LITHOSOLS AND THEIR SPRING BEAUTY

In the Miocene, about 17 million years ago, great lava flows spread across the interior of the Pacific Northwest, year after year and flow after flow, reaching depths of 6000 feet in some places. As the molten rock cooled, it left very extensive areas of basalt. Some of these rocky areas remain as cliffs, too steep to support soil. In the flatter areas, breakdown of the bedrock produces shallow volcanic soils supporting steppe and shrub-steppe habitats.

Where the soil remains shallow and rocky, there is a rich and diverse plant community, often dominated by scabland sagebrush (Artemisia rigida) and grasses. Even though they remain hot and dry for much of the year, in the spring these lithosols come alive with wildflowers, as do the sandy soils around them dominated by big sagebrush (Artemisia tridentata).

Two of the most showy lithosol species are Simpson’s hedgehog cactus (Pediocactus simpsonii) and bitterroot (Lewisia rediviva). Both of them can be locally abundant on rocky soils in the Northwest interior, adding splashes of pink or red to a landscape that features all the flower colors of the rainbow.

The cactus is typical of its group, with a deep taproot that gathers limited water and nutrients, a succulent stem protected from herbivores by fierce spines, and no leaves. The showy flowers are produced in spring when the bees that pollinate them are most likely to be active in this hot, dry environment. Many other small insects feed on the pollen and/or nectar but are probably not effective pollinators.

Bitterroot is stemless and leafless, the clumps of flowers growing up from a fleshy taproot that anchors the plant firmly among the rocks. This may be another strategy to avoid herbivory—put most energy into reproduction without supporting photosynthetic leaves that are attractive to plant-eaters. The only thing edible on the plant, the succulent root system, is well protected in the soil, but Native Americans ate it as an occasional delicacy. During the Lewis and Clark Expedition, Meriwether Lewis ate bitterroot and brought back specimens; he was honored by its generic name.

Bitterroot survives without apparent chlorophyll by utilizing CAM (Crassulacean Acid Metabolism) photosynthesis, in which leaf stomata open only at night, when water loss is reduced, to take up CO₂ that is then released the next day and converted into sugars by chemical pathways that do require light energy.

These flowers and their habitats are best seen in April and May. As the air heats and the soil dries, this large part of the Columbia Basin seems to retreat back into dormancy.

Dennis Paulson