The shimmering Quaking Aspen surrounding Mt. Humphreys (left) generally sound the opening trumpet of color for the fall symphony. The mix of trees at the Boyce Thompson Arboretum (below) often presents the last and most reliable fall colors each year—in November and sometimes even December.
addicted to fall. And as December neared, I realized I had found myself with an undiminished need to study the contrast between saturated red and sky blue. So I went to the last gasp of fall in the state—not to mention the easiest drive from Phoenix—even though I knew an oddball botanical garden in the low desert could not possibly compete with the places I had already found. I sat and savored the cool caress of the sun-softened winter breeze, balanced atop a 30-million-year-old hummock of ash. Some 80 years ago
iconoclast, miner and entrepreneur Col. William Boyce Thompson built a house atop a nearby crag and bought 350 acres of spectacular Sonoran desert, running back up into the rugged depths of Queen Creek Canyon. A longtime lover of plants, Col. Thompson hired a botanist to lay out his garden, which ultimately laid the foundation for the Arboretum. Sitting on that lichen-crusted, rusted-orange swirl of rock, I let my eye wander across the swaying treetops—enlivened by the fall colors of both the stalwarts of a
Sonoran desert stream and colorful immigrants like the Asian pomegranates or the Chinese pistachios. Fall has always struck me as a reckless waste of biological energy —as trees dropped the leaves they’d labored so hard to create, especially here in the desert, where winter consists of scattered freezes and sunny afternoons. So I’d interviewed some biologists, fitted together puzzle boxes of ecology and mounted this leisurely expedition to unravel the logic of fall in the desert. Essentially, trees all face a vexing
problem: getting through the winter. This may not sound like a big deal if you can suck water out of the ground and make all your food from sunlight. But all sorts of unpleasant things can happen during even a mild desert winter, from a tree’s point of view. For instance, it’s almost sure to freeze several times. In addition, wintering trees must cope with shorter days and less available water as the soil freezes. Many trees solve this problem by developing extremely tough, frost-resistant leaves—like the needles that adorn pines and firs. These needles take a lot of energy to produce and don’t present a very impressive surface for carrying out photosynthesis, but they can shrug off frigid temperatures for months at a time. Other trees deal with winter by mass producing large, flimsy leaves that soak up energy through the salad days of summer, but wither with winter. The drop in temperatures in the early fall triggers a cascade of physiological changes as the trees draw nutrients out of the vulnerable leaves, then seal off the now dispensable leaf. The nutrients left in the leaves still return to the system, once they drift to the ground and decay. “Evergreens invest a lot of energy per leaf and design it to last for several years,” explains Julie Stromberg, a plant ecologist at Arizona State University who specializes in cottonwoods and other desert riparian trees.“Deciduous trees make cheap leaves that don’t cost as much energy to use.” The whole process is orchestrated by the release of certain chemicals, which produce Fall 2008 17