between PAR and Pnet in C4 plant species. Although these researchers found that, generally, the edge rows received lower PAR than the middle rows, particularly in the red oak–maize system because of the higher canopy leaf area, once competition for water and nutrients was removed through polyethylene root barriers and trenching, there was no indication of yield reduction because of reduced PAR (Figure 4–3), leading them to conclude that competition for light was not a factor for these two systems. Interestingly, however, Reynolds et al. (2007) reported that competition for light was a factor in both soybean (C3 species) and maize yield reductions in a multispecies temperate agroforestry system in southern Ontario, Canada. In addition, they concluded that competition for light was more important than that for water during the study period. There are several reports from China showing reduced crop yield as a result of intercropping with trees. For example, Li, Meng, Dali, & Wang (2008) reported a 51% lower wheat (Tritcum aestivum L.) yield in a paulownia (Paulownia Siebold & Zucc.)–wheat intercropping system than sole cropping and attributed the reduction to shading. In a jujube (Zizyphus jujuba Mill.)–winter wheat–summer maize intercropping, Yang, Ding, Liu, Li, & Egrinya Eneji (2016) reported that the mean yield of winter wheat and summer maize was reduced by 35.6 and 35.2%, respectively, compared with monoculture. Zhang et al. (2017) also reported similar results from a jujube–winter wheat intercropping system.
Fig. 4–2. Net photosynthesis as a function of photosynthetically active radiation in maize (C4 plant) and cotton (C3 plant)
(based on data from Zamora et al. [2006] and Jose [1997])
Fig. 4–3. Grain yield of alley‐cropped maize at the edge (average of eastern and western rows closest to tree row) and alley center in two alley‐cropping systems involving black walnut and red oak in southern Indiana. Light transmittance (as a fraction of full sunlight) reaching the top of edge and center‐row plants is also shown
(reprinted with permission from Jose et al., 2004).
Herbivory and physical damage
Damage to trees from associated animal species in certain agroforestry practices such as silvopasture can be substantial. Browsing animals such as goats, sheep, or deer are more likely to eat foliage, while large ruminants such as cattle are more likely to trample young trees. Generally, younger animals are more likely to damage trees than older, more experienced ones (Nowak, Blount, & Workman, 2002). Any browsing of the terminal shoot may result in deformity and loss of growth. Similarly, physical damage to bark or stem can result in loss of vigor and eventual death of saplings and young trees (Jose & Dollinger, 2019).
In a young silvopastoral system in Missouri, Lehmkuhler et al. (2003) reported significant damage to tree seedlings during the second year after planting when cattle were introduced. Seedlings that received protection using electric fencing were mostly undamaged (Figure 4–4). In a study in the Swiss Alps, Mayer, Stockli, Konold, and Kreuzer (2006) assessed cattle damage on naturally regenerated young Norway spruce [Picea abies (L.) Karst] following a summer grazing period. They observed that 4% of the young trees were browsed on the apical shoot, 10% were browsed on lateral shoots, and 13% of the trees showed other damage. The percentage of browsed or damaged (physical damage such as breaking seedlings or trampling) trees was positively correlated with the cattle stocking rate (livestock units per hectare) (Figure 4–5). This suggests that higher cattle stocking rates not only increase browsing pressure but also the risk of unintentional trampling of trees.
Damage or injury to animals as a result of trees can also occur in silvopastoral systems. In a recent survey of silvopastoral farmers in the northeastern United States, Orefice, Caroll, Conroy, & Ketner (2017) reported that farmers were concerned about falling tree branches as health risks for the animals. They also reported at least two forms of livestock injuries, one resulting from cows’ tails being caught and torn off by woody vegetation and the other relating to hoof injury to pigs.
Fig. 4–4. Extent of damage to trees by cattle during second year after planting with and without electric fence protection in a silvopastoral system in Missouri
(based on data from Lehmkuhler et al., 2003).
Fig. 4–5. Relationships between cattle stocking rate (livestock units, LU) and percentage of browsed trees, otherwise damaged trees, and the sum of browsed and otherwise damaged trees
(modified from Mayer et al., 2006).
Facilitative Interactions—Aboveground
Modification of the microclimate
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
