The main concern in agroforestry practices is performance or the relationship between structure and function. In each of the natural systems, there are certain processes that are most important in determining interactions among the woody and non‐woody components and in turn the overall function of the system (Table 3–2). These same processes influence interactions among the components of analogous agroforestry practices.
Closed‐Canopy Mesic Forests
Water, nutrients, and light (energy) are the main resources for which plants compete. In more mesic (wetter) sites with adequate nutrients, light is the limiting factor. Trees and shrubs invest heavily in structural components to lift leaves above competitors and capture light before it reaches the ground. Forests often have two or three canopy layers as trees, saplings, and shrubs capture light at different levels, and this vertical stratification may increase the total energy captured by the system. Total leaf area can be quantified as leaf area index (LAI), the ratio of total leaf surface area to unit ground surface area. Depending on leaf orientation, a canopy with an LAI of 3–4 can intercept 90% of the incident solar radiation (Loomis & Connor, 1992). Mature mesic forests generally have an LAI of 8–10 (Odum, 1971), so competition for light is intense, with only 1–5% of incident solar radiation reaching the forest floor in closed‐canopy deciduous forests (Hicks & Chabot, 1985). For example, light penetration was 6% in a high‐elevation fir forest with full crown density and 18% when crown density was 50% (Smith, 1985). Light quality as well as quantity is affected by tree canopies, with radiation below the canopy relatively enriched in red wavelengths (Atzet & Waring, 1970). The light environment under plant canopies is highly variable both spatially and temporally as sun flecks shift with changes in the angle of incident sunlight.
As a result of competition for light, mesic forests often have sparse ground‐level vegetation. In a tulip tree–oak (Liriodendron–Quercus) forest in the southern Appalachians, only 2% of the total aboveground biomass consisted of herbaceous species (Harris, Sollins, Edwards, Dinger, & Shugart, 1975). However, some deciduous forests have rich herbaceous layers (Braun, 1967), and seasonal changes in LAI offer temporal niches for certain species. Spring 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