THE EFFECT OF NEIGHBORS ON ROOT DISTRIBUTION IN A CREOSOTEBUSH (LARREA-TRIDENTATA) POPULATION

We excavated and mapped the lateral extension of 32 creosotebush shrub s (Larrea tridentata) in the Chihuahuan desert of New Mexico to examin e the effect of neighborhood interaction on root distribution. The sma llest closed-angle polygon encompassing all roots of an individual was taken as a representation of its root system. Several geometrical cha racteristics of these polygons were measured and compared to interfere nce vectors based on the location and size of the neighbors. We found that root systems were more developed away from the maximum competitiv e pressure of neighbors. Relation between root system shape and pressu re from neighbors was stronger when the competitive vectors were integ rating effect from all neighbors. Size of neighbors did not appear to contribute significantly to the relation. The resulting spatial patter n tended to reduce the overlap between neighboring root systems. Two c onceptual models of root growth response to neighbors appear to explai n our results. Both models assume that when the root system of neighbo rs meet, root growth is impaired or ceases at the zone of contact. In the nonoverlapping, non-compensatory model, the decrease in root growt h between two close neighbors is not compensated elsewhere, possibly a ffecting the overall plant performance. In the non-overlapping, compen satory model, a plant with a close neighbor responds by investing in r oot growth away from the competitive pressure or simply in zones free of neighbors. Under this model, two plants can be close to each other and not compete. Competition in the population is for space and only o ccurs when a plant root system is crowded on all sides.