North American Agroforestry. Группа авторов

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North American Agroforestry - Группа авторов


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from the tree rows by a polyethylene root barrier versus alley‐grown maize with no root barrier. By quantifying competition for water in the black walnut–maize alley‐cropping system, Jose, Gillespie, Seifert, and Biehle (2000) concluded that severe competition for water was occurring between the trees and crops.

      It is important to note that interspecific competition for water becomes increasingly intense when water levels decrease throughout the soil profile (Miller & Pallardy, 2001). Factors such as drought, the water holding capacity of the soil, and irrigation all play a role in the degree to which competition for water limits plant growth and productivity. Competition for water can be minimal given adequate levels of precipitation and/or irrigation in an agroforestry system.

      Competition for nutrients

      As in the case of conventional agriculture, nutrients can often be limiting in agroforestry systems. Therefore, many agroforestry systems are subject to fertilization, which is most commonly done at the level needed for the crop component to maintain high growth and productivity. Without fertilization, inter‐ and intraspecific competition for nutrients will be high and there will likely be a decrease in crop yields with time (Jose, Gillespie, Seifert, Mengel, & Pope, 2000).

      Allen et al. (2004b) observed competition for N in a pecan–cotton alley‐cropping system in northwestern Florida, where cotton plants in a barrier treatment had a 59% higher aboveground biomass than plants in a non‐barrier treatment. Although a companion study indicated that competition for water was also a factor (Wanvestraut et al., 2004), the researchers hypothesized that because pecan trees leaf out earlier in the spring and have a high nutrient demand early in the growing season, the soil N was depleted before the cotton plants were established later in the growing season. Therefore, in this particular system, cotton plants are more likely to rely on fertilizer N to fulfill plant needs (Allen et al., 2004b). In addition, in a companion study, Allen et al. (2005) showed that N mineralization rates differed between barrier and non‐barrier treatments in this pecan–cotton alley‐cropping system. Higher rates of N mineralization were observed in the non‐barrier treatment (26.05 mg kg−1 mo−1) than the rates observed in the barrier treatment (19.78 mg kg−1 mo−1), indicating that competitive interactions for water and N in the non‐barrier treatment may have led to a decreased ability of the cotton plants to take up N (Allen et al., 2005).

Parameter Species Treatment
Intercrop Monoculture
Height (H), cm wild cherry 522 a 470 b
hybrid walnut 436 a 332 b
Diameter (D), cm wild cherry 9.2 a 447.3 b
hybrid walnut 47.8 a 444.7 b
Stem volume index (D2H), 103 cm3 wild cherry 447.2 a 431.6 b
hybrid walnut 431.3 a 448.9 b

      Note. Means followed by different letters are significantly different at α = 0.05 for each row.

      Allelopathy

Schematic illustration of average leaf N (a) concentrations and (b) amounts in wild cherry and hybrid walnut trees 6 yr after planting with unirrigated cereal crops (intercropping) or in traditional monoculture plantations (control).

       (adapted from Chifflot et al., 2006).

      It has also been reported that certain crop species may induce allelopathic effects on trees as well, including a decrease in development and growth (Smith, Wolf, Cheary, & Carroll, 2001; Todhunter & Beineke, 1979). In a study of containerized pecan trees, Smith et al. (2001) showed that allelochemical‐containing


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