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 sources and decreasing water quality (Bonilla, Muñoz, & Vauclin, 1999; Ng, Drury, Serem, Tan, & Gaynor, 2000; Tilman et al., 2002). In an agroforestry system, however, trees with deep rooting systems potentially play the role of a “safety net” by retrieving excess nutrients that have been leached below the rooting zone of agronomic crops. These nutrients are then recycled back into the system through root turnover and litterfall, increasing the nutrient use efficiency of the system (van Noordwijk et al., 1996). Additionally, because trees have a longer growing season than most agronomic crops, tree roots occupying the same rooting zone as associated agronomic crops will increase nutrient use and use efficiency in an agroforestry system by capturing nutrients before crops are planted and after crops are harvested.
Evidence supporting the safety net concept has been observed in field trials. In a pecan–cotton alley‐cropping system in northwestern Florida, Allen et al. (2004a) reported a 245% NO3–N increase at the 0.9‐m depth when pecan roots were separated from cotton roots by a root barrier compared with the non‐barrier treatment. These researchers suggested that this indicates the trees could potentially play the role of a N safety net by taking up N fertilizer from deep in the soil profile and redepositing it on the soil surface via litterfall (Allen et al., 2004a).
The safety net concept can be applied to other nutrients in agroforestry systems as well. In a silvopastoral system in Florida, Nair, Nair, Kalmbacher, and Ezenwa (2007) monitored soil P concentrations in pastures with and without 20‐yr‐old slash pine (Pinus elliottii Engelm.) trees. They found lower concentrations of P in the soil surface horizon and at the 1.0‐m depth in pastures with trees, suggesting that silvopastoral associations enhance soil nutrient retention and limit nutrient transport in surface water. Lee, Isenhart, and Schultz (2003) documented increased nutrient removal efficiency when trees were incorporated into a riparian buffer strip placed on the border of agronomic field plots in their study in Iowa. They reported that a switchgrass (Panicum virgatum L.) and woody stem buffer removed similar amounts of sediment as a switchgrass‐only buffer, but nutrient removal was increased by >20% in the switchgrass and woody stem buffer (Table 4–5).
Table 4–5. Percentage of sediment and nutrients removed by two riparian buffer systems in a study conducted in Iowa (adapted from Lee et al., 2003).
| Sediment or nutrient | Switchgrass only buffer removal | Switchgrass and woody stem buffer removal |
|---|---|---|
| —————————— % —————————— | ||
| Sediment | 95 | 97 |
| Total N | 80 | 94 |
| NO3–N | 62 | 85 |
| Total P | 78 | 91 |
| PO4–P | 58 | 80 |
The Future
It is true that we have made significant improvements in our understanding of ecological interactions in temperate agroforestry. As this chapter has revealed, we have information on above‐ and belowground interactions that define the ecological sustainability of some of the well‐known agroforestry practices such as alley cropping and silvopasture in the United States and other temperate regions of the world. We also have information on the management techniques that may reduce competitive interactions while enhancing complementarity in those systems. However, our knowledge is still limited in several areas. For example, despite much research examining resource competition, we still lack a deeper understanding of the interactive effects of multiple resources on system productivity in several agroforestry systems. Modeling has helped us understand multiple resource interactions to a great extent (Lovell et al., 2017), but continued acquisition of information at a range of scales is urgently needed. Although information on specific components and their interactions are important, we also need to pay attention to interactions of agroforestry systems with the biotic and abiotic components of the surrounding landscape matrix. Watershed‐level research and studies of agroforestry systems as wildlife habitats and corridors need to explore these relationships in detail.
Another area that needs immediate attention is the screening of species and germplasm for above‐ and belowground complementarity. Most of the improved germplasm currently used in agroforestry comes from breeding efforts for monoculture cropping systems. Breeding for crops and trees that can perform better under shade and under interspecific competition for water and nutrients needs to be initiated.
The available literature on facilitative interactions in temperate agroforestry is very limited. For example, the concept of hydraulic lift is yet to be experimentally proven in a temperate context. Information on the canopy and root architecture of many common agroforestry species is still not readily available. Dinitrogen fixation remains an unexplored and underutilized concept in many of the well‐studied temperate agroforestry systems.
A number of agroforestry practices have received little attention from the scientific community despite their popularity. For example, forest farming is an attractive agroforestry practice in many parts of the United States. However, ecological sustainability or component interactions have seldom been investigated in these systems. Similarly, incorporation of high‐value agronomic or horticultural crops into existing agroforestry practices or the design of new agroforestry
