Divergent roles of deep soil water uptake in seasonal tree growth under natural drought events in North China

Understanding the ecophysiological mechanisms underlying tree adaptation in drought-prone regions is crucial for predicting forest health in the face of future climate changes. However, little is known about how tree hydraulic adaptations modulate the responses of seasonal tree growth to water deficits along a large climate gradient. In this study, the spatially divergent responses of tree growth to natural drought events were assessed through observations of water uptake patterns, leaf water potentials, sap flow rates, and tree stem radial variations in Chinese pine (Pinus tabulaeformis) plantations along a climate gradient in North China. Higher proportions of water were absorbed from the middle and deep soil layers during dry spells compared to the growing season across all study sites. However, at the semiarid and subhumid sites, increased water uptake from deeper soil layers did not completely alleviate drought stress. Instead, the trees adopted strong stomatal control over transpiration at the expense of reduced growth. In contrast, at the humid site, seasonal shifts in water uptake effectively compensated for the negative impacts of seasonal drought on tree growth. Given that intensified climatic droughts are expected under climate warming, the establishment of an early warning drought system is increasingly urgent for plantations in regions prone to drought stress.