Press-pulse interactions and long-term community dynamics in a Chihuahuan Desert grassland

By Scott Collins, Alesia Hallmark, Tim Ohlert, Anny Chung, Lauren Baur & Jennifer Rudgers

Chihuahuan Desert grassland dominated by black grama (foreground) and Great Plains grassland dominated by blue grama (background) at the Sevilleta National Wildlife Refuge in central New Mexico, USA. Photo credit: Lauren Baur.

Understanding environmental changes requires long-term observation, and the line-intercept dataset at the Sevilleta Long Term Ecological Research site in New Mexico represents 30 years of plant community data at a very fine spatial scale. To collect these data, we laid out 400 meters of tape at two locations, and recorded what plant species or other ground cover was beneath every centimeter of each tape. One location is in an area of Chihuahuan Desert grassland, while the other is at the transition, or ecotone, between the Chihuahuan Desert and Great Plains grassland. The dominant plant species in the Chihuahuan Desert grassland is black grama (Bouteloua eriopoda), a desert grass whose range extends south into Mexico. The ecotone site contains black grama as well as its close relative, blue grama (Bouteloua gracilis), a prairie grass whose range extends north into Canada. These data have been collected in spring and fall every year since 1989.

In 2009, a wildfire burned the ecotone site, drastically reducing grass cover. Before the fire, black grama had been increasing in dominance over time. Both blue and black grama were hit hard by the fire, but blue grama quickly returned to its pre-fire abundance. However, black grama has not been able to recover its former dominance, and now the two species are about equal in cover. In other words, the sudden “pulse” event of the fire caused species reordering via its extreme negative effect on black grama.

Meanwhile, at the unburned desert site, grass cover has decreased over the past few years, while forb cover has been increasing. Specifically, black grama is declining, while blue grama has maintained a low but stable cover. These recent years correspond to a warm phase of the Pacific Decadal Oscillation. In general, we found that black grama is more sensitive than blue grama to such directional climate shifts, or “press” interactions.

As aridity in the southwestern US increases with climate change, we might expect black grama, a desert-adapted grass, to outcompete blue grama, more at home in cooler prairie regions. However, our analysis suggests that the future composition of southwestern ecosystems will depend on a complex interaction of both press and pulse events.

This is a plain language summary for the paper of Collins et al. published in the Journal of Vegetation Science ( The summary was prepared by Lauren Baur.