Differential photosynthetic responses of broad- and fine-leafed savanna trees to elevated temperatures

Abstract

South African savannas are poorly studied with respect to leaf and canopy photosynthesis making it difficult to quantify their response to global warming and contribution to the overall global carbon budget. This study addressed these issues by examining the effects of different leaf temperatures (range 25-35°C) on the photosynthesis of broad-leafed (Combretum apiculatum, Dalbergia melanoxylon, Sclerocarya birrea) and fine-leafed (Acacia nilotica, Acacia nigrescens) savanna tree species during two summer dry spells in the Kruger National Park. Results showed that photosynthetic rates of fine-leafed tree forms were significantly higher than those of broad-leafed tree forms over the entire range of photosynthetic flux densities (range: 0–2 000µmol m− 2 s− 1 PFD) but not over the entire range of external (Ca) and internal (Ci) leaf CO2 concentrations (range: 0–120Pa Ci). The lower photosynthetic rate, carboxylation efficiency (Vcmax) and electron transport capacity (Jmax) of broad-leafed forms was not reflected in their leaf N, chlorophyll or carotenoid concentrations, but did correspond with a greater foliar accumulation of non-structural carbohydrates known to inhibit photosynthesis through several routes. Despite these differences, photosynthesis in both functional types was suppressed at leaf temperatures of 35°C, the magnitude of photosynthetic acclimation equivalent in broad- and fine-leafed forms. In both forms, the decline in photosynthetic rate corresponded with a concomitant reduction in stomatal conductance, a consequence of a steep leaf-to-air vapour pressure deficit gradient with increasing leaf temperature, leading to a decrease in carboxylation efficiency as indicated by reduced Ci:Ca ratios. It is concluded that anticipated increases in temperature due to global warming may negatively impact on primary production of especially broad-leafed savannas with potential consequences for the global carbon budget.