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).