Mat saltbush thrives on Mancos Shale
Mat saltbush is not a glamorous, attractive plant, but it has the distinction of filling a salty shale environment where no other plant can live
Plant communities are usually nicknamed for the most dominant species, such as a ponderosa pine community or a pinyon-juniper woodland. Mat saltbush, also called matscale, communities live on such harsh soils that they are sometimes the only species in in the community.
Mat saltbush is found most commonly on Mancos Shale, which occurs in the Four Corner states (Colorado, Utah, Arizona and New Mexico) and Wyoming. A community composed solely of mat saltbush is an eerie sight, for the soil is mostly bare and adjacent saltbushes might be 30 feet apart. Soil color ranges from light gray to white, and the saltbush foliage is a light gray-green, so the habitat appears ghostly, unhealthy and foreboding.
Mat saltbush thrives on Mancos Shale in the Four Corner states and Wyoming. Photo by Jeff Mitton
Mat saltbush, Atriplex corrugata, is a native shrub that reaches only 6 inches tall but it grows as a mat, with new roots sprouting from peripheral branches that touch the ground, so it might be 10 to 20 times as wide as it is tall. So saltbush shrubs are clones, slowly expanding in an inhospitable environment. Saltbush is perennial, living several decades and can be diploid (two copies of each chromosome) or tetraploid (four copies). The ecological consequences of doubling the genome are unknown.
Mancos shale was formed in the salt marshes and at the bottom of the North American Inland Sea during the Mid-Cretaceous, from 64 to 144 million years ago. Virtually nothing grows on Mancos shale on steep hillsides, such as those on the Book Cliffs, from Grand Junction, CO to Green River, UT. Similar cliffs are found between Capitol Reef National Park and the Henry Mountains. Although tall, steep slopes are barren, mat saltbush grows on these same soils at the bottom of the cliffs, on flatter sites.
Mancos Shale is inhospitable not because it has concentrations of minerals or nutrients too high or too low, but due to its mechanical properties. Soil instability becomes a major problem on steep hillsides, for the top layer of sediment slides downhill at 1 to 2 centimeters per year, but the underlying layer slides more slowly, and deeper yet the rate is even slower. Differential sliding of layers snaps the roots of any plants that germinate and begin to grow. In addition, Mancos Shale weathers to very fine-textured particles, that have a tendency to become compacted, restricting the diffusion of oxygen to growing root tips.
Still another problem that haunts some — but not all — Mancos Shale sites is salt. Arid, poorly-drained sites with high rates of evaporation accumulate salts, and in valley bottoms where the water table is close to the surface, salts migrate toward the surface. High concentrations of soluble salts exacerbate osmotic stress, making it difficult for plants to extract water from the soil. Mat saltbush persists on salty, shale sites because it is one of the most salt-tolerant plants in the west.
I took the photo of mat saltbush on the western side of Lake Powell, beside the road from Big Water to Smoky Mountain. The saltbush stretched for many miles — a stark landscape sustaining just one species. To get a better sense of this strange landscape, I walked among the ghostly plants for a while. Shiny flakes, 1 to 2 inches in diameter, littered the surface of the soil — gypsum, composed of calcium and sulfur, neither of which is necessarily inimical for plants.
Nevertheless, gypsum flakes reinforced my conviction that this was a unique environment. Mat saltbush is not a glamorous, attractive plant, but it has the distinction of filling a salty shale environment where no other plant can live.
Further evidence that Mancos Shale is a unique substrate for plants has been reported since 2006. Three new species of plants have been described that are endemic to, or limited to, Mancos Shale barrens in southwestern Colorado. These include a mustard, cushion bladderpod, Physaria pulvinata, and two asters — Lone Mesa snakeweed, Gutierrezia elegans, and Mancos Shale packera, Packera mancosana. All of these occur at Lone Mesa State Park in Dolores County, and only cushion bladderpod is known to occur outside of the park. Although mat saltbush can occasionally be found on other soils, it is also considered endemic to Mancos Shale.
Apparently, adaptation of plants to Mancos Shale virtually excludes them from other soils.