Ecological and phylogenetic patterns of carbon isotope discrimination in the winter-rainfall flora of the Richtersveld, South Africa

Leaf tissues of vascular plant species from the arid Richtersveld of northe rn Namaqualand (Succulent Karoo), South Africa, show a great range of varia tion in carbon isotope discrimination. Analyses of Delta values for species with obligate C-3 metabolism demonstrated that the water use efficiency in dicated by these values varies between species and within a species populat ion both between seedlings and mature shrubs and between sites along an ari dity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among eph emerals were there taxa with high Delta values associated with low water us e efficiency, but these high values were balanced by other ephemeral specie s with low Delta values comparable to those typical of moderate to long-liv ed shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of C rassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photo synthesis, although Crassulaceae from more mesic environments may be CAM-fl exible. Succulent-leaved species of Senecio and leaves of stem-succulent Eu phorbia were CAM-flexible in their Delta values, while shrubby species of C eraria (Portulacaceae) appeared to rely primarily on C-3 photosynthesis. Va riable patterns of reliance on CAM photosynthesis were present in the ecolo gically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the s ubfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites . Within the subfamily Ruschioideae, there was a wide range of Delta values indicating species with CAM, CAM flexibility, and primary reliance on C-3 photosynthesis. Low Delta values indicative of typical CAM fixation were as sociated with species with greater longevity, suggesting that a positive co rrelation may exist between the degree of reliance on CAM photosynthesis an d plant life span.