Thinning increased fine root production, biomass, turnover rate and understory vegetation yield in a Chinese fir plantation

Quantifying the fine root (FR) dynamics of understory plants and canopy trees is essential to accurately assess the impact of forest operations on forest production and ecological stability. However, such information has been poorly quantified, and little is known about the underlying mechanism of FR dynamics in response to human disturbances such as thinning. We investigated the FR dynamics and the understory vegetation of a Chinese fir plantation in southern China under four levels of thinning intensity (30, 50 and 70% of trees removed and uncut controls). We found that the total FR biomass, production and mortality, and turnover linearly increased with thinning intensity, and the FRs were stimulated more in the deeper soil layer (10-20 cm) than in the surface soil layer (0-10 cm). Across all stands, understory plants contributed 35-42% and 34-49% to the total biomass and production, respectively. The biomass and species richness of the understory plants increased with increasing thinning intensity. The principal component analysis indicated that the increased FR production and turnover rate of Chinese fir mainly result from increased understory plant richness and diversity, whereas the increased FR production and turnover rate in understory vegetation result from warmer soil and lower soil phosphorus (P). Our findings provide evidence for the positive effects of thinning on FR dynamics. The results suggest that the FR dynamics of understory plants could play a more critical role than expected in determining forest carbon (C) and nutrient cycling. The mechanism affecting FR dynamics following thinning differed between the understory plants and the canopy trees.