Responses of soil respiration and its temperature sensitivity to thinning in a pine plantation

Abstract

Understanding the effects of forest management practice on soil respiration (Rs) and its temperature sensitivity (Q10) is crucial for the accurate estimation of the global carbon budget. However, the dynamics of Rs and Q10 resulting from plantation thinning are not well understood. To evaluate the impacts of forest thinning on Rs and Q10, we selected a pine plantation in the eastern Tibetan Plateau and applied the technique of thinning by simulating gap formation. We measured Rs monthly before (from July to November 2008) and after (from December 2008 to June 2012) thinning, combining with monthly microclimatic factors. Rs showed significant seasonal variability (P<0.001) in both control (0.30±0.05 to 2.94±0.29μmolCm−2s−1) and thinned (0.25±0.04 to 2.99±0.08μmolCm−2s−1) stands, with higher rates from July to September and lower rates from December to February. These seasonal dynamics were primarily driven by soil temperature rather than by moisture limitation, except during an especially dry season, when soil moisture significantly affected Rs. There were significant differences in Rs during the growing season between the control and thinned sites following thinning. Rs increased at the thinned sites compared with the control sites during the summer of the first and the third year following thinning; however, Rs decreased at the thinned sites in the second summer. This suggested that climatic variability, such as change in soil moisture induced from thinning, played an important role in controlling Rs. There was no significant difference (P=0.083) in Rs between control and thinned treatments after thinning (December 2008 to June 2012). For example, averages of Rs were 1.19±0.34, 1.20±0.36, 1.16±0.38μmolCm−2s−1 in the control, small gap and intermediate gap treatments, respectively. In addition, the short-term Q10 showed a significantly seasonal variability (P<0.001) and was altered by thinning (P=0.048). The long-term Q10 also showed a significant inter-annual variability (P<0.001), while thinning did not affect it. During the study period (December 2008 to June 2012), the Q10 values for the small and intermediate gaps significantly increased by 13.9% and 25.9% (P<0.05), respectively, compared with the control. These results suggest that the forest thinning by simulating natural gap formation has a relatively small impact on soil CO2 emission and its temperature sensitivity in comparison to the greater influence of inter-annual climatic variability.