Competition between three Chihuahuan desert species: evidence from plant size‐distance relations and root distribution

Abstract

Abstract. Using size‐distance data we tested the intensity and importance of competition between Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihuahuan desert. We also compared the vertical and horizontal distribution of roots to assess the potential degree of overlap in the use of soil 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 stronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition involved always Opuntia, whereas Larrea and Hilaria did not influence each other. The analysis of the symmetry of competition showed that Opuntia was adversely affected by the presence of Hilaria or Larrea. Although 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.

Spatial Associations, Size–Distance Relationships and Population Structure of Two Dominant Life Forms in a Semiarid Enclave of the Venezuelan Andes

“…When the nearest neighbour of a focal plant was outside the limits of the plot, its size and the distance between them were recorded and included in the analysis (e.g. Briones et al 1996). Cactus individuals growing around or from fallen stems of cacti were recorded as vegetative propagation.…”

“…The normality of both variables, sum of sizes and distance, was obtained through a log 10 transformation. Since plant density and size may affect the slope of the relationship between sum of sizes and distances (Wilson 1991), data (log 10 sum of sizes and log 10 distances) were standardized by dividing each value by the average calculated within each species combination; thus, each standardized variable had a mean of 1 (Wilson 1991;Briones et al 1996).…”

“…However, recent meta-analysis suggests that some results will be unexpected and largely inconsistent with this hypothesis (Goldberg et al 1999;Maestre et al 2005). The effects of competition in arid plant communities can be inferred from the study of the relationship between distance and size of neighbouring plants (Pielou 1962;Fowler 1986;Welden et al 1988;Wilson 1991;Briones et al 1996). The hypothesis is that competition between neighbours results in density-dependent growth and mortality; thus, closely spaced plants will be small and have low survivorship (Pielou 1962;Fowler 1986).…”

“…The hypothesis is that competition between neighbours results in density-dependent growth and mortality; thus, closely spaced plants will be small and have low survivorship (Pielou 1962;Fowler 1986). A positive correlation between the sum of neighbour plant sizes and the distances separating them can be interpreted as the result of competition, and the lack of such correlation as the result of the absence of competition (Pielou 1962;Yeaton and Cody 1976;Briones et al 1996).…”

“…Dominant plants of these habitats, such as thorny mimosoid legumes and/or columnar cacti, can compete for water in some stage of their growth (e.g. Yeaton and Cody 1976;Briones et al 1996). Severe water conditions (xeric zones) could favor more facilitation than competition between neighbouring plants; thus, under benign water conditions (mesic zones), competition can be more intense than in abiotically stressful conditions (stress-gradient hypothesis, sensu Callaway 1997).…”