Nutrient acquisition and physiological responses of dominant Serengeti grasses to variation in soil texture and grazing

1 Themeda triandra and Digitaria macroblephara are dominant grasses in the herbaceous layer of Serengeti National Park. Catena topography creates variation in soil texture and grazing that may have important physiological implications for these species. Through a combination of laboratory and field experiments, we tested whether variation in soil texture and grazing differentially affected growth, photosynthesis, leaf enzyme activities and nitrogen assimilation of the two species. 2 In the laboratory, simulated grazing (weekly clipping at 7 cm above soil) increased photosynthesis and enzyme activities for D. macroblephara. Clipping also increased photosynthesis for T. triandra, but did not affect enzyme activity. D. macroblephara and T. triandra were affected oppositely by soil texture; enzyme activities and photosynthesis were greater for D. macroblephara in sandy soils but greater for T. triandra in fine-textured soils. 3 Across a natural catena gradient, species abundances of D. macroblephara and T. triandra were negatively correlated with each other and related to grazing frequency in opposite ways, D. macroblephara positively and T. triandra negatively. In a field experiment, rates of (NO3-)-N-15 uptake for unclipped plants of D. macroblephara and T. triandra were similar and did not vary with soil texture. However, when clipped,(NO3-)-N-15 uptake was stimulated in both species but in opposite ways across a gradient of percentage sand, positive for D. macroblephara and negative for T. triandra. 4 These results suggest that species responded to defoliation and variation in water availability created by different soil textures. At sites where D. macroblephara and T. triandra co-occur, catena variation in soil texture and grazing may provide spatial variation in habitat suitability. Moreover, herbivory may promote species coexistence by facilitating differential physiological responses to topographic and soil heterogeneity. 5 This study illustrates how interactions between a direct biotic process and an indirect environmental variable can cause different plant physiological responses that may promote species coexistence in a heterogeneous environment.