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.