4–8. Average leaf N (a) concentrations and (b) amounts in wild cherry and hybrid walnut trees 6 yr after planting with unirrigated cereal crops (intercropping) or in traditional monoculture plantations (control). The error bars show the 95% confidence intervals of the measured means
(adapted from Chifflot et al., 2006).
There are several examples of allelopathy in temperate agroforestry systems (Geyer & Fick, 2015; Jose & Gillespie, 1998; Jose & Holzmueller, 2008; Thevathasan, Gordon, & Voroney, 1998) and most of them involve black walnut, a species that produces a phenolic compound called juglone (5‐hydroxy‐1, 4‐naphthoquinone) that restricts the growth of some other species. Although Jose, Gillespie, Seifert, and Biehle (2000) documented that competition for water was the leading factor in reduced growth of alley‐grown maize in a black walnut–maize alley‐cropping system in Indiana, a companion study (Jose & Gillespie, 1998) also indicated the possibility of juglone phytotoxicity. Jose and Holzmueller (2007) reported sensitivity of cotton (Gossypium sp.) and peanut (Arachis hypogaea L.) when exposed to juglone in hydroponic cultures. These are two common species with potential for alley cropping with pecan in the southern United States, and pecan also produces juglone (Jose, 2002). However, some species do not appear to be as susceptible to juglone (Geyer & Fick, 2015), and this should be taken into consideration when developing an agroforestry system.
It has also been reported that certain crop species may induce allelopathic effects on trees as well, including a decrease in development and growth (Smith, Wolf, Cheary, & Carroll, 2001; Todhunter & Beineke, 1979). In a study of containerized pecan trees, Smith et al. (2001) showed that allelochemical‐containing leachates added to the containers from bermudagrass [Cynodon dactylon (L.) Pers.], cutleaf evening primrose (Oenothera laciniata Hill.), and tall fescue [Schedonorus phoenix (Scop.) Holub.] decreased pecan root weight by 17%, trunk weight by 22%, and total tree dry weight by 19% compared with the control treatment.
Management techniques to reduce the effects of allelopathy have also been examined in agroforestry systems. Jose, Gillespie, Seifert, and Biehle (2000) demonstrated that by separating the root systems of black walnut and maize using a polyethylene barrier, crop yield became similar to that of a monoculture. They further showed that the juglone concentration in the soil was negligible beyond the polyethylene barrier. Juglone concentration beyond the root barrier decreased to trace levels of 0.08 and 0.01 μg g−1 soil (at distances of 2.45 and 4.25 m, respectively) in the barrier treatment compared with 0.42 and 0.32 μg g−1 soil in the non‐barrier control treatment.
A comprehensive study examining field soil juglone concentrations, sorption mechanisms, juglone production rates, and degradation rates and products (Von Kiparski, Lee, & Gillespie, 2007) showed that juglone can accumulate under field conditions, with release rates from black walnut being greater than abiotic and microbial transformation rates. In a 19‐yr‐old walnut plantation, surface soil pore water juglone concentrations approached but did not exceed the inhibition solution thresholds of typical intercrops. But substantially higher levels of juglone can be reversibly sorbed by soils, and true plant impacts may be a balance of responses to multiple stress conditions in the mixed systems. From greenhouse studies, it was determined that substantial quantities of juglone can be released into the rhizosphere, and so rooting patterns of intercrops will be of particular concern when judging allelopathic potential. However, soil chemistry will play a role in these intercrops, as this study showed that microbial activity will quickly degrade juglone and decrease persistence. Soils low in microbial activity, including subsurface horizons and acidic soils that are low in organic C and fertility, can accumulate juglone, and thus this negative interaction among interplanted species should continue to be considered in walnut and pecan agroforestry systems.
Facilitative Interactions—Belowground
Hydraulic lift
Hydraulic lift is the process by which deep‐rooted plants transport or conduct water from deep within the soil and release it into the upper, drier regions of the soil. The process has been reported to be an appreciable water source for neighboring plants in some systems (Caldwell & Richards, 1989; Corak, Blevins, & Pallardy, 1987). This phenomenon can increase plant growth, in some cases, by increasing the availability of water for shallow‐rooted plants and has important implications for ecosystem nutrient cycling and net primary productivity (Horton & Hart, 1998).
In a tropical agroforestry context, numerous studies have shown that trees can benefit associated crop plants through hydraulic lift by increasing water availability during dry periods when water would otherwise be unavailable (Burgess, Adams, Turner, & Ong, 1998; Dawson, 1993; Ong et al., 1999; van Noordwijk, Lawson, Soumaré, Groot, & Hairiah, 1996). In temperate agroforestry systems, however, research documenting the hydraulic lift phenomenon is limited. Hydraulic lift in temperate systems has been reported in Quercus sp. and Pinus sp. (Asbjornsen, Shepherd, Helmers, & Mora, 2008; Espeleta, West, & Donovan, 2004; Penuelas & Filella, 2003). These species are commonly used in temperate agroforestry systems, indicating a potential for these genera to be used in agroforestry to positively impact water relations. For example, Espeleta et al. (2004) reported hydraulic lift in longleaf pine (Pinus palustris Mill.), a species commonly used in silvopastoral systems in the southeastern United States. They reported hydraulic lift in two oak species (Q. laevis Walt. and Q. incana Bartr.) as well. They concluded that the ability of these species to redistribute water from the deep soil to the rapidly drying shallow soil has a strong positive effect on the water balance of understory plants.
Dinitrogen fixation
The incorporation of trees and crops that are able to biologically fix N2 is fairly common and well researched in tropical agroforestry systems (Nair, Buresh, Mugendi, & Latt, 1999). In temperate systems, similar accounts of incorporating N2–fixing trees into agroforestry are rare, perhaps because of the abundance and historically low cost of N fertilizer and the low value of N2–fixing trees. Despite the infrequent use of biological N2 fixation by trees in temperate agroforestry systems, there is potential for using N2–fixing tree species native to temperate environments. Species from the genera Robinia, Prosopis, and Alnus have the potential to provide N2 fixation benefits in temperate agroforestry systems (Boring & Swank, 1984; Seiter, Ingham, William, & Hibbs, 1995). Seiter et al. (1995) demonstrated this potential in a red alder (Alnus rubra Bong.)–maize alley‐cropping system in Oregon. They observed, using a 15N injection technique, that 32–58% of the total N in maize was obtained from N2 fixed by red alder and that N transfer increased by shortening the distance between the trees and crops.
There are also several leguminous herbaceous plant species capable of fixing atmospheric N2 in temperate agroforestry systems, including alfalfa, clover, hairy vetch (Vicia villosa Roth), and soybean (Troeh & Thompson, 1993). Although multiple studies have incorporated leguminous herbaceous species capable of biological N2 fixation into temperate agroforestry systems (Alley et al., 1998; Delate et al., 2005; Gakis et al., 2004; Silva‐Pando, Gonzalez‐Hernandez, & Rozados‐Lorenzo, 2002), few studies have actually quantified the effects that these species have on soil N (Dupraz et al., 1998; Waring & Snowdon, 1985). Nitrogen buildup in the soil is possible from leguminous herbaceous understory species; however, this is a slow process that does not occur immediately after herbaceous plant establishment. In a radiata pine–subterranean clover (Trifolium subterraneum L.) silvopasture in Australia, Waring and Snowdon (1985) observed a 36% increase in soil N at the end of seven growing seasons in the silvopasture, which corresponded to a 14% increase in tree diameter compared with pines growing in a monoculture without a subterranean clover understory.
