For most deciduous trees, winter is a time of metabolic dormancy. As sunlight dwindles and as snow and frozen soils leave water inaccessible, these trees cease to photosynthesize and grow. But there is one exception, a tree species full of peculiar and wonderful adaptations, not only in winter but year-round, a unique beauty and indeed a mind-boggling record-setter.
So let’s consider some of the many reasons to appreciate the quaking aspen.
Quaking aspens are distinctive in that their trunks and branches can carry out photosynthesis throughout their lives. Their thin bark allows sunlight to pass through to the cortex, where cells contain chlorophyll. This adaptation allows the aspens to convert the sun’s energy to sugars and starches even when leafless in winter.
Both the tree’s common name and its scientific name (Populus tremuloides) refer to how aspen leaves tremble even in the slightest breeze, collectively creating shimmering ripples of motion, spectacular especially when the leaves are golden in fall. The leaves move in this way because their stems (petioles) are flattened perpendicular to the leaf blades and thus lack rigidity. It is less clear why quaking aspen leaves have evolved to quake in the wind. Scientists have posited that the movement moderates the distribution of sunshine through the crown, or that it ensures air circulation for optimal photosynthesis, or that it inhibits damage to the leaves by insects.
Photo: Hannah George
Quaking aspens reproduce mostly asexually, via horizontal growth of roots and sprouting of new stems. What appears to be a grove of many quaking aspen trees is thus usually a single organism, called a clone. This explains why the many “trees” in a single aspen stand all tend to leaf out in spring and change color in autumn at the very same time.
An expansive root network allows the many trunks and crowns throughout a clone to share water and soil nutrients that may be unequally distributed. A wide-ranging mature root system also allows aspen clones to resprout after landslides and fires, which might take out most or all of the trees above ground, but rarely completely destroy the roots below.
Aspen clones can spread prodigiously if sunlight and soil conditions permit. Indeed, the most massive known living thing in the world is a quaking aspen clone in the Wasatch Mountains in Utah. Named “Pando” (Latin for “I spread”), this aspen stand has expanded from a single seed to its current extent of about 107 acres and 47,000 trunks. Scientists estimate that Pando weighs 14 million pounds. Pando is probably also extremely old – indeed, possibly the oldest known organism on Earth. Plausible conjectures of Pando’s age range from thousands to tens of thousands of years and upward. But it is currently impossible to determine precisely when Pando’s seed first germinated.
Quaking aspens reproduce sexually as well. Each aspen clone is either male or female, with outwardly similar sexual organs called catkins. Catkins of male trees each produce millions of pollen grains, which are carried away by the wind. Catkins of female trees produce ovules, which, when fertilized by the pollen, become tiny, fluffy seeds. These seeds too are dispersed by the wind. Most do not survive, but those that land on a favorable bed of soil germinate within a few days.
Given this versatile adaptive and reproductive toolkit, allowing for both in situ resiliency and opportunistic dispersal, it’s little wonder that the quaking aspen is the most widely distributed tree species in North America. It ranges from Alaska throughout Canada and the Great Lakes states to the Atlantic, and south through the Sierra Nevada and Rocky Mountains to Mexico, growing in an unusually wide range of habitats, from dry, rocky alpine slopes to wet lowlands. Throughout their range, quaking aspens are vital pillars of their ecosystems, providing high-quality habitat for a wide range of wildlife from bears to Golden-winged Warblers. They are an especially important source of sustenance for Ruffed Grouse, who feed on the budding catkins throughout the winter.
Quaking aspens are common in our Finger Lakes region. Look for them especially in relatively open areas, such as successional fields and recently logged forests. They’re easiest to spot at a distance in fall, when all the leaves in a clonal stand turn gold at once. But even now in winter, you can find them by that distinctive bark, which is light greenish on younger trees and newer branches, and white with black scars on the trunks of mature trees.
