Grass water stress estimated from phytoliths in West Africa
Article first published online: 20 JAN 2005
Journal of Biogeography
Volume 32, Issue 2, pages 311–327, February 2005
How to Cite
Bremond, L., Alexandre, A., Peyron, O. and Guiot, J. (2005), Grass water stress estimated from phytoliths in West Africa. Journal of Biogeography, 32: 311–327. doi: 10.1111/j.1365-2699.2004.01162.x
- Issue published online: 20 JAN 2005
- Article first published online: 20 JAN 2005
Vol. 40, Issue 1, 208, Article first published online: 17 DEC 2012
- Arid environment;
- vegetation and climate proxies;
- West Africa
Aim This study calibrates the relationship between phytolith indices, modern vegetation structure, and a climate parameter (AET/PET, i.e. the ratio of annual actual evapotranspiration to annual potential evapotranspiration), in order to present new proxies for long-term Quaternary climate and vegetation changes, and model/data comparisons.
Location Sixty-two modern soil surface samples from West Africa (Mauritania and Senegal), collected along a latitudinal transect across four bioclimatic zones, were analysed.
Methods Two phytolith indices are defined as normalized data: (1) humidity-aridity index [Iph (%) = saddle vs. cross + dumbbell + saddle], and (2) water stress index [fan-shaped index (Fs) (%) = fan-shaped vs. sum of characteristic phytoliths]. Vegetation structures are delimited according to Iph and Fs boundaries. Bootstrapped regression methods are used for evaluating the strength of the relationship between the two phytolith indices and AET/PET. Additional modern phytolith assemblages, from Mexico, Cameroon and Tanzania are extracted in order to test the calibration established from the West African samples. Accuracy of the AET/PET phytolith proxy is compared with equivalent pollen proxy from the same area.
Results Characterization of the grass cover is accurately made through Iph. A boundary of 20 ± 1.4% discriminates tall grass savannas from short grass savannas. Water stress and transpiration experienced by the grass cover can be estimated through Fs. AET/PET is accurately estimated from phytoliths by a transfer function: AET/PET = −0.605 Fs − 0.387 Iph + 0.272 (Iph – 20)2 (r = 0.80 ± 0.04) in the application domain (AET/PET ranging from 0.1 ± 0.04 to 0.45 ± 0.04). Phytolith and pollen estimate with similar precision (rpollen = 0.84 ± 0.04) the AET/PET in the studied area.
Conclusions This study demonstrates that we can rely on the phytolith indices Iph and Fs to distinguish the different grasslands in tropical areas. Moreover, a new phytolith proxy of AET/PET, linked to water availability, is presented. We suggest from these results that combining phytolith and pollen proxies of AET/PET would help to constrain this climate parameter better, especially when phytolith assemblages are dominated by Panicoideae and Chloridoideae C4-grass phytoliths, are devoid of Pooideae C3-grass phytoliths, and occur with a few tropical ligneous woody dicotyledon phytoliths. As AET/PET is a bioclimatic indicator commonly used in vegetation models, such a combination would help to make model/data comparisons more efficient.