Abstract
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 afluorescence started to decrease (TCrit) and had
decreased by 50% (T50). TCrit averaged
46.7°C (5th–95th percentile:
43.5–49.7°C) and T50 averaged 49.9°C (47.8–52.5°C).
Heat tolerance partially adjusted to site temperature;
TCrit and T50 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.
TCrit increased with the estimated thermal time constant
of the leaves, indicating that species with thermally buffered leaves
maintain higher heat tolerance. Among lowland species,
T50 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.