Naturalist’s Notes: Aspen Leaves

Spring Aspen Leaves

By Beth Dodd:


One of my favorite days of the year is on its way. It’s called “bud burst.” This is the day when the trees open their new leaves in the spring. It happens on a different day every year, and may occur on a different day at your house than at my house. There is no way to know exactly when it will happen, but every spring I can feel it coming in the air and in the earth.

The anticipation is maddening as I wait for the leaves to appear. Starting in April, I’ll watch the wave of new green leaves slowly wash its way up the mountains. It begins in Manitou Springs, moves up Fountain Creek and Ute Pass, until at long last in mid-May it reaches the aspen trees around my home in Divide.

The delicate new leaves glow in the sun, transforming the winter-worn forest with their bright color and dancing movements. While the calendar claims that spring begins on March 20 on the day of the equinox, it’s the little miracle of the leaves reappearing that let’s me know winter is finally over.

The previous year’s leaves drifted to the ground six or seven months ago in a shower of autumn gold. The leaves would be a liability to the aspens in the winter, so they ruthlessly sever them to avoid death by starvation and dehydration. If the trees kept their leaves, the leaves would freeze and die any way, rendering the trees incapable of making sugars for food. Also, water evaporates from the surface of leaves and is difficult or impossible to replace when the ground is frozen.

The process of the trees casting away their leaves begins when they change color in the fall. As the days shorten and nights lengthen, production of green chlorophyll drops off and stops until all of the chlorophyll is gone. Carotenoids and anthocyanins, the substances that make carrots orange and apples red, are revealed.

While the yellow or orange carotinoids are always there, and are especially abundant in aspen leaves, the amount of red anthocyanins can vary from year to year. Anthocyanins are produced when sugar is trapped inside the leaves. This is affected by the weather before and during the time that the green chlorophyll is diminishing, with sunny days and cool nights bringing the best color displays.

The job of chlorophyll is to absorb sunlight and help transform it into sugars during the process of photosynthesis. When chlorophyll production slows in the fall, the veins and stems of the leaves break down, closing the pathways for moving sugars to the rest of the tree. A layer of dead cells forms at the base of each leaf, and is easily separated like the perforations around a coupon. Once this separation layer is complete, the leaf is ready to fall. A puff of wind usually does the trick.

The countless numbers of fallen aspen leaves will decay into the soil and nourish the forest in future years. The mass of dead leaves also holds moisture and helps to insulate and protect the roots of many other plants until spring comes.

Wild animals like deer, elk, and beavers also get help from the aspen trees to survive the winter. They can eat the nutritious bark to help them survive when other foods are in short supply, leaving black scars that contrast boldly with the white bark.

Over the winter aspen trees seem lifeless, but they are only dormant. A lot is still going on inside. With their leaves gone, the trees begin slowly processing the sugars they stored over the summer. Their smooth bark is photosynthetic, so they can produce a small amount of sugar on warm winter days to help them survive. The aspen’s stems, twigs, and buds are equipped to survive extreme cold so the trees can reawaken when spring brings a new growing season.

But how do the trees know when to wake up? Warm temperatures alone are not enough to inspire the bud burst. Otherwise, the trees might expose their fragile leaves too soon during an early warm spell in January or February, only to have them frozen when the cold returned. Oddly, the key factor actually seems to be the cold. Depending on the species, deciduous trees (a.k.a. the leaf droppers like aspens, maples, and oaks) require an extended period of dormancy before they open their buds in the spring. If it doesn’t get cold enough for long enough, the trees won’t revive.

However, when a tree’s dormancy requirements have been met and the time is right, a surge of hormones and enzymes will start the tree growing again. Before their leaves emerge, aspens produce dangling, fuzzy catkins. These are the tree’s flowers, which are not colorful like the spring blossoms of apple and dogwood trees, but resemble furry caterpillars. The air temperature must be in the 50s for about a week for the flowers to bloom, which usually happens in late April or early May in Teller County.

Aspens grow in groups called groves, which may consist of many genetically identical trunks called clones sharing a single root system. Cloned groves are either all male or all female, with pollen producing catkins on male trees and seed producing catkins on female trees. Aspen pollen and seeds are carried by the wind, and if a pollinated seed lands in sunny, damp soil, a new grove can start.

Once the business of reproduction is underway, the aspen trees are ready to open their precious new leaves at last. The bud scales that protected the tender branch tips all winter are shed, and suddenly the forest is renewed with brilliant green leaves bursting from every branch. The tiny leaves grow quickly, and their green color darkens as they bask in the sun and send welcome surges of sugar flowing through the trees once again.