“I would not sacrifice a single living mesquite tree for any book ever written. One square mile of living desert is worth a hundred ‘great books’ – and one brave deed is worth a thousand.” — Edward Abbey

Across the grasslands of the Chihuahuan Desert in New Mexico and Texas, mesquite is loathed, reviled. Mesquite has been invading those grasslands, reducing the forage capacity for herds of cattle. An internet search for research on honey mesquite ecology is dominated by articles describing methods to rid those rangelands of mesquite.

What is missing in those tirades is an analysis of what might be causing this invasion. Could it be over grazing, reducing the ability of bunchgrasses to compete with mesquite? Increasing aridity favoring deep-rooted shrubs over shallow-rooted grasses? Or could it be that the natural fire cycle has been disrupted by the ranchers’ fire suppression actions?

In contrast to the current levels of antipathy directed at mesquite by cattle ranchers in other western states, here in the Colorado and Mojave Deserts mesquite has been treasured for thousands of years. Historically, indigenous people here have depended on the abundant and nutrient-rich bean pods of honey and screwbean mesquite as a dietary staple. The sweet, nutty, protein-rich pods are ground and then cooked into an oatmeal-like gruel or formed into compact balls and dried to be used later as a high-energy meal along the trail. Mesquites are very deep rooted; they need to be tapped into year-round ground water to survive. The indigenous peoples’ other food resources were dependent on rainfall, which was unpredictable due to the vagaries of El Niños, La Niñas, or atmospheric rivers. With their roots ensconced in a stable water supply, mesquite flowering and fruiting is independent of weather or climate. Mesquite provided a dependable, easily stored food supply. Although these people had no need to define land ownership, families or clans would have exclusive access to their particular patches of mesquite. That is how critically important the annual harvest of mesquite pods was to their family’s survival.

Desert wildlife enjoy the dependable resources provided by mesquite as well. Away from mesquite patches, the desert’s fecundity follows a boom or bust pattern. Ample rainfall at the right time of year yields booms: carpets of wildflowers and explosions of insects, pocket mice, kangaroo rats, lizards, and birds. When rain fails to arrive, which is increasingly common, wildlife populations crash. Not so in mesquite thickets. Wildlife populations are far more constant because the food resources are more constant. You might then expect there to be a super abundance of wildlife in those thickets, but while stable, the populations tend to be far less abundant than during boom years in the surrounding desert. Why? The resource consistency and cover provided by mesquite thickets results in a much higher predator density than you might find in the surrounding desert. Snakes, kit foxes, coyotes, bobcats, and badgers take up permanent residence in mesquite thickets, culling populations of prey species, mice, ground squirrels, wood rats, and lizards, and so keeping them at lower densities. Away from mesquite thickets, the populations of those prey species fluctuate predictably but irregularly, correlating with abundance of rain; predators cannot build populations where prey abundance is so variable. In mesquite thickets prey populations are more constant and so are the predators. If droughts are especially severe and prolonged, mesquite thickets may act as refugia, providing a sanctuary from which species can then repopulate the surrounding landscape once the rain returns.

In the Colorado and Mojave Deserts, honey and screwbean mesquite are restricted to areas where the water table is fairly close to the surface. We typically find them along the margins of palm oases and riparian forests. They are deep-rooted, with roots reaching water at least 40 feet and as much as 200 feet or more below the surface and 50 feet beyond their canopy perimeter. So, mesquite, along with other deep-rooted desert trees (ironwood, palo verde) are particularly important at pulling carbon dioxide out of the air and transferring it deep underground. Referred to as carbon sequestration, this is one of the best ways to reduce the current overabundance of carbon dioxide in the atmosphere, storing it permanently underground. We often hear of the tropical forests being so vital to protect due to their biodiversity and ability to sequester carbon. That is undeniably true, especially for their biodiversity. However, tropical trees are more shallow-rooted and so their stored carbon is less permanent. And tropical soils are filled with microorganisms that use that carbon for food then respire that carbon right back into the atmosphere. Carbon sequestered by mesquite is a far more permanent solution. Their contribution to biodiversity is also undeniable. Not just for desert wildlife, migratory songbirds align their northward migration from the tropics to more northern latitudes to coincide with the flowering of mesquite (and ironwood and palo verde). Those flowers provide dependable (again, not dependent on spring rainfall) nectar and abundant insects, acting much like a fast-food restaurants (but with healthier food!) for those northward migrants. Without mesquite, ironwoods, and paloverde, far fewer songbirds would be successful in reaching their northern breeding grounds.

When my family and I moved to the Coachella Valley in the 1980s, there were vast groves of honey mesquite on the valley floor, especially in the central and east valley. There were also healthy mesquite thickets along the traces of the San Andreas earthquake fault along the entire northern edge of the valley floor. Almost all of that mesquite is gone or in decline. Transitioned to agriculture, golf courses, suburban landscapes or unable to still reach a declining aquifer, we have lost most of these mesquite. I remember walking through the mesquite thickets in what is now La Quinta and Indio and finding countless pottery fragments (sherds) testimony of the thousands of years the Cahuilla people thrived here. Across what is now the Coachella Valley National Wildlife Refuge, we can find the rotting stumps of what were mesquite thickets. There the land is protected, but the mesquite can no longer reach the aquifer. There too I have found pottery sherds, last touched by Cahuilla people gathering mesquite pods that would have sustained them throughout the following years. Even along the San Andreas fault, where water is at or near the surface, some of the mesquite is in decline, especially near areas where groundwater is being harvested to support more development.

For all these reasons, for desert biodiversity, for providing “fueling stations” for migrating birds, and for carbon sequestration, we should protect mesquite thickets. The Natural Community Conservation Plan for the Coachella Valley, the State of California component of the Coachella Valley Multiple Species Conservation Plan, names mesquite dunes and hummocks as one of 27 natural vegetation communities that received protection under that conservation agreement. Mesquite dunes and hummocks are mesquite thickets growing on aeolian sand dunes. Protection in this case requires more than buying real estate. First, how does a mesquite grow on sand dunes? It turns out they don’t, at least that is not how they start. Mesquite seedlings get established during large flood events when the ground becomes water saturated for months. For the seedlings to survive they need to send down a tap root as fast and far down as possible before the surface layers dry out. Assuming that they reach depths that are perpetually wet, they can then survive. If the seedling is in the path of a moving, wind-blown sand dune, the seedling will slow the wind causing sand to accumulate. To keep from being buried the seedling must grow faster than the sand accumulates. While it appears that the seedling is growing on a sand dune, actually the seedling is growing up through a sand dune. Long term protection includes enabling floods to occur, maintaining sand sources and aeolian sand movement, and it also means protecting aquifers. Ultimately that means living sustainably on a landscape blessed with a large but ultimately finite aquifer. The Cahuilla figured that out, but we still have much to learn.

Nullius in verba 

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