Grasslands at Chaco, Petrified Forest and Wupatki, 2007–2021

Penstemon blooming near Fajada Butte, Chaco Culture National Historical Park. NPS.

Semi-arid grasslands and shrublands cover much of the land in the parks of the Southern Colorado Plateau, and they are central to those parks' natural and cultural landscapes. In many of them, long protected from livestock grazing, these grasslands may look much as they did before European expansion. Grasslands are declining worldwide; in these parks they shelter birds, mammals, reptiles and insects, add to the parks' diversity, and — by holding the soil in place — protect fossils, archeological sites and cultural artifacts.

The National Park Service's Southern Colorado Plateau Inventory and Monitoring Network (SCPN) began monitoring these systems in 2007, recording the structure, abundance, species and richness of upland vegetation, the soil surface and soil stability. This brief reports the first 15 years, 2007–2021, at three parks:

• Chaco Culture National Historical Park (CHCU);
• Petrified Forest National Park (PEFO);
• Wupatki National Monument (WUPA).

Crews measured 150 plots — 30 in each of five grassland and shrubland ecological sites, classified using the Natural Resources Conservation Service system.

Key findings

• Warm-season grasses dominate, making up most of the cover.
• Nonnative plants are uncommon; the most frequent were Russian thistle and cheatgrass.
• Perennial grasses declined — slightly, but significantly — over the 15 years.
• Climate had mixed effects: drier, hotter springs tended to reduce plant cover, while hot, dry Julys tended to increase it.

Vegetation

Crews identified 283 plant species. Live vegetation covered 12–24% of the ground.

• Warm-season (C4) grasses, adapted to warmer conditions than cool-season (C3) grasses, made up more than two-thirds of the vegetation at four of the five sites (over 70% of live foliar cover). At one Wupatki site, shrubs shared dominance.
• Forbs (flowering plants) were a small share of cover but supplied most of the diversity.
• Under 4% of species were nonnative. Russian thistle (Salsola tragus) was the most often found, in more than half the plots at Wupatki's volcanic upland site; cheatgrass (Bromus tectorum) came second but appeared in fewer than 10% of plots at every site.

Snakeweed. NPS.

Soils

Soils at Petrified Forest and Chaco were deeper than at Wupatki. Physical and rudimentary soil crusts covered most of the soil surface, except at Wupatki, where surface gravel dominated. Biological soil crust — cyanobacteria, lichens and mosses — was sparse, and soils were moderately stable.

Trends over 15 years

Bayesian hierarchical models, accounting for year and each site's seasonal water stress and topography, found:

• perennial grass cover fell at all five sites, the clearest result;
• shrub and forb trends were mixed;
• species richness fell at Chaco's sandy loam site but rose at Petrified Forest's sandy loam site and Wupatki's volcanic site.

Direction of trends in plant cover by site: green, increasing; red, decreasing; gray, not significant; blank, not modeled. NPS figure.

Climate effects

The climate variables affected plant cover in different directions. The most consistent patterns:

• more water stress in July and more spring days warm enough for growth (accumulated growing degree days) increased perennial grass and shrub cover in the same year;
• more water stress in spring reduced many measures.

Direction of climate effects on modeled responses: green, positive; red, negative; gray, not significant; blank, variable not in that model. NPS figure.

Hotter, drier conditions from human-caused climate change are predicted to reduce perennial grasses and increase shrubs and weedy forbs in Southwestern grasslands. The perennial grass declines seen here fit those predictions, and continued drought will likely make the changes worse.

What it means for management

With grass cover declining but nonnative plants still scarce, limiting disturbance — including trespassing livestock and new roads or trails — is important. Protecting these sites from further decline, together with careful restoration, will help keep the ecosystems stable as the climate changes.

Sources

• National Park Service, Southern Colorado Plateau Inventory and Monitoring Network: "Change Over Time in Semi-Arid Grassland and Shrublands at Three Parks: Chaco Culture National Historical Park, Petrified Forest National Park, and Wupatki National Monument," resource brief prepared by Christopher Calvo, January 2024, with the full report it summarizes.