Influence of precipitation and species composition on phytomass of a semi-arid African grassland

Abstract

1. The influence of botanical composition on annual phytomass production of a semi-arid grassland in response to precipitation was tested with a 19-year experiment. Three compositional states reflecting good, medium and poor rangeland condition, whose basal cover increased from poor through to good, were created in 1978. 2. Multiple regression models were developed for assessing whether phytomass was influenced by precipitation, composition, phytomass of the previous year, basal cover, abundance of individual species, or diversity. Composition and precipitation accounted for 66% of the variation in phytomass, but a separate model of basal cover by precipitation was equally successful. The linear effect of rainfall on phytomass was enhanced as composition improved from poor (0.076 g m-2 mm-1) through medium (0.197 g m-2 mm-1) to good (0.277 g m-2 mm-1). Phytomass increased for low precipitation if it had been high the preceding year. Phytomass was more variable over time with deteriorating condition. 3. Species' complementarity ensured greater and more stable production. Setaria sphacelata, Eragrostis chloromelas and Digitaria eriantha increased phytomass on the good or medium condition grasslands during drier years, whereas Themeda triandra had this effect during wetter years. 4. Precipitation-use efficiency (PUE) was influenced mostly by composition and a linear and quadratic effect of precipitation (63% of variance). Optimum PUE of 0.308, 0.203 and 0.096 g m-2 mm-1 for the good, medium and poor condition grasslands, respectively, occurred at intermediate amounts (±680 mm) of precipitation. PUE was increased if phytomass had been high the previous year. 5. Species' complementarity of PUE in response to precipitation was evident for all compositional states. Ten, mostly uncommon, species and their interaction with precipitation explained an extra 21-42% of the variance. Stability of production was related to PUE for medium and poor condition grassland. Uncommon species therefore ensured growth efficiency and stabilized production as condition deteriorated. 6. Diversity had no influence on phytomass or PUE except for a small to moderate effect, respectively, for the medium condition grassland. 7. Vegetation structure, through limiting runoff and promoting infiltration, is an important control on the amount and efficiency of plant production under variable precipitation, whilst composition further influences the amount and stability of production.