C. australe:
Consistent with our expectations, Tcrit and Tleaf were higher in the upland provenance of C. australe compared to the lowland provenance. Leaves of the upland provenance had substantially shallower leaf angles than those from the lowland provenance. This likely resulted in an increased radiative load, resulting in warmer Tleaf, driving Tcrithigher for the upland provenance. Leaf angle is often ignored in applications of leaf energy balance modelling, despite the strong effect it can have on modifying the net radiation term (Ponce De León & Bailey, 2024; Woods et al., 2018; Yang et al., 2023). This effect is partially reflected in the modelled leaf temperatures (Tmod); however, covariation of other thermal traits dampened the influence of leaf angle on Tmod resulting in no statistically significant differences in Tmodbetween provenances.
However, Tcrit (measured on leaves sampled at the top of the canopy) still showed strong provenance differentiation, suggesting Tleaf at the top of the canopy in the upland provenance was indeed warmer than for the lowland provenance. While this could indicate that leaf angles at the top of the canopy still differed sufficiently between provenances to impact leaf energy balance, another explanation is that the provenance-differentiation observed in Tleaf was also driven by systematic differences in microclimate due to plant height. The upland population, being smaller in stature compared to the lowland population, had a hotter thermal environment due to its proximity to the exposed ground with higher sensible heat/longwave radiation, as well as lower turbulence associated with wind speed. During our experiment, with the 3 × 3 m spacing between trees, the ground is exposed, and spot measurements revealed ground temperatures around 50 to 60 °C. In a provenance trial of sagebrush, Olsoy et al. (2023) also found that sub-species differences in Tleaf were associated with plant height, with shorter stature plants having higher T­leaf. With no compensatory changes in leaf traits with height within the canopy observed for C. australe , this could lead to higher Tleaf and Tcrit in the smaller, upland provenance. This spatial pattern contrasts with what is observed in trees in a closed canopy where the lower canopy and ground are shaded and thus cooler than the upper canopy (Rey-Sanchez et al., 2017).