Saturday, October 2, 2004

Plant for the Day: Phillip’s Agave, Agave phillipsiana (the plant shown in Agave parryi, a close relative that was also cultivated in central Arizona during pre-Columbian times)

Native people inhabiting north-central Arizona before Europeans arrived cultivated several species of agave that are restricted to this area. Phillip’s agave was discovered at the Grand Canyon and officially named just a few years ago. It was found near pre-Columbian ruins, presumably cultivated by the Kayenta Anasazi, who lived and farmed in the canyon about 1000 years ago. Originally thought to occur only in the canyon, it has recently been found in several other pre-Columbian sites in north-central Arizona. The plants are nutritionally rich, and provided a dependable food source at a time of year when other edible plants were scarce. Although agaves were not cultivated in this area after about A.D. 1450, living plants remain in the landscape today because of their clonal spread and longevity. Along with several colleagues here at UGA and the Desert Botanical Garden in Phoenix, I am studying genetic variation in this and several other cultivated species—trying to determine where the plants originated, what their progenitors were, how many times they were introduced from farther south in Mexico, and whether there is any genetic variation in the existing populations. These plants are extremely rare, and we hope that our research will help protect the few populations that remain.

We left the city lights of Las Vegas behind in the evening and headed south through the desert to cross the Colorado River. The river forms the boundary between Nevada and Arizona, and we paralleled it on Route 95 down to Davis Dam, where we crossed the river into Arizona. The dam, which impounds Lake Mojave, was completed in 1953 as part of the system of dams along the lower Colorado River that regulate flow, supply water, and generate hydroelectric power. The main purpose of Davis Dam is to regulate flow from Hoover Dam 67 miles upstream, in order to meet downstream needs established in the 1944 water treaty with Mexico. It also controls flooding from side washes joining the Colorado River below Hoover Dam. With tight regulation of flow from 20 dams on the Colorado River and its tributaries, the river typically runs dry before it reaches its mouth at the Gulf of California. Water is so scarce in this arid region that apportionment of the water supplied by the Colorado River is legally controlled, amidst a great deal of controversy.

Once we crossed the river, we began the climb up into Kingman, Arizona. I’ve traveled the road between Kingman and Lake Mojave many times as I’ve visited my brother’s family and we’ve driven down to the lake to swim. That stretch of highway offers a great example of typical Basin and Range topography, with alluvial fans forming the transition between the fault-block mountains and the intervening valleys. Alluvial fans accumulate when mountain streams leave their confining, rocky valleys in the uplands and their capacity to transport sediment suddenly decreases, causing them to deposit sediment at the base of the mountain. Over thousands of years, a fan-shaped deposit is constructed as the braided streams shift across the fan, leaving a veneer of sediment. Where many adjacent canyons occur along a range, alluvial fans often coalesce into a bajada, or a continuous apron of sediment.

On this cross-country trip, our only brush with the Sonoran Desert, where I’ve spent many months doing field research on desert plants, is in this stretch between the Colorado River and Kingman. One of the most conspicuous plants is the saguaro (Carnegiea gigantea), an impressive columnar cactus that lives to about 150 years of age. It occurs on upper bajadas and alluvial fans, where the coarse soil is well suited to a plant that copes with drought by taking up water quickly with its shallow roots after rain for storage and later use. Fine-textured saline soils of lower bajadas provide drier, more stressful habitats for plants, and the plantlife inhabiting valley bottoms is much less diverse than on upper bajadas. Some of the higher elevation bajadas around Kingman support saguaro and Joshua tree growing together—a mingling of plants from the Sonoran and Mojave Deserts. Saguaro was an important plant for native peoples, who collected the bright red fruits to make wine and jam.

East of Kingman, we climbed from the lower elevations of the Basin and Range up the Mogollon Rim, which marks the southern edge of the Colorado Plateau. While surrounding terrain underwent intense folding and faulting, the Colorado Plateau remained relatively intact structurally throughout the last 600 million years. Sedimentary rock layers, formed from aquatic sediments deposited when that area was inundated by tropical seas, as well as from terrestrial sediments, were gently uplifted without significant structural deformation. Its characteristic signature today is horizontal layers of sedimentary rock—in many areas visible in deeply incised canyons, while in others, in the eroded cliffs of mesas and buttes.

On the plateau where moisture is more abundant and plant productivity is higher, we began to see ponderosa pine forests, leaving low desert behind for the last time. Before Europeans settled this area and instituted a policy of fire suppression, ponderosa pine occurred in fairly open forests that experienced low-intensity surface fires every 1-5 years. With a century of fire suppression, these forests have become much denser than they were in pre-European settlement times; and when fires do occur, the fuel buildup causes much more intense, often catastrophic, fires.

About 30 miles west of Flagstaff, Arizona, we turned north towards the Grand Canyon—one of the more spectacular sites on this scenery-studded trip. The Grand Canyon is only one of several gorges cut deeply into the Colorado Plateau by the Colorado River and its tributaries. As the Colorado Plateau rose during its formation, ancestral streams carved deeply into the rock layers, shifting their courses in some cases, forming spectacular canyons. A goldmine of historical information is revealed in the walls of the Grand Canyon. Rock layers are progressively older from the rim of the canyon to the floor. Many of the layers contain fossils that indicate what plants and animals were alive at the time, as well as environmental conditions leading to the deposition of the sediment later indurated into rock. The oldest rocks at the bottom of the canyon are 2 billion years old. The limestone layer that makes up the south rim of the Grand Canyon is approximately 250 million years old, and is the youngest layer exposed in the canyon! But the canyon walls in Zion and Bryce National Parks to the north include several younger layers above this same limestone layer—a piece of the historical record that is missing from the Grand Canyon. While the rocks that make up the Grand Canyon are ancient, the canyon itself has likely been incised much more recently—in the last 5-6 million years. At its deepest point, the canyon is 6000’ deep. Such an elevation span causes the microclimate, and hence biotic communities, to vary substantially from the rim to the river below. Stately conifer forests occur along the canyon rim, while desert plants eke out their existence on the hot, dry canyon floor far below.

On our way eastward from the Grand Canyon, we passed through the Painted Desert, a much less dissected, nonetheless subtly beautiful, section of the Colorado Plateau. The Painted Desert occupies the western part of the Navajo Nation, which extends up to the Four Corners. The Navajo came to the Southwest from the plains of Canada around the 15^th century. The Spanish had already introduced sheep and horses into this area, which the Navajo adapted to their hunting and gathering way of life. Just off the main road lies Navajo National Monument, which preserves ruins of native people living in this area long before the Navajo—the Anasazi. In the late 1200s, the Anasazi lived in protected alcoves in sandstone cliffs, above fields they farmed on the canyon floors below. The Anasazi suddenly disappeared from this area around A.D. 1280. Archaeologists debate the cause of their demise. Some speculate that warfare was to blame; others pointed to drought or other causes.