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) and creosotebush (Larrea tridentata). The initial causes of shrub encroachment are thought to be a combination of drought and overgrazing. However, in the JRN-LTER, evidence suggests that maximum shrub cover is limited by shrub-shrub competition. This research explores shrub-shrub competition across the Jornada Basin, providing new estimates of its intensity and distribution. Building on previous studies of lateral root distributions and a detailed shrub map (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 potential intensity of competitive interactions. We calibrate values of the CI that represent intense competition for mesquite and creosotebush using values at 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 different communities of the JRN-LTER, and explore patterns emerging for different species on different soils and landforms.
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 the distinct physiological and morphological traits of creosotebush and honey mesquite translate to different CI values representing "intense competition" and limitations to shrub density and canopy cover. We explore the impact of parameterizing the CI model using data from 50-100 cm imagery (that can't resolve the smallest shrubs) with data from UAV (that can resolve shrubs < 50 cm). CI values calculated using UAV and NAIP are largely similar, suggesting the primary importance of adult shrubs in driving landscape scale competition, and that use of imagery with a resolution as low as 1 meter may be sufficient. Maps of CI across the Jornada Basin show that shrub-shrub competition is generally low, but with some locations where intense competition is limiting further shrub expansion.
