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

Jurgensen, Martin, Tarpey, Rachel, Pickens, Jim, Kolka, Randy, Palik, Brian
Soil Science Society of America journal 2012 v.76 no.4 pp. 1418-1425
Pinus resinosa, basal area, bioenergy, carbon, correlation, forest litter, hardwood forests, long term effects, mineral soils, nitrogen, nitrogen content, risk reduction, wildfires, Minnesota, Wisconsin
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 (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 ha 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 ha) that were thinned five times over 50 yr. Our results showed that the heaviest-thinned (13.8 m ha) 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 ha) 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.