C3 Poaceae and Restionaceae phytoliths as potential proxies for reconstructing winter rainfall in South Africa

Abstract

The main objective of this research is to identify graminoid phytolith morphotypes with potential as proxies for reconstructing past winter rainfall in South Africa. The main argument of this study is that phytolith proxies for winter-rainfall maxima should be found among native graminoids abundant in the Cape Region, where most of the precipitation occurs in the cooler part of the year. Vegetation surveys indicate that C3 Poaceae (cool-season grasses) and Restionaceae (restios) are abundant graminoids in the winter rainfall zone (WRZ). Therefore, the frequencies of diagnostic graminoid morphotypes of each of these two groups are correlated independently with both percent and amount of winter rainfall (defined here as the sum of April-September precipitation). The phytolith assemblages used for this study were collected from soils along two transects across the winter (WRZ), all-year (ARZ), and summer (SRZ) rainfall zones of South Africa.The study shows that the highest frequencies of the diagnostic C3 grass silica short-cell (GSSC) morphotypes increase with winter rainfall. Although considerable numbers of C3-GSSC morphotypes occur in samples at elevation above 1500m in the SRZ, their frequencies are lower than in the samples of the WRZ and ARZ. Correlation is noteworthy only when all the C3-GSSC morphotypes are grouped. Individual C3-GSSC morphotypes and groups of GSSC morphotypes linked to particular C3 grass subfamilies (Pooideae, Ehrhartoideae and Danthonioideae) correlate poorly with winter rainfall incidence. The results of this study also suggest that Restionaceae phytoliths are good indicators of winter rainfall, but because Restionaceae occur mainly in oligotrophic soils in association with fynbos vegetation, the use of their phytoliths in paleorainfall reconstruction should be taken with care. Despite certain limitations, this research shows that the use of C3-GSSC and Restionaceae phytolith has a strong potential for reconstructing the extent of winter rainfall during the colder stages of the Pleistocene. However, to achieve this objective a study with a larger number of reference material and tighter geographical sampling is needed.