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

Abstract

<jats:title>Abstract</jats:title><jats:p>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 (<jats:italic>F</jats:italic><jats:sub><jats:italic>v</jats:italic></jats:sub>/<jats:italic>F</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub>) of photosystem II was measured in seven dominant tree species using leaf discs exposed to varying levels of heat stress. <jats:italic>T</jats:italic><jats:sub>50</jats:sub> was calculated as the temperature at which <jats:italic>F</jats:italic><jats:sub><jats:italic>v</jats:italic></jats:sub>/<jats:italic>F</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub> was half the maximum value. <jats:italic>T</jats:italic><jats:sub>5</jats:sub> is defined as the breakpoint temperature, at which <jats:italic>F</jats:italic><jats:sub><jats:italic>v</jats:italic></jats:sub>/<jats:italic>F</jats:italic><jats:sub><jats:italic>m</jats:italic></jats:sub> decline was initiated. Leaf thermotolerance in the rapidly warming southern Amazonia was the highest recorded for forest tree species globally. <jats:italic>T</jats:italic><jats:sub>50</jats:sub> and <jats:italic>T</jats:italic><jats:sub>5</jats:sub> varied between species, with one mid‐storey species, <jats:italic>Amaioua guianensis</jats:italic>, exhibiting particularly high <jats:italic>T</jats:italic><jats:sub>50</jats:sub> and <jats:italic>T</jats:italic><jats:sub>5</jats:sub> values. While the <jats:italic>T</jats:italic><jats:sub>50</jats:sub> values of the species sampled were several degrees above the maximum air temperatures experienced in southern Amazonia, the <jats:italic>T</jats:italic><jats:sub>5</jats:sub> values of several species are now exceeded under present‐day maximum air temperatures.</jats:p>