Black locust under long-term rainfall exclusion treatment changed its drought-resistance strategy to an adventurous pattern that potentially raised the risk of hydraulic damage

The mechanism of plant response to water availability is a crucial research area in ecosystem adaptation to dry environments. Global change in precipitation patterns exacerbates drought risks in many regions, necessitating investigations of tree responses to drought. However, compared with seasonal drought events, how water use characteristics respond to long-term rainfall reduction remains poorly characterized. Here, we assessed the effects of multi-year rainfall exclusion treatments (in the 4th and 5th years) on sap flow, canopy conductance (G_C), and the response pattern to varying transpiration driving force (vapor pressure deficit (VPD)) and soil moisture conditions in a black locust (Robinia pseudoacacia) plantation in the China's Loess Plateau. The experimental platform includes paired plots of a 30% throughfall exclusion treatment and a control within the plantation. The reduction of soil water content (SWC), either as temporal variations or as a result of rainfall exclusion treatment, generally lowered sap flow, G_C, and the sensitivity to VPD. The difference in each index between plots was substantially greater than that among temporal variations in SWC within a plot. The trees in the control plot showed strong responses of G_C to temporal SWC changes, exhibiting an isohydric behavior. However, those subjected to long-term rainfall exclusion treatment had lower variations in G_C and the sensitivity across different SWC, exhibiting an anisohydric behavior under limited water conditions. Additionally, rainfall exclusion treatment decreased basal area (BA) increment but increased water use efficiency (WUE). The transition to anisohydric behavior suggests an adventurous pattern of water use strategy that has risks of hydraulic damage and shoot mortality. These findings provide insights into the different adaptation strategies within a species to diverse intensity and duration of drought conditions, which is crucial for sustainable forest management under climate change conditions.