Photosynthetic quantum efficiency in south‐eastern Amazonian trees may be already affected by climate change

Abstract

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.

Tree mode of death and mortality risk factors across Amazon forests

“…However, this seems not to be the case in our data as we find no relationship between the species growth rates and their drought affiliation for Southern Amazonia (R 2 = 0.0001, P value = 0.1). Thus, the selective mortality of the most vulnerable species indicates that these communities are already experiencing climatic extremes that go beyond the conditions to which these species are adapted 40,41 . Surprisingly, drought-tolerant species were at greater risk of death in East-Central Amazonia.…”

Photosystem II heat tolerances characterize thermal generalists and the upper limit of carbon assimilation

“…Reported values for T max in tropical plant species range from 40.1 to 41.8 C and are comparable to the temperatures that cause the first signs of damage in F v /F m (i.e. T crit ) for tropical species (Figure 1a; Perez & Feeley, 2020a;Slot, Krause, Krause, Hernández, & Winter, 2018;Tiwari et al, 2020). Coordination between T crit and T max would provide support for the hypothesis that PSII heat tolerance constrains the upper limit of carbon assimilation by limiting electron transport (Slot & Winter, 2017a).…”

“…Higher heat tolerance of PSII photochemistry is generally assumed to allow for improved growth, reproduction and/or survival in hot environments, presumably by facilitating photosynthesis at high temperatures (Feeley, Martinez-villa, Perez & Duque 2020;Krause, Winter, Krause, & Virgo, 2015;Tiwari et al, 2020; but see Perez & Feeley, 2020a, 2020b. However, these assumptions have not been widely tested, and it is unclear how PSII heat tolerance integrates with different thermal strategies for understanding the effects of climate change on plants.…”

Leaf heat tolerance of 147 tropical forest species varies with elevation and leaf functional traits, but not with phylogeny

“…Our approach was based on the protocol that Krause et al (2010) developed to study leaf heat tolerance of (Panamanian) tropical trees. The protocol yields results that are very similar to those obtained in classical necrosis tests (Krause et al, 2010), and is now commonly used (e.g., Feeley, Martinez-Villa, et al, 2020;Leon-Garcia & Lasso, 2019;Slot et al, 2019;Tiwari et al, 2021). We measured F v /F m on leaf disks 24 hr after they were incubated for 15 min in a temperaturecontrolled water bath, using 8-12 incubation temperatures between…”