Mapping Gila Cliff Dwellings’ Plants in the Wake of a Wildfire

A hillside of shrubs and trees with burnt trees in the foreground. Image from the National Park Service's page on the project.

Vegetation mapping at Gila Cliff Dwellings National Monument was scheduled to begin in 2011, exactly one week after the Miller Fire ignited. The fire burned most of the project area and pushed the work back a year, so the crew ended up mapping plant communities that change quickly as they recover. Communities the fire had drastically altered got classes of their own, so the map can be updated as the area recovers.

What a vegetation map is for

A vegetation map shows how plant communities are spread across a landscape. Knowing what grows where, and what kinds of habitat a park holds, helps park managers with planning, resource monitoring, interpretive programs, prescribed fire and climate-change response, among other things. Such maps also give other ecological studies a baseline. Not every park used to have a current one, and the National Park Service (NPS) Vegetation Inventory Program aims to finish baseline mapping and classification inventories at more than 270 NPS units. Each map represents hundreds to thousands of hours of work by ecologists, field technicians, GIS technicians, data managers, editors and park staff.

How a map is built

The work starts in the field. Botanists visit a series of plots and survey every vascular plant species within different height categories. For each species they judge how much of the plot it covers and assign a cover class from 1 (under 1%) to 9 (96-100%), and the most abundant and important species also get a dominance rank. The plot data are used to sort vegetation into "associations", assemblages of particular plant species that repeatedly turn up in similar habitats. Aerial imagery then shows where each type occurs: the areas are drawn as polygons and organized into map classes, which are generally named for the plants that dominate them. Finally the team assesses the accuracy of the results, builds a geodatabase and a map, and writes a final report.

Botanists mapping vegetation at Gila Cliff Dwellings National Monument. Image from the National Park Service's page.

The Gila Cliff Dwellings project

The Sonoran Desert Network (SODN) mapped the monument from 2012 to 2015. The monument, in southwestern New Mexico, has two small units separated by two kilometers of U.S. Forest Service land. To make the products more useful, the project area took in both units plus an ecologically meaningful buffer, 1,047 hectares (2,587 acres) in all. The crew divided it into 203 map polygons sorted into map classes, and recorded 349 plant species. The communities ranged from lush riparian woodlands to drier upland woodlands and tree savannas with a grassland understory. The most widespread association was Two-needle pinyon - (Oneseed juniper, Alligator juniper) / Blue grama Open Woodland, at 34% of the project area, followed by Ponderosa pine Woodland and Tree Savanna at 14%.

The monument's vegetation map. Image from the National Park Service's page.

The associations on the map:

• Ponderosa pine Woodland and Tree Savanna
• Douglas fir - Gambel oak Forest
• Two-needle pinyon - Oneseed juniper (Gray oak) / Blue grama Tree Savanna
• Two-needle pinyon - (Oneseed juniper, Alligator juniper) / Blue grama Open Woodland
• Narrowleaf cottonwood / Dewystem willow Riparian Woodland
• Regenerating Narrowleaf cottonwood / Dewystem willow Riparian Woodland
• Fourwing saltbush Shrubland
• Alderleaf mountain mahogany Shrubby Rock Outcrop
• Rubber rabbitbrush Shrubland
• Gambel oak (Gray oak) Shrubland and Shrub Savanna
• Gray oak / Mixed Perennial Grass (Tarragon) Post-Fire Shrubland and Herbaceous
• Gray oak / Fendler's globemallow Post-fire Shrubland and Herbaceous
• New Mexico locust Shrubland
• Annual Forb - Mixed grama Herbaceous
• Post-fire Riparian Mixed Herbaceous
• Blue grama Grassland

The Miller Fire

The fire was first sighted around midnight on April 28, 2011, in the adjacent Gila National Forest. It burned across the park on May 10 and was officially contained on June 14, after burning a total of 35,950 hectares (88,835 acres). It burned the majority of the project area and delayed the mapping by a year.

Burned ground is hard to classify. Many plants died during or after the fire and were sometimes replaced by other species, and the first species to move in are not always the ones that stay, so the same area can pass through several stages in a short time. Waiting for full recovery was not a reasonable option, though: woody species can take up to a decade to reestablish and more years after that to mature. The team still felt confident in its classifications. It validated the mapping results with data from multiple years after the fire. Communities drastically altered by the fire stay in a post-fire "regenerating" state for many years, and these were mapped and described separately from their unburned or lightly burned counterparts.

How fire treats different communities

Woodlands. Fires in pinyon-juniper savannas are thought historically to have come every 10-30 years, usually as widespread, low-intensity fires that kept the stands open and grassy with scattered trees. Pinyon-juniper woodlands are now more common in the U.S. Southwest, however, and they often burn large areas as crown fires, with high overstory mortality. Healthy, open ponderosa pine stands are likewise largely maintained by repeated lower-intensity fires, which reduce surface fuels and the density of small trees while leaving the large trees. Both communities are adapted to fire and should recover naturally. They will likely gain shorter-term benefits too: nutrient cycling stimulates sprouting, and dead snags give cavity-nesting birds habitat.

Riparian areas. Fire is much less common along streams, and its effects are less well documented. In the Miller Fire the riparian area along the West Fork of the Gila River burned at moderate and high severity, and many overstory trees were completely consumed or killed. The two main species affected, narrowleaf cottonwood and dewystem willow, have since sprouted vigorously from their bases; after six years of growth the trees are 3-5 meters tall. Every five years SODN samples permanent monitoring plots in the upland and riparian areas of the park. They were sampled in 2012, one year after the fire, and again in 2017, and the plan was for a trend report, after sampling in 2022, to document vegetation change over time and highlight the effects of the 2011 fire.

Fire policy

In the last century fire suppression reduced the number of fires and the acreage allowed to burn, which is thought to have contributed to denser stands and fuel build-up in many western forests. The naturally ignited Miller Fire, by contrast, was allowed to burn with limited suppression apart from protecting human life and structures, a practice called "wildland fire use" or "wildfire use for resource benefit." Some research shows that fire severity is significantly lower in areas that have burned recently (less than 22 years between fires) than in areas that have not, which suggests that severity may fall as normal fire cycles are restored. SODN will keep monitoring vegetation and soils at the monument as part of its long-term vital signs monitoring program.

Sources

• National Park Service, Vegetation Mapping at Gila Cliff Dwellings National Monument; the brief summarizes S. E. Studd and J. Galvin, 2019, Vegetation Inventory, Mapping, and Characterization Report, Gila Cliff Dwellings National Monument; rewritten in hubnx's own words.