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Trees can modify the microclimate of an agroforestry system, which, in turn, may benefit associated crop species. Despite the previous examples of competition for light, moderate shading can have a positive effect on crop growth. For example, Lin et al. (1999) found that because of shade tolerance, Desmodium canescens (L.) DC. and D. paniculatum (L.) DC., two warm‐season legumes, had significantly higher dry weight under 50 and 80% shade than full sunlight in Missouri. Burner (2003) found that, across six harvesting periods, orchardgrass (Dactylis glomerata L.) yields did not differ among 8–10‐yr‐old loblolly pine (Pinus taeda L.) and shortleaf pine (Pinus echinata Mill.) silvopastures compared with yields in open pastures in Arkansas. Additionally, in the loblolly pine system, orchardgrass persistence was greater than in the open system (72 vs. 44% stand occupancy, respectively).

Shading can also have a positive effect on forage quality. Lin, McGraw, George, and Garrett (2001) reported that under an 80% shade treatment, the crude protein content of most of the cool‐season forage grasses studied was greater compared with the full sun treatment (Table 4–2). In a study of a 6–7‐yr‐old walnut–hybrid pine [pitch (Pinus rigida Mill.) × loblolly] and annual ryegrass (Lolium multiflorum Lam.) and cereal rye (Secale cereale L.) mixture silvopasture in Missouri, forage yield was slightly decreased in the silvopasture compared with forage yield in a nearby open pasture; however, forage quality was greater (Figure 4–6) and beef heifer average daily gain and gain per hectare were similar for the silvopasture and open pasture treatments (Kallenbach, Kerley, & Bishop‐Hurley, 2006). Ford et al. (2019) observed similar results in Minnesota, where forage yield was lower but quality was greater in silvopastoral systems than open pastures. In a recent synthesis of information from several existing studies, Pang et al. (2019a, 2019b) showed that for a number of forage species (warm‐season and cool‐season grasses, forbs, and legumes), a moderate level of shading (45% of full sun) yielded the highest crude protein. Forage biomass yield also was either highest or similar to 100% sun for most of the studied species.

Table 4–2. Crude protein of selected introduced cool‐season grasses when grown under three levels of shade during 1994 and 1995 in Missouri (modified from Lin et al., 2001).

Species	Crude protein
Full sun	50% Shade	80% Shade
	——————— % ———————
Kentucky bluegrass	20.3 b	20.7 b	22.7 a
‘Benchmark’ orchardgrass	12.6 c	15.7 b	19.6 a
‘Justus’ orchardgrass	19.8 a	16.7 a	18.5 a
‘Manhatten II’ ryegrass	15.3 b	16.0 b	18.8 a
Smooth bromegrass	16.7 c	18.1 b	20.2 a
‘KY31’ tall fescue	14.0 b	15.0 b	18.1 a
‘Martin’ tall fescue	14.3 b	15.5 b	18.5 a
Timothy	15.4 c	17.6 b	20.4 a

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).