Limited thermal acclimation of the photosynthetic heat tolerance of tropical tree species along an elevation gradient in Rwanda

Climate warming increases the risk of harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. Heat sensitivity may be particularly high in tropical tree species adapted to a thermally stable climate. Heat tolerance thresholds of photosynthetic reactions were investigated in sun-exposed leaves of 12 tropical montane tree species with different growth and water-use strategies. Chlorophyll a fluorescence, leaf gas exchange, leaf morphology and lipid composition were measured at three common gardens along an elevation and temperature gradient. Photosynthetic heat tolerance partially acclimated to increased growth temperature; on average 0.31°C increase in heat tolerance per 1°C increase in growth temperature. Consequently, thermal safety margins were narrower for species at the warmer, lower-elevation sites. Moreover, although tree species with traits that predispose them to higher leaf temperatures (low stomatal conductance and big leaves) had higher photosynthetic heat tolerance, their thermal safety margins were smaller. Heat tolerance and its acclimation were linked to the adjustment of fatty acid composition and membrane fluidity. Our results suggest that tropical trees have some capacity to thermally acclimate to increasing temperatures, but not strong enough to prevent increased heat stress in a warming climate.