Intraspecific biogenic silica variations in the grass species Pennisetum pedicellatum along an evapotranspiration gradient in South Niger
Résumé
Climate influences terrestrial Si and C cycles. To evaluate how drought influences Si dynamics in African savannahs we studied 12 sites along a climatic gradient of precipitation and drought-stress in South Niger. We analysed plant traits and Si (phytolith) content in 12 specimens of a drought adapted grass species (Pennisetum pedicellatum Trin.), and in surface soils. Phytolith/amorphous silica (ASi) was quantified by the 1%Na2CO3 (nc) digestion method, and the acid plus heavy-liquid extraction method (bz). Phytolith morphotypes were also analysed under the microscope. Our results show that with increasing drought-stress (decreasing AET/PET and precipitation, and increasing temperature), ASi in P. pedicellatum (ApSinc and ApSibz) increases, while ASi in soils (AsSibz) decreases. ApSinc shows higher correlation with precipitation of the wettest month (Pwet, r = −0.84, p < 0.001), AET/PET (r = −0.73, p < 0.01) and length of dry season (r = 0.83, p < 0.001) than with AsSibz (r = −0.59, p < 0.05). Plant height increases with increasing precipitation (r = 0.61, p < 0.05). Leaf area is positively correlated with AsSibz (r = 0.81, p < 0.01). Bulliform phytoliths in P. pedicellatum growing under driest climate represent up to 12% of the leaf phytolith assemblage and just 2% in specimens sampled south of ∼13°N at more humid sites. Thus, the abundance of silicified bulliform cells is strongly negatively correlated with Pwet (r = −0.92, p < 0.001) and AET/PET (r = −0.93, p < 0.001). Other intra-specific variations, e.g. the length of bilobate phytoliths, are not observed along the climatic gradients documented here. We conclude that drought and evapotranspiration rates influence plant height and leaf size, increase Si uptake by the C4 grass P. pedicellatum, and favour silicification of the bulliform (motor) cells triggering leaf movements. In surface soils, greater phytolith accumulation is likely due to higher plant productivity and weaker soil erosion at humid sites.