© 2018 by the authors. Despite the great importance of fine roots, which are referred to as roots smaller than 2 mm in diameter, in terms of carbon and nutrient cycling in terrestrial ecosystems, how fine root biomass, production, and turnover rate change with stand development remains poorly understood. Here we assessed the variations of fine root biomass, production, and morphology of trees and understory vegetation in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations at the ages of 7 years old, 17 years old and 25 years old in southern China, representing the sapling, pole and mature stage, respectively. Fine roots of trees and understory vegetation were sampled with sequential coring method to a depth of 60 cm and sliced into 4 layers (0-15, 15-30, 30-45 and 45-60 cm). Fine root biomass and necromass were highest in the pole stages among these three different aged Chinese fir plantations, although the significant differences were only detected for fine root necromass between 25-year-old and 7-year-old plantations. Fine root biomass of Chinese fir was heterogeneous in both temporal and spatial dimensions. Seasonal variation of fine root biomass in three age groups showed a similar pattern that the standing fine root biomass reached a peak in January and fell to the lowest in July. Vertically, the fine root biomass decreased with the increase of soil depth, but this extinction rate decreased with stand development. The effects of stand age on either total fine root length and surface area, or specific root length were not significant. However, the root tissue density increased significantly with Chinese fir stand ages, which suggested that the fine roots on Chinese fir may resort more to the mycorrhizal associations for the nutrient and water acquisition in the later stage of Chinese fir plantations. In addition to the stand age effect, the fine roots exhibited highly spatial and temporal variations in Chinese plantations, indicating different root foraging strategies for soil nutrient and water acquisition. Therefore, the fine root research not only helps to understand its role in carbon sequestration in terrestrial ecosystem under global climate change, but can also improve our understanding of nutrient management in forest ecosystem. At the same time, the research on the productivity of the Chinese fir growth stage provides guiding significance for the construction and management of Chinese fir.
