Within most grassland and savanna ecosystems, water availability is a key regulator of ecosystem dynamics. Species persistence in these ecosystems require tradeoffs that balance competitive efficiency for resources, and an ability to tolerate (or avoid) frequent periods of low water availability. C4 grasses tend to have higher leaf-level water use efficiency compared to woody species. In addition, grasses tend to have a greater portion of their overall root biomass concentrated in surface soils. Maximum rooting depth is often an ineffective predictor of competition for water because root type, distribution by depth, and functional conductivity vary. For this reason, more effective traits related to water uptake and use are required. Here, we present a conceptual overview linking the physiological and morphological characteristics among C4 grasses and C3 woody plants using long-term data from the Konza Prairie LTER, Kruger National Park, and experimental manipulations at Wits Rural Facility in South Africa.
Based on a dynamic template of water availability, grasses and woody plants exhibit distinct physiological traits when competition for water is high, compared to coexistence when water is plentiful. Grasses exhibit a fixed reliance on water from surface soils, while forbs and woody plants utilize surface soils when wet, and shift reliance to deeper soils during dry periods. Many C4 grass species exhibit physiological and morphological leaf and root traits that confer drought tolerance, while many woody species are more susceptible to drought stress. Recent assessments of sap-flux in mixed species communities illustrate that grasses transpire a large percentage of their daily water flux at night during periods of high moisture in surface soils. This strategy may negatively impact neighbors through resource-depletion and more frequent dry soil conditions. Ultimately, these physiological traits and ecohydrological strategies among grasses and woody plants influence landscape patterns, ecosystem processes, and susceptibility to drought in many grasslands and savannas.