WATER AVAILABILITY AND THE COMPETITIVE EFFECT OF A COLUMNAR CACTUS ONITS NURSE PLANT

A field study was conducted in a semi-arid tropical ecosystem in Mexic o to test whether competition for soil water is the causal mechanism u nderlying the negative effect of the columnar cactus Neobuxbaumia tete tzo on its nurse plant Mimosa luisava and to examine how this relation ship varies over time. The effect of irrigation was evaluated by recor ding the production of leaves. modules (i.e. internodes with an auxill ary bud), inflorescences and fruits in shrubs growing either isolated or associated with juvenile or adult columnar cacti. 4 001 of water, i n five doses of 801 each every 15 d. were added to the treatment plant s; no water other than rainfall was added to control plants. Additiona lly, to evaluate how the effect of the columnar cacti on the shrubs ma y vary among years we made a comparison of the production of plant str uctures between 2 years of contrasting rainfall. The irrigation treatm ent increased the production of modules, inflorescences and fruits, bu t not of leaves. Shrub response to watering was also dependent on clas s of association: those associated with juvenile cacti showed a higher response to irrigation than any other treatment. Our results show tha t water addition increases the production of structures and partially reduces the negative effect of the cactus on nurse shrub, thus support ing the hypothesis of competition for water. The negative effect of th e cacti on their nurse plants was present during both years of observa tions. but the intensity of the negative effect varies from relatively wet to dry years. The results are discussed in relation to how tempor al changes in resource availability affect the results of competitive interactions and the importance of this mechanism in the structure and dynamics of this dryland community. (C) Elsevier, Paris.