Experimental design
To determine if leaf thermal traits, leaf temperatures and thermal tolerance differed between the lowland and upland provenances of the four selected species, we identified six replicate individuals per group (n = 6 individuals × 2 provenances per species) that had healthy and fully sun-exposed canopies on which to conduct all measurements. Plant heights ranged from 0.7 m to 9.0 m (mean 4.46 m). Leaf morphological traits and thermal tolerance, which requires destructive leaf sampling, were measured on leaves sampled from the top of the canopy using a pole pruner. Leaf temperature, stomatal conductance, and leaf angle were measured in situ , with the aid of a stepladder. As a result, these measurements were conducted at canopy heights ofc. 2.5m, except in smaller stature trees which were measured at the top of their canopy.
Two of the species, C. australe and H. novo-guineensis, were previously shown to have significant provenance-level differences in plant growth (Middleby et al., 2024b), with trees from the lowland-origin provenance taller than trees from the upland-origin provenance in our common garden. In contrast, plant height did not differ between provenances in the other two species, M. elleryana and T. microcarpa . However, considering only the subset of individuals measured for this study, the lowland provenance of T. microcarpa also had greater tree heights than the upland provenance (Supporting information S3). To determine if potential provenance differences in leaf morphological traits and thermal tolerance (measured at the top of the canopy) were due to the greater plant heights of lowland provenances, we chose a subset of individuals (n =5 per species) from C. australe and H. novo-guineensis to measure leaves from mid-canopy (but still fully sun-exposed) positions, at canopy heights similar to the upland provenance trees.
In situ measurements of leaf temperatures took place daily from the 19th until the 25th of October 2023, and then again from the 6th to the 8th of November 2023. Stomatal conductance was measured over three days, combined with leaf temperature measurements on the 25th of October 2023 and 7th and 8th November 2023. Destructive leaf sampling for thermal tolerance assays and leaf morphological traits began on the 19th of October 2023 and ended on the 25th of October 2023. All leaves measured and sampled represented healthy, mature leaf material of similar positioning and aspect, on the North side of the canopy. It rained frequently in the lead-up to our measurement campaign, so plants were not expected to be moisture-limited.