Читать книгу North American Agroforestry - Группа авторов - Страница 93

Although belowground competition has been documented in many agroforestry systems, it is difficult to separate the belowground competition for water from that for nutrients. Competition for water, however, has been detected in some temperate agroforestry systems. In a silvopastoral study in the Canterbury region of New Zealand, Yanusa et al. (2005) reported a slightly lower water potential in radiata pine (Pinus radiata D. Don) trees planted in an alfalfa (Medicago sativa L.) pasture compared with trees growing in a vegetation‐free control treatment. Competition for 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.