Re. Miller et Lf. Huenneke, The relationship between density and demographic variation within a population of Larrea tridentata, SW NATURAL, 45(3), 2000, pp. 313-321
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.