Controls on the heterogeneity of soil respiration in a tolerant hardwood forest

The spatial and temporal distribution of soil respiration (Rs) in forested landscapes and its control by environmental conditions and carbon pools has not been sufficiently investigated. A Rs monitoring strategy targeting different topographic features within a sugar maple forest on the Algoma Highlands on the Canadian Shield revealed that the critical transition zones (transiently wet areas adjacent to wetlands, streams, and lakes) yielded significantly larger Rs than adjacent upland or wetland portions of the catchment. Environmental conditions (soil temperature and soil moisture) explained the majority of variance in Rs. An exponential relationship in which the exponent is a polynomial expression that is linear with respect to temperature (degrees C) and quadratic with respect to moisture (% by volume) explained 57.0% of the variance in Rs (mu mol CO2 m(-2) s(-1)). However, inclusion of the quantity of carbon and substrate quality (carbon: nitrogen ratio) in the freshly fallen leaves, and the organic and/or mineral soils into the polynomial regression model explained an additional 15.3% of the variance in Rs (mmol CO2 m(-2) s(-1)). This study highlighted the critical transition zone as major sites of Rs because of the synchronicity in optimal temperature and moisture conditions during the growing season and the large pool of high quality substrate that accumulates in topographic depositional areas.