Systems
Forest canopies are heterogenous. In regions where the climate is mesic enough to support closed‐canopy forests, disturbances such as wind, avalanches, or fire create gaps that support herbaceous vegetation for a brief period of time. In old‐growth forests of the eastern United States, 9.5% of the land area historically was in small gaps (created by the death of one to several trees) (Runkle, 1982). New gaps formed at a rate of 1% of the land each year while an equal area of gaps closed due to sapling growth, making this a landscape‐level, steady‐state process. Less frequently, larger areas are disturbed by hurricanes, fires, insect outbreaks (e.g., gypsy moth), and other large‐scale events (Spies & Franklin, 1989). Since European settlement, most U.S. forests have been logged at least once.
The smaller the gap, the greater the edge effects on increasing competition for water and nutrients, shading, and reduction of windspeed. Edges are also zones of increased diversity and activity for many species of insects, birds, and mammals; at the landscape scale, the size and distribution of gaps is an important determinant of many forest functions, and edge “presence” in the landscape is often enhanced through the adoption of agroforestry systems. Swidden or slash‐and‐burn agriculture mimics the process of gap formation and succession in many tropical forests and has been called the most sustainable form of agriculture when practiced appropriately (Kleinman, Pimentel, & Bryant, 1995). An analogous form of temperate shifting agriculture was practiced by Native Americans in the New England region (Davies, 1994). In some temperate U.S. forests, logging of small patches to mimic the natural processes of gap formation offers a more sustainable alternative to large‐scale clear‐cuts (Maser, 1994).
Grasses and forbs dominate a gap immediately following disturbance but are soon replaced by trees or shrubs. This transition is known as succession, the “orderly process of community development that involves changes in species structure and community processes with time, and results from modification of the physical environment by the community” (Odum, 1971). Keever (1950) described a typical succession pattern for abandoned farmland in the North Carolina Piedmont with crabgrass (Digitaria spp.), asters (Aster spp.), and ragweed (Ambrosia artemisiifolia L.) dominating the first 2 yr, followed by broomsedge (Agropogon virginicus L.), which was gradually replaced in 10–15 yr by shortleaf (Pinus echinata Mill.) or loblolly (Pinus taeda L.) pines. A hardwood understory develops by 60 yr and forms the climax oak–hickory forest by 150 yr.
Succession results from the gradual modification of microclimate as the expanding canopy intercepts increased amounts of solar radiation each year. Enough light reaches the ground early in the successional process to support significant forage production. With proper management to limit damage to young trees, livestock can be rotationally grazed as part of a silvopastoral agroforestry practice. Stocking rates are reduced as tree growth reduces light levels until canopy closure eliminates forage production. Successional silvopastoral practices are particularly well developed in New Zealand and Australia (Anderson, Moore, & Jenkins, 1988) and in many parts of temperate Europe (Dupraz et al., 2018).
Successional principles can also be applied in cropping systems. Perhaps the best known temperate example is alley cropping with black walnut (Garrett & Kurtz, 1983; Garrett & Harper, 1998; Williams & Gordon, 1992; Thevathasan & Gordon, 2004). Black walnut is planted at wide spacings (e.g., 12 m), and row crops are grown in the alleys for up to 10 yr. When shading reduces row crop yields, forage crops are substituted either for haying or for direct grazing. By the time canopy closure ends profitable forage production, nut production provides income until the trees are cut for timber, and the process begins anew.
Xeric and Transitional Forests
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