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

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 m 2 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 m 2 ha −1 ) that were thinned five times over 50 yr. Our results showed that the heaviest‐thinned (13.8 m 2 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 m 2 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.