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.
Root plasticity
Many plant species show some degree of plasticity (the ability to respond to changes in local nutrient supplies or impervious soil layers) in their vertical (as well as lateral) root distribution (Kumar & Jose, 2018). Plants also exploit plasticity to avoid competition (Ong et al., 1996; Schroth, 1999). Belowground niche separation in response to competition can help component species in an agroforestry system to avoid competition. This can lead to complementary or facilitative interactions that help increase the production potential of the system.
It is possible to apply treatments such as repeated disking, knifing of fertilizer applications, or trenching, applied while trees are young, to force tree roots to grow deeper. Wanvestraut et al. (2004) observed pecan roots displaying plasticity by penetrating deeper soil strata, thereby avoiding a region of high cotton root density. This enhanced the overall water use efficiency of the system because the cotton plants were able to capitalize on the water available in the topsoil layer while the pecan trees exploited the moisture available in the deeper soil layers. Zamora et al. (2007) corroborated the findings of Wanvestraut et al. (2004) and confirmed the morphological plasticity of cotton roots in response to competition from pecan trees.
Dawson, Duff, Campbell, and Hirst (2001) demonstrated that cherry (Prunus avium L.) tree root distribution was influenced by grass competition in a silvopastoral system in Scotland. Cherry roots increased within the upper soil surface horizon after grass competition was removed with herbicides, and in areas where grass competition was not removed, the average depth of the tree roots increased with time.
Safety net role
In conventional agricultural systems, less than half of the applied N and P fertilizer is taken up by crops (Smil, 1999, 2000). Consequently, excess fertilizer is washed away from agricultural fields via surface runoff or leached into the subsurface water supply, thus contaminating water