Figure 5: Photosynthetic thermal tolerance of adult evergreen trees measured across tropical forest biomes. Except for Indian data for the Western Ghats, all other studies including current study did the heat treatment for 15-minutes
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 metric Tcrit 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.