Woody plant encroachment facilitated by increased precipitation intensity

Global circulation models and empirical evidence suggest that precipitation events are likely to become more extreme across much of the globe(1,2). As most plant roots are in shallow soils(3-5), small but pervasive changes in precipitation intensity could be expected to cause large-scale shifts in plant growth, yet experimental tests of the effects of precipitation intensity are lacking(6-9). Here we show that, without changing the total amount of precipitation, small experimental increases in precipitation intensity can push soil water deeper into the soil, increase aboveground woody plant growth and decrease aboveground grass growth in a savannah system. These responses seemed to reflect the ability of woody plants to increase their rooting depths and competitively suppress grass growth. In many parts of the world, woody plant abundance has multiplied in the past 50-100 years, causing changes in fire, forage value, biodiversity and carbon cycling(10). Factors such as fire, grazing and atmospheric CO2 concentrations have become dominant explanations for this woody encroachment and semi-arid structure in general(10-12). Our results suggest that niche partitioning is also an important factor in tree-grass coexistence and that the woody plant encroachment observed over the past century may continue in the future should precipitation intensity increase.