Heat tolerance in relation to leaf construction costs
T50 scaled with LMA, supporting the hypothesis that heat
tolerance increases with increasing construction costs of the leaves.
This relationship was strong among lowland species, but the
(considerably smaller) variation in T50 among
mid-elevation and pre-montane species was driven by other, unidentified
factors. Previous links between heat tolerance and leaf cost traits LMA
and LDMC were obtained at sites with considerable temperature
seasonality (e.g., Knight & Ackerly 2003; Sastry & Barua 2017). In the
current study temperature seasonality was minimal, with the difference
in mean temperature between the hottest and the coolest month ≤2.0°C at
all sites, including the lowland sites. In these lowland forests LMA
correlates strongly with leaf lifespan (Kitajima & Poorter 2010). While
greater leaf lifespan does not significantly increase the likelihood of
exposure to higher air temperature, leaf temperature may be considerably
higher in the dry season, when incoming solar radiation at the canopy
level is greater (Rey-Sánchez et al . 2016). If dry-season water
deficit forces stomatal closure, limited transpirational cooling leads
to further increases in leaf temperature, especially in the warmer
lowlands with prolonged dry seasons. Increased heat tolerance may thus
be advantageous for leaves that persist through the dry season.
Evergreen species did not have higher heat tolerance than deciduous
species in a subset of 99 species for which this information was known
(Fig. S5), but more detailed information on leaf phenology may be
required to evaluate the effects of potentially high dry season leaf
temperatures.