Группа авторов

North American Agroforestry


Скачать книгу

spatial and temporal relationships of their woody and herbaceous components. Figure 3–2B uses the same framework to identify structurally analogous agroforestry practices found in this region.

      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.

Schematic illustration of (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.
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