Spend even a little time in Southern California and you're likely to have an experience that's not all that common anywhere else in the country. You're outside in your shirt sleeves, feeling comfortably warm -- or maybe even uncomfortably warm -- and looking at a couple feet of snow a few miles away. It happens out here in the desert as well; snow often remains on San Jacinto and San Gorgonio peaks well after temperatures on the desert floor have climbed into the triple-digits. Southern California's geographical combination of warm climate and tall mountains creates an everyday object demonstration of a phenomenon the meteorologists call "environmental lapse" -- the fact that the higher you go, the colder you get.
Southern Californians have taken advantage of this fact for a long time. Take for instance the people we now know as Serrano, who loved throughout the southern Mojave and in its fringing mountain ranges. Rather sensibly, the Serrano undertook seasonal migrations within their lands, heading for the relatively warm desert during winter and climbing up into the mountains during the summer months. (The name "Serrano," in fact, comes from the Spanish word for "highlander.")
I've come to understand this phenomenon a bit more viscerally in the last few weeks: my fianceé and I are moving from our apartment in Palm Springs, with an elevation of around 400 feet above mean sea level, to a new place in the unincorporated community of Joshua Tree. Our new house is at about 3,400 feet above sea level, and is thus significantly cooler. On average, Joshua Tree seems to be somewhere around 10° Fahrenheit cooler than the Coachella Valley, a fact that is making my thermophobic fianceé very happy about our move.
That's about the difference predicted by mathematical models based on an idealized "standard atmosphere:" moving from roughly sea level to roughly 3,000 feet means your local air temperature should drop about 10.7°F. This is as a result of greater air pressure at lower elevations: the more atmosphere you have sitting on top of you, the higher the local barometric pressure will be. Higher pressure equals more energetic air molecules equals more heat energy per cubic fot of air. Joshua Tree's got about 90% the air pressure that Palm Springs does, therefore fewer air molecules per cubic meter, therefore it's cooler.
On average, that is. Altitude makes a difference in defining your local microclimate, but so do local topographic variations and vegetation and wind patterns and that sort of thing. Our place in Palm Springs is surrounded by asphalt and concrete and glass, and is a quarter mile from a 3,200-foot east facing rock wall that absorbs solar heat throughout the day to re-radiate later. Temperatures in the little driveway outside our apartment building regularly top 110° even when Palm Springs' official temperature is only 104° or so.
The new place is less paved and more vegetated, so the urban heat island effect will likely be lessened. We have a north-facing flank of the Little San Bernardinos half a mile to our south that may cool our yard as much as San Jacinto warms our old one: in evening, cooler, denser air tends to run down slopes. But who knows? We'll find out over the course of the next 12 months.
Some desert denizens don't have the luxury of picking up and heading somewhere else when the temperature isn't to their liking. This creates a problem when we raise the temperature, as we seem to be doing by dumping carbon into the atmosphere: unless wildlife can migrate either uphill or northward or both, trouble looms. Witness the American pika, likely doomed as its mountaintop habitats grow ever warmer, and the pika's habitable areas smaller.
But inability to migrate and temperature variations at altitude have also made the California desert what it is. The designations "Low Desert" for the California portion of the Sonoran Desert and "High Desert" for the Mojave and northward broadly reflect this. Once you get to know California desert plants, you can often tell to within a few hundred feet what altitude you're at by seeing what's growing around you. The distinctive ocotillo, for instance, indicates that you're below 5,000 feet in the southern parts of the California Desert. (North of Joshua Tree it gets too cold for Ocotillo to thrive even below that altitude.) Creosote bush, which grows throughout the Mojave and Sonoran deserts but peters out at the southern edge of the Great Basin desert, thrives from sea level (or below, in Death Valley and the Salton Basin to around 4,000 feet. At the upper limits of its altitudinal range it's often replaced by blackbrush, which grows pretty well up to about 6,000 feet above sea level.
The desert's prominent and picturesque Joshua trees, a few of which are growing in our new yard, do best between about 2,500 and 5,000 feet. Higher-altitude Joshua trees often grow in the company of Utah junipers, which can grow at 3,000 feet in moister parts of the desert but really prefer altitudes between 5,000 and 8,000 feet. Higher parts of the desert are often covered in a mixed forest of juniper and pinyon pine, the latter tree doing best most places in the desert between 6,000 and 9,000 feet.
All of these ranges are subject to fudging around the edges, of course. Give a highlands plant more water than it usually finds in the desert and it may well thrive significantly downslope of where you expect it. A few miles from the house I lived in in Nipton in 2008 there stood a mature, healthy pinyon pine, miles away from its nearest kin, surrounded by Joshua trees and blackbrush at around 4,000 feet. A flash flood had evidently brought its seed down off the mountains to the west a century or two previous, and a broad wash a few yards away kept its roots just slightly wetter than they would have been, and it had obviously done just fine. West toward the Antelope Valley more moisture leaks into the desert off the Pacific, and Joshua trees, junipers and even a few straggly pinyon pines all live together on the valley floor between 2,500 and 3,000 feet.
Despite the blurry edges, though, the desert's vegetation has sorted itself out into a web of overlapping ranges, and hiking across one species' upper timberline and into the range of its next-higher neighbor is a lesson in perspective. At first, hiking toward the upper edge of creosote's range, you may notice the first few blackbrush scattered here and there among the creosote. Some time later you notice the creosote has thinned dramatically and the blackbrush density similarly thickened , and you're standing in a mixed "forest" of the two. As you keep hiking, one species slowly dwindles and the other increases. Eventually you realize that you've long since passed the uppermost creosote and have been hiking in solid blackbrush for some time. You turn and look, and the boundary between the two startles you. It took you forty-five minutes to walk across the boundary, but from a half mile or so away it looks as sharp as if someone had engineered it.
Chris Clarke is an environmental writer of two decades standing. Director of Desert Biodiversity, 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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