Litter decomposition was retarded by understory removal but was unaffected by thinning in a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation

Thinning and understory removal, which are commonly used in forest management, are expected to affect litter decomposition. However, little is known about how litter decomposition rates change in stands subject to both thinning and understory removal. This knowledge gap is urgent to be filled because the two practices are often applied simultaneously. In the present study, an experiment containing a complete factorial combination of thinning and understory removal was arranged in a Chinese fir (Cunninghamia lanceolata) plantation. Litter decomposition rate was determined by using litter bags, and litter bags with different mesh sizes (0.1 mm and 2 mm) were used to identify the effect of soil mesofauna on decomposition. The temperature, moisture, and microbial abundance of surface soil were also measured. Results showed that thinning did not change litter decomposition rate, partly because of the unresponsive soil temperature, moisture, and abundance of microbial groups following thinning. Understory removal markedly restricted litter mass loss by more than 20%. Despite an obvious decrease in soil microbial abundance following understory removal, this should not account for why understory removal lowered litter decomposition rate, because this negative effect on litter mass loss disappeared when soil mesofauna was excluded. In addition, no interaction between thinning and understory removal was detected on litter decomposition. We emphasized that (1) Understory removal retard litter decomposition by suppressing the activity of soil mesofauna rather than by reducing microbial abundance; (2) Understory removal caused an equal reduction in litter mass loss regardless of thinning treatment. We believe that these results provide an important guidance for evaluating the effect of thinning and understory removal on some ecological processes associated with litter decomposition (i.e. soil formation and CO2 emission from the soil to the atmosphere) in a fir plantation.