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).