Читать книгу North American Agroforestry - Группа авторов - Страница 73

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.