Leaf wax n-alkane distributions in arid zone South African flora: Environmental controls, chemotaxonomy and palaeoecological implications

The environmental controls on leaf wax n-alkane distributions and associated interpretation of such distributions in geological archives have long remained rather enigmatic. Studies of contemporary vegetation often conflate changing environmental conditions and species differences between biomes, making it difficult to assess the extent to which variation is driven by plant adaptation to prevailing environmental conditions and/or more fixed chemotaxonomic patterns. We present a case study of arid and semi-arid regions of South Africa that considers these issues. We anticipate that such conditions may impart strong controls upon leaf wax synthesis. Leaf wax n-alkane data from 215 plants and 93 soils from the Succulent Karoo and Fynbos biomes of South Africa revealed yield and distributions to be highly variable. While many plants exhibited concentrations comparable with previous reports, several succulent plants produced n-alkane yield up to 100x the modal n-alkane concentration for their biome. The data demonstrate that, on average, leaf wax n-alkane distributions in the Succulent Karoo are different from those of the Fynbos biome, with the former associated with longer maximum chain length, less dispersed distributions and stronger odd numbered chain length preference. The patterns were closely mirrored in the soils. Average chain length was weakly correlated with climate and this is interpreted to be a function of multiple factors. We hypothesise that a key control is the shifting proportion of plant functional type between biomes, with a greater abundance of succulent growth forms in the more arid Succulent Karoo (which tended to produce longer and less dispersed n-alkane chain length distributions), differentiating this biome from the Fynbos, in which woody shrubs are more dominant. Inter-biome variability was also apparent, with some same taxonomic or functional groups common to both biomes tending to produce longer chain length n-alkane distributions under more arid conditions. There was, however, considerable individual plant variability and we observed both insensitivity to environmental conditions (i.e. consistent n-alkane distributions) and marked variability in chain length distributions. A key finding is the high yield of n-alkanes from multiple succulent plants, many of which use the CAM photosynthetic pathway. Compound specific delta C-13/delta D analyses (e.g. estimates of C-3 and C-4 biomass) for regions with potential succulent plant contributions should therefore be carried out cautiously. (C) 2013 Elsevier Ltd. All rights reserved.