Prolonged drought due to global warming can have significant effects on tree growth and sustainability by changing physiological traits. Agroforestry is considered climate-smart and can help buffer the effects of extreme climates. However, it remains unclear as to how trees in agroforestry physiologically adapt to prolonged drought in semiarid regions. Here, we report results from a three-year rainfall exclusion experiment (an extreme natural drought occurred during the experiment) designed to understand the physiological mechanism of young apple trees responding to different degrees of prolonged drought (moderate drought by reducing rainfall by 15% and severe drought by reducing rainfall by 25%) in an alley agroforestry system constituting of apple trees and oil crops. We found that the measured physiological traits for young apple tree under agroforestry were not clearly different from those under monoculture, although soil water content in the top 80 cm was reduced by 9.3%. Under moderate drought conditions, the apple trees in agroforestry implemented a conservative water-use strategy. Stomatal conductance, photosynthesis, and leaf transpiration were reduced by 15.5%, 3.8%, and 12.6%, respectively, whereas pre-dawn (Ψ_pd) and midday (Ψ_md) leaf water potential stabilized, indicating a clear isohydric behavior. Under severe drought conditions, however, the apple trees still maintained normal stomatal opening to significantly (p<0.01) increase photosynthesis (9.8%) and leaf transpiration (12%) at the expense of reducing Ψ_pd (58.1%) and Ψ_md (25.4%), showing anisohydric behavior which can place apple trees at risk. Reduction of aboveground biomass and greater numbers of fine roots in deeper soils to explore deep-layer soil water could explain such risky behavior by apple trees suffering severe drought. The findings here provide new insights into the mechanism through which the coordination between canopy water consumption and aboveground/belowground biomass redistribution of trees underpin the physiological adaptation of trees to different degrees of prolonged drought in semiarid regions.

全球变暖导致的长期干旱会改变生理特性，从而对树木的生长和可持续性产生重大影响。农林业被认为是气候智能型的，可以帮助缓解极端气候的影响。然而，目前尚不清楚混农林业中的树木如何在生理上适应半干旱地区的长期干旱。在这里，我们报告了一项为期三年的降雨排除实验（实验期间发生了极端自然干旱）的结果，旨在了解苹果幼树对不同程度的长期干旱（通过减少降雨量减少 15% 和中度干旱）做出反应的生理机制在由苹果树和油料作物组成的小巷农林系统中，降雨量减少了 25%，导致严重干旱。我们发现，尽管表层 80 cm 的土壤含水量降低了 9.3%，但农林复合经营下苹果幼树的生理性状与单一栽培没有明显差异。在中度干旱条件下，混农林业中的苹果树实施了保守的用水策略。与黎明前 (Ψ _pd ) 和中午 ( Ψ _md ) 叶水势稳定，表明明显的等水行为。然而，在严重干旱条件下，苹果树仍然保持正常的气孔开放，显着 ( p <0.01) 增加光合作用 (9.8%) 和叶片蒸腾 (12%)，但代价是减少了Ψ _pd (58.1%) 和Ψ _md(25.4%)，表现出可能使苹果树处于危险之中的异水行为。地上生物量的减少和深层土壤中更多数量的细根以探索深层土壤水可以解释遭受严重干旱的苹果树的这种危险行为。此处的研究结果为了解冠层耗水量与树木地上/地下生物量再分配之间的协调机制提供了新的见解，从而支持树木对半干旱地区不同程度的长期干旱的生理适应。