Читать книгу North American Agroforestry - Группа авторов - Страница 74

On more xeric (drier) sites, moisture is limiting and competition for resources is greater belowground than aboveground. Forest canopies become more open as trees become more widely spaced, and a greater proportion of light reaches the ground. Higher light levels may allow the development of significant amounts of ground‐level vegetation. Ponderosa pine (Pinus ponderosa Laws.) forests throughout the Rocky Mountains and longleaf pine (Pinus palustris Mill.) forests in the southeastern United States frequently have dense grass understories that are maintained in part by periodic fires (Daubenmire, 1978). In still drier areas, tree density decreases until scattered individuals in a grassland matrix form a savanna such as the blackjack oak (Quercus marilandica Muenchh.)–post oak (Q. stellata Wangenh.) savanna in eastern Texas, pinyon‐juniper (Pinus sp.–Juniperus sp.) savanna in the southwestern United States, and the oak–hickory savanna in western Missouri. The oak savanna, characterized by a sparse overstory of oaks and an understory of herbs and grasses, is a transitional zone between the eastern forest and the grasslands (Packard, 1988). Oak savanna was once a major community across the Midwest—although it became severely diminished after the Euro‐American settlement of the 1800s. Prior to settlement and overgrazing, large areas of sagebrush steppe in the Intermountain West also showed a co‐dominance of shrubs (Artemisia) and perennial bunchgrasses (West, 1988).

As the preceding examples suggest, disturbance (e.g., grazing, browsing, drought, fire) is a critical mediator of the competition that occurs between trees and grasses (Belsky, 1994; Hamerlynck & Knapp, 1996; Jeltsch, Milton, Dean, & Van Rooyen, 1996). In the southeastern Coastal Plain, longleaf pine forests with a grassy understory are maintained by fires of 3–10‐yr frequency that allow regeneration of the pines but prevent establishment of hardwoods, which have denser canopies than the pines and would inhibit grasses (Daubenmire, 1978). Most savannas are maintained by fire, and if fire is prevented or overgrazing leaves insufficient fuel to carry a fire, succession proceeds to a denser forest. Grasses are physiologically and morphologically adapted to burning. The ecological message is that a particular balance between grasses and trees can often be maintained only through regular disturbance.

As water becomes more limiting, trees disappear and grasses or shrubs dominate. Grasses have a high root/shoot mass ratio, which provides an advantage in competing for water and nutrients. However, a shrub such as mesquite (Prosopis L.) also has an extensive lateral and vertical root system that allows it to compete effectively for water as well as nutrients against grasses in the arid grasslands in which it occurs (Tiedemann & Klemmedson, 1973). In other cases, competition for belowground resources is reduced by the exploitation of different soil layers by different species. In the blue oak (Quercus douglasii Hook. & Arn.) savanna, competition for water between trees and grasses is reduced by vertical stratification of the two root systems, with grasses occupying mainly the top meter of soil and oak roots penetrating >25 m (Jackson et al., 1990). This stratification also promotes more efficient cycling and retention of N in the ecosystem.

In addition to competition for resources, trees and grasses in these mixed systems may compete through direct interference. An example of this would be the allelopathic suppression of understory plants in oak forests in Oklahoma (McPherson & Thompson, 1972). Alternatively, some interactions may be positive. Survival of grass seedlings was three times greater within a California blue oak savanna than in adjacent open grassland (Jackson et al., 1990) due to the more favorable environment for seedling establishment (i.e., higher relative humidity, decreased evaporation, and increased near‐surface soil moisture and nutrient levels).

Within a particular climatic region, topographic and soil patterns may have a strong influence on spatial patterns and interactions of woody and non‐woody species. Throughout much of the Great Plains grasslands, trees and shrubs are restricted to riparian areas, rocky escarpments, mesic north‐facing slopes, and other sites offering increased moisture availability and protection from fire. Rockier soils also provide better opportunity for tree seedling establishment in competition with the thick root mass of grasses (Wells, 1965). At the northern edge of the prairie, grasses on the uplands form a mosaic with groves of poplar (Populus sp.) located in depressions or on protected slopes (Daubenmire, 1978).

Significant grass production in a forest matrix allows timber production and grazing to coexist on >69 million ha (170 million acres) in the United States (U.S. Forest Service, 1981). The dual functions of these silvopastoral practices can be enhanced by management based on ecological principles. On mesic sites, thinning and pruning of trees maintains forage production while promoting high‐quality timber. Prescribed burns can prevent invasion by undesirable species while maintaining an open and productive understory. In semiarid areas, avoiding overgrazing is the most effective means of preventing the replacement of grasses by shrubs.