Importance of Crown Position and Morphological Plasticity in Competitive Interaction in a Population of Xanthium canadense

Abstract

Interactions between individuals in terms of asymmetrical display of crowns and reduction of growth were followed in an experimental population of Xanthium canadense . Competitive interaction was analysed using a neighbourhood interference model. For the analysis of morphological interaction, crown-vector, a two-dimensional vector that joins the stem base position of a plant with the centroid of its projected crown area, was used to express the direction and magnitude of the asymmetry in crown display. A model was developed in which the change in crown-vector during a growth period was expressed as a function of both the size of the focal plant and the size and location of its neighbours. There was strong morphological interaction between crowns; they repelled each other. The competitive interaction was detectable by the competition model with crown-centres used as plant locations, but not detectable with stem bases used as plant locations. It was concluded that the position of neighbours' crowns were important in determining the growth and crown asymmetry, which in turn determine the crown location; and morphological plasticity in crown shape works to reduce interference between individuals.