Root architecture links water source plasticity and growth stability in subtropical trees under seasonal drought

Abstract

Global climate change has led to frequent extreme drought events, introducing substantial uncertainty regarding how tree water source utilization strategies regulate forest growth. This study integrated tree-ring chronologies, stable isotope (δ13C, δ2H, δ18O) analyses, and photosynthetic physiological data to examine the effects of water source plasticity on radial growth (Rg) in Cunninghamia lanceolata and Quercus acutissima from subtropical plantations. C. lanceolata exhibited densely distributed shallow fine roots but poorly developed deep roots, and relies predominantly on shallow soil water in both dry and wet seasons, with limited access to intermediate depth water. This species displayed a shallow-dominated water use pattern with low plasticity, which failed to mitigate drought-induced physiological constraints, leading to a suppressed photosynthetic rate (Pn) and slower radial growth. In contrast, Q. acutissima possesses a well-developed deep coarse root system and moderately distributed shallow fine roots, enabling efficient uptake of shallow water in wet seasons and flexible utilization of stable deep water sources in dry seasons, thus showing a deep–shallow synergistic water use strategy with high plasticity. Under drought stress, Q. acutissima maintained a higher photosynthetic rate (Pn)and leaf water potential, thereby reducing carbon loss caused by frequent stomatal closure, and had significantly greater growth resistance and resilience than C. lanceolata. Our findings highlight that species-specific root architecture is closely associated with interspecific differences in water source plasticity, (as reflected by the shallow water availability elasticity index, SWEI), which is closely linked to drought resistance and growth stability. This integrated framework (combining tree-ring, stable isotope, and physiological data) offers an empirical foundation for forecasting the responses of subtropical forest tree species to ongoing climate change.