Soil CO2 efflux in uneven-aged managed forests:: temporal patterns following harvest and effects of edaphic heterogeneity

Forest management is expected to influence soil CO2 efflux (FCO2) as a result of changes in microclimatic conditions, soil properties, and root dynamics. We measured FCO2 during the growing seasons of 2003 and 2004 in both gap and non-gap locations within stands ranging from 0 to 10 years after the most recent harvest in a selection-managed northern hardwood forest in central Ontario, Canada. Soil temperature, moisture, pH, depth to bedrock, and organic matter content were also measured to examine relationships between FCO2 and soil properties. Timber harvesting resulted in large changes in FCO2 that varied with time since harvest. Immediately following harvest (year 0) FCO2 in gaps increased by -55%, declined to 20-40% below pre-harvest levels during years 1-3, and recovered to pre-harvest levels 5-6 years post-harvest. A similar temporal pattern, but with smaller changes, was found in non-gap locations within harvested stands. We suggest that the initial increase in FCO2 is due to a combination of root decay, soil disturbance, and increased soil temperature in gaps, while the subsequent decrease and recovery are caused by the gradual regrowth of active roots. We also found strong effects of soil temperature, depth to bedrock, and soil water content on FCO2; however, soil pH and soil organic matter concentration had no detectable effects. Our results suggest that selection harvests mainly influence FCO2 through changes in tree root respiration, and that the net result is a decrease in FCO2 through the entire felling cycle.