Species-specific leaf thermoregulation provenance effects
In our study, we observed provenance effects in at least one thermal
trait (i.e., leaf width, leaf angle, or g s) in
three of the four species, and provenance effects in measured
Tleaf in two species. Assuming the expression of leaf
thermal traits under common garden conditions would reflect underlying
genetic adaptation associated with provenance, we expected higher
Tleaf for cool-adapted, and lower Tleaffor warm-adapted provenances, which would be consistent with selective
pressure to limit plant exposure to extreme leaf temperatures (Michaletz
et al., 2016). However, patterns of provenance-differentiation in
thermal traits and Tleaf followed expected trends (H1)
only in one species, C. australe , with the opposite pattern
observed for T. microcarpa , and no variation associated with
provenance for the other two species. While this contrasts with other
studies that have found warm-adapted plants to have a lower
Tleaf compared to cool-adapted plants (Blasini et al.,
2022; Kitudom et al., 2022; Kullberg et al., 2023), species-specific
patterns in intra-specific variation of thermal traits have been
reported previously (Kullberg et al., 2023; Manishimwe et al., 2022).
This suggests that different species may have different selection
pressures, genetic history, or trait trade-offs resulting in contrasting
patterns of provenance-associated variation in leaf thermoregulation. It
is also interesting to note that the species where we did observe the
expected direction of provenance-differentiation in leaf
thermoregulation, C. australe , was also the species that had the
coolest realized thermal niche (S 1). As a result, the mean annual
temperature of the lowland provenance collection site, and common garden
planting location were at the warmer limit of the species distribution.
This is similar to the provenance-trial set up and findings of Blasini
et al (2022), and partly supports the theory that selection pressures
leading to local adaptation are stronger at the climate extremes of a
species distribution (Rehm et al., 2015; Zimmermann et al., 2009).
However, further work assessing variation in thermoregulation in
multiple provenances, across sites, and in a broader range of species
would be needed to establish if this could explain the contrasting
patterns observed across species tested here.