The encroachment of shrubs into adjacent grasslands is a phenomenon that has been reported across the world, especially in recent decades. A well-documented example of this encroachment is the expansion of creosotebush (Larrea tridentata), a native perennial shrub, into the arid and semi-arid grasslands of the southwestern United States. Invasions and encroachments such as this can be modelled as a moving wave that propagates through space and time, with patterns of conspecific density-dependence playing a key role because they are an important factor in determining whether the wave is pushed, pulled, or stagnant. Here, we apply this wave model to the creosotebush-grassland ecotones of the Sevilleta National Wildlife Refuge, an LTER site in central New Mexico. Using transects longitudinally placed along these ecotones, we collect yearly data on recruitment and demographic rates such as growth and reproductive efforts along a gradient of conspecific densities. These data are then used to investigate the relationship between spatial distribution of creosotebush and how its populations expand, with density-dependent demographic rates and recruitment being examined as a function of conspecific density.
Results/Conclusions
Our analyses indicate that annual growth and number of reproductive structures in creosotebush at the study site have a significant negative correlation with conspecific density, indicating that plants at the lower-density vanguard of the wave grow and reproduce more than plants that are located further back. There was little evidence for a significant relationship between density and probability of flowering, but plant size was found to be much a more accurate predictor of whether or not flowering occurs. Plant size was also significantly positively correlated with number of reproductive structures, but negatively correlated with annual growth. Recruitment patterns were similar to what was observed regarding how density relates to growth and reproductive structures, with the probability of a recruit being found at lower densities rather high and decreasing as conspecific density increases. However, at very low shrub densities, there actually appeared to be a positive correlation between density and probability of locating a recruit, indicating that Allee effects may play a role in the spatial distribution of these new plants. These results provide better knowledge of demographic patterns at the low-density vanguard of the invading creosotebush population and will play an important part in the application of the moving wave model to this system, helping to improve our understanding of how populations of this shrub spread.