T. microcarpa:
In contrast, T. microcarpa had the opposite pattern to C.
australe , with the lowland provenance having higher
Tmod, Tleaf and Tcritthan the upland provenance. This was likely driven by substantially
lower g s rates of the lowland provenance compared
to the upland provenance.
Although an assumption of local adaptation of plant thermoregulation led
to an expectation of higher g s in lowland
provenances, it is not surprising that T. microcarpa showed the
reverse pattern. This is due to the likely trade-offs in gas exchange
with temperature and vapour pressure deficit, which covary. A recent
glasshouse experiment (Middleby et al., 2024a) compared acclimation of
gas exchange (including the stomatal slope parameterg 1) under different temperature and VPD
conditions, revealing tropical trees may decrease water use efficiency
in response to increased air temperature but increase water use
efficiency in response to increased vapour pressure deficit. Supporting
this, a study assessing water use in a lowland and upland forest in this
region showed the lowland forest with high VPD had lower canopy
conductance and annual transpiration than the upland forest with low VPD
(Binks et al., 2023). It is therefore possible the lowerg s of the lowland provenance in T.
microcarpa is an adaptation to the higher evaporative demand of the
seed source location, rather than temperature itself.
This pattern of provenance differences in Tleaf traits
for T. microcarpa is particularly interesting considering, likeC. australe and H. novo-guineensis , plants from the
lowland provenance were taller than the upland provenance and thus
experience a cooler microclimate to the upland plants. The close
relationship between Tleaf and Tmodsuggests the measured variation in leaf thermal traits was adequate to
explain these patterns, although it is also possible that the variation
in thermal tolerance and Tleaf was due to some
unmeasured variable, as canopy architecture, including leaf clumping,
can also play a large role in determining Tleaf (Gauthey
et al., 2023; Leuzinger & Körner, 2007; Woods et al., 2018).