root quantity were negatively correlated, with 80% of tree roots being found in the A soil horizon. That study indicates, however, that soil cultivation suppresses tree root growth in the top portion of the soil, further establishing that appropriate tree management interventions can minimize competition with crops. Peng, Thevathasan, Gordon, Mohammed, & Gao (2015) also found a reduction in soybean yield when it was grown in a 26‐year‐old tree‐based intercropping site with silver maple (Acer saccharinum Marsh.), hybrid poplar (Populus deltoides × nigra), and black walnut (Juglans nigra L.). In addition to belowground competition, the trees, now roughly 15–20 m tall, also reduced incident photosynthetically active radiation, thereby reducing net assimilation, growth, and soybean yield. Obtaining optimal benefits from agroforestry requires knowledgeable selection, placement, and management of the woody and non‐woody components. Thinning and/or outright removal of some trees, as well as planting shade‐tolerant crops, are management options to consider as the system ages. A random mixture is unlikely to perform well.
Unfortunately, there is no single optimal agroforestry design that interested farmers and ranchers can be encouraged to adopt. Differences in climate, topography, soils, crops, and livestock exist at scales that range from the local to the continental. Agroforestry practices must be designed to fit the particular ecological, social, and economic context of the farm in question. Component interactions in agroforestry practices have been investigated to a small extent (Ong & Huxley, 1996), and while the emphasis has been on tropical systems (e.g., Rao, Nair, & Ong, 1997), some information is also available for temperate agroforestry systems (Thevathasan & Gordon, 2004). Whether we are considering temperate or tropical agroforestry, Muschler, in An Introduction to Agroforestry (Nair, 1993), pointed out “that the complexity and lifespan of agroforestry makes investigations of mechanisms and processes extremely difficult.” Leaving consideration of socioeconomic issues for later, how can we obtain the ecological knowledge necessary for the optimal design of a wide variety of temperate agroforestry practices?
Fig. 3–1. Hypothetical relationship between perennialism and sustainability in selected natural ecosystems and agroecosystems (based on Van Andel et al., 1993).
Table 3–1. The four key criteria that characterize agroforestry practices (modified from University of Missouri Center for Agroforestry, 2018, pp. 9–10).
| Criteria | Description |
|---|---|
| Intentional | Combinations of trees, crops, and/or livestock are intentionally designed, established, and/or managed to work together and yield multiple products and benefits, rather than as individual elements that may occur together but are managed separately. Agroforestry is neither monoculture farming nor is it a mixture of monocultures. |
| Intensive | Agroforestry practices are created and intensively managed to maintain their productive and protective functions and often involve cultural operations such as cultivation, fertilization, irrigation, pruning and thinning. |
| Integrated | Components are structurally and functionally combined into a single, integrated management unit tailored to meet the objectives of the landowner. Integration may be horizontal or vertical, above‐ or belowground, simultaneous or sequential. Integration of multiple crops utilizes more of the productive capacity of the land and helps to balance economic production with resource conservation. |
| Interactive | Agroforestry actively manipulates and utilizes the interactions among components to yield multiple harvestable products while concurrently providing numerous conservation and ecological benefits. |
The answer lies, at least in part, in the native ecosystems upon which U.S. agriculture is built. Highly sustainable, these systems were locally adapted to the environmental conditions under which they evolved. Natural ecosystems can provide models for the design of sustainable agroecosystems (Davies, 1994; Soule & Piper, 1992; Woodmansee, 1984). We believe that it is possible to identify structural and functional characteristics of natural ecosystems that contribute to their sustainability and then retain or incorporate these into agroecosystems while maintaining production. Regional and local differences in natural ecosystems can serve as guides for tailoring agroforestry practices that best fit a particular farm’s environmental conditions. Our goal in the remainder of this chapter is to illustrate some of the structural and functional relationships among woody and herbaceous vegetation in natural ecosystems of the United States and to show how these relationships apply to agroforestry practices.
Ecological Interactions in Mixed Tree and Herb Systems
Categories of Systems
Ecosystem function is determined not just by species composition, but also by the spatial and temporal arrangement of the component species. Figure 3–2A identifies eight main types of North American temperate ecosystems based on the 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.
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
