Fine root nutrient foraging ability in relation to carbon availability along a chronosequence of Chinese fir plantations

In the quest to reveal the mechanisms of yield decline during the late growth stage of tree plantations, there remains a knowledge gap regarding the changes of the fine root nutrient foraging strategies during stand development and their underlying mechanisms. Fine root traits were studied using the soil core method in a typical chronosequence of six first-generation Chinese fir plantations: 2- (recently planted), 7- (sapling), 16- (middle-aged), 21- (pre-mature), 40- (mature), and 88-year-old (over-mature) stands. For the fine roots of the 0–1 mm diameter class, the specific root length (SRL), specific root area (SRA), and specific root tip density (SRT) decreased with stand age, indicating that the fine root nutrient foraging ability decreased in older stands. Fine root nitrogen (N) concentration decreased with stand age, but it significantly increased by the over-mature stage, likely as a result of physiological compensation for the decline in nutrient foraging ability. The concentrations of non-structural carbohydrates (NSCs), their components, and the NSC : N ratio (RNN) were lower in older stands, but the ratio of soluble sugar concentration to starch concentration (RSS) and the root δ13C values were higher. This reflected decreased carbon (C) availability and increased utilization of C reserves in the fine roots during late stand development. The root NSC concentration was positively correlated with SRL, SRT, and N concentration, indicating a positive effect of root C availability on nutrient foraging ability. The root δ13C value was positively correlated with the RSS and negatively correlated with the RNN, indicating that δ13C can accurately reflect the balance of carbohydrate supply and demand in fine roots. It was concluded that fine root nutrient foraging ability decreased in older stands, likely due to increasing fine root C limitation. These results provide a new perspective for understanding the mechanism of age-related yield decline in tree plantations.