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The continued functioning of tropical forests under climate change depends on their joint resilience to drought and heat. However, there is little understanding of how tropical forests will respond to a combination of these stresses, and no field studies to date have explicitly evaluated whether sustained drought alters their sensitivity to temperature. In this study, we measured the temperature response of net photosynthesis, foliar respiration and the maximum quantum efficiency of photosystem II (Fv/Fm) of eight hyper-dominant Amazonian tree species at a 17-year tropical forest rainfall exclusion experiment, to investigate the effect of long-term reduction in soil water availability on forest thermal sensitivity. Additionally, we compared thermal traits across tree species that had previously been classified as drought-tolerant or intolerant, based on their mortality response after exposure to the same experimental drought, testing whether the direction and magnitude of thermal trait adjustment in response to sustained soil moisture deficit is influenced by drought tolerance, and if there is co-ordination, independent of treatment, between drought and thermal sensitivity. Despite 0.5 – 2 ˚C higher canopy air temperatures in the drought plot compared to the control, no change in average thermal sensitivity of net photosynthesis or respiration was observed. However, photosystem II tolerance to extreme-heat damage (T50) was reduced from 50 ± 0.3 ˚C to 48.5 ± 0.3 ˚C under drought. Surprisingly, drought-induced thermotolerance declines were stronger in drought-tolerant species compared to drought-intolerant. However, drought-tolerant species were further characterised by a wider thermal breadth of photosynthesis (Tspan), conferring a greater photosynthetic stability to moderate rise in temperature, and were more able to maintain high stomatal conductance rates at high leaf temperatures (higher gsTL46 and lower gsdiff) compared to drought-intolerant species, making them less likely to experience critically high leaf temperatures. Our results suggest that long-term reductions in precipitation, as projected across much of Amazonia by climate models, are unlikely to greatly alter the response of tropical forest communities to rising mean temperatures but may increase the risk of leaf thermal damage during heatwaves.

Keywords:

Drought, heat-stress, photosynthesis, respiration, thermotolerance, fluorescence, thermal sensitivity, tropical trees

Emma Docherty, Emanuel Gloor, Daniela Sponchiado, Martin Gilpin, Carlos Pinto, Haroldo Junior, Ingrid Coughlin, Leandro Ferreira, João Junior, Antonio da Costa, Patrick Meir, David Galbraith

Presentation within symposium:

S-19 Tropical forest response to temperature: a pantropical synthesis of elevation gradients, and leaf thermoregulation studies

Long-term drought effects on the thermal sensitivity of Amazon Forest trees

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