Variations in fine root biomass, morphology, and vertical distribution in both trees and understory vegetation among Chinese fir plantations

The nutrient limitation hypothesis has often been used to explain the late stage decline in productivity of tree plantations. However, few studies have considered the role of fine roots (≤ 2 mm) may play in the process, and even fewer have considered the impact of understory vegetation. Here we assessed how fine root morphological traits, such as specific root length (SRL) and root tissue density (RTD), as well as fine root biomass (FRB) and root length density (RLD), varied in Chinese fir (Cunninghamia lanceolata) and associated understory plants with stand development. Results showed that, for Chinese fir fine roots, the SRL significantly decreased, while the RTD significantly increased, with stand development. There was no significant increase in the relative belowground biomass allocation (indicated by the ratio of total FRB or total RLD to stand basal area) of Chinese fir. This may reflect decreased nutrient foraging by Chinese fir fine roots during the later stage of stand development. Interestingly, understory vegetation had both significantly higher SRL and lower RTD than Chinese fir in every plantation. The weighted mean depth of FRB or RLD of understory vegetation was significantly lower than that of Chinese fir. In the later stage of stand development, understory vegetation tended to dominate the total RLD of the stand in the topsoil, reflecting intense belowground competition of understory plants on Chinese fir. Our study emphasizes that, besides the declined nutrient foraging ability of Chinese fir itself, the intense nutrient competition from understory vegetation could exacerbate nutrient limitation of Chinese fir, which may be an overlooked factor contributing to the decline in plantation productivity during the later stage of stand development.