Living on the edge: warmer climate reduces leaf thermal safety margins and causes gas exchange decoupling in Mediterranean shrubs

Abstract

Global warming pushes plants close to their leaf thermal limits, altering carbon uptake, growth and survival. Therefore, investigating leaf thermal tolerance adjustment is essential to understand future vegetation dynamics, especially in high-risk Mediterranean shrubs facing hot and dry summers. We measured dark-adapted leaf fluorescence (Fv/Fm), thermal thresholds (Tcrit, T50, Tmax), optimal assimilation temperature (Topt), thermal safety margin (TSM), leaf and air- temperatures (Tair, Tleaf) and gas exchange (A, gs, E) in six shrub species in six sites along a climatic gradient in Catalonia, Spain. We found that Topt increased as conditions warmed, while Fv/Fm and Tmax showed quadratic responses decreasing in the warmest sites. Hence, TSM declined and approached 0 at the hottest sites, indicating that shrubs were operating close to their thermal limits. Warmer Tair under high levels of solar radiation raised Tleaf, though some species maintained Tleaf < Tair during daytime, suggesting a passive or active leaf cooling at high temperatures exceeding solar energy. Moreover, E remained high despite low A at higher Tleaf, revealing a decoupling of gas exchange at high temperatures. Incorporating leaf thermal tolerance adjustments and gas exchange decoupling at extreme temperatures into vegetation models could improve predictions of shrub function and dynamics under warmer climates. HighlightOur study highlights that Mediterranean shrubs have limited capacity to adjust leaf thermal tolerance at warmer sites, resulting in narrower thermal safety margins. The decrease in leaf temperatures through evaporative cooling may not compensate for the more frequent and intense heatwaves, leading to irreversible damage. These findings highlight the importance of incorporating leaf thermal tolerance adjustment and gas exchange decoupling into future vegetation models.