Effect of nitrogen availability on spatial distributions of creosotebush (Larrea tridentata) in two desert ecosystems

Background/Question/Methods

In many desert plant populations, individuals appear to be uniformly distributed.  Creosotebush (Larrea tridentata) has been shown to exhibit this uniform distribution; however, little work has been done to determine the factors that lead to this pattern. We sampled two creosotebush populations in the Mojave Desert and two populations in the Sonoran Desert to confirm distribution patterns and to investigate the possible influence nitrogen availability may have on spacing patterns. We sampled twenty-five 10x10 meter plots at each site.  In each plot, we measured height, two diameter measurements, and distance to nearest neighbor for each plant in order to determine distribution, cover, and relationship between cover and nearest neighbor. The distribution was determined using a dispersion index and statistical significance was determined using a computer simulation.  Leaves were collected from each bush to estimate productivity during the previous year. We measured leaf and soil carbon and nitrogen content using an elemental analyzer.

Results/Conclusions

Three out of the four sites were significantly more uniform than would be expected by chance alone. The exception was one site in the Sonoran Desert that exhibited a random distribution. Our findings suggest that as distance to nearest neighbor increases, cover also increases, likely as a result of competition between plants. Average leaf carbon to nitrogen ratios were higher in the Mojave Desert than in the Sonoran Desert (30 vs. 18, respectively), indicating higher N availability in the Sonoran Desert. Average productivity was also higher in the Mojave Desert than in the Sonoran, suggesting productivity in creosote is not limited by nitrogen availability. These findings suggest that competition for nitrogen is not causing the uniform distribution of creosote bush. We conclude that creosote are in general uniformly distributed, however, we found little evidence that competition for nitrogen is the factor causing this distribution. Other possible factors we did not measure that are likely to influence creosote distribution include water scarcity, root system structure, and allelopathy.