Note. Means followed by the same letter within a row are not significantly different (Tukey’s Studentized range test, α = 0.05).
In addition to their effect on solar radiation, trees can also influence the microclimate of the surrounding area in terms of wind speed and humidity. Serving as windbreaks, trees slow the movement of air, thereby reducing evaporative stress. For example, in a silvopastoral system in Australia, wind speed was reduced up to 80% in a zone that extended 5H upwind and 25H downwind of the windbreak (where H is the height of the windbreak) (Cleugh, 2002). Windbreaks have also been shown to reduce evapotranspiration, improve the distribution and utilization of irrigation water, and improve crop water use efficiency (Davis & Norman, 1988). As shown in several studies, the wind reduction and improved microclimate resulting from planting windbreaks or shelterbelts in crop fields may translate into improved crop quality and yield within the sheltered areas (10–15H), (Brandle, Hodges, & Zhou, 2004; Kort, 1988). These effects, however, may vary with annual rainfall conditions (Rivest & Vézina, 2015).
Shading from trees can lower temperatures and reduce heat stress of crops in agroforestry systems. For example, in a silvopastoral system in west‐central Spain, the presence of trees significantly lowered the air and soil temperature beneath the canopy on warm days and significantly increased both air and soil temperature beneath the canopy on cold days (Figure 4–7) (Moreno Marcos et al., 2007). Due to the air and temperature modifications caused by the tree shading, forage under the tree canopies began growing earlier in the growing season and continued growing later in the growing season in this system (Gómez‐Gutierrez & Pérez‐Fernández, 1996; Moreno Marcos et al., 2007). Similar results have been reported in other agroforestry systems. In their study of a pecan–cotton alley‐cropping system in northwest Florida, Ramsey and Jose (2002) observed cotton plants germinating earlier in the growing season under pecan canopy cover compared with the cotton‐only system, which was attributed to moister and cooler soil conditions. Tomato (Lycopersicon esculentum Mill.) and snap bean (Phaseolus vulgaris L.) showed earlier germination, accelerated growth, and increased yields under simulated narrow alleys than wider alleys in an alley‐cropping study in Nebraska (Bagley, 1964; Garrett et al., 2009).
Fig. 4–6. Acid detergent fiber, neutral detergent fiber, and crude protein of annual ryegrass–cereal rye in Open and Tree pastures at the Horticulture and Agroforestry Research Center near New Franklin, MO. Bars indicate standard errors at each sampling
(adapted from Kallenbach et al., 2006).
Enhancing beneficial insect populations
Variations in tree–crop combinations and spatial arrangements in agroforestry have been shown to have an effect on insect population density and species diversity (Altieri, 1991; Pardon et al., 2019). Agroforestry helps reduce pest problems because tree–crop combinations provide greater niche diversity and complexity than monoculture systems of annual crops (Martin‐Chave, Béral, & Capowiez, 2019). This effect may be explained in one or more of the following ways: (a) wide spacing of host plants in the intercropping scheme may make the plants more difficult for herbivores to find; (b) one plant species may serve as a trap crop to detour herbivores from finding the other crop; (c) one plant species may serve as a repellent to the pest; (d) one plant species may serve to disrupt the ability of the pest to efficiently attack its intended host; and (e) the intercropping situation may attract more predators and parasites than monocultures, thus reducing pest density through predation and parasitism (Root, 1973; Jose et al., 2004).
Reports of agroforestry practices enhancing beneficial insects are limited in the temperate agroforestry literature. Studies with pecan, for example, have looked at the influence of ground covers on arthropod densities in tree–crop systems (Bugg, Sarrantonio, Dutcher, & Phatak, 1991; Smith et al., 1996). Bugg et al. (1991) observed that cover crops (e.g., annual legumes and grasses) sustained lady beetles (Coleoptera: Coccinellidae) and other arthropods that may be useful in the biological control of pests in pecan (Bugg et al., 1991; Garrett et al., 2009). However, Smith et al. (1996) found that ground cover had little influence on the type or density of arthropods present in pecan. Brandle et al. (2004) summarized the beneficial effects of windbreaks on natural enemies of crop pests. According to them, windbreaks influence the distributions of both predator and prey. Greater diversity of the edges provides numerous microhabitats for life‐cycle activities and a variety of hosts, prey, pollen, and nectar sources. As windbreak structure becomes more complex, various microhabitats are created and insect populations increase in both number and diversity.
Fig. 4–7. Effect of trees on air and soil temperature of holm oak dehesas: (a) air temperature as a function of distance from the tree, and (b) soil temperature (ST) with respect to the air temperature both beneath and beyond the tree canopy
(adapted from Moreno Marcos et al., 2007).
Improving Wildlife Habitat
Agroforestry practices, by increasing structural and compositional plant diversity on the landscape, provide improved wildlife habitat for many species. In some agriculture‐dominated landscapes, windbreaks and riparian buffers offer the only woody habitat for wildlife (Johnson & Beck, 1988; Söderström, Svensson, Vessby, & Glimskär, 2001). Brandle et al. (2004) reported that in Nebraska, landowners identified wildlife as a primary reason for the establishment of windbreaks on agricultural land. In a comparison of maize monoculture with riparian buffer plantings of ladino clover (Trifolium repens L.) and orchardgrass with three tree species—green ash (Fraxinus pennsylvanica Marsh.), black walnut, and red oak—Gillespie, Miller, & Johnson (1995) showed that the riparian strips provided better habitat for birds than maize monoculture, with both higher bird densities and diversity.
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