The causal role of soil water in asymmetric sensitivity of forest growth from a filed precipitation manipulation experiment

Globally and regionally, large efforts had been made on spatial relationships between forest growth and precipitation. Yet, substantial uncertainty still surrounds generalities describing their temporal relationships. A lack of experimental evidence that combines increased and decreased precipitation treatments to verify their relationships at site-level. Additionally, we do not know whether water in the deep soil profile drives the discrimination of their relationships when decreased vs. increased precipitation changes become extreme. To obtain generalities describing patterns of ecosystem sensitivity to altered precipitation, our experiment was manipulated precipitation throughfall through gravity-driven transfer from the decrease precipitation treatment plots (–30%, –50%, –65%) to the increase precipitation plots (+30%, +50%, +65%) from mild, moderate to extreme level in a temperate deciduous forest (planting Black locust) on the Chinese Loess Plateau. Over the 3 years, the decreased and increased precipitation treatment caused the largest reduction and increment in soil water by 0.9% and 1.4%, soil water variability by 5.2% and 8.8%, leaf area index (LAI) both by 0.4 m³/m³, diameter at breast height (DBHPPT) by 0.18 cm and 0.34 cm, and forest biomass change ratio (FBCR) by 18% and 33%, respectively. Soil water showed nonlinear positive responses to the precipitation change, while forest growth (i.e., LAI, DBHPPT and FBCR) had linear positive responses to the soil water change. The mean sensitive of forest growth was higher to altered increase precipitation than decrease precipitation. Planting forest of Black locust showed high drought tolerance but rapid growth pattern and soil water uptake to decreased vs. increased precipitation. The growth pattern of forest corresponded to the large depleted soil water in the deep soil profile. We conclude that the strategies of forest responses to the soil water condition play an essential role in regulating the asymmetric response. Our findings emphasize that soil water will play an essential role in regulating forest growth along precipitation gradients. The positive asymmetric response of forest growth to altered precipitation indicates that the intensified interannual variability in the future may positively affect the variability of forest growth.