Forest Ecosystem Topics

Diversity and Age Class

Physical and biological diversity are important attributes of ecosystems. In forests, one measure of biological diversity is the diversity of tree species. A forest with many kinds of trees also supports many kinds of other organisms: more kinds of animals, plants, fungi, lichens and so on. Diversity of tree species is measured at different scales: within a stand, the number of species is counted. Many stands of different species mixes can make up a biogeoclimatic zone, each of which has a characteristic complex of tree communities. Across the landscape, several biogeoclimatic zones are characteristic of each ecozone.

The diversity of forest age classes gives physical structure to a forest, and also contributes to biological diversity, because some organisms require young forests, while others like them old. As well, some animals need a young forest for some functions (like browsing), and old forests for others (like hiding from predators). Information on important wildlife species and endangered species in the Columbia Basin is available elsewhere. This page presents information on the diversity of tree species and age structure within the forest types of the Columbia Basin.

Tree Diversity

The federal government aggregates forest statistical data by ecozone, and in some cases by ecoregion (see Terrestrial Ecozones of Canada in the Ecosystem Diversity section). All of the Columbia Basin is in Canada's Montane Cordillera ecozone. At right is a graph of the area of forest types represented by the dominant tree species. The large area dominated by spruce (mostly Englemann spruce), pine (mainly lodgepole pine, although large areas of ponderosa pine occur in the southeast) and true fir (mainly subalpine fir) reflect the extent of high elevation forest of this mountainous region. Lodgepole pine is an early successional species, meaning that it sprouts thickly and grows quickly following disturbance, such as fire. Stands of pure lodgepole pine have typically have a low biodiversity and offer scant support for wildlife that require a well-developed understory. At the stand ages, the pines begin to die and spruce comes up. The openings left by dead trees provide space and sunlight for deciduous trees and shrubs. The dying and fallen trees themselves provide habitat for wildlife, such as insects and woodpeckers. Therefore, as forests age in this high-elevation zone, they become much more biologically diverse.

The "other conifer" category includes western red cedar, a very important tree of lower elevation forests in the Interior Cedar-Hemlock biogeoclimatic zone (see Ecosystem Diversity) of the Columbia Basin. This biogeoclimatic zone also has the highest diversity of tree species of all the biogeoclimatic zones in the province. Besides western cedar and hemlock, the ICH zone hosts sizable numbers of Douglas-fir, Pacific yew, western larch, Englemann spruce and western white pine. Poplars, including both aspen and cottonwood species, and paper birch are common throughout the low- and mid-elevation forest types. These and other broad-leafed species are critically important for wildlife, and are becoming increasingly recognized as essential to forest nutrient cycling.

The volume of commercially harvestable wood in the tree species of the Montane Cordillera ecozone is different from the area dominated by each species. For example, pine, mainly lodgepole pine, covers large areas of high elevation land, but these thin trees produce comparatively little lumber (and provide relatively little wildlife habitat). By contrast, forests dominated by big, old hemlock trees are not big by area, but produce large volumes of lumber (and high non-timber values). High quality, large trees like western white pine do not influence the volume data significantly because they are widely scattered within the forest. Throughout the region, western white pine suffers from an exotic (introduced; non-native) disease, white pine blister rust, that will limit its place in future forests unless resistant varieties can be found and widely planted.

Old Growth Forest

Diversity of forest age classes is another important measure of forest ecosystem health. All older forests are not necessarily "old growth." Specialists have defined a number of "attributes" that describe "old growth" forest. They include large trees for species and site, wide variation in tree sizes and spacing, large dead standing and fallen trees, multiple canopy layers, canopy gaps and understory patchiness and decadence in the form of broken trunks and root decay. The age of "old growth" forests vary by forest type, ranging from about around 250 years for coastal old growth, to 350 years or more for interior cedar-hemlock forests. British Columbia has always had, and still retains, a major proportion of old growth forests in western North America. As well, a sizeable proportion of northwestern North America's protected old growth is in British Columbia.

Mature Forests in the Columbia Basin

The amount of mature versus immature forest is important to biodiversity because (a) a mosaic of age classes across the landscape supports the highest biodiversity values, and (b) it is one of the ecosystem attributes most susceptible to change following settlement and accompanying wildfires and timber harvest. In settled regions, the amount of mature forest tends to decline, along with populations of plants and wildlife that require older forest types. In the graph above, old growth is included in the "mature" age class used by the Ministry of Forests, which is 80+ years for lodgepole pine and 120+ years for other coniferous forests. In most regions of British Columbia, there is more "mature" than "immature" forest. In the Columbia Basin, however, logging and fires have reduced the amount of mature forests to less than the amount of immature forest within the productive forest land base (i.e., not including non-forest lands and unproductive forest lands).

References

Lowe, J.J., K. Power and M.W. Marsan, 1996. Canada's forest inventory 1991: Summary by terrestrial ecozone and ecoregions. Canadian Forest Service, Ottawa.

Ministry of Forests, 1998. Annual Report 1996/97.

Forest Ecosystem Topics