Impacts of forest post thinning residues on soil chemistry, fauna and roots: Implications of residue removal in Finland

Abstract

There is increasing interest in removing greater amounts of non-timber residues from forests for use in energy production. As part of a study aimed at elucidating the ecological impacts of these practices in Finland, piles of thinning residues of contrasting mass per unit area were established in 2004 around the base of individual Scots pine trees. As part of an NSF funded undergraduate training program, we have sampled the soil under these residue piles to evaluate the effect of leaving different amounts of residue on site to supply nutrients for remaining trees. Aboveground tree biomass production was significantly higher with higher levels of post-harvest residue retention. However, changes in soil chemistry were minimal and inconsistent with increasing residue amounts. Fine (<1mm) tree root biomass showed little response to residue levels, but specific root length (length per unit mass) and ectomycorrhizal root tip abundance showed significant increases in the highest residue addition compared to the no-addition control. This may suggest a greater root searching strategy under high residue levels. There was no evidence of residue effect on soil nematodes but a consistent increase in abundance of 0–2mm length enchytraeids with increased residue. This may be a result of higher rates of asexual reproduction under residues. Soil arthropod abundances did not correlate with residue levels. Even in this cold boreal forest ecosystem, it is possible that sampling 5years after residue placement we may have missed some of the early influence of decomposition of more labile fractions of the residues on soil organisms and processes. It is likely that the current phase of woody material decomposition will have lesser effects on soil biology.