Alterations in rainfall amounts change productivity but not competition of key grasses in southern Africa

Background/Question/Methods

An important threat to grassland ecosystems today is global climate change. Predictions for many regions of the world are for either an increase or decrease of mean rainfall amounts. Changes in rainfall regimes can have important implications for African grasslands, which house much of the remaining large mammalian herbivore diversity globally and sustain livelihoods through the farming of livestock. With changes in rainfall amounts, these grassland communities may alter if competing plant species differ in their ability to respond to changing climate conditions, which can have important implications for their ability to support both wild and domestic herbivores. To explore how future rainfall regimes may influence the establishment and growth of two key native C₄ grass species in South African grasslands, we conducted a greenhouse experiment in which  we simulated five rainfall scenarios (+/- 0, 25 and 50% of mean rainfall)  and assessed establishment, growth and competitive interactions of different combinations (100/0%, 75/25%, 50/50%) of Chloris gayana and Digitaria eriantha. We expected to observe plant establishment, growth and productivity to be enhanced with increased rainfall amounts and to be reduced with decreased rainfall amounts. Further, we expected to observe a change in the competitive abilities of the two grass species under the different rainfall conditions.

Results/Conclusions

For both grass species, the number of germinated seeds and seedling survival increased with increasing rainfall, i.e. -50% up to +50%. Additionally, rainfall positively affected plant growth and plant aerial cover of grasses. We found that across all rainfall treatments, the presence of C. gayana negatively affected the establishment and growth of D. eriantha. D. eriantha only thrived when C. gayana was absent. A possible explanation is that as C. gayana is a fast-growing pioneer species, it is capable of establishing itself quickly regardless of the climate conditions. Consequently, it suppressed the growth and productivity of the slow-growing D. eriantha. These results suggest that even though productivity may change with rainfall, the competitive ability of grasses in Southern African grasslands may remain stable irrespective of climate-driven rainfall changes.