Absorptive roots trait plasticity explains the variation of root foraging strategies in Cunninghamia lanceolata

The architecture and morphology of absorptive roots show substantial plasticity in response to forest management practices. These traits are known to play important roles in the acquisition of soil resources by trees. However, the effects of nutrient addition, thinning and pruning on absorptive root traits and their feedback to root foraging strategies remain unclear. We investigated the values and plasticity of traits related to nutrient foraging (root architecture, growth and morphology) for first- and second-order roots (absorptive roots) following nitrogen (N) addition, phosphorous (P) addition, thinning and pruning treatments in a young Chinese fir ( Cunninghamia lanceolata ) plantation. We measured twelve traits of absorptive roots under the five treatments (control, N addition, P addition, thinning and pruning) and determined relationships between the values and plasticity of root traits and stem growth rate. We demonstrated clear patterns of root traits and their plasticity in response to the treatments. N and P addition increased root biomass (B) and root tissue density (RTD). Thinning and pruning led to larger specific root length (SRL) and root nitrogen concentrations (N), but resulted in lower root length (L) and root length density (RLD). Principal component analysis of the measured traits and plasticity provided evidence for two suites of traits related to resource acquisition and conservation strategies among treatments. The trait syndromes exhibiting resource acquisition strategy (SRL and N) is arrayed well along the positive part of the first axis, whereas the opposite trait syndromes with resource conservation strategy (root diameter and RTD) is along the negative part of the first axis. The first axis also separates nutrient-induced treatments (N and P addition) from light-mediated treatments (pruning and thinning). Furthermore, first-order root exhibited higher foraging sensitivity and precision (expressed as relative fine root length difference) in response to P addition than to the other treatments. The foraging sensitivity and precision, plus B, L and RLD showed that first-order roots are more responsive to environment than second-order roots across the treatments. Stem growth rate was correlated positively with absorptive root traits (biomass, root surface area index, root length density, and root tip number) after thinning, but negatively with these traits after N and P addition. These findings demonstrate that C . lanceolata finely tuned root foraging strategies between first- and second-order root traits and their plasticity at the intraspecific level in response to forest management practices. Further studies may explore nutrient-induced and light-mediated foraging strategies between absorptive roots across root branch orders in mature Chinese fir plantation.