Degradation Ecological Versus Socioeconomic

Degradation associated with "desertification" or "xerification" in arid environments (West, 1986; Rapport and Whitford, 1999) or "deforestation" in humid environments is in sharp contrast to that associated with "thicketization" of grasslands and savannas in mesic environments (Fig. 2). Desertification and deforestation typically have negative consequences both ecologically and socioeco-nomically. "Thicketization" has some adverse socioeconomic implications, as it reduces the capacity of rangelands for subsistence or commercial livestock production. However, it does not necessarily represent a degraded system with respect to biodiversity, productivity, nutrient cycling, and other important ecological characteristics.

Today's La Copita landscape is clearly different from that of 100-200 YBP, but is it "degraded"? The conceptual model in Figure 10, based on the La Copita case study, proposes a degradation phase (Fig. 3) followed by an aggradation phase (Figs. 6 and 9, Table 2) that begins when unpalatable woody plants establish, grow, modify microclimate, and enrich soil nutrients. Present-day landscapes at La Copita are a rich mosaic of productive woodlands and tree-shrub patches interspersed with remnant grass-dominated patches. Current plant and soil C and N mass is substantially greater than that which occurred under "pristine" conditions. In addition, these landscapes are highly resilient following disturbance (Scanlan, 1988; Flinn et al, 1992) and provide habitat for numerous wildlife species, both game and nongame. So, in this case, the system that has developed following an initial degradation phase is now ecologically diverse, productive, and functional. It would seem that it is "degraded" or "dysfunctional" (Tongway and Ludwig, 1997) only with respect to its socioeconomic value for cattle grazing. However, it has other potential socioeconomic values whose realization would necessitate a change from traditional land

• heavy, continuous grazing • elimination of fire • minimal browsing

FIGURE 10 Conceptual model of ecosystem changes accompanying grazing-induced succession from grassland to woodland based on the La Copita case study. Dashed lines depict hypothesized upper and lower bounds of Y values; (i.e., lower bound is that which might occur in absence of woody plant encroachment (Fig. 3)); upper bound is that which might occur when woody plant communities mature (e.g., forward projection of Figs. 6 and 9). Values for biodiversity will vary substantially from system to system; in some cases (e.g., Juniperus systems) low-diversity monocultures of woody vegetation may develop. At the La Copita, this degradation-aggradation cycle has occurred over ca. 100-150 years.

• heavy, continuous grazing • elimination of fire • minimal browsing

FIGURE 10 Conceptual model of ecosystem changes accompanying grazing-induced succession from grassland to woodland based on the La Copita case study. Dashed lines depict hypothesized upper and lower bounds of Y values; (i.e., lower bound is that which might occur in absence of woody plant encroachment (Fig. 3)); upper bound is that which might occur when woody plant communities mature (e.g., forward projection of Figs. 6 and 9). Values for biodiversity will vary substantially from system to system; in some cases (e.g., Juniperus systems) low-diversity monocultures of woody vegetation may develop. At the La Copita, this degradation-aggradation cycle has occurred over ca. 100-150 years.

uses. These include alternative classes of livestock (e.g., goats), lease hunting, charcoal production, and ecotourism.

Given the demonstrated potential for nutrient sequestration in the conversion from grassland or savanna to woodland (Johnston et al., 1996; Scholes and van der Merwe, 1996; Scholes and Bailey, 1996; San José et al., 1998), these lands may also have "carbon credit" value to society (e.g., Glenn et al, 1992). Government or industry subsidies and payments for management practices that promote or maintain woody plant cover on rangelands would stand in sharp contrast to past rangeland management practices that have sought to eliminate or reduce woody vegetation cover using costly and often short-lived chemical or mechanical treatments that may not produce desired results (Belsky, 1996) and may convert landscapes from sinks to sources of greenhouse gases (De Castro and Kauffman, 1998). Thus, the perspective on woody plants in rangelands may shift from negative (an expensive management problem) to positive (a potential commodity).

From a biogeochemical perspective, potential benefits of C and N sequestration should be weighed against the potentially undesirable absolute increases in NO and NMHC fluxes that may accompany increases in woody plant biomass. The effects of vegetation change on the hydrological cycle must also be considered. The extent to which shifts from herbaceous to woody plant domination might reduce stem flow and groundwater/aquifer recharge remains controversial. In addition, potential increases in ecosystem transpiration associated with woody communities with high LAI and deep root systems might increase atmospheric water vapor and either offset (due to radiative properties of water) or augment (via cloud formation) benefits of C sequestration on greenhouse gas budgets. From a biodiversity perspective, shrubland/woodland communities may be more (La Copita scenario) or less diverse (many Juniperus communities)

than grasslands. In either case, the diversity in terms of the kinds of organisms present would be very different. The development of shrublands and woodlands would be at the expense of grassland habitats and the plants and animals characteristic of such habitats. From a socioeconomic perspective, the promotion or maintenance of woody plant biomass on grasslands and savannas would necessitate a radical change in traditional land use and range management perspectives. Revenues generated from marketing of soil and plant carbon stocks could help fund needed restoration efforts and spur economic development in many sectors, but may be socioeconomi-cally disruptive on other fronts (Trexler and Meganck, 1993). Thus, there are many important issues to be resolved when evaluating the merits of C and N sequestration associated with vegetation change.

Climate & Atmospheric Processes

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