Life at Pueblo La Plata

A short walk from the tumbled-down walls of Pueblo La Plata is a boulder-strewn field that offers uninterrupted views of some of Arizona’s most open and rugged country. On this blustery day in early March, the clouds cast shadows on the surrounding mountains. Their flanks appear purple and as deeply pleated as the bellows of an accordion.

Botanist Wendy Hodgson and her fellow researchers don’t break stride to take in the beautiful view. Littered with rocks and prickly pear cactus, this is ankle-twisting, shin-stabbing terrain. And when the weather warms up, you’re apt to bump into a rattlesnake or two. Take your eyes off your feet here and you do so at your peril.

Besides, this is no pleasure hike. Hodgson is on a mission. Clustered on a gentle rise are tall, woody flower stalks as thick as a man’s wrist. Hodgson makes lots of forays into Arizona’s back country. She scans hillsides and ridge tops for spikes just like these with their prominent peduncles, or stems, laddered against the sky. This corner of Perry Mesa is the kind of botanical find most likely to stop her in her tracks.

As we approach the ridge, the ground underfoot grows noticeably rockier. We have entered an ancient agave garden. More than 600 years ago, the occupants of Pueblo La Plata moved volcanic rocks ranging in size from cobbles to boulders to the edge of the mesa. They heaped the rocks into piles and then planted agaves among them.

No one knows for sure what advantages were to be gained from such techniques. Hodgson suggests that the rock piles may have served to conserve precious moisture around the plants during the hot, dry summers or to provide a source of radiant heat during cold snaps in winter.

Hodgson completed her master’s degree in botany at ASU in 1982. Since that time, she has focused on the food plants of Arizona’s indigenous people, among them, agaves. She currently serves as a senior research botanist and director of the herbarium at the Desert Botanical Garden in Phoenix.

As part of her work at DBG, Hodgson has tracked agave gardens located near archaeological sites all over the central and northern part of the state. She’s trudged the slopes of Sonoran Desert mountains north of Phoenix and clambered up the hillsides of the Verde Valley. She’s also explored the narrow side canyons of the Grand Canyon. Agaves are low-slung plants with fleshy lancets of leaves. Given that they were a staple of food and fiber, Hodgson isn’t surprised to find that early people farmed them.

But upon closer examination of the plants in these scattered agave fields, the botanical picture has grown far more complicated—and interesting. Arizona boasts 20 species of native agaves. In recent years, however, Hodgson and her colleagues have identified five additional species of non-native agaves. At least three of these may have originated in northwestern Mexico.

One thousand years ago or more, the offspring of these Mexican plants likely began to make their way northward in the packs of native people and into their agricultural fields. The agave plant’s main mode of reproduction is vegetative. Underground rhizomes spread out from the mother plant and sprout tiny plants known as pups. The plants that survive today are genetic copies of agaves that were planted and tended by prehistoric people. Hodgson calls them living archaeological features.

These imported agaves exhibit differences from their native cousins in ways that undoubtedly were prized by early people. The thick fibrous leaves of these cultivars were more easily removed from the nutrient-laden heart of the plant. The hearts of the plants were roasted in covered pits and eaten. The leaves also generally have smaller teeth. This was a boon to harvesters who hacked them away from the plant’s center with crude stone tools, exposing themselves to serrated edges that could cut into human flesh like knives.

The non-native cultivars also mature at different intervals throughout the year. This ensured that fresh agaves would be available for longer periods of time, especially during lean seasons such as winter.

But these cultivars are not simply of interest to botanists alone. Agave pups can easily survive long-distance transport in a backpack. Researchers think that the plants probably were part of a vigorous network of trade in the Southwest. Agaves may be as reliable as styles of pottery in helping archaeologists to reconstruct the social structure and pattern of trade routes among prehistoric people.

Hodgson and her colleagues may get their first tangible clues soon. In a project funded by the National Science Foundation, botanists from the University of Georgia are analyzing snippets of plants from both native and non-native cultivars. The goal is to determine their molecular structure. These genetic markers may help researchers to match plants from the Grand Canyon, say, with their progenitors in Sonora, Mexico.

Molecular fingerprinting also may lend insight into the depth of agricultural knowledge of these early people. Hodgson says that some agaves appear to have been hybridized in order to breed desirable characteristics from one species into another. Non-native cultivars are susceptible to killing frosts and some insect pests such as the Agave snout-weevil. By hybridizing these plants with native species, native farmers may have given them greater abilities to cope with these environmental threats.

For example, the agave gardens at Agua Fria appear to contain the native Agave chrysantha and the non-native cultivar Agave parryi. But they also contain hybrids that exhibit features of both species. Hodgson wonders if these ancient farmers knew enough about the reproduction of plants to deliberately tinker with their genetic makeup. Or were these plants hybridized by the accidental forces of nature?

Asking tantalizing questions is the scientist’s job.

“This work is part of a growing trend in field studies to observe plants in the cultural as well as ‘natural’ landscape,” the botanist says. “We are realizing that more plants than we had thought—particularly those useful to humans—have been influenced by human intentional or unintentional activities. We are now looking at the landscape in a different way.”—Adelheid Fischer