Thermal tolerance is linked to anatomical but not morphological leaf traits in woody species of Andean tropical montane forests

The world is experiencing a rise in global temperatures as a result of climate change. Higher temperatures, along with more frequent heat waves, negatively impact physiological levels and ultimately lead to plant death. Although woody species have different strategies to regulate leaf temperature, the relative importance of structural and anatomical traits in leaf temperature regulation remains unclear. We evaluated the variation in thermal tolerance (T50 and Tcrit) among 17 woody species in an Andean Tropical Montane Forest in Colombia. We explored how 18 leaf structural and anatomical traits can explain the variation in thermal tolerance. Our findings revealed high variability in thermal tolerance, structural, and anatomical leaf traits among species. Thick layers of upper cuticle, epidermis, and hypodermis, increased T50. However, increases in Tcrit were only associated with upper hypodermis thickness. Higher stomatal index, a trait related to increased stomatal conductance and leaf transpiration, increased T50, while traits associated with leaf water transport did not correlate with temperature tolerance. Our results highlight the contrasting influence of leaf tissues on leaf tolerance and the diverse strategies plants employ to cope with high temperatures. These findings hold significant implications for the climate-smart restoration of Andean Tropical Montane forests.

Methods
We estimated leaf thermal tolerance using T50 and Tcrit in four to six individuals per species. We measured 18 leaf structural and anatomical traits in the same individuals for which we assessed thermal tolerance. The structural traits were measured according to the standardized protocols of Cornelissen et al. (2003) and Salgado-Negret et al. (2016). The anatomical leaf traits were recorded for three leaves per individual for each species where structural traits were measured. The leaves were then fixed in FAA (formaldehyde: acetic acid: 70% ethanol, 10:5:85) and processed following the protocol by Toro-Tobón et al. (2022).