Warming accelerates Chinese fir root turnover and alters root production distribution in response to soil temperature and moisture

Fine root production (FRP) and fine root turnover (FRT) are pivotal regulators of forest biogeochemical cycles and productivity; yet, the effects of warming on FRT and the spatiotemporal dynamics of FRP remain poorly understood, particularly in subtropical forests with pronounced seasonal rainfall variability. A large-scale in situ soil warming experiment (ambient +4°C) was conducted in a Cunninghamia lanceolata plantation, employing minirhizotrons to monitor fine root dynamics over a 4-yr period. Results showed that warming consistently accelerated FRT, but increased FRP only during the driest year. Warming effect on monthly FRP was fully mediated by concurrent changes in soil temperature (ST) and soil moisture (SM). In the control, FRP distribution became shallower with increasing SM, reflecting a moisture-dependent trade-off strategy between nutrient acquisition in the topsoil and water uptake from the subsoil. Under warming, however, FRP distribution became less responsive to SM and exhibited a consistent deepening trend with increasing ST, indicating enhanced subsoil water acquisition at the cost of reduced topsoil nutrient uptake. These findings indicate that warming accelerated FRT and reduced sensitivity of FRP vertical distribution to SM, potentially impairing tree nutrition. These insights are essential for improving predictions of productivity and climate resilience in subtropical forest ecosystems.