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
species. 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)
and had decreased by 50% (T). T
averaged 46.7°C (5–95
percentile: 43.5–49.7°C) and T averaged 49.9°C
(47.8–52.5°C). Heat tolerance partially adjusted to site temperature;
T and T decreased with elevation
by 0.40°C and 0.26°C per 100m, respectively, while mean annual
temperature decreased by 0.63°C per 100m. The phylogenetic signal in
heat tolerance was weak, suggesting that heat tolerance is more strongly
controlled by environment than by evolutionary legacies.
T 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 increased with leaf mass per area, so species with
structurally more costly leaves reduce the risk of leaf loss during hot
spells. These results provide insight in interspecific variation in heat
tolerance at local and regional scales.
Temperate and Tropical Forest Canopies are Already Functioning beyond Their Thermal Thresholds for Photosynthesis