Sand dune shrub species prioritize hydraulic integrity over transpirational cooling during an experimental heatwave

Abstract

Plant thermoregulation through transpirational cooling is important when facing heat stress and is suggested as an active mechanism to maintain lower leaf temperature during heatwaves. However, whether the role of cooling through transpiration is constrained in drought-prone habitats for the sake of hydraulic safety remains largely unknown. To evaluate the influence of plant water use characteristics on transpiration-based thermoregulation, we compared responses to natural and experimental heatwave conditions of two sand dune shrub species commonly used in dune revegetation projects but inherently differ in xylem hydraulics and water-use strategies. Salix gordejevii adopts a conservative water-use strategy and has low xylem native embolism and Caragana microphylla uses water less frugally and suffers high degrees of xylem native embolism. Under natural conditions, S. gordejevii exhibited lower transpiration levels, higher leaf temperature and had greater leaf thermotolerance with regard to cell membrane thermostability than C. microphylla. During the experimental heatwave, neither species increased cooling through transpirational water loss and maintained stable leaf water status and xylem embolism levels, suggesting that water conservation for hydraulic integrity is possibly a higher priority than transpirational cooling in sand dune shrubs experiencing heatwaves. This study provides evidence that the water-use behavior of shrubs from water-limited sand dunes prioritizes protection against water stress over heat stress.