North American Agroforestry. Группа авторов

1,464.0 a 1,586.0 a 1,133.0 a 1,370.0 a 1,045.0 b 1,558.0 a Yield, t ha⁻¹ 2.51 a 2.59 a 1.04 b 1.97 a 1.29 b 2.00 a 1998 soybean PAR, mmol s⁻¹ m⁻² 1,405.0 a 1,158.0 a 746.0 b 1,296.0 a 670.0 b 1,336.0 a Yield, t ha⁻¹ 2.24 a 2.25 a 1.15 b 1.67 a 1.55 b 2.85 a 1997 maize PAR, mmol s⁻¹ m⁻² 1,528.0 a 1,579.0 a 952.0 b^* 1,407.0 a^* 1,075.0 b^* 1,525.0 a^* Yield, t ha⁻¹ 4.21 a 4.83 a 2.89 b 4.61 a 2.07 b 4.64 a 1998 maize PAR, mmol s⁻¹ m⁻² 1,422.0 a 1,200.0 a 794.0 b 1,117.0 a 481.0 b 1,420.0 a Yield, t ha⁻¹ 5.70 a 5.88 a 0.69 b 5.29 a 3.79 b 7.07 a

      Note. Soybean and maize intercrops, July 1997 and July 1998. Within each treatment (control, poplar, maple), values in each row followed by the same letter are not significantly different (Tukey’s HSD, P < 0.05).

      ^* Significant at the 10% level.

      The physiological basis of yield reduction due to shading has been investigated by several studies in temperate agroforestry systems (Albaugh et al., 2014; Ehret, Graß, & Wachendorf, 2015; Miller & Pallardy, 2001; Reynolds et al., 2007). Shading changes the quality of light reaching the understory canopy (Krueger, 1981). Since overhead canopies absorb both the longest and shortest wavelengths of the light spectrum (red and blue), diffuse radiation is primarily composed of medium‐wavelength light (orange, yellow, and green). Growth regulating hormones and, therefore, growth are influenced by the interactions of the plant phytochrome system with red and infrared wavelengths (Baraldi, Bertazza, Bogino, Luna, & Bottini, 1995). Inadequate exposure to red light is known to influence stem production in clover (Trifolium sp.) (Robin, Hay, Newton, & Greer, 1994), tillering in grasses (Davis & Simmons, 1994a), flowering (Davis & Simmons, 1994b), and other basic plant growth processes (Sharrow, 1999).

Theoretically, one physiological response to shading depends on the pathway used by crop species to fix C (C₃ vs. C₄). In C₃ plants, as PAR increases from complete shade to approximately 25–50% of full sun there is a corresponding increase in the photosynthetic rate (P_net); however, as more light becomes available, P_net does not increase but rather levels off despite the additional increase in PAR (Figure 4–2). In contrast, in C₄ plants, P_net does not level off as PAR increases to full sunlight but rather continues to increase with increasing PAR (Figure 4–2). The difference between C₃ and C₄ plants is related to the pathway by which these two types of plants fix CO₂ (Kozlowski & Pallardy, 1997; Lambers, Chapin, & Pons, 1998). Theoretically, because of the ability of C₃ plants to maximize P_net growing under partial shade, they should be better suited for agroforestry practices than C₄ plants. However, field studies have produced mixed results.

In accordance with the theory that C₃ plants would not have reduced growth under shaded conditions, Wanvestraut, Jose, Nair, and Brecke (2004) reported no effects on the growth and yield of cotton (Gossypium hirsutum L.), a C₃ plant species, when grown under moderate shading in a temperate pecan [Carya illinoinensis (Wangenh.) K. Koch]–cotton alley‐cropping system in Florida. Contrary to an anticipated yield decrease in maize, a C₄ species, in response to shading, Gillespie et al. (2000) reported no effect of shading in both black walnut (Juglans nigra L.)–maize and red oak (Quercus rubra L.)–maize alley‐cropping systems in Indiana, which was not the expected result given the known strong positive correlation between PAR and P_net 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