Data analysis
All values are reported as a plant-level mean ± 1 SD. The thermal safety
margin (TSM) of each plant was calculated with Tcrit or
T50 as the upper threshold. From this value, we
subtracted the mean maximum temperature at this site, 32 °C, and then
subtracted either the mean observed leaf-air temperature differences
measured with the handheld IR sensor (∆Tleaf), the mean
observed canopy-air temperature differences measured with UAV thermal
imagery (∆Tcan), or the modelled leaf-air temperature
differences calculated under common conditions from the traits, leaf
width, stomatal conductance, and leaf inclination angle
(∆Tmod).
To assess whether there was provenance differentiation in
∆Tleaf we used a linear mixed effects model including
fixed effects for ‘provenance’ and random effects for measurement days
‘doy’ as well as individual leaves nested within individual plants
‘unique_id/leaf’. The underlying data was based on individual spot
measurements of Tleaf minus measured
Tair at that time. Models were fitted to each species
separately and pairwise comparisons between provenances were assessed
using ‘emmeans’ (Lenth, 2022).
To assess whether there was provenance differentiation in leaf traits
(width, angle, conductance, thermal time constant), leaf-air temperature
differences (∆Tmod, ∆Tcan) and thermal
tolerance (Tcrit and T50), as well as
the thermal safety margins (TSM), we performed a two-way unpaired
student’s t-test on each species separately. We used plant averages as a
single observation so that all tests had balanced designs with n = 6 plants for each group. Homogeneity of variance was tested using the
levene test and normality was tested using the Shapiro test.
To determine whether provenance-differentiations in traits were a result
of systematic differences in plant height between provenances, we
performed two analyses. First, we tested for within-canopy differences
in leaf traits and thermal tolerance in the lowland provenance ofC. australe and H. novo-guineensis. To do so we performed
paired t-tests on 5 replicate plants per group. The leaf trait, LMA, did
not meet assumptions of normality so data was log transformed for
analysis. In addition, we examined the relationship between modelled and
observed leaf temperatures, and plant height using linear regression
with either Tleaf, or Tmod as the
response variable, and plant height as an explanatory variable. All
analyses were performed using R version 4.2.2.