Woody Plant Rooting Depth and Ecosystem Function of Savannas: A Case Study from the Edwards Plateau Karst, Texas
Savannas are characterized by the co-dominance of grasses and deep rooted shrubs or trees (e.g., Walter, 1971) in seasonally dry environments with a limited and fluctuating water supply. The two dominant plant functional types have contrasting strategies of drought avoidance, which determine the ecosystem-wide responses of savannas to precipitation variability. Grasses typically avoid drought by focusing their physiological activity in periods when soil water availability is high and become dormant as soon as the shallow soil, the sole source of their water supply, dries out. Shrubs and trees typically avoid drought by switching to deeper water sources, below the root zone of the grasses. Although this pattern has been observed in savannas, there are also variations on the theme, depending on two key factors: rooting depth and water availability in the lower root zone of the trees.