Variation in Local Density Results in a Positive Correlation between Plant Neighbor Sizes

A positive correlation between the sizes of plants and the sizes of their nearest neighbors has been observed in many plant populations and is most often attributed to variation in microsite quality. We demonstrate another potentially important cause, using a simple spatial simulation model in which a plant's size is determined by its available area plus stochastic variation. If the spatial arrangement of individuals is nonuniform and local crowding reduces individual size, a positive correlation between the size of a plant and the size of neighboring individuals is to be expected. If a plant is small because it is crowded, its nearest neighbors will also tend to be crowded and therefore small. Such positive correlations will be observed whenever any trait of an object is related to the area available to it. Looking at the performance of neighboring objects without explicitly considering distance can give a very misleading signal.