of development after harvesting Humphreys, Elyn R.

Abstract

Forest harvesting and succession have a major impact on the dynamics of carbon exchange between forests and the atmosphere. This thesis examines the differences in stand-level fluxes of carbon dioxide (C02) and the biophysical factors, which affect respiration and photosynthesis in three different coastal Douglas-fir stands at different stages of development after harvesting. The eddy covariance technique was used to measure stand-level CO2 fluxes in Douglas-fir stands established in 2000, 1988, and 1948 on the east coast of Vancouver Island, British Columbia. A portable soil chamber system was used to measure CO2 efflux from the soil in each stand. One year of measurements (2002) was used to contrast net ecosystem production (NEP) between the three stands, while three consecutive years (September 2000 to September 2003) were used to examine interannual variability in NEP in the recently clearcut-harvested stand. Total annual NEP, ecosystem respiration (Re) and gross ecosystem production (GEP) increased with increasing stand age. In 2002, the 3-year-old stand was a large carbon (C) source (NEP = -610 g C m~2 y_ 1), the 14-year-old stand was a small C source (NEP = -130 g C m - 2 y_ 1), and the 53-year-old stand was a moderate C sink (NEP = 250 g C m~2 y- 1). Seasonal variations in both soil and ecosystem respiration were related to soil temperature, while within- and between-stand variability in soil respiration was related to the concentration of soil nitrogen. Differences in annual Re in these forests were better related to productivity than to soil temperature. Seasonal variations in NEP differed between stands and were related to differences in stand structural characteristics such as canopy roughness, leaf area, biomass, species composition and phenology. The effect of advection on NEP estimates was greatest in the 53-year-old stand and was also related to these structural characteristics as well as site characteristics. In the first three years after harvesting, leaf area increased with the growth of Douglas-fir seedlings, and pioneer and understory species in the youngest stand. Changes in the relationships between respiration and soil temperature and between GEP and radiation were linked to leaf area index. Results from this study emphasize the importance of stand structure and succession as well as site characteristics and weather variability on C exchange processes.

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