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Competitive Interactions—Aboveground Competition for light
ОглавлениеThe incorporation of trees or shrubs in an agroforestry system can increase the amount of shading that plant species, primarily those in the understory, experience compared with growing in a monoculture. Green plants are photoautotrophs, and both the fraction of incident photosynthetically active radiation (PAR, 400–700‐nm wavelength) that a species intercepts, and the ability of that species to convert radiation into energy (through photosynthesis) are important factors in plant biomass growth (Ong, Black, Marshall, & Corlett, 1996). Furthermore, these biomass growth factors are influenced by a number of additional factors including temperature, available water and nutrients, CO2) level, aspect, time of day, photosynthetic pathway (C3 vs. C4), plant age and height, leaf area and angle, canopy structure, species combination, and transmission and reflectance traits of the canopy (Brenner, 1996; Kozlowski & Pallardy, 1997).
Numerous studies have examined shading and its effects on crop growth (Artru et al., 2017; Gillespie et al., 2000; Reynolds, Simpson, Thevathasan, & Gordon, 2007), and many of those studies have indicated that shading by tree species is a factor in reducing crop yield. For example, lower PAR levels resulting from overhead shading by hybrid poplar (Populus sp. clone DN‐177) and silver maple (Acer saccharinum L.) significantly reduced the yield of maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] in a temperate alley‐cropping system in southern Ontario, Canada (Table 4–1) (Reynolds et al., 2007). The yields of soybean and maize were reduced by 49 and 51%, respectively, when PAR levels decreased by 29% at 2 m from silver maple tree rows. Similar results have also been reported for temperate silvopastoral systems. In Missouri, significant decreases in the mean dry weight of warm‐season grasses was observed as the amount of available light declined (Lin, McGraw, George, & Garrett, 1999). In a silvopastoral aspen (Populus tremuloides Michx.) stand in Alberta, Canada, a decrease in canopy cover resulted in a significant increase in understory production, while understory production was only slightly affected by decreased belowground competition (Powell & Bork, 2006). When the canopy was removed, understory net primary production increased up to 275% compared with the control stands with full canopy.
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 result given the known strong positive correlation 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).
