“No one will protect what they don’t care about, and no one will care about what they have never experienced.” — David Attenborough

By Dr. Cameron Barrows

In 1898, C. Hart Merriam published a hypothesis, describing the “life zones of the United States." In essence, he described life zones within which similar plants and animals could be found. These life zones followed a pattern dictated by changes in latitude, mirrored by changes in elevation, such that life zones ultimately were described based on changes in climate. At the highest elevations, highest latitudes, he defined the “Arctic-Alpine Zone," a largely treeless landscape comprised of dwarf willows and alpine flowers. Below the Arctic Zone was the Hudsonian Zone, a land where spruce and firs, high elevation conifers, could be found. Next was the Canadian Zone, also comprised of conifers, but with greater diversity and where pines would be dominant. Below the Canadian Zone was Merriam’s Transition Zone. Here was a region of overlap between species whose affinities were more northern (boreal) and those species with southern (austral) origins. Douglas fir, big cone spruce, redwoods, and sequoias, along with madrones, maples, alders, and azaleas find their homes in this Transition Zone. Next, lower in elevation and latitude, is the Upper Sonoran Zone. Live oaks, sagebrush, piñon pines, and junipers thrive here. Finally, Merriam defined the Lower Sonoran Zone, what we call desert, the land of agaves, yuccas, creosote bushes and cacti. Without crediting Merriam, if you ride the Palm Springs aerial tram and listen to the recorded narration on your upward journey, you will hear how your 10-minute tram ride duplicates the climatic zones and associated plants and animals you would encounter if you drove the more than 1,000 miles from the Mexican to Canadian borders.

Merriam’s life zones are too broad, too coarse and generalized to be of much value to a naturalist. For example, in Merriam’s Lower Sonoran Zone, today we fit the Mojave, Colorado, Sonoran, Chihuahuan, and Peninsular (Baja California) Deserts, each with a unique assembly of species. What he got right was the importance of climate in determining what can live where. In Whitewater and Mission Creek and Morongo Canyons at the western edge of the Coachella Valley, you can find, by nature’s standards, an almost knife-edged ecotone separating desert and coastal life forms. This is especially apparent on the east-west trending side canyons. The north-facing slopes are populated by live oaks, sugar bushes, junipers, and ceanothus shrubs, what Merriam would have called his “Upper Sonoran Zone.” These are fire-adapted plants that quickly resprout after wildfires. On the south-facing slopes are familiar desert plants, creosote bush, brittlebush, burro bush, Mojave yuccas, and if the winter-spring rains are just right, carpets of desert wildflowers. This is Merriam’s “Lower Sonoran Zone.”

These species have not adapted to fire, but the juxtaposition with the north-facing slope means fire can occur here nonetheless. When it does, you can see the “ghost of fires past” by the absence of species such as creosote bushes at lower elevations and blackbrush (Coleogyne ramosissima) at higher elevations. Here, the differences between the north and south facing slopes are not due to elevation or latitude; they are due to the amount of solar radiation that they receive. In the northern hemisphere south-facing slopes receive more sun, and so are hotter and drier than north-facing slopes. It’s all about climate.

These western canyons are of particular attraction to naturalists because so many different species occur in such close proximity. Western fence lizards (otherwise coastal and at higher elevations) almost side by side with desert spiny lizards and chuckwallas. Coastal bushtits, Nuttall’s woodpeckers, California towhees, and California thrashers occur with desert-loving ladder-backed woodpeckers, rock wrens, and phainopeplas nearby. Those phainopeplas live and breed in the desert in the winter and spring, then shift coastward in the summer where they can breed a second time.

There are other curious patterns to be found. There are two typical desert shrubs of the same genus (Encelia) called brittlebushes. Encelia farinosa occurs at lower elevations, while E. actonii (Acton’s brittlebush) occurs at elevations above 3,000 feet. Except in Mission Creek Canyon. There, Acton’s brittlebush occurs in the canyon bottom, while E. farinosa occurs higher on the south-facing slopes. Why the difference? I think I may have figured it out. Cold air sinks, and Mission Creek receives especially cold air sinking off the slopes of 11,503’ Mount San Gorgonio. Acton’s brittlebush is more cold-adapted, and so is better able to handle sub-freezing temperatures in winter. The lower elevation E. farinosa can’t handle frosts, and so is relegated to the warmer, even though higher elevation south-facing slopes. It’s a hypothesis, but there may be validation for that hypothesis in a side canyon off of Whitewater Canyon. There Acton’s brittlebush is absent, but E. farinosa is abundant on the south-facing slopes. We all know that the 2023 winter and spring have been much colder than average. Interestingly, in this side canyon the Encelia on the canyon bottom were mostly dead, while the ones on the south-facing slopes were thriving. It could be that the cold air sinking off of San Gorgonio frost-killed the canyon bottom brittlebushes. Climate matters.

Because climate matters, the rapidly changing and erratic climate we are experiencing is of concern. Animals and plants can usually adjust to slow, incremental change. Fast, erratic change is different. Slow changes allow natural selection to favor those individuals capable of thriving – or moving – in the face of that change. Fast or erratic change has more unpredictable consequences.

Nullius in verba                                                                                                                                                      Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe