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.