Large-leaved species and slow-cooling species have higher heat
tolerance
Leaf width is an important driver of leaf temperature (Leigh et
al . 2017; Fauset et al . 2018), as is overall leaf size (Leighet al . 2017), with wide and large leaves warming up more than
narrow and small ones under similar conditions. Leaf width did not
affect heat tolerance, but TCrit increased with leaf
size. This supports the idea that species with a large boundary layer
that are more likely to experience high temperatures should have higher
tolerance to those temperatures. Indeed, Perez & Feeley (2020a)
recently showed that T50 scaled with the highest
measured leaf temperature among 19 species in the Fairchild Tropical
Botanical Garden in Florida.
Heat tolerance—specifically TCrit, the temperature
associated with the onset of thermal damage—increases with the
estimated thermal time constant τ, while it was hypothesized to
decrease. Species with low τ can rapidly heat up, so they were expected
to have a selective advantage of high heat tolerance. However, species
with low τ also cool down again quickly, so while they may experience
higher temperatures, these high temperatures are unlikely to be
sustained for long periods of time. The protocol we used to determine
heat tolerance involves 15-minute exposure to set temperatures, and it
is the exposure to 15 minutes of TCrit that causes the
initiation of irreversible heat damage. Species with high τ are more
buffered against temperature fluctuations and thus more likely to
experience a temperature for 15 minutes continuously than species with
low τ, even if the peak temperatures are lower than those low-τ species
experience. We could not assess whether high τ leaves in the current
study indeed experienced more sustained high temperatures and low τ
leaves higher but shorter peak temperatures, nor whether large-leaved
species were consistently warmer than small-leaved species, as
high-resolution, long-term monitoring of leaf temperatures of 147
tropical forest species was not feasible. New developments in wireless
sensor networks and infrared thermometry provide avenues for detailed
monitoring of canopy microclimates and leaf temperatures in the near
future (e.g., Egea et al . 2017; Jin et al . 2018; Websteret al . 2018), to determine variation in the thermal safety
margins across species, and their underlying morphological and
physiological traits.