No. 81 Spring 2015
Photo: Steve Bier
Anza-Borrego’s Formidable Desert: Implausible Richness By Evan MacKinnon, Environmental Scientist, Colorado Desert District The desert is a land of extremes — dry alkali soils, searing temperatures and relentless winds — it’s the last place you’d expect to find a rich biological system teeming with life. And yet in Anza-Borrego Desert State Park, receiving barely seven inches of rain per year, there exists an intact ecosystem, part of a large desert preserve complex in Southern California where ecological processes go largely unimpaired by human exploits. AnzaBorrego is extraordinary, not just as a local conservation achievement in an increasingly urbanized corner of the country, but as a unique and biologically diverse wildland of global conservation importance. My experience of other arid areas of the West didn’t prepare me for the “lushness” of the Anza-Borrego Desert. It certainly didn’t resemble the sagebrush scrub of the Great Basin Desert, the creosote flats of the Mojave Desert, or the alkali scrub habitats of the rarely recognized “San Joaquin Desert” (Germano et al. 2011). Instead, vegetation is abundant and of all types, including annuals, shrubs and succulents, as well as the gangly ocotillo, the emblem of the Sonoran Desert (Schoenherr 1995). The desert’s lushness is seen best in desert washes, where we find trees like desert willow, mesquite and ironwood. These arborescent forms seem implausible given such meager rainfall, yet their fortuitous geographic location and exceedingly deep roots allow them to flourish. Fan palms, another bizarre and seemingly out-of-place form, conjure up
images of the desert’s tropical past. Anza-Borrego’s plant diversity is not unusual by California standards. Indeed the Park lies in San Diego County, quite possibly the most botanically diverse county in the United States (Rebman & Simpson 2014). Much of the diversity of Anza-Borrego has to do with its geological and topographic complexity. For example, moist and shady canyons favor a suite of organisms very different from those growing on rocky mountaintops or silty valley bottoms. Also, the bimodal (winter and summer) rainfall pattern favors different plant communities at different times of year, with essentially two ephemeral communities being supported in one area. Around half of the desert’s plant diversity consists of annual species that withstand periodic dry periods in their dormant seed form (Phillips and Comus 2000). In other words, during most years this species diversity exists only in the form of a soil “seed bank.” The California desert is a place of discovery. One would be hard pressed to locate another region where an inquisitive botanist is more likely to describe a new species than in the under-documented and rugged terrain of California’s desert. Already the desert has provided us with classic ecological lessons on topics like ecological facilitation among species, as well as information on the physiological adaptations and biochemical pathways that allow plants to not only endure the desert,
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