LEAF AREA INDEX, MAMMALIAN SPECIES RICHNESS AND MAMMALIAN COMMUNITY STRUCTURE IN SOUTH AMERICA TODAY AND IN THE MIDDLE CENOZOIC

Leaf Area Index (LAI), the one-sided area of leaves/area of ground (m²/m²), is a variable used in climate models that captures insolation and light absorption in photosynthesis. LAI correlates with primary productivity and the architecture of vegetation, and as such, is of interest to vertebrate paleontology. LAI can now be reconstructed in the fossil record by tracking light dependent changes in morphology of leaf epidermal cells that are preserved as phytoliths. Preservation of phytoliths in paleosols reflects in situ vegetation and since phytoliths are microfossils, they can occur at any stratigrahic level resulting in high resolution and continuous records of vegetation change. Moreover, rLAI quantifies vegetation structure and can distinguish open vs closed vegetation in the absence of grasses. We compiled MODIS based LAI data from 80 sites in modern South America along with climate variables and mammal inventories to assess how LAI correlates with diverse precipitation and temperature variables, mammal species richness and faunal composition. LAI is most strongly correlated with potential evapotranspiration (adjR²=0.71, p<0.0001) and mean annual precipitation (adjR²=0.58, p<0.0001). For living mammals of South America, LAI correlates with species richness (adjR²=0.51, p<0.0001), macroniche diversity (adjR²=0.40, p<0.0001), the prevalence of terrestrial (adjR²=0.54, p<0.0001) and arboreal species (adjR²=0.50, p<0.0001), the ratio of browsers to grazers (adjR²=0.25, p<0.0001), cenogram slope (adjR²=0.61, p<0.0004), and the proportion of hypsodont species (adjR²=0.29, p<0.0001). Comparison of middle Cenozoic (50-11 Ma) rLAI values from fossil vertebrate sites in Patagonia reveals that relatively open habitats (rLAI values from 0.5-2.0) were more species rich than South American open habitats today with values more similar to those of modern African savannas, despite low abundances of grasses in South America during the middle Cenozoic. We hypothesize that the Great American Biotic Interchange (GABI) preferentially altered the mammalian community structure of open habitats in South America and that potentially, the diversity of pre-GABI endemic mammals maintained open habitats in Patagonia during the middle Cenozoic.