ephemerals in forest understories are able to leaf out and capture substantial light energy before overstory canopy development occurs. Goldenseal (Hydrastis canadensis L.), an understory forb native to many eastern deciduous forests, produces >95% of its aboveground biomass within the first month of its growing season, well before the overstory canopy is developed (Eichenberger & Parker, 1976). Other species are adapted to full‐shade conditions and experience peak development in late summer (Greller, 1988). The co‐occurrence of these strategies results in increased capture of light energy as well as more efficient use of other resources. Nutrient uptake by spring ephemerals may sequester up to 90% of the N and K that could potentially be leached during the spring from some Midwest forests (Blank, Olson, & Vitousek, 1980; Peterson & Roelf, 1982). Thus, temporal as well as spatial stratification plays a role in system function.
Fig. 3–2. (A) Categorization of ecosystems in terms of the spatial and temporal relationships of the woody and herbaceous components; and (B) categorization of temperate agroforestry practices in terms of the spatial and temporal relationships of the woody and herbaceous components.
Table 3–2. Summary of the most important processes in interactions between woody and herbaceous species in natural ecosystems of the United States and in analogous agroforestry practices.
| Natural system category | Key processes in interactions among woody and herbaceous species | Analogous agroforestry practices in which these processes are important |
|---|---|---|
| Mesic forest, closed canopy | canopy interception of solar radiation and modification of microclimate | mushroom productionginseng production |
| Disturbance patchiness in forest landscape | gap‐creating disturbancesedge effectslandscape processes | none (in tropical areas this would be swidden agriculture) |
| Early successional systems | progressive modification of microclimate as tree canopy closes | black walnut alley croppingsilvopasture—grazing of early successional stages |
| Xeric forest, open canopy | competition for waterlocalized interception of solar radiation | silvopastoral practices |
| Mixture of forest and grass patches in transition zones | topographic patterns often serve as templatechronic stress and disturbance | silvopastoral practices |
| Ribbon forests | windspeed reductionsnow distribution | windbreaks |
| Riparian forests in grasslands | corridors for movement of wildlifespecialized wildlife habitatinterception of sediment and nutrients | riparian forests in cropland or pasture matrix |
| Isolated grasslands | no interactions | not agroforestry |
Forest canopies modify other aspects of microclimate in addition to radiation. During the day, interception of solar radiation by the tree canopy creates a temperature maximum at the height of maximum foliage density (Oke, 1987). This creates a temperature inversion that increases the atmospheric stability in the canopy relative to open terrain, partially decoupling the local atmosphere from the external environment. Windspeed decreases rapidly with distance into the canopy, while daytime humidity increases and CO2 concentration decreases due to transpiration and photosynthesis by the foliage. At the forest floor, this altered environment affects seed germination, plant establishment, litter decomposition, and the population dynamics of microorganisms, insects, and other organisms (Belsky, 1994; Jackson, Strauss, Firestone, & Bartolome, 1990; Tiedemann & Klemmedson, 1973; Vetaas, 1992).
Agroforestry options for closed‐canopy forests are limited to crops that are adapted to a low‐light environment, such as shade‐tolerant flowers. Shiitake mushrooms [Lentinula edodes (Berkeley) Pegler; Harris, 1986] and ginseng (Panax quinquefolius L.; Duke, 1989) fit perfectly in this situation, both requiring the protected environment of the forest floor. Shiitake is grown by inoculating logs with mushroom spawn and then stacking the logs under a hardwood or conifer canopy. If the site is a deciduous forest, shade cloth can be used to provide protection during leafless months. Ginseng, a medicinal herb, is cultivated in a variety of temperate deciduous forests, although most often associated with maple (Acer saccharum Marsh.) and beech (Fagus grandifolia Ehrh.). It grows well at light intensities from 5–30% and is sometimes intercropped with goldenseal to deter root rot (Duke, 1989).
Disturbance Patches and Early Successional 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
