Effects of elevation and site temperature on heat tolerance and
leaf traits
T50 and TCrit both decreased
significantly with elevation (F1,150=35.1 and 46.1, for
T50 and TCrit, respectively,p <0.001) (Fig. 1). Including CAM and C4plants in the analysis did not change the slopes, but decreased the
variance explained—all CAM and C4 plants were
collected from lowland sites and heat tolerance was higher in CAM
plants, and lower in the C4 plant than in
C3 plants. T50 and TCritdecreased by 0.26°C and 0.39°C per 100 m, respectively (0.28°C and
0.41°C per 100 m when including non-C3 species). The
lapse rate across the study sites was 0.63°C decrease in mean annual
temperature (MAT) per 100 m increase in elevation.
Heat tolerance increased significantly with MAT
(F1,150=30.0 and 38.0, for T50 and
TCrit, respectively, p <0.001) (Fig. 1).
For every 1°C increase in MAT, T50 increased by 0.41°C
and TCrit increased by 0.60°C. T50 and
TCrit also correlated significantly with the mean
maximum temperature of the warmest month and with the mean minimum
temperature of the coldest month (Fig. S4). The b parameter, the
steepness of the decline in Fv/Fm around
T50, significantly decreased with elevation and
increased with MAT, so Fv/Fm declined
less steeply in cooler, higher elevation species than in lowland
species. However, elevation and MAT only explained a small amount of
variance in b (r2≤0.06).
Consistent with the patterns across species, within species there was a
tendency for heat tolerance to decrease with elevation and increase with
site temperature (irrespective whether minimum, mean, or maximum) (Fig.
2). Among lowland sites (between 0 and 200 m elevation, with MAT
differences <1°C) patterns in heat tolerance with elevation
and MAT were not consistent and the confidence intervals of the two
values tended to overlap (Fig. 2). T50 and
TCrit decreased with elevation by 0.4 and 0.7°C per 100
m, respectively, based on a regression weighted by elevation difference
between sites—to reduce the weight of species measured at two lowland
sites. T50 and TCrit increased with site
temperature by 0.9 and 1.5°C per °C MAT, respectively.
To enable comparison of heat tolerance among plant categories we
standardized T50 and TCrit to sea-level
values using the trendlines in Fig. 1. Standardized T50and TCrit were not significantly different between
native and non-native species, or between evergreen and deciduous
species (Fig. S5). Gymnosperms tended to have moderately higher
T50 than angiosperms (p <0.1, two-sample
t-test) because of the high heat tolerance of three Zamiaspecies, but the sample size of gymnosperm species (n=5) was
insufficient to draw meaningful biological conclusions from these
apparent differences. For similar reasons, cycads (i.e., Zamiaspecies in our study), had higher T50 than lianas (n=6),
shrubs (6), and trees (134), and higher TCrit than
lianas (Fig. S5). When including the CAM species Agave americanaand Furcraea cabuya , forbs had higher T50 than
most other functional groups (not shown).