Diversity, metabolic types and δ13C carbon isotope ratios in the grass flora of Namibia in relation to growth form, precipitation and habitat conditions

Abstract

The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in δ¹³C values along a rainfall gradient (50-600 mm) and in different habitat conditions. However, there are significant differences in the δ¹³C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest δ¹³C values (-11.7 ‰) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower δ¹³C values (-13.4 ‰) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (-12.5 ‰) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative δ¹³C values than PCK species and δ¹³C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, δ¹³C values decreased from -11 ‰ in the inland region (600 mm precipitation) to -15 ‰ near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed.

Keywords: C4 photosynthesis; Geographic distribution; Grass flora of Namibia; Poaceae; δ13C values.