In the Chihuahuan desert there are two distinct vegetation types – black grama grassland and creosote shrubland (dominated by Bouteloua eriopoda and Larrea tridentata, respectively). Over the last century, extensive areas of black grama grassland have been invaded by creosote, resulting in shifts in plant species composition, alterations in biogeochemical cycles and forage quality and quantity for herbivores, and increased erosion potential. Although the effects of this invasion are fairly well-known, the mechanisms by which they occur are not. We conducted a field study that was designed to address multiple questions. First, does plant species diversity differ between the two communities and what are the temporal dynamics of these communities? Second, how does the heterogeneity of soil resources differ in shrubland vs. grassland, and can this heterogeneity can be reduced by the addition of nutrients to intershrub spaces? Third, how will species diversity and the spatial distribution of soil resources respond to removal of the dominant plant species (L. tridentata from desert shrubland and B. eriopoda from desert grassland)? Fourth, why is L. tridentata able to invade grassland, but B. eriopoda is unable to invade the shrubland (is there seed limitation or problems with establishment)?
Results/Conclusions
Species diversity (measured as species richness) did not differ statistically between shrubland and grassland sites, even though species composition of each site was distinct. Forb richness was highest in grassland removal plots and lowest in shrubland removal plots while percent cover of forbs was highest in grassland removal plots and lowest in grassland control plots. The addition of nutrients resulted in an increase in soil NH4-N and NO3-N in all plots, as well as forb and overall vegetative cover in the grassland plots but not the shrubland plots. In general, soil resources (NH4-N, NO3-N, organic matter) were greater in the grassland site, while soil moisture was greater in bare (unvegetated) soils than beneath vegetation at both sites. Seed addition did not increase cover of either species.
These results indicate that black grama may compete with forb species while creosote may facilitate the establishment/persistent of forbs. Following disturbance, species richness at the grassland site increased while richness at the shrubland site decreased, indicating that intact black grama grassland is more resilient to disturbance than desert shrubland.