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

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. Across all species, 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 occasionally maintained Tleaf < Tair during daytime, suggesting a passive or active leaf cooling at high temperatures. Moreover, E and, to a lesser extent, gs remained relatively high despite low A at higher Tleaf (at a given VPD), revealing a partial 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.