Heat tolerance in relation to leaf thermoregulation
Regression models of T50 and TCrit were
consistently better (based on AIC) when they included elevation rather
than any of the temperature metrics (TMin, MAT, or
TMax). Because temperature and elevation were tightly
correlated, we did not include both in the same models. We tested the
effects of various leaf traits associated with thermoregulation on heat
tolerance. Leaf width did not correlate with T50 or
TCrit. However, leaf area and elevation combined yielded
a good model for TCrit(R2adj =0.27,
F2,118=23.4, p <0.001). Very similar
results were obtained for a model containing leaf length in addition to
elevation, with R2adj of 0.29
(F2,118=25.5, p <0.001). Species with a
higher τ, i.e., species with leaves that take longer to heat and cool,
had higher TCrit when accounting for elevation
(F2,118=22.6, p <0.001,
R2adj=0.27), without an interaction
effect of elevation—TCrit was greater at low than at
high elevation (Two-sample t test, t = 6.27, P <0.001),
whereas τ did not differ between sites (Fig. 5). When substituting MAT
for elevation τ also significantly affected TCrit(F2,118=19.6, p <0.001,
R2adj=0.24). T50 and b
were not affected by τ, and elevation did not interact significantly
with τ’s relationship with heat tolerance traits.