Thinning promotes litter decomposition and nutrient release in poplar plantations via altering the microclimate and understory plant diversity

Thinning is widely employed in forest management to improve productivity, protect forest biodiversity and maintain ecosystem functions. Here a thinning experiment with four treatments (unthinned, CK; 30% tree removal from below, MB; 50% tree removal by interlaced thinning, HI; and 50% tree removal from below, HB ) was set up in the poplar plantation, while a followed decomposition experiment with four litter types was conducted under the poplar plantations of undergoing four thinning treatments using the litterbag technique. Thinning affected the microclimate, but only the heavy thinning (HI and HB) significantly enhanced photosynthetic photon flux density (PPFD) and soil moisture in the plantations during the growing season. Thinning promoted understory vegetation biomass and vegetation diversity via modifying the microclimate parameters. Pearson correlation analysis showed that PPFD and understory herbaceous biomass were significantly correlated to Shannon-Weiner diversity index. Both thinning intensity and litter type significantly affected the litter remaining mass over times. Overall, increasing thinning intensity and litter complexity enhanced decay rate, while reduced half live (t0.5) and t0.95 values. Correlation analysis showed that air relative humidity, soil temperature, air temperature and soil moisture significantly influenced the litter mass loss rates. Non-additive (synergistic) effects were observed when different litters were mixed, but the non-additive effect was most pronounced when more herbaceous species litter were mixed with poplar leaves and 50% thinning intensity was applied. Dynamics of nutrient release from different litter types were similar to those on the litter mass lose, depending on the litter quality and microclimatic conditions. Our results suggest that a thinning operation with 50% tree removal from below (HB) would maintain the structural and functional features of the poplar plantations at the similar sites.