Only five studies in the literature reported data for adult evergreen trees in tropical forests. O’Sullivan et al. (2017) and Zhu et al. (2018) report the F0 rise inflexion point metricTcrit whereas the remaining tropical studies viz., G. Heinrich Krause et al. (2010), Sastry and Barua (2017), Slot et al. (2018) and the present study report T50 based on Fv/Fm decline. Although the number of studies is limited, this dataset includes rainforests in Australia, India and the NeoTropics (Table 1). BothT50 (Figure 5) and Tcritshowed very closely located modes around 47–48°C. While most of theT50 was > 45°C and the mode was around 47°C; the range of T50 (45–52°C) was much narrower than Tcrit , which showed very high interspecific variability within sites. Mixed effect models showed that the choice of metric (F = 24.32, P < 0.001) with season (F = 52.89, P < 0.001) and species (F = 13.04, P < 0.001) explained significant variation in the data indicating that both metrics are different given different underlying mechanisms. The two metrics were therefore compared separately.
The T50 values for the Central America and Southern Amazon species in the wet period were 50.4±0.36°C and 49.4±0.73°C respectively whereas for the Western Ghats it was significantly (t31 =2.40, p =0.02) lower at 46.9±0.31°C. Tcrit was found to vary much more across species and seasons than T50 . WhileTcrit ranged from 37.2–66.7 °C (n=96),T50 ranged from 45.5–56.42°C (n=54). A linear mixed effect model fitted to explain T50 showed season (F =12.96, P <.0001) and biogeographical region (F =15.26, P <.0001) as significant variables (pseudo R2 = 0.885). In contrast,Tcrit showed no effect of season, however the variability and limited number of sites represented, limits any meaningful comparison.