Phytoliths of East African grasses: An assessment of their environmental and taxonomic significance based on floristic data
Résumé
Relations between phytolith occurrences. taxonomy, and habitat are assessed for 184 East African grass species through the re-analysis of two qualitative surveys of phytolith types in leaf epidermis. This is done in conjunction with data on grass subfamily, photosynthetic pathway, and requirement for light and moisture compiled from floras and the literature. This survey includes ca 79% of the grass genera listed in the flora of tropical East Africa. It aims to further investigate the potential for grass short cell phytoliths to characterize the environment, and therefore improve reconstructions of past vegetation and climate in Africa. In this analysis, we identified ca 60 phytolith types (within the main categories Rondel, Trapeziform Short Cell, Bilobate, Cross, Polylobate, Saddle, and Trapeziform Sinuate) reported to occur in 10 grass subfamilies (Pharoideae, Bambusoideae, Ehrhartoideae, Pooideae, Danthonioideae, Arundinoideae, Chloridoideae. Centothecoideae, Panicoideae, and Incertae Sedis Streptogyna). These subfamilies include hydrophytic, helophytic, mesophytic and xerophytic species, with C3 or C4 photosynthetic pathways, and with affinities for shade, open, or semi-shade habitats. Analysis of phytolith occurrences shows that few morphotypes are restricted to some species only. However, there are morphological variations (of size and number of lobes) within the main phytolith categories Rondel, Bilobate, and Cross, which could additionally be considered to improve environmental and taxonomical interpretation of phytolith assemblages. The Rondel phytolith with a base diameter >15 mu m was only reported in C3-Pooideae, while the Rondel with a base diameter of <15 mu m occurs in several grass subfamilies (including Pooideae). Bilobates with long shanks between the two lobes are most frequently reported in xerophytic species, while Bilobates with short shanks are most frequently reported in mesophytic grass species. Finally, three-lobed crosses are reported only in Panicoideae and Chloridoideae, all being C4, light-loving species. A correspondence analysis confirms already known relationships between 1) Saddle forms, C4 pathway, open and xeric habitats, 2) Bilobates, Crosses, Polylobates, shaded and hydric habitats, 3) Trapeziform Sinuates and Pooideae. Of major implication for palaeoenvironmental reconstructions in East Africa, we found that the Trapeziform Sinuate phytoliths mark the presence of C3-grasses in the Afromontane zone, whereas the Rondels alone do not because they also occur in many C4 species of the Chloridoideae subfamily. We also establish that collapsed saddles are not diagnostic for Bambusoideae closed-habitat grasses since they occur in xerophytic species of the Chloridoideae, characteristic of several open habitats. In conclusion, this study contributes to better characterize Afromontane vegetation and better discriminate mesic and xeric vegetation types in East Africa. It also brings caution for future phytolith studies that rely on the presence of diagnostic types instead of phytolith assemblage analysis to trace the presence of particular taxa and/or environments in East Africa.