A shift from isohydric to anisohydric water-use strategy as a result of increasing drought stress for young apple trees in a semiarid agroforestry system

Abstract

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 (Ψpd) and midday (Ψmd) 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 Ψpd (58.1%) and Ψmd (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.