Print PageThe vegetation of the Sonoran and Mojave deserts is characterized by widespread shrublands dominated by Larrea tridentata (creosote bush) and Ambrosia dumosa (burrobush). These shrub species create fertility islands that influence the distribution of native and invasive annual plants. Spatial patterns in the distribution of shrubs may subsequently play an important role in the spread of wildfire. Non-native species often grow further from the influence of shrub canopies, potentially enhancing fire risk by creating a more continuous matrix of flammable material.
As a first step in characterizing species distributions and fire risk within these desert systems, we sampled approximately one hectare of creosote shrub land in each desert, recording the GPS locations of all Larrea and Ambrosia shrubs. Canopy volumes for each plant were also determined. We then applied univariate and bivariate second-order point pattern analyses to characterize the spatial patterns and interactions among shrubs. The relationships between shrub volume and density were further explored through the construction of Voroni tessellations and mark-correlation analysis, an extension of standard point-pattern analyses that that takes differences in size into account.
Results/Conclusions
In the Sonoran site, Ambrosia is rare. The average canopy volume of Larrea was 1.85 m3, with a density of 0.066 shrubs/m2 and a significant pattern of inhibition to about 2.5m. In the Mojave site, Ambrosia and Larrea were both found in abundance. Here, Larrea were much larger (mean volume of 4.84m3) than in the Sonora, less dense (0.0281 shrubs/m2) and exhibited inhibition at larger scales (significant to about 3.5 m). Ambrosia, a smaller shrub (mean volume of 0.082 m3) was more abundant (density of 0.101 shrubs/m2) and was significantly clustered at ranges under 4m. Bivariate analyses of Larrea and Ambrosia in the Mojave revealed a pattern of inhibition to about 3 m. In the construction of Voronoi cells and the mark-correlation analyses, we found a significant positive relationship between the size of a shrub and its neighborhood size for both sites.
The overall interpretation of our findings is that the spatial pattern in the distribution of Larrea and Ambrosia suggests that competition for resources plays a major role in structuring the distribution of shrubs in these desert systems. This should also have significant effects on the distribution of native and invasive annual plants and the potential for fire risk.
