Effect of Neobuxbaumia Tetetzo on Growth and Fecundity of its Nurse Plant Mimosa Luisana

A field study was conducted in a semiarid basin located in the Tehuacan Valley in Central Mexico to determine the effect of the columnar cactus Neobuxbaumia tetetzo on the growth and fecundity of its nurse plant Mimosa luisana. The production of leaves, modules (internodes), inflorescences and fruits was recorded in shrubs of M. luisana growing either in isolation, or associated with small or large cacti. The effect of association with cacti was evaluated for three size-classes of shrubs. To test whether the cactus is likely to replace the nurse plant, a projection matrix was used to estimate the finite growth rates of the canopies of isolated large shrubs and of shrubs growing in association with large cacti. The production of leaves, internodes, inflorescences and fruits of shrubs was negatively affected by the presence of columnar cacti growing beneath the nurses' canopies. The negative effect of the cacti on nurse growth was significant in all cases, but it was more important in larger shrubs. The interaction term was also statistically significant in all cases, implying that the effect of the association varies depending on the size of both the shrub and the cactus. The finite growth rate of the canopy estimated for isolated shrubs implied positive growth (lambda = 1.402), while associated shrubs had negative canopy, growth (lambda = 0.933). The results show that large columnar cacti can indeed replace their nurse plants by increasing branch mortality and decreasing the production of canopy parts in the nurse. The importance of this association in the structure and dynamics of arid plant communities is discussed.