Land, Water and People: Mixed conifer—Our most diverse forests
Variety is a good word to describe the mixed conifer forests that grow part way up on the slopes of the mountain surrounding the San Luis Valley. A variety of trees live in this life zone, as do a wide variety of shrubs, herbaceous plants, and wildlife. Even the natural return interval for wildland fire varies greatly.
The mixed conifer forest can start as low as 7,800 feet in elevation in some areas and may extend up to more than 10,000 feet. The lower elevations of this zone are often dominated by majestic orange-barked ponderosa pine. These trees are always a favorite with kids as the bark smells like vanilla.
As one travels up in elevation, Douglas-fir mixes in and then white fir, and maybe a few limber pine, blue and Engelmann spruce, subalpine fir, and aspen. Aspen are deciduous, rather than coniferous trees, but that hasn’t kept them out of the mixed conifer zone. Or more accurately, as we will learn later, that hasn’t kept the variety of conifers from moving in with the aspen.
There are a great variety of shrubs that live among the trees in the mid-elevation. The creeping stems and leathery leaves of low growing bearberry, also known as kinnikinnick, often forms green mats covering the forest floor. Their tiny red berries provide food for birds and other wildlife. Oregon grape also grows low to the ground in this zone. Its display of bright yellow flowers stand out in the early summer and its holly-shaped leaves turn orange-red in the autumn.
Not to be outdone, many of the herbaceous plants put on their own spectacular display. The large purple blooms of pasque flowers are one of my personal favorites. This beauty will begin poking up from the forest floor in another month and then show off its white interior and circle of brilliant yellow stamens when the flower opens. Then, as summer begins to unfold, red columbines and wild geraniums will add their colorful touch to the forest floor.
The variety of trees, shrubs and herbaceous plants attract a diversity of birds and other wildlife in the mixed conifer forest. One exceptionally charismatic bird that lives in this area is the northern goshawk. This fairly large raptor nests and hunts in the forest, able to quickly maneuver through the trees to nab other birds, rabbits and small mammals. Elk and deer, of course, are found in the mixed conifer as is the somewhat secretive American marten. This two foot long member of the weasel family spends much of its time in trees seeking out pine squirrels and nesting birds, but doesn’t hesitate to slink around on the ground to dine on snowshoe hares and small mammals.
The mixed conifer forest has a complex fire regime. On drier slopes, large ponderosa pine and Douglas-fir may dominate where fire is a frequent visitor. These fires often burn at a low intensity as they kill young trees and clean up woody debris on the forest floor. The thick bark of the older trees protects them from the heat of the fire. It is in these warm-dry mixed conifer stands that fire suppression has had the biggest impact by allowing young trees and shrubs to grow dense, thus adding fuel that leads to bigger, hotter fires.
Fire burns less often on the higher northern slopes of mixed conifer stands. This cool-moist environment allows thin barked trees, such as white fir, subalpine fir, spruce and limber pine to take hold and thrive. When conditions are dry enough for these areas to burn, they often burn at a higher severity killing all or most of the trees. If there were some aspen in the mix, they will sprout from their roots creating pure stands of aspen that are then slowly invaded by conifers. Eventually, in the absence of fire, a mixed conifer forest will dominate again.
The mixed conifer forests make up only 10 to 15 percent of the Rio Grande National Forest, but the sheer variety of vegetation and wildlife make them a joy to visit.
Mike Blakeman is the public affairs officer for the San Luis Valley Public Lands Center. He spends much of his free time scrambling around the mountains with a camera in his hand.