Leaf heat tolerance of 147 tropical forest species varies with elevation and leaf functional traits, but not with phylogeny

Exceeding thermal thresholds causes irreversible damage and ultimately loss of leaves. The lowland tropics are among the warmest forested biomes, but little is known about heat tolerance of tropical forest plants. We surveyed leaf heat tolerance of sun-exposed leaves from 147 tropical lowland and pre-montane forest species by determining the temperatures at which potential photosystem II efficiency based on chlorophyll a fluorescence started to decrease (T-Crit) and had decreased by 50% (T-50). T-Crit averaged 46.7 degrees C (5th-95th percentile: 43.5 degrees C-49.7 degrees C) and T-50 averaged 49.9 degrees C (47.8 degrees C-52.5 degrees C). Heat tolerance partially adjusted to site temperature; T-Crit and T-50 decreased with elevation by 0.40 degrees C and 0.26 degrees C per 100 m, respectively, while mean annual temperature decreased by 0.63 degrees C per 100 m. The phylogenetic signal in heat tolerance was weak, suggesting that heat tolerance is more strongly controlled by environment than by evolutionary legacies. T-Crit increased with the estimated thermal time constant of the leaves, indicating that species with thermally buffered leaves maintain higher heat tolerance. Among lowland species, T-50 increased with leaf mass per area, suggesting that in species with structurally more costly leaves the risk of leaf loss during hot spells is reduced. These results provide insight in variation in heat tolerance at local and regional scales.

Automated summary

The study surveyed the leaf heat tolerance of 147 tropical lowland and pre-montane forest species, finding that heat tolerance partially decreases with increasing environmental temperature, but is positively correlated with the thermal time constant of leaves. Additionally, species with structurally more costly leaves have a reduced risk of leaf loss during hot spells.