The relationship between density and demographic variation within a population of Larrea tridentata

Larrea tridentata is a dominant desert shrub throughout the warm deserts of North America. We studied a Larrea population we believed to be in the process of invading adjacent grasslands. To determine whether our hypothesis was correct we developed a set of predictions about characteristics of invading populations and examined the relationship between Larrea density and demographic variation. For an invading Larrea population we predicted that: 1) individuals in low density areas would exhibit attributes indicating higher intrinsic rates of increase - individuals would have greater fruit production, 2) individuals in high density areas would exhibit the effects of negative density-dependence - plants would have canopies in poor condition (with at least 20% dead branches) and be small in size, and 3) low density areas would be below carrying capacity - plots would have a lower total Larrea biomass than high density areas. We measured height, average width, canopy condition, and fruit set for 2,000 plants sampled from 40 plots varying in density. We also estimated the total Larrea biomass for each plot. We collected data on two measures of microenvironmental variation, cover of the common grassland species, and cover of gravel on the soil surface. All plant characters were significantly correlated with Larrea density and both measures of microenvironmental variation were significantly correlated with density. In contrast, results for total Larrea biomass did not conform to our predition. Although the relationship between Larrea density and the plant characters supported our predictions for an invading Larrea population, results for total Larrea biomass did not. An alternative explanation that the pattern of demographic variation within the study population was determined by microenvironmental variation was more strongly supported.