water led to a 44% decrease in instantaneous CO2 assimilation, a 48% decrease in stomatal conductance, and a 64% decrease in growth between trees growing in the alfalfa pasture and trees growing in the alfalfa‐free control treatment (Yanusa et al., 2005). In a recent study in a temperate silvopasture system in Patagonia, Argentina, Quinteros, Bava, Bernal, Gobbi, and Defosse (2017) studied the competition effects of herbaceous vegetation on the survival, growth, and plant water relations of planted Nothofagus pumilio seedlings and observed higher survival and growth and better plant water status where competition from herbaceous vegetation was controlled, indicating strong interspecific competition otherwise.
Water stress has also been found to affect crop plants. Multiple studies have reported large reductions in crop plant height (NeSmith & Ritchie, 1992a; Wanvestraut et al., 2004) and leaf area (Jose, Gillespie, Seifert, & Biehle, 2000; NeSmith & Ritchie, 1992b) and yield (Gillespie et al., 2000; Wanvestraut et al., 2004) when water is a limiting factor. Wanvestraut et al. (2004) also observed competition for water in a pecan–cotton alley‐cropping system in Florida. By the end of the growing season, cotton plants in the barrier treatment were 26% taller and had 48% greater leaf area than non‐barrier plants. Therefore, it is not surprising that cotton lint yield was 35% higher in the trenched barrier treatment than the control. Results similar to those of Wanvestraut et al. (2004) were reported in a black walnut–maize alley‐cropping study conducted in Indiana (Gillespie et al., 2000; Jose, Gillespie, Seifert, & Biehle, 2000). Jose, Gillespie, Seifert, and Biehle (2000) reported a 21% increase in maize leaf area in the barrier treatment compared with the non‐barrier treatment. In a companion study, Gillespie et al. (2000) reported a decreased maize yield when comparing alley‐grown maize separated 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).
Table 4–4. Comparison of height, diameter, and stem volume index of wild cherry and hybrid walnut trees 6 yr after planting for two treatments, intercropped or monoculture with weed control only (modified from Chifflot et al., 2006).
| 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.
If water is not a limiting factor, trees also benefit from increased nutrient availability in agroforestry systems. For example, Chifflot, Bertoni, Cabanettes, and Gavaland (2006) showed that 6 yr after planting, wild cherry (Prunus avium L.) and hybrid walnut trees (J. nigra × J. regia) grown in an intercropped agroforestry system with unirrigated cereal crops in France were significantly larger (Table 4–4) and had significantly greater N content and concentration (walnut hybrid only) than trees grown in a traditional plantation with weed control (Figure 4–8). They concluded that the trees benefited from N fertilization that was applied to the cropped alleyways.
Allelopathy
In addition to increased competition, the mixing of multiple species in a single system can potentially bring about other negative effects. One of those effects is allelopathy, which is caused by allelochemicals that are produced by some plant species and released into the soil by root exudation and aboveground litterfall. Allelochemicals have been documented to affect germination, growth, development, distribution, and reproduction of numerous plant species (Inderjit & Mallik, 2002). However, production rates of these chemicals in a given system depend on a variety of factors, including age of the species, density of the species, and the time of year. These factors, in combination with the residence time of the chemicals in the soil and plant resistance and/or tolerance to the chemicals, play a role in the degree to which these chemicals inhibit growth. It should also be noted that under certain soil conditions, such as low soil water regimes, these chemicals may be oxidized into a nontoxic form (Fisher, 1978).
