Soil water availability induces divergent ecosystem water-use strategies to dry-heat conditions in two poplar plantations in North China

Terrestrial ecosystems are increasingly threatened by more frequent and intensified compound dry-heat conditions, exacerbated by land-atmosphere feedback under changing climate. The water-use strategy (WUS), which determines the trade-off between carbon assimilation and water consumption, is a key metric for exploring how terrestrial ecosystems respond to environmental stresses. However, how water availability influences their water-use strategies in response to dry-heat conditions is still not well understood. Here, we compared the response of WUS to dry-heat conditions in two poplar plantations with different soil water availability (SWA) in North China, represented by the Priestley-Taylor coefficient (α), and applied an interpretable machine learning algorithm to disentangle the independent and interactive effects of the main biophysical factors on WUS under dry, heat, and their compound conditions. We found that the plantation with higher SWA (high-SWA plantation) showed a more permissive WUS (i.e., increased marginal water cost of carbon gain) under dry-heat conditions than the low-SWA one due to the decoupling of evapotranspiration and gross primary productivity, and less sensitive to α and vapor pressure deficit (VPD) variations. During dry conditions, heat made the WUS more permissive in the high-SWA plantation and more conservative in the low-SWA one. Moreover, the α-VPD interaction was progressively strengthened as intensified dry-heat stresses until reaching a threshold (α = 0.7, VPD = 2.2 kPa in the low-SWA plantation and α = 0.85, VPD = 2.6 kPa in the high-SWA plantation). Thus, our study emphasizes that land-atmosphere feedback exacerbates the vulnerability of poplar plantations to dry-heat conditions.