Desert annuals flower profusely in shale barrens

By Jeff Mitton

From my tent site on Little Wild Horse Mesa, I was driving, ever so carefully, south along Little Wild Horse Road with the intent of crossing the Muddy River to continue on to Factory Butte and then the Waterpocket Fold.

I was in for a pleasant surprise.

Before leaving my tent site, I had taken photos of the wildflowers brought out by the wet winter and spring. Although it was an arid site, with no trees or bushes nearby, the mesa was lit up by wildflowers. The deep purple flowers of scorpionweed and the tiny yellow flowers of desert trumpet were in every direction, and prairie wild onion, prince's plume, yellow cryptanth and globe mallow were also conspicuous.

After breaking camp, I drove south, descending from Little Wild Horse Mesa to a multicolored landscape of bentonite clays called the Moroni slopes, fascinating and bizarre for their erosion gullies exposing layers of contrasting colors. After crossing the Moroni slopes, I entered a grey landscape of Mancos shale dominated by a towering formation named Factory Butte. I refer to this area as the shale barrens, for it supports almost no plants.

Due to their lack of stability and high mineral concentrations, bentonite clays and Mancos shale are pernicious to most species of plants. Bentonite clays are derived from the weathering of volcanic ash in the presence of water, and they are placed into groups depending on whether they have high levels of potassium, sodium, calcium or aluminum. Mancos shale is composed of mudstone and has high concentrations of selenium toxic to most plants.

Bentonite is infamous for swelling when wet and shrinking when dry, and the shale is inimical to many plants because it slides so much; measurements taken around Factory Butte revealed that the shale on the hillsides is migrating about 5 centimeters per year. Soil swelling, shrinking and sliding breaks roots, excluding perennials.

Descending from a hill devoid of plants into a small valley in the shale barrens, I was stunned to find a profusion of bright yellow and purple flowers. I stopped to enjoy the spectacle. The flowers were only on the gentlest slopes, and the bottom of the valley and several valleys were graced by the same bright but limited plant community.

The yellow flower was Rocky Mountain stickweed, Cleomella palmeriana, which is endemic to the Colorado Plateau. Its bright flowers contrasted with the shades of grey in the shale sediments, turning shale barren valleys yellow for miles.

The plant bearing rich purple flowers was brittle phacelia, Phacelia demissa, and it reminded me of the African violets that were so popular as houseplants decades ago. But brittle phacelia does not look like a plant adapted to toxic soils and an arid environment, for it has relatively large, round leaves, rather than the very narrow leaves of many desert plants adapted to retain as much moisture as possible. Round, soft leaves work well because this plant germinates and blooms in moist springs but shrivels and dies by mid-summer and thus has no need for adaptations to endure the hot, dry summer.

Web blogs for the area around Factory Butte documented the explosion of these flowers in the past, but mentioned that this pair of annual plants only appears occasionally, perhaps two out of every 10 years, following exceptionally moist winters. So perhaps they are more aptly intermittent annuals.

Shale soils seem to foster the evolution of endemic plants. In the last 15 years, two new species (Lone Mesa snakeweed, Gutierrezia elegans, and cushion bladderpod, Physaria pulvinata) have been described from the shale soils at Lone Mesa State Park, near Dolores. More locally, Bell's twinpod, Physaria bellii, is limited to the tiny outcroppings of Niobrara and Pierre shale between Denver and Fort Collins. These three species plus Rocky Mountain stickweed and brittle phacelia are specifically adapted to and limited to shale soils.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Daily Camera.

June 5, 2014