Using aerial imagery and a novel landscape competition index to quantify shrub-shrub competition in the Jornada Basin

Background/Question/Methods

The Jornada Basin Long-Term Ecological Research Site (JRN-LTER, or JRN) is a semi-arid grassland-shrubland in southern New Mexico, USA. The JRN has seen significant shrub encroachment during the last 150 years, particularly by honey mesquite (Prosopis glandulosa). The initial causes of shrub encroachment are thought to be a combination of drought and overgrazing, with controls on shrub establishment largely due to competition with grasses and other demographic bottlenecks. However, evidence suggests that at later stages of shrub encroachment in a system, maximum shrub cover could be limited by shrub-shrub competition. Building on previous studies of lateral root distributions and a detailed map of shrub sizes and density (Ji et al., 2019), we developed a simulation model quantifying a landscape-scale (1 ha) metric (the "competition index", CI). The CI quantifies root overlap and thus potential intensity of competitive interactions. We calibrate values of the CI that represent intense competition for honey mesquite using values at selected sites where density-dependent limitation to growth was inferred earlier (Ji et al., 2019). We then map and compare intensity of shrub competition across the JRN-LTER and explore patterns across the geomorphic provinces within the basin.

Results/Conclusions

While the competition index (CI) is computed using location and size of individual shrubs, we find a close correlation between CI and shrub cover, indicating that the spatial distribution of individual shrubs is less important than the total cover. Based on high density sites likely undergoing self-thinning competition, we conclude that at said high density sites, the distinct physiological and morphological traits of honey mesquite translate to CI values representing "intense competition" and limitations to shrub density and canopy cover. Maps of CI across the Jornada Basin show that shrub-shrub competition is generally quite low, but with isolated locations where intense competition is limiting further shrub expansion. Comparing CI across physiographic provinces show distinct differences in average CI values. Low basin-wide levels of competition and the associated relationship to landforms suggest that demographic bottlenecks are more likely to provide constraints on honey mesquite growth than intraspecific competition.