Saltbush evolved to filter dust from wind to form mounds
The wind that sculpts the stones also conspires with plants to transform a valley floor to a landscape of mounds with embedded plants
Cathedral Valley, in the northern, primitive portion of Capitol Reef National Park, features magnificent sandstone walls and spires. During a recent visit, I found that the wind sculpting the stones also conspires with plants to transform a valley floor to a landscape of mounds with embedded plants.
The mounds are one to three feet tall, and they bristle with vertical branches bearing leaves and tiny flowers.
I checked the lists of plants for Capitol Reef NP and found that these were Gardner's saltbushes, Atriplex gardneri, which grow on salty and alkaline soils from British Columbia and Saskatchewan to Nevada, Utah, Colorado and New Mexico.
The process of entrapment of dust to mounds around plants eventually causes the demise of the plants, for as the mound grow taller, salts become concentrated at the top of the mound, excluding plants. After the plants die, wind erodes the mound, exposing crust again. This is a recurrent, cycling process.
A deep taproot supports a woody base that branches profusely, producing many vertical branches with grayish green, rounded leaves. This plant has remarkable variation, both within populations and across its wide geographic range.
Seven varieties (similar to subspecies) are recognized, and most are separated geographically and adapted to different habitats. Plants can be annual or perennial, exclusively male, exclusively female, or bearing both types of flowers. Gardner's saltbush can have two copies of each chromosome (as is the case for humans), which is denoted 2n, or they can be 4n, 6n, 8n or 10n. These different chromosomal forms are usually adapted to different habitats.
Perplexing variation within populations is attributable to crossing among chromosomal forms and hybridization with other saltbush species. In Capitol Reef National Park, Gardner's saltbush grows with four-wing saltbush, Atriplex canescens, shadscale, A. confertifolia, and mat saltbush, A. corrugata.
It hybridizes extensively with shadscale and mat saltbush, so intermediate forms occur as well. Incidentally, shadscale has multiple chromosomal forms as well: 2n, 4n, 6n, 8n, 10n, and 12n. All these sources of variation make identification of saltbush species a challenge, and even the professional systematists maintain different opinions of the formal name of Gardner's saltbush—some call it Atriplex gardneri, others insist it is Atriplex cuneata.
In northwestern Nevada, ecologists studied a salt-tolerant plant community occupying mounds around playas, which are ephemeral lakes in shallow desert basins from which water evaporates quickly. One species typically on the mounds is Torrey saltbush, A. lentiformis.
This empirical study found that wind-blown or eolian dust is very common, and it has high salt content. They found that plants, including Torrey saltbush, germinate in crevices in barren crust. As the plants grow, they entrap wind-blown or eolian dust that accumulates around the plants.
At the top of the page: Gardner's saltbush transforms landscapes by forming mounds. Above: The landscape as seen from a broader perspective. Photos by Jeff Mitton.
As the plants grow in height and diameter, the mounds grow to the point that they coalesce. The process of entrapment of dust to mounds around plants eventually causes the demise of the plants, for as the mound grow taller, salts become concentrated at the top of the mound, excluding plants. After the plants die, wind erodes the mound, exposing crust again. This is a recurrent, cycling process.
Similar processes form mounds in Cathedral Valley. Some of the areas around the Temple of the Moon are barren soil crusts with numerous, conspicuous cracks reaching about 3/8 inches into the crust. When occasional snowfall or thunderstorm brings moisture, it would seep into these cracks, where it would evaporate more slowly than on crustal sheets exposed to sun and wind.
A seed blowing into the crack could get a good start, sending up a shoot, developing leaves and sending roots down to find moist soil. Gardner's saltbush has a taproot, and its roots can reach a depth of 4.5 feet. As the leaves proliferate, they baffle the wind, causing dust to accumulate beneath the plant. When a twig contacts the accumulating soil, it develops adventitious roots, allowing the plant to grow in diameter.
The mound would grow in diameter as the root system expanded outward and branches became more numerous. The soil, shaded by twigs and leaves, would retain snowmelt and rainwater much longer than the exposed desert crust, helping the plant survive and grow.
I was unable to find studies recording ages of individual plants, but repeat photography in the Grand Canyon identified four-wing saltbushes over 100 years old.
Plants that are more effective in baffling wind to collect dust and pile it as soil would grow larger and probably be able to produce more seed. Natural selection, driven by differences in reproductive success, makes the next generation of plants similar to the plants that had the highest reproductive success in the previous generation.
Gardner's saltbush has evolved to manipulate the wind, improve water retention and to continue to grow larger as it harvests soil from the wind.