An Introduction to Desert Sagebrush and its Evolution | KCET
An Introduction to Desert Sagebrush and its Evolution
Author Stephen Trimble calls the Great Basin desert the "Sagebrush Ocean," and points out that if you choose your path carefully you can walk all the way across that great northern desert in any direction without leaving sagebrush too far behind. With the shrubs filling all but the lowest, most saline parts of Great Basin valleys, sagebrush country can indeed seem a great gray-green sea dotted with mountainous "islands."
This sea may appear monotonous to the casual observer, but it turns out sagebrush is a pretty complex thing. Far from being a wild botanical monoculture, sagebrush has a surprising amount of biodiversity, recently evolved -- and still evolving as the Western landscape changes.
Sagebrushes as a group may well be the most common shrub in the Western United States. Somewhere around 100,000 square miles of western North America is clothed in sagebrush habitat. In parts of California's northern deserts, sagebrush can seem to mantle every square foot of land in sight, either in solid stands or interspersed with grasses, buckwheats and rabbitbrush, or piñon and juniper woodlands.
Sagebrush grows in other parts of California as well. In fact, in California it almost seems a stretch to think of sagebrush as a desert shrub. The most common species, big sagebrush, is found throughout the interior of California, in the Sierra Nevada and Cascade ranges as well as the northern deserts. It grows close to the coast in the mountains above Santa Barbara, Los Angeles, and San Diego. And though the presence of sagebrush is generally taken as a sign that you're in the cold Great Basin Desert rather than the hotter Mojave or Sonoran deserts, you can find Artemisia tridentata in those warmer deserts as well, at higher elevations in mountain ranges such as the New York Mountains in the Mojave Preserve and the Pinto Mountains in Joshua Tree National Park.
There are about a dozen species of sagebrush, the precise number depending on which botanical authority you adopt, and a number of distinct subspecies among them. They range from huge shrubs that can reach ten feet high where there's a little water, to the diminutive pygmy sagebrush no more than a few inches tall.
Despite the common name they share, none are closely related to true sages -- nor do they really resemble the sages in odor all that closely, if you ask me. All of them belong to the genus Artemisia, which they share with wormwood and tarragon. Big sagebrush, or Artemisia tridentata, has a number of subspecies, of which the revered Jepson Manual lists four as native to California. The USDA's Plants Profile web database adds a fifth. Jepson considers that one a full species in its own right.
Along with Artemisia tridentata out there in the California desert are its close cousins black sagebrush, little sagebrush, and silver sagebrush. Rothrock sagebrush grows in the upper elevations of deserty mountain ranges like the Whites, Inyos, and San Bernardinos. Bigelow sagebrush is known from the Inyo and Last Chance mountains near Death Valley, and from a few ranges in the Mojave Preserve -- and from suitable places farther east through the Four Corners region into the Texas Panhandle.
There are likely other species in the California desert as well, perhaps even well-documented ones. If you haven't figured it out already from the tortured prose in the preceding two paragraphs, the status of most sagebrush species is subject to revision and argument. It doesn't help much that many of the species hybridize readily with one another, creating plants that are intermediate in form and appearance and which can often set fertile seed, muddying the desert ground even further. Three different references can refer to the same population of sagebrushes with three different Latinate monickers. It's enough to give a beginning sagebrush aficionado fits.
Some of what distinguishes the different species and subspecies is that each tends to be adapted to a distinct set of environmental conditions. Of three subspecies of Artemisia tridentata, subspecies wyomingensis does better in low, arid elevations, vaseyana prefers higher-elevation, better-watered places; and tridentata -- whose full name is Artemisia tridentata tridentata -- splits the difference Goldilocks style, preferring locales that are neither too hot nor too cold but just right. Black sagebrush tends to like higher elevations and alkaline soils. Rothrock sagebrush needs deep mountain soils. Each taxon has its preferred digs.
It isn't just temperature and water that determines where sagebrush grows. Other environmental factors play a role as well, often by working to exclude sagebrush. Sagebrush used to grow thick in the Mid-Hills area in the Mojave Preserve. I drove through there along Wild Horse Canyon Road about ten years ago, keeping my pickup at 15 miles an hour so as not to kick up road dust, and the big sagebrush lined the road in drifts three feet tall. I slowed even more so that I could smell it, sun-warmed and wafting. A coyote crossed in front of me, sleek and gorged on cottontails: it bounced across the dirt road in seeming slow motion, a wary sidelong grin on its muzzle.
On June 26, 2005 all that sagebrush went away: it was consumed in the devastating Hackberry Fire, and hadn't yet re-sprouted the last time I visited.
I've talked to some who argue that sagebrush -- and other native plants in the Hackberry Fire area -- probably won't recover as long as the area is grazed. As it turns out, grazing is an issue for sagebrush across the west. Sagebrush isn't great for cattle or other livestock: its characteristic aroma comes from chemicals evolved to poison herbivores. Cattle will eat it if they have to, but enough of it will make them sick, kill off their gut bacteria, and generally cause them to Fail To Thrive.
About the only large herbivore that really does well on a sagebrush diet is the American pronghorn, which evolved along with the swarm of sagebrush species in the western half of North America and was able to evolve tolerance for the sagebrush's chemical warfare arsenal. Once abundant in California's Central Valley, the pronghorn was extirpated from most of the state in the 19th and 20th centuries. Some still hold on in the Modoc Plateau and the backcountry in Mono County, and a few have been transplanted from Modoc to Carrizo Plain and similar spots. Very few animals are as closely associated with sagebrush as the pronghorn. Driving the dirt roads between Bridgeport, CA and Hawthorne, Nevada some time ago, I came upon a lone pronghorn standing in a field of Artemisia tridentata, looking utterly unconcerned at my presence. With no sign of human activity around other than my truck and the road it was on, the scene could have come from the California of centuries ago. That kind of thing is regrettably uncommon now.
Other animals depend on sagebrush as well, notably the endangered but unprotected sage grouse, of whose diet of sagebrush leaves constitute about two thirds, with the rest consisting of other leaves and the occasional insect. A sage grouse population in California, in the Mono Basin, turns out to be genetically distinct from other sage grouse. It may eventually be declared a new species.
As for other animals, rodents and smaller birds eat sagebrush seeds with abandon, and many seem to enjoy young leaves.
Even though sagebrush isn't your typical bovine's preferred meal, heavy grazing can damage sagebrush cover not only through direct consumption but through trampling -- sagebrush tends to have weak, easily splintered wood -- as well as through compaction of soil and altering of surface hydrology. Sheep grazing in the Warner Mountains has been implicated in the decline of sagebrush meadows, which are being "invaded" by fir trees. And of course, the usual threats to desert landscape elsewhere -- energy extraction, urbanization, and other industrial development -- threaten swathes of sagebrush as much as they do any other type of desert vegetation.
Industrial interference in the world of sagebrush is about a century and a half old. But the wild, wooly, ungainly tribe of sagebrushes may be better equipped to adapt to disruption than most other desert plants. It's thought these days that the ancestors of today's sagebrush species arrived from Asia at some point during the Pleistocene -- or less than 2.5 million years ago, which would make the tribe pretty much newcomers to the North American flora. (If you ignore the many species added in the last 500 years, that is.)
When the ancestors of today's sagebrushes got to what we now call the Great Basin, they would have found before them a broad country growing increasingly arid as the Sierra Nevada and Cascades grew taller, cutting off the flow of moisture from the Pacific. Armed with adaptations to cold that would have proved handy against drought, with small seed that could be quickly dispersed by wind, water, and animals, the paleo-sagebrushes would have exploded across the west, finding new habitats to shape their descendants' evolution. What resulted was what the plant ecologists call a center of diversity for the sagebrush tribe: an engine of evolution, distilling potential new species and subspecies from each different type of habitat. The fact that sagebrush is wind-pollinated helped keep isolated populations from dying out, remixing their uniquely evolved traits back into the main gene pool.
The result? A nearly continent-wide swarm of plants whose differences and similarities are so complex that scientists will likely be arguing about species and subspecies divisions within that swarm until there are no more scientists. And the sagebrush will live on as the scientists argue, scenting the Great Basin air with its defensive weaponry, and providing an example of evolution in action to make the most ardent creationist stammer.
Chris Clarke is an environmental writer of two decades standing. Author of Walking With Zeke, he writes from Palm Springs regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.
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