Letter from the desert: Thresholds
Posted by Chris Clarke on October 8, 2008
Creosote bush, Larrea tridentata, grows in the lower elevations of the Mojave. At least it does so in places where the soil is not too alkaline. In the flattest part of this valley nothing grows, and the fringes of the dry lake are the domain of gray-leaved Atriplex, but gain a little altitude above the playa and a little bit of drainage in the stony soil, and you come into the creosote.
Creosote has an altitudinal range running more or less from the valley floors to around 4,000 feet above sea level. It just so happens that most of the Mojave’s surtface is somewhere between the valley floors and the 4,000-foot line. Creosote thus is probably the most common large plant in the Mojave. The Mojave’s mountains are mainly discontinuous blocks of crust, some of them reaching truly prodigious heights, but with few ranges stretching more than a couple dozen miles. There are the high peaks, few of them surpassing eight thousand feet, clothed in piñon and juniper and a few white fir, and then a swift descent down steep alluvial slopes and past the 4,000-foot level.
The Mojave’s mountains are islands in a sea of creosote.
Where it can survive, creosote dominates the landscape. The plants secrete a chemical that is toxic to most other vegetation. Creosote flats tend to be relatively monotonous-looking, mature plants separated by expanses of more or less bare soil.
Joshua trees seem not to be particularly susceptible to this allelopathic chemical. Creosote is often the “understory” plant in Joshua tree forests growing below 4,000 feet.
The upper boundary of a creosote stand is abrupt, as such boundaries go, but like any boundary in nature it is near-impossible to pinpoint it exactly. Above 4,000 feet the most common understory plant in Joshua tree forests is blackbrush, Coleogyne, a low-growing dark-gray and very scratchy plant that grows in thick patches. Stand at 4,000 feet and look uphill, and then look downhill, and the difference in the way the ground looks will be remarkable. Upward the forest soil is mantled in a rather dreary gray. Downhill a wave of deep green seems to flow up toward you.
But near at hand, it’s harder to distinguish the boundary. There is certainly a point at which the Larrea thrives so well that the Coleogyne succumbs to allelopathic poisoning. Were it not for the creosote, the blackbrush would extend significantly farther downhill, to the 3,000-foot level or below. The creosote outcompetes it between three and four thousand feet. And there is also an obvious point above which no creosote grows. Those points may be separated by a quarter mile of desert, the creosote growing more sparsely as you climb and blackbrush gaining a toehold. Where the blackbrush is thick it casts shade on the soil. Creosote seeds — set in abundance each year — fall beneath the blackbrush and wither, if they sprout at all.
After a bit more uphill walking in the Joshua tree forest you will pass the last creosote bush. You probably won’t realize it at the time. Other things will be commanding your attention: the pebble in your boot, the possibility of snakes, the junipers a thousand feet up on the mountain. You will likely walk by the uppermost creosote bush after several minutes of slowly thinning creosote, other things on your mind and no particular sense of change in your surroundings. Walking through blackbrush takes some concentration. Paths that seem sensible and clear will lead you nowhere, or past Scyllas and Charybdises composed of snakes and cholla, and you will be backtracking and sidestepping and watching the ground, preoccupied, covering a mile for every half-mile gained.
And then you stop to breathe and turn, and below you you see a wall of creosote as well-defined, from your vantage point, as a painted line on a highway, and you suddenly realize you’ve missed the resinous smell and the sound of bees, which passed out of your senses while your mind was otherwise occupied.
