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 pro cess of invading adjacent grasslands. To determine whether our hypothesis w as correct we developed a set of predictions about characteristics of invad ing populations and examined the relationship between Larrea density and de mographic variation. For an invading Larrea population we predicted that: 1 ) individuals in low density areas would exhibit attributes indicating high er intrinsic rates of increase-individuals would have greater fruit product ion, 2) individuals in high density areas would exhibit the effects of nega tive 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 biom ass than high density areas. We measured height, average width, canopy cond ition, and fruit set for 2,000 plants sampled from 40 plots varying in dens ity. 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 chara cters were significantly correlated with Larrea density and both measures o f microenvironmental variation were significantly correlated with density. In contrast, results for total Larrea biomass did not conform to our predit ion. Although the relationship between Larrea density and the plant charact ers supported our predictions for an invading Larrea population, results fo r total Larrea biomass did not. An alternative explanation that the pattern of demographic variation within the study population was determined by mic roenvironmental variation was more strongly supported.