Limited thermal acclimation of the photosynthetic heat tolerance of tropical montane tree species along an elevation gradient.

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Climate warming increases the risk of harmful leaf temperatures in terrestrial plants, particularly in tropical tree species that have evolved in warm and thermally stable environments. We examined heat tolerance thresholds of photosynthetic light reactions in sun-exposed leaves of 12 tropical montane tree species with different strategies for growth and water use. Leaf chlorophyll a fluorescence, gas exchange, morphology, and thylakoid membrane lipid composition were measured at three common gardens along an elevation and temperature gradient in Rwanda. Tree species with traits predisposing them to higher leaf temperatures, such as lower stomatal conductance and big leaves, had higher photosynthetic heat tolerance, but narrower thermal safety margins (TSMs). Photosynthetic heat tolerance partially acclimated to increased growth temperature. Heat tolerance increased by 0.31°C on average for every 1°C increase in growth temperature. Thus, TSMs were narrower for trees grown at the warmer sites. Heat tolerance and its acclimation were linked to the adjustment of thylakoid membrane lipid composition. Moreover, TSMs were larger in species with high leaf mass per area. Our results show that (i) leaf temperature is more important than heat tolerance in controlling interspecific variation in TSMs, and that (ii) tropical trees have limited ability to thermally acclimate to increasing temperatures.

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