Thermal sensitivity thresholds for photosystem
II
A key question concerns how close tree species are to their thermal
sensitivity thresholds in the warmest region of the Amazon? The data
presented here show that T50 for seven focal
species is at least 4 °C above the highest air temperatures ever
experienced at the study site (43.9 °C). However, for most species, T5 , the breakpoint
temperature, is already surpassed in the peak of
the warm/dry season (Figure 6). Thus, the site temperatures are already
reaching levels where incipient irreversible loss of PSII activity
occurs during the dry/warm periods. Such periods are predicted to become
more frequent with future warming (Yao, Luo, Huang, & Zhao, 2013).
However, these inferences are based on air temperatures, while leaf
temperatures are arguably a more meaningful measure of proximity to
thermal sensitivity thresholds. During the dry/warm season, limited
access to soil moisture restricts stomatal opening. This is likely to
lead to leaf temperatures that exceed air temperatures (G. Heinrich
Krause et al., 2010; Slot & Winter, 2016), at least for part of the
day. Under these circumstances, plants in our study area, especially
those exposed to full sunlight, may operate closer to thermal
sensitivity thresholds than the simple examination of air temperatures
suggests.