Soil CO2 efflux and net ecosystem exchange following biomass harvesting: Impacts of harvest intensity, residue retention and vegetation control

Abstract

Biomass harvesting removes more woody material than would be taken with conventional forest harvesting. Harvesting residues, left on site are an important substrate for micro-organisms that maintain nutrient cycles essential for future forest productivity by mineralizing organic matter, and releasing carbon dioxide (CO2) as a respiratory bi-product. We assessed the impact of biomass removal intensity (stem-only [SO], full-tree biomass [FT], full-tree biomass plus stumping [FT+S], full-tree biomass plus stumps and forest floor removed [FT+B]), and herbicide application on soil respiration and net ecosystem exchange of carbon (C) in a harvested 40-yr-old jack pine stand. Soil respiration (surface CO2 efflux) normalized to 15°C (R15) was lower in biomass harvest treatments than in the uncut stand and a mature 80-yr-old fire-origin natural stand. Among harvest treatments, R15 was positively related to the amount of C retained, with the general pattern of FT+B<FT+S<FT≈SO. Differences in R15 among treatments were primarily related to residue and soil organic matter quantity and quality (i.e., presence of mineral soil and forest floor polysaccharide). Herbicide application further reduced R15 by diminishing root respiration, although herbicide treatments in the SO, FT and FT+S resulted in greater net CO2 fluxes to the atmosphere in August because herbaceous photosynthesis was greatly reduced. We suggest that criteria for determining site-specific biomass retention should take into account the amount and type of residue required to maintain microbial soil respiration driving nutrient cycling.