“The desert floras shame us with their cheerful adaptations to the seasonal limitations. Their whole duty is to flower and fruit, and they do it hardly, or with tropical luxuriance, as the rain admits. ... One hopes the land may breed like qualities in her human offspring, not tritely to 'try,' but to do.” — Mary Hunter Austin

California has by any measure the highest overall biodiversity of any State in the U.S.A. As an example, nearly 8,000 species of native vascular plants live in California; over 2,300 of those are what we call wildflowers, non-grass annual plants. One third of California is classified as desert. If our deserts are making at least a proportional contribution to California’s biodiversity riches, then we should expect that about ⅓ (33%) of California’s wildflowers would be found in our deserts. It turns out that over 1,300 species of wildflowers occur in our deserts, or 58%. The Mojave Desert alone includes 1,286 species of wildflowers (57% of California’s total). So rather than contributing ⅓, the deserts contribute over ½, and nearly ⅔ of California’s wildflowers. Why are our deserts so rich in wildflowers?

The answer lies in the one thing deserts have the least of, rain. In the North American warm deserts, no two years have the same rainfall patterns. Sometimes there are summer monsoons, sometimes there is snow, sometimes there is little or no winter rain, sometimes the winter rains wait until spring to show up, sometimes the summer monsoons wait until fall to dampen the desert. Each species of desert wildflower has what might be called a “rainfall niche,” meaning each will only germinate if their species-specific rainfall needs are met. Some have broad rainfall niches. For these species, any rain, no matter when or how much rain falls upon the desert, as long as the ground gets soaked, is enough for them to germinate. Others are specialists, needing the right amount of rain at the right time of year.

In September and October of 2022, we were drenched by rare late summer-early fall rainstorms, spawned from Pacific Ocean hurricanes. The resulting wildflowers included many we rarely see, and some common ones that were in greater abundance than I have seen before. Those otherwise uncommonly seen blossoms included desert filaree (Erodium texanum), specter phacelia (Phacelia pedicellata), silk dalea (Dalea mollis), soft prairie clover (Dalea mollisima), glandular ditaxis (Ditaxis clariana, and California ditaxis (D. serrata ssp. californica). One of the common species that is in never-before-seen abundance is the rock daisy (Perityle emoryi). By germinating earlier than the more typical spring wildflowers, these species have less competition for water, space, and pollinators and so are able to flower, produce abundant seeds that then bide their time in the sand until the next hurricane-generated fall rains start this cycle again.

Rain specific responses are not limited to annual wildflowers. There are perennial plants that only occur within regions where the probability of summer monsoon rains is high. Ocotillo, desert agave, elephant trees, paloverde, and ironwood live within this rainfall niche. All are abundant within Anza Borrego State Park and south into Baja California, both places where summer rains are more likely to fall every year. Each of these species reaches its northwestern-most distribution in the Santa Rosa Mountains. The elephant trees, (Bursera microphylla) with their northernmost occurrences making it to Martinez Canyon, drop out first. Ironwood (Olneya tesota) drop out in about the same area. Paloverde (Parkinsonia florida) make it to about Palm Desert and Deep Canyon. Ocotillo (Fouquieria splendens) occur just a bit west of Highway 74 before they too are absent from the hillsides. Finally, the desert agave (Agave deserti) can be common right up to the Palm Canyon stream channel where they suddenly disappear. Each has a different tolerance and requirement for those erratic summer monsoons. When their summer water needs are not sufficiently met, they can no longer survive. For the same reason large cactus such as saguaros (Carnegia gigantea) barely occur in California. They depend even more on summer monsoons (plus long-nosed bats, their primary pollinators, do not occur in California).

For other desert plants, the connection to summer monsoons is more subtle, but still critical. Shrubs such as creosote (Larrea tridentata) and brittlebush (Encelia farinosa) occur well north of the zone of regular, dependable summer-fall rain yet their existence may still be dependent on those warm rains occasionally occurring. Both of these species occur in sometimes dense populations of individuals that are almost all the same size, and so presumably all of similar ages. After those September-October rains of 2022 seedlings of both brittlebush and creosote appeared and were in some places abundant. The wager these seedlings are making is that, with the help of some winter rain, over the next five months of the winter and early spring they will have been able to extend their roots deep enough to reach cooler-damper soil layers that will allow them to survive the heat and drought of mid-summer. If they germinated in the spring, they might have just a few weeks or at most a month before the summer drought hits, and so would have almost no hope of survival. Those even-aged stands of adult creosote and brittlebushes may be the result of a previous rare but critically important hurricane-spawned wet summer and fall. A rare event, but essential for the creation and maintenance of our deserts’ high levels of biodiversity.

Nullius in verba                                                               

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