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

Quantifying soil respiration (R _s ) and its components [autotrophic respiration (R _a ) and heterotrophic respiration (R _h )] in relation to forest management is vital to accurately evaluate forest carbon balance. Thus, R _s , R _a , and R _h 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 R _a and R _h and thus R _s , and these decreases will change with time following UR and thinning. Mean values of R _s , R _a , and R _h 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 R _h /R _s 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 R _h (R ² =0.69-0.96) than R _a (R ² =0.51-0.86). HT and LT increased Q ₁₀ for both R _a and R _h , except for R _h 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 CO ₂ emission. To get firewood for local people and to reduce soil CO ₂ emissions under global warming, canopy trees are needed to be protected and understory shrubs may be allowed to be used in the subtropical China.
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