Asymmetrical growth of the crowns of neighbouring desert shrubs

To study the influence of nearest neighbours on the geometry of crown growth I calculated the difference between the distances from the centres of the base and crown of a plant to the base centre of the nearest neighbour. The calculations were replicated in two populations of the desert shrub Ambrosia deltoidea [(Torr.) Payne]. In both, the difference was positive, indicating that neighbouring plants grew apart. As a consequence, the dispersion of crown centres was greater than the dispersion of base centres. Also, the correlation of proximity and summed sizes of nearest neighbours was greater when proximity was measured between crown centres rather than base centres. Least squares regression analysis indicated partial dependence of the difference between base- and crown-centre distance on neighbour size and proximity. Previous studies of the dispersion of desert shrubs that used nearest-neighbour crown-centre distances might have erroneously concluded that greater dispersion of larger plants was due to proximity-dependent mortality when it was actually due to asymmetrical crown growth.