Ecotypic variation in leaf thermoregulation and heat tolerance but not thermal safety margins in tropical trees

To avoid reaching lethal temperatures during periods of heat stress, plants may acclimate either their biochemical thermal tolerance, or leaf morphological and physiological characteristics to reduce leaf temperature (T leaf ). While emerging evidence indicates that plants from warmer environments have a greater capacity to regulate T leaf , the extent of intraspecific variation and contribution of provenance is relatively unexplored. We tested whether upland and lowland provenances of four tropical tree species grown in a common garden differed in their thermal safety margins by measuring leaf thermal traits, midday leaf-to-air temperature differences (∆T leaf ), and critical leaf temperature defined by chlorophyll fluorescence (T­ crit ). Provenance variation was highly species- and trait- specific. Higher ∆T leaf and T­ crit were observed in the lowland provenance for Terminalia microcarpa , and in the upland provenance for Castanospermum australe , with no provenance differences observed in the other two species. Within-species covariation of T crit and ∆T leaf led to a convergence of thermal safety margins across provenances. These findings suggest that when grown under common conditions, lowland and upland provenances may not differ substantially in their vulnerability (defined here as thermal safety margin) to heat stress, despite differences in operating temperatures and T crit .