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Analysis of environmental and biological controls on soil respiration in boreal deciduous and coniferous stands Gaumont-Guay, David

Abstract

This thesis examines the environmental and biological controls on respiration processes in mature trembling aspen (established in 1919) and black spruce (established in 1879) stands located at the southern edge of the boreal forest in Saskatchewan, Canada. The seasonality of soil (Rs) and bole respiration (Rb) in the aspen stand was primarily driven by temperature. Rs and the maintenance component of Rb (Rbm) were also strongly reduced during drought conditions. The annual and Rbm at 10 °C (Rs\o and i?bmio) and the annual temperature sensitivity of Rs and Rbm (2IORS and gioRbm) decreased in response to drought from 2001 to 2003. Daily Rs\o and gioRs values derived from half-hourly measurements of Rs and soil temperature during individual nights in 2001 differed from the annual parameters and were also reduced during the late summer drought conditions. On average over the three years, Rs, Rb and estimated foliage respiration (Rf) accounted for 70, 13 and 16% of scaled-up chamber estimates of ecosystem respiration (Re). The integrated scaled-up chamber estimates of Re were 23% higher than the estimates obtained from the eddy covariance technique. On average, drought resulted in a 26% reduction in Rs from 2001 to 2003 and explained most of the interannual variability in Re. The exclusion of live roots in the black spruce stand in 2003 caused a gradual reduction in Rs that reached 25% three weeks after the application of the treatment. Annual estimates of rhizospheric (Rr) and heterotrophic respiration (Rb) were 324 and 230 g Cm⁻² y⁻¹ in 2004, which accounted for 53 and 38% of Rs, respectively, the remainder originating from feather moss respiration. Rb dominated during winter, spring and fall when soil temperature was low. Rr dominated in summer when trees were fully active. Soil temperature and precipitation were the dominant climate variables controlling the seasonality of Rs in the black spruce stand. However, a lagged correlation was observed between gross ecosystem photosynthesis measured with the eddy covariance technique and RT at seasonal and diurnal time scales, which suggests a link between photosynthate production, transport and Rs. Overall, the results from this study clearly showed that respiration processes in forest stands are not only driven by the physical climate system but also incorporate a strong biological control.

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