Disturbance history influences downed woody debris and soil respiration

In boreal forests, disturbances such as stand-replacing fires and clearcut harvesting create large amounts of downed woody debris (DWD), which release considerable amounts of CO₂ to the atmosphere during decomposition. Harvesting and fire differ with respect to DWD characteristics (e.g., size), decomposition environment (temperature and moisture), and abundance; hence, differences in piece- and stand-level DWD respiration rates between stands of different disturbance at origin can be expected. From July to September 2008 (every 5-9 days), we measured temperatures, moisture content, weight, and respiration of 45 standardized black spruce logs placed in three old-growth (>140 years), three clearcut harvested (3 years prior), and three burned (23 years prior) black spruce forest stands in Labrador, Canada. Concurrently, soil temperatures, soil moisture potential, and soil respiration were measured next to each log. Moisture content was the dominant environmental control of DWD respiration, followed by temperature. Within-log differences in moisture and temperature resulted from drying of the log surface and water transfer from the live moss layer to the lower log parts. Downed woody debris temperatures and respiration rates were higher in harvested and burned stands compared to old-growth stands, and piece-level DWD respiration rates were highest in harvested stands. Downed woody debris moisture levels were lower in disturbed stands, particularly in burned stands, where respiration was limited by low moisture content. Compared to soil respiration, stand-level DWD respiration (based on estimated DWD surface area per hectare) reached similar (83%; following fire) or equal (100%; following harvest) rates. Disturbance-induced changes in forest temperature and moisture regimes are reflected by DWD respiration rates; hence, for purposes of forest C modeling, decomposition rates of DWD should be differentiated by the decomposition environment created by different disturbance types.