Long-term Effect of Silvicultural Thinnings on Soil Carbon and Nitrogen Pools

Abstract

The effects of long-term silvicultural thinning on soil C and N content are not well known. We evaluated the impact of periodic thinnings on soil C and N pools in a 134-yr-old red pine (Pinus resinosa Ait.) forest in Minnesota, and a 104 yr-old northern hardwood forest in Wisconsin. The red pine stands had five thinning regimes (13.8, 18.4, 22.7, 27.6, 32.1 m2 ha-1 residual basal area [BA]), which were cut five or seven times over 46 yr. The northern hardwood stands had three residual basal area treatments (13.8, 17.2, 20.6 m2 ha-1) that were thinned five times over 50 yr. Our results showed that the heaviest-thinned (13.8 m2 ha-1) and uncut control red pine stands had higher C and N contents in the mineral A horizon, as compared to the other four thinning treatments. Multiple thinning did not affect C and N pool size in the forest floor and surface mineral soil (30-cm depth) in either red pine or hardwood stands. Within stand BA variability was positively correlated to C and N pools in the forest floor of the lightly-thinned (32.1 m2 ha--1) red pine treatment, but was negatively correlated to C and N pools in the A horizon. Our study and the literature indicate that stem-only removal for wildfire risk reduction and bio-energy production would have little impact on total soil C and N pools. However, more information is needed on the effects of whole-tree thinning regimes on soil C and nutrient contents.