There is a very good reason why the trunks of aspen trees are white. And if you go for a Saturday snowshoe trip with Karen Vail, she will provide the answers to that riddle and many more.
Vail is a naturalist working for the nonprofit educational organization, Yampatika. Vail and her colleagues are dedicated to teaching residents and visitors alike about the natural and cultural resources of Northwest Colorado.
Every Saturday morning during the winter, a Yampatika naturalist meets interested people at Yampatika's office in the U.S. Forest Service Building on South U.S. Highway 40, across from the Holiday Inn.
After signing waiver forms, the group heads downtown to Emerald Mountain for a guided snowshoe tour. The tour is free, but donations are accepted. Snowshoe rentals are $3, and a ticket for a single ride up the Howelsen Hill chairlift is $2.
Even longtime locals might be surprised at how much they learn about their back yard during a Saturday morning spent with Vail.
She has boundless knowledge of the plant and animal communities in the city park on Emerald Mountain, just above the Howelsen Hill Ski Area. She shares it with remarkable enthusiasm.
"Can you tell me why an aspen tree's trunk is white?" Vail quizzed participants during a hike last winter. "Here's a hint: Notice that the part of the tree with leaves on it is relatively small. Think of how many plants there are that in the middle of the winter can turn themselves on."
The answer to Vail's question lies in the fact that aspen are able to continue photosynthesizing all year round, even after their leaves fall off in autumn.
Brush the waxy powder off an aspen, Vail said, and you begin to see a green layer underneath (she refrains from actually doing this because it could harm the tree). The powder gives the aspen's trunk its whitish appearance, but it serves a more important function. The green layer beneath the powdery surface craves sunlight, but it is sensitive to heat. So, even in winter, the color of the tree trunk deflects heat.
"It's a really good adaptation for the climate the trees live in," Vail said. "They need the light, but can't take the heat."
On unusually warm winter days, Vail said, it's possible to press your ear against the tree trunks and listen to the sound of fluids carrying nutrients through the aspen trunks.
Vail leads her snowshoe tours to an overlook of Mount Werner and Buffalo Pass. Even where alpine ski trails have interrupted the forest on Mount Werner, it's easy to watch the progression of species up the mountain. Gambel oak in the foreground give way to lodgepole pine on north facing slopes near the bottom of the mountain, then stands of aspens finally transition to a forest of fir and spruce.
"Plants can tell you so much about an area," Vail said.
She points to an abrupt change on the east flank of Emerald Mountain where a bench drops away to a cool trough and the heat-seeking Gambel oak suddenly yield to aspen.
There also is an unseen environment beneath the 4 feet of snow on Emerald Mountain. Scientists call it the sub-Nivean zone. Colorado's relatively dry snowfall creates temperature gradients in the accumulated drifts. Older snow settles and becomes more dense and is followed by insulating blankets of drier snow. Trapped heat forms layers of depth hoar that can be used by rodents to travel beneath the surface.
In some places, the heat emanating from the earth actually creates an area free of snow known as a "lens pocket."
"It can be 10 feet in diameter," Vail said. "It's like a greenhouse."
Small plant life can continue to grow in a lens pocket and insects remain active. Red-backed voles can raise successive litters all winter long in the unseen environment. However, they must maintain an air path to the surface, and that can be their undoing.
The air passages represent chimneys of scent to predators. With their pronounced sense of smell, coyotes and foxes can locate their prey by the passages.
It's not unusual to see the tracks of brown weasels (ermine), squirrels, foxes and occasional coyotes and elk on a Saturday snowshoe. But the small animals remain in their dens on the stormiest days because they must expend so much energy to move across the fresh snow. Instead, they wait for the snow to settle and develop a top layer that can support their weight.
It's just one of many lessons to be learned about the natural world of the Rocky Mountains in the wintertime.