Neighbor interactions among herbaceous plants in a perennial grassland.

Abstract

Individual clumps of the perennial grass Bouteloua gracilis from which 25% or 75% of neighbors had been removed to a radius of 50 cm displayed significantly higher biomass production over one summer than did control plants. Neighbor removal also resulted in increased tiller production and flowering as compared to control plants. These results clearly demonstrate the importance of competition to B. gracilis in this system. In contrast, significant differences in predawn leaf water potential were found on only two of four sampling dates during the growing season. On the driest sampling date, increased variability in leaf water potential within treatments obscured the effect of treatment, even though the difference in mean leaf water potential between 75% removal and control treatments (0.35 MPa) was quite high. On the wettest sampling date, all plants appeared well-hydrated, and there was only negligible difference among treatments. In a second study, I examined the effect of near neighbors on the fecundity of an annual herb, Machaeranthera tanacetifolia (Asteraceae). Fecundity was negatively related to the number of neighbors within a biologically meaningful neighborhood radius for both conspecific neighbors and grass neighbors in 1984, and for grass neighbors in 1985. However, in no case did neighbors explain more than 6% of the variance in fecundity of M. tanacetifolia, and most plants produced few or no seeds regardless of the number of neighbors. I present a simple graphical model in which competition from neighbors serves as a necessary but not sufficient condition for high fecundity; other factors must also be favorable if a plant is to be successful. Application of this model to my data revealed that the amount of area occupied by neighbors had an important effect on fecundity for the subset of the population for which other conditions were inferred to be most favorable. As other conditions became less favorable, neighbors became less relevant to fecundity. I then discuss these results in terms of a general model of the interacting effects of competition and other factors, and distinguish four possible classes of outcome.