chapter  23
20 Pages

CENTURY-SAVANNA Model for Tree–Grass Ecosystems

ByWilliam J. Parton, Robert J. Scholes, Ken Day, John O. Carter, Robin Kelly

At least a sixth of the global land surface is covered by ecosystems in which both woody plants and grasses contribute to the NPP (Scholes and Hall, 1996); these ecosystems are broadly referred to as savannas. Classical theory suggests that trees have an advantage over grasses as a result of the tree’s ability to root throughout the soil profile, whereas grasses predominantly root in the surface horizons (Walker et al., 1981). Although a vertical separation between tree and grass roots and their exploitation of sources of soil water can be observed (Brown and Archer, 1990; Dodd et al., 1998; Midwood et al., 1998), savanna soils are often shallow (<1 m); and in semiarid regions, soil moisture seldom penetrates to the depth where trees have an advantage. The primary niche difference between trees and grasses may instead be primarily temporal rather than a result of rooting patterns (Scholes and Walker, 1993; Sala et al., 1997). Savanna climates have a distinct seasonality: a hot, wet growing season and a warm, dry nongrowing season. Trees in savannas reach their full leaf area in the first weeks of the wet season, sometimes earlier if soil moisture is sufficient; whereas grasses reach peak leaf area later in the growing season with a decline before trees begin to

CONTENTS

Introduction .........................................................................................................443 Savanna Models ..................................................................................................444 SAVANNA-CENTURY Model Description .....................................................446 Model Testing and Calibration ..........................................................................449 Model Results ...................................................................................................... 451 Discussion ............................................................................................................455 References ............................................................................................................. 459

shed their leaves (Archibald and Scholes, 2007). Therefore, trees have almost exclusive access to resources at both the beginning and the end of the growing season; whereas grasses may be more competitive during the middle.