Tree-grass interactions in savannas: Investigating competitive and facilitative mechanisms across a precipitation gradient

Background/Question/Methods

Recent research indicates the importance of both facilitative and competitive mechanisms in determining plant community structure. The stress-gradient hypothesis (SGH) predicts an increasing importance of facilitative mechanisms relative to competition along gradients of increasing environmental stress. Though developed across a variety of ecosystems, the SGH’s relevance to the dynamic tree-grass systems of global savannas remains unclear. In savannas, trees may facilitate grass growth in dry, water-limited regions by reducing sub-canopy evapotranspiration rates and increasing soil moisture availability through hydraulic lift. Conversely, competition for water, light, and nutrients may play a larger role in shaping community structure in more mesic savannas. Contrary to tree impacts on grass growth, the effect of grasses on growth of adult trees has rarely been quantified, although both competitive suppression (i.e. through competition for water and nutrients) and facilitation (through grass-related increases in infiltration of rainfall) are possible. Here we present a meta-analysis that quantifies the net effect of the presence or absence of woody plants on herbaceous production with respect to annual rainfall. We also analyze new and previously published data to quantify grass effects on tree growth as well as explore potential abiotic and edaphic underlying mechanisms driving the observed patterns.

Results/Conclusions

Our meta-analysis revealed a shift from net-competitive to net-facilitative effects of trees on sub-canopy grass production with decreasing annual precipitation, consistent with the SGH. We also found a significant difference between savanna sites from Africa and North America, suggesting differences in tree-grass interactions in the savannas of tropical and temperate regions. Field experiments in Mali, West Africa, similarly indicate that trees facilitate grass growth in drier regions and suppress grass growth in wetter regions, consistent with the SGH. By contrast, the presence or absence of grass had no effect on adult tree growth over one growing season for sites distributed along the West African rainfall gradient (~300-1300 mm MAP). Experimental observations of light availability and soil moisture suggest a complex of interacting abiotic factors driving regional tree-grass interactions. Nutrient analyses indicate no change in nutrient ratios along the rainfall gradient, but consistent nutrient enrichment in the presence of tree canopies. The results of this meta-analysis coupled with our field experiments in Mali advance our understanding of tree-grass interactions in savannas and contribute a valuable dataset to the developing theory behind the SGH.