TREE CHEERS FOR ANDREW DOUGLASS

A RAINY WINTER DAY IN TUCSON is as good as any to stay inside and explore the Special Collections at the University of Arizona Libraries. I pull on a latex glove and pick up a bundle of spindly sticks from a tan envelope. Each piece of wood, cored from the center of a tree, is marked with pencil etchings. The lines highlight the trees' knowledge of weather history in the American Southwest — and the wood samples unlock a memory in my vault as well.

In 2019, I volunteered with a survey team, assembled by historian Fred Blackburn, to document modern human history written on the walls at Aztec Ruins National Monument. The walls, built more than 1,000 years ago by the Ancestral Puebloans, are a palimpsest: Between 1890 and 1920, tourists and local visitors signed their names on Aztec's wooden beams in charcoal, pencil and knifepoint.

Cold, damp air sank into the stone masonry room we were surveying, one of hundreds in this fortress hugging the Animas River in Western New Mexico. Dim light entered through four windows, but headlamp beams were necessary to reveal the stories. At each mark, we made photos and measurements, followed by a hypothesis. Some names were illegible scrawl, others signed in gorgeous script.

The room's sinewy wooden window lintels were scarred with circular holes, each plugged with a piece of wood stamped with a serial number. The meaning of these was conclusive: They were tree-ring core samples drawn a century ago by Andrew Ellicott “A.E.” Douglass, a pioneer of dendrochronology. These samples launched what Douglass called “the ancient tree-ring calendar,” which helped lead to a more accurate understanding of human and climate history in the Southwest.

Since that time, I’ve realized I encounter Douglass’ work frequently. You probably do, too. Pull out any article from the past 20 years about weather, rainfall and river flows in the Southwest, and it’s likely to include a reference to the “worst drought in 1,200 years” — often in reference to the drawdown of the Colorado River’s major reservoirs, Lake Mead and Lake Powell, in 2021 and 2022. Both reservoirs rebounded somewhat after the record-setting winter of 2023, but as of June 2024, they remain just over 30 percent full.

Such cycles of drought and deluge, over the past 12 centuries, are part of the story that the trees shared with Douglass.

DOUGLASS WAS BORN IN 1867, among the pines in Vermont. Before his fascination with trees took root, he gazed above them at the stars and fell in love with astronomy. After college, Harvard hired him to climb mountains in the Andes and Northern Arizona in search of high places to construct observatories. He helped establish Lowell Observatory in Flagstaff in 1894, and the then-rural mountain town honed his interests in alpinism and tree-ring studies.

The connection between trees, astronomy and climate sounds far-fetched, but at that time, emergent theories were revealing how variations in solar activity affected weather. The tree rings, Douglass and other scientists found, displayed not only the age of the tree, but also the weather conditions in which each ring had formed. Larger bands appeared in wet years, thinner ones during droughts. Other indicators helped correlate tree growth and climate with solar changes. In a 1929 National Geographic article, Douglass wrote, “Every year the trees in our forests show the swing of Time’s pendulum and put down a mark. They are chronographs, recording clocks, by which the succeeding seasons are set down through definite imprints. Every year each pine adds a layer of new wood over its entire living surface of trunk and branches.”

Fragrant yellow pines in Flagstaff were the first trunks into which Douglass bored holes. Standing in the snow, he used a steel ruler and a magnifying lens to measure the rings. Before long, he transitioned his life and research south to Tucson, where he worked as an astronomy professor at the UA. In his free time, he continued to roam Arizona, taking tree samples and developing new tools for his trade.

His work stirred interest from Clark Wissler, curator of the American Museum of Natural History, who requested that Douglass take tree-ring samples from archaeological sites in Arizona and New Mexico. This, he hoped, could improve the accuracy of the archaeological record in the American Southwest. Until then, dating estimates had been based on pottery and stratified remains.

After some experimentation that yielded poor results, Douglass took high-quality beam samples at Aztec in 1920 with the help of archaeologist Earl Morris. He also developed a method of cross-dating, or comparing the samples from dwelling beams with those from living trees in the area. To improve this work, Douglass created a tool to sample wooden beams from structures in situ, and by removing only a l-inch-diameter core, the new tool obtained readable samples without damaging structural integrity. It was an important step toward protecting cultural sites from being damaged by research.

Although the 1908 Antiquities Act was in effect to ensure that all archaeological study was done by permitted professionals, Douglass' time was still the dark ages of American archaeology, with a dissonance between places being studied and those places' Indigenous peoples. A requirement for tribal consultation on archaeology projects would not go into effect until 1990. Ever the pioneer, Douglass took a different, more respectful approach to his work than did many of his peers. When he sampled beams at the Arizona village of Oraibi with the permission of the Hopi Tribe, he plugged the sample scars with turquoise at the tribe's request, “to appease the spirits... [and] to prevent the lodgment of the ‘spirit of decay’ in the timber.” The sampling success launched expeditions throughout the Four Corners region. Douglass sent sampling tools to archaeologists, who shipped the wood cores back to Douglass' lab. For nearly a decade, the core samples helped to piece together the timeline of Ancestral Puebloan habitation, and in 1929, a piece of charcoal found at a site in Show Low offered enough “ringed integrity” to fill a final gap. The 1,200-year “tree-ring calendar” extended back to A.D. 700.

For National Geographic, Douglass wrote, “By learning how to read these records, specifically those of the pines, we have discovered a magic key to open mysterious books and interpret the meaning of their writings.” He continued, “We have established in our Southwest a chronology for that period more accurate than if human hands had written down the major events as they occurred.... The collection of an unbroken series of tree rings has made clear the chronology of the Southwest.”

FOR DOUGLASS, the study helped crack a climate code that would inspire the remainder of his career: “When a real theory of climate has been developed and we can predict drought and flood over a period of years, this Arizona story in tree rings will have played a creditable part in developing that climatic foresight, which is perhaps the most valuable economic advantage yet lying beyond our reach.”

To look forward, Douglass' studies highlighted the importance of looking back. For example, the trees revealed that during the years 1632, 1379, 1067 and 840, excessive drought reigned. Douglass concluded, “From this combination of climatic facts and human movements, we have unearthed evidence of a human cycle, the time during which village Indians of the Southwest could live in one place till they depleted its resources and were compelled to move to new localities.”

To support Douglass' work, the UA's Laboratory of TreeRing Research was created in 1937. Four years later, the Los Angeles Department of Water and Power contracted the lab to use tree rings to examine historic Colorado River flows to predict future water supply for “national defense purposes.” Los Angeles, which already benefitted from electricity generated by the river as it passed through Hoover Dam (then Boulder Dam), was about to complete construction of the Colorado River Aqueduct to import water to Southern California. The study took samples from the Upper Colorado River Basin to examine river flows between 1366 and 1941. The resulting report, written by Douglass' student and protégé Edmund Schulman, revealed significant droughts in one out of every five years, and roughly the same for years of excess rain, during every century studied.

The results did nothing to temper California's draw of the river. Scientific information about drought had already been ignored in 1922, when the Colorado River Compact divvied up the waterway among Western states. Via a simple accounting equation, the compact allocated more water than existed in the river even during its wettest periods. And it ignored the deep knowledge of Indigenous peoples who had lived in the region's arid landscape since time immemorial. In fact, the compact didn't offer tribal governments — or Mexico, for that matter — river water at all.

Instead, the U.S. government, states and private developers siphoned the Colorado to grow a cash crop: cities. They mined water to expand the megalopolis of Phoenix in the desert. They harnessed the river behind dams, drowning Glen Canyon in exchange for electricity. And they wrangled water to groundwater-depleted Tucson via a concrete canal. There was never enough water to build the American Southwest we live in now, yet development continued despite the known consequences, resting on the theory that someone else could solve those problems later.

Later is now. This year, a new study using dendrochronology revealed that in the past 500 years, there has been no drought as severe as that of the past two decades, which also has been the warmest 20-year period in half a millennium. These conditions have been heightened by fossil-fuel consumption. The scenario which scientists now describe as "aridification," rather than drought — is exacerbating the drawdown of the Colorado, which is predicted to lose 30 percent of its annual flow in the next 25 years.

Some parameters of the Colorado River Compact will be up for renegotiation before they expire at the end of 2026. These include how much water is allocated to each Colorado River Basin state — Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming — and to Mexico. Twelve of the 30 tribal nations with ancestral ties to the river will be looking for resolution of their water rights, too. The question is: How do we share even less water among even more stakeholders — and the 40 million people who depend upon it?

NEWSPAPER CLIPPINGS and magazine articles about Douglass' scientific career fill several boxes at the UA's Special Collections facility, and Arizona Highways published feature stories about him in 1940 and 1946. He continued his research into his early 90s and died in 1962, at age 94. In the years since, his name and remarkable career fell off the page, and his work is rarely cited by name in articles about the West's "megadrought."

In the rooms of Aztec, Fred Blackburn encouraged our team to "document, document, document." These instructions did far more than record modern human history at Aztec. They helped preserve a link between Douglass' punctures in window lintels and the climate stories the trees have told. They helped us remember how, and why, we learned to read the history recorded in trees.

Outside Aztec's walls, the Animas River pulses over polished stones on its way to meet the San Juan River before that waterway converges with the Colorado. Its steady flow is a poorer indicator of drought than what the trees know. Although Ancestral Puebloans inhabited the region for millennia, Aztec was a boomtown: Douglass' tree-ring records show that humans occupied these buildings only from 1100 to 1200. A great drought followed.

The modern-day community of Aztec, like many Southwestern cities, is less than two centuries old. A great drought is here. The trees continue to take note.