Atmospheric and climate change will expose tropical forests to conditions they have not experienced in millions of years. To better understand the consequences of this change we studied photosynthetic acclimation of the neotropical tree species Tabebuia rosea to combined 4°C warming and twice-ambient (800 ppm) CO2. We measured temperature responses of the maximum rates of ribulose 1,5-bisphosphate carboxylation (VCMax), photosynthetic electron transport (JMax), net photosynthesis (PNet), and stomatal conductance (gs), and fitted the data using a probabilistic Bayesian approach. To evaluate short-term acclimation plants were then switched between treatment and control conditions and re-measured after 1–2 weeks. Consistent with acclimation, the optimum temperatures (TOpt) for VCMax, JMax and PNet were 1–5°C higher in treatment than in control plants, while photosynthetic capacity (VCMax, JMax, and PNet at TOpt) was 8–25% lower. Likewise, moving control plants to treatment conditions moderately increased temperature optima and decreased photosynthetic capacity. Stomatal density and sensitivity to leaf-to-air vapor pressure deficit were not affected by growth conditions, and treatment plants did not exhibit stronger stomatal limitations. Collectively, these results illustrate the strong photosynthetic plasticity of this tropical tree species as even fully-developed leaves of saplings transferred to extreme conditions partially acclimated.

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 (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.

Tropical forests are experiencing unprecedented high temperature conditions due to climate change that could limit their photosynthetic functions. We studied the high temperature sensitivity of photosynthesis in a rainforest site in southern Amazonia, where some of the highest temperatures and most rapid warming in the Tropics have been recorded. The quantum yield (Fv/Fm) of photosystem II was measured in seven dominant tree species using leaf discs exposed to varying levels of heat stress. T50 was calculated as the temperature at which Fv/Fm) was half the maximum value. T5 is defined as the breakpoint temperature, at which Fv/Fm) decline was initiated. Leaf thermotolerance in the rapidly warming Southern Amazonia was the highest recorded for forest tree species globally. T50 and T5 varied between species, with one mid storey species, Amaioua guianensis, exhibiting particularly high T50 and T5 values. While the T50 values of the species sampled were several degrees above the maximum air temperatures experienced in southern Amazonia, the T5 values of several species are now exceeded under present-day maximum air temperatures.