Neighborhood Interference among Velvet Leaf, Abutilon theophrasti, and Pigweed, Amaranthus retroflexus

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Abstract
Spatially local interference among pigweed (Amaranthus retroflexus) and velvet leaf (Abutilon theophrasti) individuals was studied in experimental two species stands. Although survivorship was density-independent for both species, individual fecundities were affected by the local population density of velvet leaf and pigweed near each plant. We estimated the functional forms of the spatially local density dependence and the distances over which plants interfere with one another. For both species hyperbolic functions provided better fits than either linear or log-linear functions. The apparent significance of higher-order interaction terms from the linear and log-linear regressions appears to be an artifact of the hyperbolic form of the data. We explain aspects of the functional forms of the relations between fecundity and local densities in terms of morphological attributes of each species. We also describe some statistical problems associated with estimating the effects of spatially local interindividual interference.