Transient Root Plasticity and Persistent Functional Divergence in Pine and Oak Forests in Response to Thinning

The mechanisms through which forest thinning influences the fine root foraging strategies of coexisting tree species in mixed forests remain unclear, limiting our ability to manage mixed forests for long-term productivity. We employed a space-for-time substitution approach in a Qinling pine-oak forest, in which fine roots of Pinus tabuliformis (Pt) and Quercus aliena var. acuteserrata (Qa) were sampled from unthinned plots and plots thinned 6 years (transient phase) and 14 years (persistent phase) prior, respectively. We analyzed the morphological and chemical traits of both absorptive and transport fine roots (0–20 cm depth) to decipher their distinct adaptation strategies. The results showed that Pt enhanced morphological plasticity in both absorptive and transport roots at T2010 (e.g., specific root length: +44% and 37%, p < 0.05). In contrast, Qa showed minimal changes in absorptive root morphology and chemistry (p > 0.05). Redundancy analysis indicated that thinning intensified functional divergence in root strategies between the two tree species and between the two root functional types (i.e., absorptive vs. transport fine roots). Hierarchical variance partitioning analysis indicated that root functional type was the primary driver (51.9%), with tree species identity (Pt vs. Qa) and thinning practices being secondary. Critically, soil properties significantly shaped absorptive root traits (explaining 10.4% of the variance) but did not affect transport root traits, whereas thinning was a dominant factor for transport roots (21.6%). This insight enables the tailoring of silvicultural interventions to tree species-specific foraging strategies, optimizing belowground resource acquisition in mixed forests.