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Abstract

Photosynthesis rates and photosynthesis–leaf nutrient relationships were analysed in nine tropical grass and sedge species growing in three different ecosystems: a rain-fed grassland, a seasonal floodplain, and a permanent swamp, located along a hydrological gradient in the Okavango Delta, Botswana. These investigations were conducted during the rainy season, at a time of the year when differences in growth conditions between the sites were relatively uniform. At the permanent swamp, the largest variations were found for area-based leaf nitrogen contents, from 20 mmol m–2 to 140 mmol m–2, nitrogen use efficiencies (NUE), from 0.2 mmol (C) mol–1 (N) s–1 to 2.0 mmol (C) mol–1 (N) s–1, and specific leaf areas (SLA), from 50 cm2 g–1 to 400 cm2 g–1. For the vegetation growing at the rain-fed grassland, the highest leaf gas exchange rates, high leaf nutrient levels, a low ratio of intercellular to ambient CO2 concentration, and high carboxylation efficiency were found. Taken together, these observations indicate a very efficient growth strategy that is required for survival and reproduction during the relatively brief period of water availability. The overall lowest values of light-saturated photosynthesis (Asat) were observed at the seasonal floodplain; around 25 µmol m–2 s–1 and 30 µmol m–2 s–1. To place these observations into the broader context of functional leaf trait analysis, relationships of photosynthesis rates, specific leaf area, and foliar nutrient levels were plotted, in the same way as was done for previously published ‘scaling relationships’ that are based largely on C3 plants, noting the differences in the analyses between this study and the previous study. The within- and across-species variation in both Asat and SLA appeared better predicted by foliar phosphorus content (dry mass or area basis) rather than by foliar nitrogen concentrations, possibly because the availability of phosphorus is even more critical than the availability of nitrogen in the studied relatively oligotrophic ecosystems.