Response of Soil Respiration to Thinning of a Temperate Pine Forest in Southern Ontario

English Abstract:

Soil respiration (Rs) is the largest component of ecosystem respiration, and the second largest amount in ecosystem carbon budgets after gross primary production. Soil temperature and soil moisture can explain most of the temporal and spatial variations seen in soil respiration. Forest thinning can change soil temperature and water content, organic matter, root biomass and microbial activity, thus changing soil respiration. This study will investigate the impact forest thinning will have on the forest soil respiration, with the use of an automated soil CO2 chamber system, in a 75-year old temperate pine (Pinus strobes L.) forest, near Lake Erie in southern Ontario, Canada. The chamber system was installed and started collecting data in the summer of 2008 with four chambers operating. In the spring of 2009, another four chambers were installed.

During the winter of 2012, the forest was selectively thinned and approximately 30% of the trees were removed to improved light and water availability and stimulate growth of the remaining trees. The chamber system was removed during this winter for precautionary measures and as heavy machinery needed to access the site to remove selected trees. This disturbance of soil layer and decomposition of debris left behind is expected to increase soil respiration. Gaps, resulting from chamber malfunctions, calibrations, winter snow and ice problems, were filled using empirical models that used a Q₁₀ relationship for soil temperature and soil CO₂ efflux, and a logistic relationship for soil water content of the root zone and soil CO₂ efflux.

Total annual Rs (average of 930 g C m² y^-1 over seven years) accounted for the majority (approximately 76%) of the total ecosystem respiration measured from the Turkey Point Flux Station. Distinct highs and lows were observed in Rs that closely tracked soil temperature. During the winter months, variability in Rs decreases with a decrease in soil temperature and a decrease in the variability of soil moisture. In the years following the thinning, Rs shows little difference to the pre-thinning years, and is most likely influenced by both interannual climate variability and thinning effects.