Creosote germination across a grassland transition at the Sevilleta National Wildlife Refuge, NM

Shrub encroachment alters community dynamics and ecosystem functioning worldwide. Grazing increases shrub encroachment as herbivores selectively consume grasses, easing the advancement of shrubs. Diversity decreases with shrub invasion and species richness drops by half when semiarid grasslands become shrublands. A shift in dominance from grasses to shrubs changes community structure and lowers community stability by increasing species turnover. Transition to shrubland also increases regional winter low temperatures by nearly 2˚C. Soil resource distributions change in invaded grasslands as shrubs create islands of fertility and alter nutrient cycles. Soil under plants has higher infiltration rates than interspaces and at the SNWR shrublands have twice as much interspace area than grasslands, leading to higher runoff. Greater runoff also leads to greater soil and nutrient removal. Shrub encroachment in the Southwest has increased dramatically over the past 150 years and is presumably linked to intensified human impacts. One local example of shrub encroachment is the expansion of creosote (Larrea tridentata) within Chihuahuan Desert grasslands. Chihuahuan Desert grasslands stretch from central Mexico to central New Mexico and are dominated by black grama. Winter low temperatures limit northward creosote expansion. Over the past century winter low temperature have increased 2˚C within some areas of the Southwest. With nighttime temperatures predicted to continue increasing, temperature limitations of creosote distribution may be relaxed. Within central New Mexico lies the transition between Chihuahuan Desert grassland and Colorado plateau shortgrass steppe, dominated by blue grama. Although temperature and climate are similar at the boundary between these ecosystems, creosote is only found in black grama grasslands and not in blue grama grasslands. Soil texture varies between the two grasslands and has been suggested as a major factor restricting creosote expansion along the transition. However, the affect of soil texture on creosote expansion has not been directly tested. The main research questions were: i) Can creosote germinate in blue grama grasslands? ii) Does grazing influence creosote germination in semiarid grasslands?