Thinning but not understory removal increased heterotrophic respiration and total soil respiration in Pinus massoniana stands.

Quantifying soil respiration (Rs) and its components [autotrophic respiration (Ra) and heterotrophic respiration (Rh)] in relation to forest management is vital to accurately evaluate forest carbon balance. Thus, Rs, Ra, and Rh were continuously monitored from November 2013 to November 2016 in Pinus massoniana forests subjected to four different management practices in China. We hypothesized that understory removal and thinning decrease Ra and Rh and thus Rs, and these decreases will change with time following UR and thinning. Mean values of Rs, Ra, and Rh in light thinned plots (LT=15% of tree basal area thinned) and heavily thinned plots (HT=70% of tree basal area thinned) were significantly higher than in control (CK) and understory removal plots (UR). The annual Rh/Rs ratio ranged from 58% to 70% across all treatments, and this ratio was significantly higher in HT and LT than in UR and CK. Only HT significantly increased soil temperature. Soil temperature could better explain Rh (R2=0.69-0.96) than Ra (R2=0.51-0.86). HT and LT increased Q10 for both Ra and Rh, except for Rh in UR. Soil moisture content (W; %) was significantly higher in HT than in other treatments, but W had limited effects on soil respiration in that rain-rich subtropical China. This result suggests that global warming alone, or in combination with clear-cutting or canopy tree thinning will markedly increase soil heterotrophic respiration and thus the total soil CO2 emission. To get firewood for local people and to reduce soil CO2 emissions under global warming, canopy trees are needed to be protected and understory shrubs may be allowed to be used in the subtropical China.