"Look closely at nature. Every species is a masterpiece, exquisitely adapted to the particular environment in which it has survived. Who are we to destroy or even diminish biodiversity?" - E.O. Wilson

By DR. CAMERON BARROWS

(This essay includes paraphrased portions of an essay originally written by Dr. Michael Allen, a retired University of California Riverside Distinguished Professor whose specialty is soil microbiology)

When we speak about biodiversity, the focus is typically on vascular plants or animals, or both. The number of lizards or cacti living in a defined area. Using such a barometer, California’s deserts more than hold their own compared with most other bioregions across North America. Still, as impressive as the numbers of desert reptiles or vascular plants are, such an assessment ignores where the real wealth of biodiversity can be found, below ground. In a recent survey of a few soil plots at the Boyd Deep Canyon Research Station near Palm Desert over 700 species of fungi and 1,000 species of bacteria were located within one plant species’ roots alone.

Functionally, there are two types of organisms in California's deserts, primary producers-those that fix atmospheric CO2 into complex organic molecules that drive life, and decomposers that use the organic molecules to sustain themselves, respiring fixed CO2 back to the atmosphere. Plants and crust photosynthetic organisms undertake most of the primary production. Nearly all other organisms, including humans, are part of the channel returning the CO2 back to the atmosphere.

Measurements of productivity, or how much carbon is drawn from the atmosphere, have shown that both creosote bush (Larrea tridentata) uplands and microphyll woodlands (mesquite, paloverde, and ironwoods – all deep-rooted plants) can take up nearly half a pound of carbon per meter2 per year. Multiply that value across the total area of creosote or microphyll woodland and the importance of deserts for removing carbon dioxide from the atmosphere becomes clear. This is greater than some forest and grasslands ecosystems. Wet forests, such as those in tropical climates or areas with ample warm season rains, are typically shallow rooted. While those forests’ productivity, their consumption of CO2, can be higher than desert plants during the deserts’ dry season, they return most of that carbon back into the atmosphere due to the rapid surface decomposition of fallen leaves and branches by bacteria and fungi. In contrast, desert plants with deep roots, along soil fungi and bacteria associated in symbiotic relationships with those roots, can sequester organic carbon deep within the soil where it can stay for centuries, perhaps millennia, and so incrementally reduces CO2 in our atmosphere.  

Primary producers also include a smattering of photosynthetic bacteria such as the purple- and the green-sulfur bacteria and a diversity of chemolithotrophs. These organisms are descended from the earliest life forms on earth. They can be found in extreme environments in places like Death Valley and the Salton Sea. Chemolithotroph microbes undertake a role in two particularly important processes: formation of desert varnish critical for petroglyph formation, and volatilization of selenium, both a required vitamin but also a toxin in high concentrations. There is an entire microbial consortium controlling manganese precipitation and mineralization that cements clay materials of manganese and iron-oxides on rock surfaces comprising rock varnish.

Desert crusts, previously called cryptobiotic crusts, can form from cyanobacteria, also known as blue-green algae. These prokaryotic organisms produce long (for microorganisms) silica sheaths as many individual linear chains grow (when wet). As they dry, these cells within these sheaths contract, leaving structures that bind soil, reducing wind erosion and the dust storms that result when bare, fine textured soils are blasted by the wind. Crusts also include cyanobacterial lichens, a mutualistic symbiosis between cyanobacteria and fungi. In both cases, cyanobacteria will fix atmospheric nitrogen, a critical process for sustaining desert ecosystems. Green algal (chlorophytic) lichens, mosses, and fern-allies such as Selaginella also form crust-like structures. Cyanobacterial and cyanobacterial-lichen crusts are especially interesting in California deserts. In some regions, particularly on the Colorado Plateau of Arizona and New Mexico with high summer precipitation, they cover vast areas and control soil erosion. In California deserts with less monsoonal, summer rain, they are less dominant. Nevertheless, although of lesser intensity, there is almost nowhere in the California Deserts where these crusts do not exist, covering the soil surface in patches between plants. Also important, if you pick up a small piece of crust, on the underneath side and look at it microscopically, you often see the fungal hyphae of mycorrhizal fungi, scavenging ions of ammonium that are fixed by the cyanobacteria. These mutualistic fungi then transfer ammonium to host plants in exchange for plant carbon.

The majority of desert atmospheric nitrogen fixation occurs in legume plants from the mutualistic symbiosis of rhizobia bacteria and legume plants, such as those tree species that comprise microphyll woodlands. Here the plant provides both the energy in the form of sugars and produces a nodule which creates the anaerobic microsites (nodules) where atmospheric N2 (unavailable) can be converted to NH4 (ammonium) that can be used by the plant to build proteins. When you examine a nodule, you can tell whether N2 is being fixed. Gently rub the nodule. If it turns red, a compound called leghemoglobin is present and active. Leghemoglobin, like our hemoglobin, scavenges oxygen allowing the N-fixation, an anaerobic (no oxygen) reaction, to occur.

Mycorrhizae, or fungus-root, is another type of mutualistic symbiosis existing with the vast majority of desert plants, where the mycorrhizal fungal hyphae stretch out exploring the soil for nutrients like phosphate and ammonium and for water, and exchanging those resources with the plant for the carbon fixed. There are many types of mycorrhizae in deserts. The most common are arbuscular mycorrhizae found with most desert plants. These include relationships among fungi, legumes, and rhizobia bacteria, with the fungus providing phosphate and water, the plant carbon (energy), and the rhizobium nitrogen.

Those microphyll woodlands, with their countless essential microbial associates, are a critical component of the California deserts. Their importance in carbon sequestration cannot be overstated. Microbial associations also play a critical role in ensuring that these woodlands have access to water and soil nutrients, even in dry years, so that the trees can produce leaves and flowers every spring. This yearly bloom and the pollinating insects they attract provides an essential fuel for western northward migrating birds. These birds follow a flyway that they have traveled thousands and perhaps millions of years. Leaving their wintering grounds in Mexico, Central and South America, the birds fly up the Gulf of California and then up the course of the Colorado River. In a region somewhere around the border of the Colorado and Mojave Deserts, the birds then abruptly turn left and cross the desert, stopping in the microphyll woodlands to refuel and hydrate before continuing. The desert leg of their migration is necessary for them to continue north through California, Oregon, Washington, and points north. Otherwise, the Sierra Nevada and Cascade Mountain Ranges create an insurmountable barrier from accessing those western forests. Without the microphyll woodlands, especially in dry years, they may not have the energy to reach their breeding habitats.

Nullius in verba

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