Effects of weed community species richness on the performance of spring wheat: A neighborhood competition approach

Competition is an important process that affects individual plants, plant populations, and plant communities, and is of particular concern in agricultural settings.  It has long been recognized that the presence of weeds in a crop field could result in yield reductions, but the effects on crop performance of increasing weed species richness while holding weed density constant have not been fully examined.  This study tested the null hypothesis that, in addition to the effects of neighborhood density, increased neighborhood species richness would have no effect on spring wheat (Triticum aestivum L.) target plant performance via increased inter-specific competition.  Measurements were taken from circular neighborhoods of 16.5 cm radius in which a single target spring wheat plant was surrounded by various combinations of green foxtail (Setaria viridis (L) Beauv.), wild oat (Avena fatua (L)), kochia (Kochia scoparia (L) Schrad.), field pennycress (Thlaspi arvense (L)), and spring wheat plants, representing all possible combinations of neighbor species at four levels of density.  Using a series of regression models, we found that species richness had no significant effect on spring wheat biomass, yield, or relative growth rate (RGR), and that there were no significant neighbor species interaction terms indicating reduced competitive effects on the target plant due to inter-specific competition between neighbor species.  Thus, the null hypothesis could not be rejected.  However, additional analysis suggested that increased species richness may limit the competitive ability of dominant weed species, potentially creating a more stable weed community.