Effects of thinning on forest soil and stump respiration in a subtropical pine plantation

Thinning is an important forest management practice that can greatly affect ecosystem carbon budgets. However, few studies have reported the instant and prolonged effects of thinning under the same climate conditions, which is the scientific issue addressed by this study. The effects of thinning on soil physicochemical properties, soil respiration (RSoil), stump respiration (RStump) and carbon (C) release were quantified in a subtropical plantation based on parallel observations in Unthinned, 1-year post-thinning (PT1-yr) and 3-year post-thinning (PT3-yrs) treatments. Generally, the pH and NO3--N decreased, and the NH4+-N and soil water contents (SWC) increased in PT1-yr and PT3-yrs. Soil temperature (T) decreased in PT1-yr and increased in PT3-yrs. Thinning promoted RSoil, but a slowdown of the promoting effects was found in PT3-yrs. Thinning continuously promoted RStump significantly; therefore, RStump in PT3-yrs > PT1-yr > Unthinned. The responses of RSoil and RStump to T and SWC changed after thinning. Among Unthinned, PT1-yr and PT3-yrs, the fitting surfaces for modeling the relations between RSoil and TSoil, SWCSoil became flatter. In contrast, the fitting surfaces become more curved for RStump and TStump, SWCStump. These results could be attributed to the complicated changes in the composition of respiration and the changing soil environment. The C release from the underlying surface increased year by year during the three years after thinning. The C released through RSoil was the dominant component. However, the C released through RStump determined the significant difference between C release in PT1-yr and PT3-yrs, which indicated the potential importance of RStump over time after thinning. As thinning is an important way to optimize the carbon sink function of global forests, extensive and intensive studies on thinning effects and the underlying mechanisms are necessary, which would be beneficial for an accurate estimation of forest carbon budgets under future climate change.