Читать книгу North American Agroforestry - Группа авторов - Страница 92

Despite the ability of many tree species to develop taproots that extend deep into the soil profile, typically most of the tree and crop roots in an agroforestry system are within the top 30 cm of the soil profile (Jose, Gillespie, Seifert, Mengel, & Pope, 2000; Jose, Williams, & Zamora, 2006; Zamora, Jose, & Nair, 2007). Consequently, there is intense competition for soil, water, and nutrients within the top 30 cm of the soil profile, which can affect crop production (Alley, Garrett, McGraw, & Blanche, 1999; Delate, Holzmueller, Mize, Frederick, & Brummer, 2005; Jose, Gillespie, Seifert, Mengel, & Pope, 2000; Zamora et al., 2007). For example, maize yields were 35 and 33% lower for black walnut and red oak alley‐cropping systems, respectively, than for conventional crop systems (Jose, Gillespie, Seifert, Mengel, & Pope, 2000). Zamora et al. (2007) reported that belowground competition reduced the total root length in cotton plants by 33% when grown in a pecan–cotton alley‐cropping system in Florida compared with cotton plants grown in a monoculture. Furthermore, they reported that cotton plants in the alley‐cropped system had lower plant biomass production and root/shoot ratios than cotton plants grown in the monoculture system (Table 4–3). Competition between trees and crops can negatively affect tree growth as well. Delate et al. (2005) reported that, 4 yr after planting, trees planted with forage crops exhibited significantly less growth than the same tree species growing in a competition‐free environment in a bottomland hardwood alley‐cropping system in central Iowa.

Table 4–3. Dry weight (DW) growth parameters of cotton grown in non‐barrier, barrier, and monoculture treatments in the 2002 and 2003 growing seasons (modified from Zamora et al., 2007).

Treatment	Whole plant DW	Root DW	Shoot DW	Root/shoot ratio
	———————— g plant−1 ————————
2002
Non‐barrier	29.78 b (2.67)	13.53 c (0.43)	26.25 b (1.81)	0.12 c (0.02)
Barrier	54.04 a (3.76)	17.60 b (0.23)	46.44 a (2.54)	0.16 b (0.01)
Monoculture	57.33 a (3.71)	18.70 a (0.69)	48.63 a (3.07)	0.19 a (0.01)
2003
Non‐barrier	25.13 c (1.88)	12.83 c (0.21)	22.29 c (1.71)	0.13 c (0.01)
Barrier	50.76 b (3.85)	16.88 b (0.41)	43.88 b (3.56)	0.16 b (0.01)
Monoculture	72.60 a (5.71)	12.19 a (0.96)	60.40 a (4.95)	0.21 a (0.01)

Note. In a given column for every growing season, means followed by a different letters are significantly different at α = 0.05. The standard error of the mean is given in parentheses.