Print PageClimate change is expected to produce fewer larger precipitation events. Changes in the timing and amount of water availability are likely to change plant productivity and the nature of competition among grasses and woody plants. Decreasing precipitation frequency may induce water stress, but may also increase infiltration and water availability, especially where overland flow is rare. Understanding the balance of these contrasting effects on grass and woody plant growth is likely to require direct experimentation. To test the effects of fewer, larger precipitation events we constructed 8m by 8m shelters that retained 50% of precipitation and deposited this collected precipitation only as 1cm (i.e., large) precipitation events. This was done in six shelters on clay soils in a subtropical savanna, Kruger National Park, South Africa. Soil water potential and grass and woody plant growth aboveground and belowground were measured during three years of manipulations.
Results/Conclusions
The rain-out shelters increased infiltration of rainwater throughout the soil profile (0-150cm). This resulted in a 35% increase in root biomass, but did not increase grass or woody plant production aboveground. These surprising results suggest a strong nutrient limitation to both grass and tree growth at the study site. Results also demonstrate the potential for a strong negative feedback to CO2-induced warming because large increases in belowground carbon sequestration were observed. Results did not support our original hypothesis that greater soil water infiltration would benefit more deeply rooted woody plants and therefore provide a potential explanation for woody plant encroachment observed in many semi-arid areas of the world in the past century. Continued manipulations will be needed to determine if increased root production will eventually lead to increases in aboveground production and shifts in the competitive interactions between grasses and woody plants.
