Even though there are examples of violations to the fundamental conditions of the CEP, using the concept as the initial inquiry of ecological thought has elucidated many other mechanisms that contribute to our knowledge of how species coexist in nature. For example, the resource‐ratio hypothesis, proposed by Tilman (1980, 1982, 1990), has been used to explain species coexistence (for a review of examples in which the resource ratio theory has been tested, see Miller et al., 2004). According to this hypothesis, coexistence occurs where resource requirements differ among species. Greater capture of a limiting resource would be accompanied by an increased ability to utilize nonlimiting resources, which, by definition, are available but underutilized. In an agroforestry setting, based on the differences in physical or phenological characteristics of the component species, the interactions between tree and crop species may lead to an increased capture of a limiting growth resource. The system as a whole could then accrue greater total biomass than the cumulative production of those species if they were grown separately on an equivalent land area (Cannell et al., 1996).
Hubbell (2001) has challenged the notion that trade‐offs are necessary for understanding broad patterns of species diversity and relative abundance. In contrast to trade‐off‐based theories, Hubbell developed a neutral model (united neutral theory) that explains plant species coexistence without any trade‐offs. Neutral theories focus on “community drift” and explain the maintenance of biodiversity at large spatial and temporal scales by a balance between speciation and stochastic extinction events. These are caused by random drifts in population size in communities of ecologically identical (hence neutral) species, that is, without invoking any species‐specific traits or interspecific trade‐offs.
Finally, spatially explicit models of plant species coexistence have been developed (e.g., Gravel, Mouquet, Loreau, & Guichard, 2010; Isabelle, Damien, & Wilfried, 2014). They do not require trade‐off or neutrality assumptions to explain plant species coexistence, and they predict coexistence if interactions among conspecifics (individuals of the same species) occur across larger distances than interactions among heterospecifics (individuals of different species). Moreover, they lend themselves to more direct experimental tests than the more general trade‐off or neutral theories.
Analysis of ecological interactions has shown both competitive and facilitative (complementary) interactions in agroforestry systems (Jose et al., 2004), which occur both above‐ and belowground (Ong, Corlett, Singh, & Black, 1991; Singh, Ong, & Saharan, 1989). As stated by Shainsky and Radosevich (1992), mechanisms of competition for resources should at least include documentation of: (a) depletion of resources associated with the presence and abundance of plants; (b) changes in physiological and morphological growth responses associated with changes in the resource environment; and (c) correlations between the presence or abundance of neighbors, depression in resource availability, and physiological performance. In contrast, according to Kelty (2000), facilitative interactions are those in which one species benefits another and occur under four mechanisms: (a) increased nutrient cycling efficiency, e.g., increasing N availability by planting an N2–fixing species with non‐N2–fixing species; (b) increased water and nutrient retention through improved soil structure; (c) increased water availability for understory species because of reduced evaporative demand or “hydraulic lift” of moisture from the lower levels in the soil by overstory species; and (d) decreases in productivity losses from insect pests, pathogens, and weeds.
Competition and facilitation are not necessarily independent of each other (Holmgren, Scheffer, & Huston, 1997); the balance between these factors may vary along a resource gradient (Brooker & Callaghan, 1998). Proper management of an agroforestry system that increases facilitative interactions and limits competitive interactions requires an understanding of the possible interactions in these systems. Therefore, an examination of both the effect that plants have on the shared resources and their response to the changed environment must occur in order for proper management to take place (Casper & Jackson, 1997; Goldberg, 1990).
Competitive Interactions—Aboveground
Competition for light
The incorporation of trees or shrubs in an agroforestry system can increase the amount of shading that plant species, primarily those in the understory, experience compared with growing in a monoculture. Green plants are photoautotrophs, and both the fraction of incident photosynthetically active radiation (PAR, 400–700‐nm wavelength) that a species intercepts, and the ability of that species to convert radiation into energy (through photosynthesis) are important factors in plant biomass growth (Ong, Black, Marshall, & Corlett, 1996). Furthermore, these biomass growth factors are influenced by a number of additional factors including temperature, available water and nutrients, CO2) level, aspect, time of day, photosynthetic pathway (C3 vs. C4), plant age and height, leaf area and angle, canopy structure, species combination, and transmission and reflectance traits of the canopy (Brenner, 1996; Kozlowski & Pallardy, 1997).
Numerous studies have examined shading and its effects on crop growth (Artru et al., 2017; Gillespie et al., 2000; Reynolds, Simpson, Thevathasan, & Gordon, 2007), and many of those studies have indicated that shading by tree species is a factor in reducing crop yield. For example, lower PAR levels resulting from overhead shading by hybrid poplar (Populus sp. clone DN‐177) and silver maple (Acer saccharinum L.) significantly reduced the yield of maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] in a temperate alley‐cropping system in southern Ontario, Canada (Table 4–1) (Reynolds et al., 2007). The yields of soybean and maize were reduced by 49 and 51%, respectively, when PAR levels decreased by 29% at 2 m from silver maple tree rows. Similar results have also been reported for temperate silvopastoral systems. In Missouri, significant decreases in the mean dry weight of warm‐season grasses was observed as the amount of available light declined (Lin, McGraw, George, & Garrett, 1999). In a silvopastoral aspen (Populus tremuloides Michx.) stand in Alberta, Canada, a decrease in canopy cover resulted in a significant increase in understory production, while understory production was only slightly affected by decreased belowground competition (Powell & Bork, 2006). When the canopy was removed, understory net primary production increased up to 275% compared with the control stands with full canopy.
Table 4–1. Effects of tree (poplar and maple) competition on photosynthetically active radiation (PAR) and crop (soybean and maize) yield at two distances from tree rows for two growing seasons (modified from Reynolds et al., 2007).
| Parameter (N = 6) | Crop | Control | Poplar | Maple | |||
|---|---|---|---|---|---|---|---|
| 2 m | 6 m | 2 m | 6 m | 2 m | 6 m | ||
| 1997 | soybean | ||||||
| PAR, mmol s−1 m−2 |