Discussion

Provenance trials in tropical tree species are relatively rare, and as such our understanding of how tropical forests acclimate or adapt to environmental change is limited. Here we established and utilised a provenance trial of tropical rainforest species in tropical Australia to assess whether trees originating from the cooler upland provenances and warmer lowlands differed in their leaf thermoregulatory traits, thermal tolerance, and as a result operate with different thermal safety margins. With contrasting patterns of provenance-differentiation observed across the four species tested, we found only partial support for our first hypothesis that the lowland provenances have cooler leaf temperatures than upland provenances when grown in a common garden (H1). Despite this, we did find that variation of leaf thermal traits and plant height was consistent with variation in Tleaf, supporting our second hypothesis (H2). Finally, although we found limited within-species association between Tleaf and T­crit, these variables covary in such a way that thermal safety margins did not significantly differ between provenances supporting our third hypothesis (H3). Together, these results suggest that despite variation in either Tcrit or observed Tleaf, there was no provenance-level difference in vulnerability to thermal stress as defined by the thermal safety margin.