Conclusion

The present study quantified the photosynthetic thermal tolerance of seven dominant evergreen trees from the hottest and most rapidly warming forest site in the Amazonia. The key findings of this study are firstly, that evergreen trees from the site exhibit a high level of PSII thermotolerance. Although variations in thermotolerance was observed between species, this was largely caused by data from one mid-storey species, which showed exceptionally high thermotolerance (the highest recorded in the tropics). A significant but weak seasonal acclimation ofT50 was observed that is consistent with the literature (Sastry & Barua, 2017; Zhu et al., 2018). AlthoughT50 values were much higher than the absolute maximum air temperature of the region, there is some evidence of incipient loss of PSII function even under current conditions. Daily maximum air temperatures during the dry periods at our study site in recent years has reached levels that overlap with the range ofT5 measured in our study. The trees in the hottest Amazonian forest site are therefore already sensitive and experiencing temperatures close to the thresholds of their high-temperature sensitivity.