Taxonomic, ecological and palaeoecological significance of leaf phytoliths in West African grasses

Grass short cell phytoliths (GSCP) are a valuable tool for the reconstruction of former grass-dominated ecosystems, especially in Africa. GSCP are highly diverse and most of them show affinities with certain taxonomic groups within the Poaceae. Taxonomic affiliation of the GSCP can be exploited for palaeoecology because many subfamilies, tribes and genera have specific ecological requirements. Finer classification of GSCP can improve the taxonomic resolution and palaeoecological reconstructions, but the numerous existing GSCP classification schemes have not been standardized yet, and are hardly comparable. It is therefore important to identify clearly defined morphotypes with taxonomic and ecological significance. We studied GSCP from 51 West African common grass species belonging to 32 genera, representing the entire ecological gradient from the Sahara to the Guinean savanna zones. We used 153 detailed GSCP morphotypes, considering variations in shape, size and 3D morphology, and eventually merged them into 20 supertypes. Correspondence analysis (CA) was applied to reveal correlations between morphotypes and taxonomic groups; ecological parameters were introduced as passive variables. In spite of redundancy among bilobates, crosses and saddles, the subfamilies Panicoideae, Aristidoideae, Chloridoideae and Ehrhartoideae can be clearly distinguished by their GSCP assemblages. Aristidoideae are characterized by very long bilobates with convex ends and very long shanks, Chloridoideae by squat saddles, Ehrhartoideae by scooped bilobates, and Panicoideae by Stipa-type bilobates and polylobates. The three Chloridoideae tribes can be separated, but not the panicoid tribes Paniceae and Andropogoneae. Rondels do not seem to have any taxonomic or ecological significance. Except for very long bilobates with very long shanks which are most common in xerophytic grasses, there is no direct correlation of certain morphotypes with the habitat requirements of the species. In the aquatic and mesophytic species, statistically significant GSCP rather mirror the taxonomic affiliation, e.g. the scooped bilobates in the aquatic Oryza species, or polylobates in mesophytic Panicoideae. The results of the Canonical CA for the detailed morphotypes and the merged supertypes are comparable. We conclude that the easily recognizable 20 supertypes are appropriate for future ecological and palaeoecological phytolith studies in Africa. For the supertypes of bilobates and crosses, the size of the particle and the length of the shank are important features whereas 3D morphology and the shape of the lobe ends can be neglected.