COMPETITION BETWEEN 3 CHIHUAHUAN DESERT SPECIES - EVIDENCE FROM PLANTSIZE-DISTANCE RELATIONS AND ROOT DISTRIBUTION

Using size-distance data we tested the intensity and importance of com petition between Hilaria mutica (a tussock grass), Larrea tridentata ( a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihu ahuan desert. We also compared the vertical and horizontal distributio n of roots to assess the potential degree of overlap in the use of soi l resources. The relationships between sizes and distances of nearest- neighbour plants suggested that intraspecific competition is generally more important than interspecific competition. However, evidence of s tronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition inv olved always Opuntia, whereas Larrea and Hilaria did not influence eac h other. The analysis of the symmetry of competition showed that Opunt ia was adversely affected by the presence of Hilaria or Larrea. Althou gh differences were found in the distribution of roots, the results of the size-distance study support the idea that, (particularly) Opuntia , below-ground niche differentiation is not sufficiently important to totally avoid the negative effects of plant competition.