rel="nofollow" href="#ulink_d6b9dffb-41c3-5804-9243-035e88c4e68e">Table 4–1. Effects of tree (poplar and maple) competition on photosynthetically active radiation (PAR) and crop (soybean and maize) yield at two distances from tree rows for two growing seasons (modified from Reynolds et al., 2007).
| Parameter (N = 6) | Crop | Control | Poplar | Maple | |||
|---|---|---|---|---|---|---|---|
| 2 m | 6 m | 2 m | 6 m | 2 m | 6 m | ||
| 1997 | soybean | ||||||
| PAR, mmol s−1 m−2 | 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 (Pnet); however, as more light becomes available, Pnet does not increase but rather levels off despite the additional increase in PAR (Figure 4–2). In contrast, in C4 plants, Pnet 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 Pnet 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
