Trees in Grasslands Biogeochemical Consequences of Woody Plant Expansion

Introduction 115

Woody Plant Encroachment in Grasslands and Savannas 116

The La Copita Case Study 118

Degradation: Ecological versus Socioeconomic 127

Implications for Ecosystem and Natural Resources Management 128

Summary 129

Steve Archer, Thomas W. Boutton, and K. A. Hibbard

Texas AeOM University

College Station. Texas

Key Words: carbon, disturbance, grazing, fire, global change, hydrocarbons, land cover, modeling, nitrogen, NOx, savanna, sequestration, soil respiration, succession, tree-grass interactions, vegetation dynamics, woody plant encroachment

1. Introduction

The term "savanna" typically denotes plant communities or landscapes having a continuous grass layer with scattered woody plants. Although savannas are not the only vegetation type where contrasting plant life forms codominate, they are one of the most striking, geographically extensive (ca. 20% of global land surface; Scholes and Hall, 1996) and socioeconomically important examples in tropical (Tothill and Mott, 1985; Young and Solbrig, 1993) and temperate (Burgess, 1995; McPherson, 1997; Anderson et al., 1999) regions. Tropical savannas cover about 1600 million ha of the terrestrial surface (Scholes and Hall, 1996), including more than half the area of Africa and Australia, 45% of South America, and 10% of India and southeastern Asia (Werner, 1991). Temperate savannas in North America occupy an estimated 50 million ha (McPherson, 1997). More importantly, savannas contain a large and rapidly growing proportion of the world's human population and a majority of its rangelands and domesticated animals. As such, they have received substantial and ever-increasing anthropogenic land-use pressure.

Many savannas are dynamic ecotones between woody plant (shrub-steppe, desert scrub,woodland or forest) and grassland formations. Savannas vary substantially with respect to the stature

1 Current Address: Climate Change Research Center, GAIM Task Force, Institute for the Study of Earth, Oceans, and Space (EOS), Morse Hall, 39 College Road, University of New Hampshire, Durham, N H 03824-3323

(shrub vs tree), canopy cover (e.g., 5-80%), functional form (evergreen vs deciduous; broad-leaved vs needle-leaved; shallow vs deeply rooted), and spatial arrangement (random, regular, or clumped) of the woody elements that compose them. Similarly, the grass layer may consist of short versus tall-statured species, bunch versus rhizomatous growth forms, and C3, C4, or mixed C3/C4 photosynthetic pathway assemblages. This variation in structural/functional characteristics reflects a rich array of interactions between climate (especially the amount and seasonality of rainfall), soils (notably, depth and texture), and disturbance (particularly grazing, browsing, and fire), as shown in Figure 1 (Walker, 1987; Backeus, 1992).

Much of the literature on savanna ecology has been devoted to describing and classifying vegetation structure. Static classification schemes minimize the importance of temporal change and divert attention from functional processes that might explain dynamic spatiotemporal variation. Grasses and woody plants may coexist in a dynamic equilibrium when climatic, edaphic, and disturbance factors interact temporally such that neither life form can exclude the other. However, a directional change in one or more of these primary controlling factors may shift the balance in favor of one life form over the other and move the system toward either grassland or shrubland / woodland. The probability, rate, and extent of such a shift may depend on local topoedaphic factors and the life-history traits and autecology of the growth forms or species involved.

Human population growth and widespread Anglo-European settlement during the 18th and 19th centuries have influenced the balance of grass-woody plant interactions worldwide. For example, extensive clearing of trees for fuel, lumber, and cropland has fragmented forests and produced anthropogenic or degraded savannas (Gadgill and Meher-Homji, 1985; Sinclair and Fryxell, 1985; Cline-Cole et al., 1990; Schule, 1990; Young and Solbrig,

GLOBAL BIOGEOCHEMICAL CYCLES IN THE CLIMATE SYSTEM

Copy:ight 200! by Aaidcniic 1'rcss. All rights ' ; : ■ r. ■ ■: : : ■:■ in Liny lonn reserved.

CLIMATE

subtropical location

SOILS

high solar radiation

CLIMATE

subtropical location high solar radiation

Biogeochemical Cycling Trees

HUMAN INFLUENCE

FIGURE 1 Numerous factors interact to affect the abundance of grasses and woody vegetation in drylands (from Scholes and Walker, 1993). The balance between trees and grasses (innermost level) is affected by determinants of structure and function (water, nutrients, fire, and herbivory). The outermost level contains the factors that give the determinants their characteristics. Over the past century, human influences have shifted the balance to favor woody plants through selective utilization of grasses by livestock maintained at high concentrations, elimination of browsers, and fire suppression (see Archer, 1994).

HUMAN INFLUENCE

FIGURE 1 Numerous factors interact to affect the abundance of grasses and woody vegetation in drylands (from Scholes and Walker, 1993). The balance between trees and grasses (innermost level) is affected by determinants of structure and function (water, nutrients, fire, and herbivory). The outermost level contains the factors that give the determinants their characteristics. Over the past century, human influences have shifted the balance to favor woody plants through selective utilization of grasses by livestock maintained at high concentrations, elimination of browsers, and fire suppression (see Archer, 1994).

1993; Mearns, 1995). Following forest clearing, pyrophytic grasses may establish and restrict woody colonization by accelerating fire cycles and maintaining low-fertility soils (Hopkins, 1983; Mueller-Dombois and Goldammer, 1990; D'Antonio and Vitousek, 1992). In the Brazilian Cerrado, rates of agricultural expansion and clearing of savanna and woodland trees rival those reported for Amazon rain forest (Klink et al., 1993). In other areas, fire suppression, eradication of indigenous savanna browsers, and the introduction of grazing livestock and exotic trees and shrubs have caused a progressive increase in woody plant density, known as bush or brush encroachment (Adamoli et al., 1990; Archer, 1994; Gardener et al, 1990; Miller and Wigand, 1994; Noble, 1997). As a result, areas that were once forest may become savanna-like, while areas that were once grassland or open savanna may progress toward a shrubland or woodland physiognomy. The biogeochemical consequences of this latter phenomenon are the focus of this chapter.

0 0

Post a comment