Winter Journey Part III — Living in a Mesquite Bosque
So here we are, in our small RV, sitting in the middle of the Avra Valley of the Sonoran Desert, a 20-minute drive west of Tucson. The Sonoran Desert is actually a large thing, stretching from Phoenix, AZ, into northern Mexico and across the breadth of Arizona. But it is not the sand-dunes-desert that may first come to mind when you hear “desert.” That image may fit the Mojave Desert to the west, better.
This picture from the Ajo Highway, running west from Tucson to the town of Ajo, shows a mesquite bosque in the Sonoran Desert. In other places the desert is a natural grassland, almost like a prairie, and the land is comprised of rocky hillsides leading dominated by Saguaro cactus and prickly pear cactus. Apparently the mesquite trees invaded some of the grasslands over the past 100 years, especially in flood plains like this one. Now the grasses are sparse and the landscape is filled with scrubby mesquite trees and cholla cactus.
During our stay from early November through mid-February, daytime high temperatures were frequently in the 60s and 70s but dropped to the low 40s and into the 30s at night. Most days were sunny and it took a week or two for us to adjust to the day/night temperature variation. We learned to wear layers, so we could peel them off as the morning chill became midday warmth, but put them back on as the sun went down and the cold returned.
Harmony and Health Foundation owns 60 sparsely populated acres in this mesquite bosque, and is surrounded on 3 sides by “state land” that, similar to federal Bureau of Land Management land elsewhere, is leased to ranchers for cattle grazing. I was able to walk in the desert daily and learned a little bit about the desert plants and animals.
In this area, mesquite grows up to 10 or 12 feet high with a good water source, but most are 6 to 8 feet high. Each tree has a “shrubby” look to my Eastern eyes, with multiple trunks spread wide and tiny compound leaves. The young branches have thorns and the mesquite seed pods look like dried bean pods dangling from above.
The mesquite has two types of roots. There are widespread roots near the surface that stabilize the tree and gather water, when it arrives, from as wide a space as possible. The other root is pretty unique. A tap root reaches as far down as possible, up to one hundred feet underground. Not only does the tap root pull water from underground water, it actually transfers surface water underground where the tree retrieves it during drought conditions!
So the mesquite is the most prevalent shrub/tree in the area and is a nursery tree. Each individual tree forms a canopy that shelters other plants, frequently sprouting from seeds dropped by birds making their daily rounds. So it is not unusual to see a cholla cactus or a prickly pear growing at the base of a mesquite, entwined in its branches. The most consistent child plant of the mesquite, however, is the stately Saguaro cactus. This is the striking, sometimes 3-armed, cactus that is a state symbol of Arizona. When you walk upon a Saguaro in the mesquite bosque, 9 times out of 10 there is a mesquite tree, or the remains of a mesquite tree, around the base of it.
The Saguaro grows very slowly and despite the dangerous-looking thorns of the mature specimen, a three year-old Saguaro may be only the size of a person’s fist. The slow growth of the Saguaro allows it to build a circle of firm, woody ribs just under its skin. The ribs support that great height and even expand during rains to absorb water in the pulpy interior of the plant. And those reaching arms? A Saguaro will be at least 50 years old before it starts to sprout branches.
And what about those mountains in the background? This particular picture shows you the west side of the Tucson Mountains, forming the western boundary of the valley that is home to the city of Tucson. This picture is a good illustration, too, of how the Sonoran Desert is not the sand-dunes type desert that we learn about in 3rd grade. That description better fits the Mojave Desert farther north.
The mountains through out the Sonoran Desert form the “ranges” that give this area its geologic designation, “basin-and-range.” Over the millennia, large rock outcroppings pushed up through the earth’s crust to form mountain ranges scattered across the landscape. The mountains are separated by “basins,” which are typically relatively flat stretches of desert. The original basins have been filled in by sand washing down off the mountains over the centuries. These accumulating sands form the slight hills and low river washes of the desert landscape.
The Avra Valley, where we are living, is a “basin” that is bounded by the Tucson Mountains on the east and the Baboquivari Mountains to the west. This is part of the territory of the ancient Hohokom and the contemporary Tohono O’odham. The current territory of the Tohono O’odham Nation, almost 3 million acres, is also a short drive west.
The mountains form “islands” of habitat for animals, birds, and plants that need cooler temperatures, which they find at the higher elevations. We enjoyed several trips to the Sonoran Desert Museum, in the foothills of the Tucson Mountains where we saw many of the native plants and animals of the Sonoran Desert. Here are a few our our pictures.
But where are the people? This has become a natural history blog post, I guess. Here is the Pond Family outside the Sonoran Desert Museum on Christmas Day 2015. It was our first Christmas picnic, outdoors, with a raptor free flight over our heads. A special day, indeed.