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by Guy R. McPherson, School of Renewable Natural Resources, University of Arizona, Tucson
The effective management of rangeland vegetation requires knowledge of the morphology and physiology of individual species and an understanding of relationships between species and environmental factors. This is because each species responds differently to the environment and to each neighboring species. For example, above-average summer precipitation enhances germination of several species; other species do not respond to this climatic phenomenon because their seeds are not physiologically synchronized with summer precipitation. Thus, germination and growth differ between species, even within groups that appear to be similar (e.g., perennial warm-season bunchgrasses).
Plants may differ with respect to morphology, physiology, or both. These differences help explain each species' different response to environment or management manipulations. For example, grasses are comprised primarily of leaves that initiate growth each year from near the soil surface; grass roots are largely restricted to the upper 30 cm of the soil; and many species of grasses, particularly those in rangelands of the southern United States, are capable of growing at very high temperatures (they are 'warm-season' plants, i.e., they possess the C4 pathway of photosynthesis). In contrast, woody plants are comprised primarily of highly-lignified tissue, and growth is initiated from points well aboveground; individual leaves may persist for several years; roots may penetrate several meters belowground; and many woody plants grow year-around (most woody plants are 'cool-season' plants, i.e., they possess the C3 pathway of photosynthesis).
Despite differences between species, all plants require the same basic resources for growth and development: light, water, carbon dioxide, and soil nutrients. Plant productivity is constrained by low amounts and availability of these resources. On rangelands, water is often the most-limiting resource; thus, plant water relations make an important contribution to vegetation structure. Soil nitrogen and phosphorous may limit plant productivity on some rangeland sites. The relative abundance of different species ultimately is controlled by the different abilities of species to extract limited resources from belowground (water, soil nutrients) or aboveground sources (light, carbon dioxide). Inherent differences in morphology and physiology dictate the ability of different species to extract resources; whether these differences in plant properties are beneficial or detrimental for resource extraction depends on the properties of neighboring plants and on environmental conditions.
Fortunately, rangeland managers can manipulate plant communities to encourage desirable plants and discourage undesirable plants (See: Example). Determination of which species are "desirable" or "undesirable" depends on management objectives: for example, mesquite is a desirable source of wood for fuel, furniture, and javelina habitat, but mesquite is an undesirable plant for pronghorn habitat or livestock forage. Societal goals dictate an active approach to management, and management decisions necessarily involve trade-offs between various objectives (e.g., production of wood for fuel or furniture vs. production of forage for livestock).
Clearly-stated goals and objectives will facilitate management and allow selection of appropriate tools to accomplish these goals and objectives. Conversely, selection of goals or objectives that are poorly defined or quantified may actually impede management. For example, use of the term 'ecosystem health' implies that there is an optimal state associated with an ecosystem, and that any other state is abnormal; however, the optimal state of an ecosystem must be defined, and clearly stated, quantifiable objectives must be developed to achieve that state. Similarly, 'ecosystem integrity' is not an objective, quantifiable property. The use of terms such as 'health' and 'integrity' as descriptors of ecosystems implies that managers or scientists can identify the state that is optimum for the ecosystem (vs. optimum for production of specific resources) and that the preservation of this state is scientifically justifiable. These terms are not supported by empirical evidence or ecological theory, and should be abandoned in favor of other more explicit descriptors. Appropriate goals and objectives should be identified on a site-specific basis and linked to ecosystem structures or functions that can be defined and quantified.
Although knowledge of individual species and their environments is important, this information is insufficient for effective management. Thus, books, courses, and other sources of information can not make explicit recommendations for resource managers--they can not serve as a "how-to" guide for management--for two primary reasons. First, management decisions must be temporally-, spatially-, and objective-specific. Second, specific management activities, though based on scientific knowledge, are conducted within the context of relevant social, economic, and political issues. These issues vary over time and between individuals, which precludes general recommendations. Thus, the role of knowledge derived from personal experience should not be underestimated: because management is necessarily site- and objective-specific, management decisions should be made by managers most familiar with individual systems.
Once specific objectives are clearly articulated, knowledge of rangeland ecology can facilitate the selection of effective tools to accomplish management objectives. In particular, knowledge of vegetation dynamics, coupled with information about plant response to defoliation and other disturbances, can contribute to informed decision-making. A simple example, which acknowledges the broad-scale trade-off between woody plants and grasses, illustrates how various tools can be used to enhance production of one of the two life-forms.
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