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.
Highlights
Temperatures have increased substantially in the Tropics in recent decades, by as much as 0.5C per decade in some regions (JiménezMuñoz, Sobrino, Mattar, & Malhi, 2013)
The following questions were addressed: (a) does the high-temperature tolerance of sun-exposed evergreen trees in the hottest Amazonian site vary across species? (b) what is the extent of seasonal plasticity in photosynthetic thermal tolerance over dry and wet seasons? In addition, (c) how does thermotolerance of southern Amazonian species compare with published data for trees from other tropical regions where maximum temperatures are typically lower? In addition to T50, as a proxy for the first heat effects, we assessed two further parameters: (a) T5, the temperature associated with the onset of the temperature-induced decline in Fv/Fm and (b) T95, the temperature at which Fv/Fm decreased below 95% of the maximum level, taken to be the limit, where photosystem II (PSII) functions are effectively lost
Tree species measured in Nova Xavantina showed a very high thermotolerance, with one species, Amaioua guianensis, recorded the highest T50 documented for any tropical evergreen tree far (52.7 ± 1.05C)
Summary
Temperatures have increased substantially in the Tropics in recent decades, by as much as 0.5C per decade in some regions (JiménezMuñoz, Sobrino, Mattar, & Malhi, 2013). Forests in the southern Amazon experience the highest temperatures in Amazonia, with monthly maximum air temperatures during dry periods frequently reaching >40C. They are amongst the earth's most rapidly warming tropical forests (Gloor et al, 2018; Jiménez-Muñoz et al, 2013) making them a natural laboratory for studying the effects of global warming on tropical forests. Heat-induced changes in Fv/Fm were used to characterise thermal tolerance for seven dominant tree species in a rapidly warming southern Amazonian rainforest over two different seasons, that is, the end of the wet season and end of the dry season. The following questions were addressed: (a) does the high-temperature tolerance of sun-exposed evergreen trees in the hottest Amazonian site vary across species? (b) what is the extent of seasonal plasticity in photosynthetic thermal tolerance over dry and wet seasons? In addition, (c) how does thermotolerance of southern Amazonian species compare with published data for trees from other tropical regions where maximum temperatures are typically lower? In addition to T50, as a proxy for the first heat effects, we assessed two further parameters: (a) T5, the temperature associated with the onset of the temperature-induced decline in Fv/Fm and (b) T95, the temperature at which Fv/Fm decreased below 95% of the maximum level, taken to be the limit, where PSII functions are effectively lost
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