Aspen, birch and black cottonwood are an important component of the forest environment in the Columbia Basin, both as pure stands interspersed through the landscape and as individual or small groups of trees within coniferous forests. Our objectives in this project are to:
1. provide an overview of the life strategy attributes of hardwoods and their importance for wildlife;
2. provide an assessment, using forest cover data, of the distribution and age class structure of hardwood stands in the Columbia Basin Fish and Wildlife Compensation Program (CBFWCP) area;
3. assess disturbance history as it relates to the current distribution and abundance of hardwoods;
4. provide an overview of regional land use policies and stand management strategies that affect hardwoods;
5. identify the long term risks faced by hardwoods in the CBFWCP area; and finally,
6. recommend and prioritize conservation and restoration actions that the CBFWCP can undertake with respect to hardwoods.
Hardwood distribution is strongly related to site condition and they are found in most biogeoclimatic zones. They are well adapted to fire and respond well to other disturbance factors such as logging, fluvial process, avalanche activity and construction activities that result in exposed soils. Flooding of riparian areas by dams along the major rivers in the Columbia Basin has been a major factor in the decline of cottonwood stands and related riparian habitats. Browsing by ungulates and livestock has likely had a major impact on aspen and cottonwood recruitment in the Rocky Mountain Trench, Elk Valley and Robson Valley.
Hardwood stands have several attributes important for wildlife. Hardwoods:
- produce exceptionally high biomass in the early years of stand development that is utilized as forage by ungulates and other browsers;
- have a relatively short life span and provide vertical structure, cavity sites, snags, and down wood more quickly than do conifers;
- are more susceptible to heartwood rot at a younger age than conifers and thus provide for cavity creation earlier;
- provide cavity creation situations in live trees more often than do conifers;
- are more susceptible to insect herbivory and thus support larger insect populations than do conifers;
- are more palatable than conifers and thus are used by a range of herbivorous insects and mammalian browsers and finally;
- support a more productive shrub layer and herb layer than generally occurs under conifers, thus increasing the complexity and diversity of bird habitat provided.
As a result, hardwoods are used by a wide range of wildlife species. They provide high forage value to ungulates and other browsers in the early seral stages. They support a diverse songbird fauna in early seral stages, and a different but equally diverse and abundant range of songbirds in mid and later seral stages. There is a high incidence of use by cavity-using species in later seral stages. Birch and aspen are very important to a range of smaller cavity nesters and insectivorous woodpeckers while cottonwoods provide larger cavity sites, created by pileated woodpeckers, that are used by an array of larger birds and small mammals such as fisher. Hardwoods do not provide habitat attributes that do not occur in conifer forests, but they do provide those attributes in greater abundance and at a younger age. As a result, many species show a marked preference for hardwood stands. Where hardwoods occur in riparian areas, very high species diversity and abundance occur. The habitat complexity and richness found in all hardwood stands is amplified by the richness of riparian sites, the larger size and longevity of cottonwood stands and the increase in vertical structure provided by generally taller stands. They also provide habitat for a range of riverine species such as beaver, osprey and cavity nesting ducks. These areas are also critical for songbirds as resting and feeding areas during migration. Hardwoods also play a critical role where they occur as individual trees or as small stands within coniferous stands, primarily by providing cavity nesting sites. We found seven listed species that are dependent to some degree on hardwoods. Most of these occur use hardwoods in riparian areas.
Pure cottonwood stands occur primarily in floodplain riparian areas along the major rivers and tributaries. Aspen stands are found on south-facing slopes and valley bottoms in major valleys in the Rockies and in some parts of the Arrow Forest District (Trail area). Pure stands of birch are very uncommon. All three species are found in mixed wood stands; and as a minor component in the extensive coniferous forests of the area. Pure hardwood stands make up only 1.1% of the total forested area of the study area, however all stands with a hardwood component cover >500,000 ha and 10.8% of the total forested area of the study area. Stands that contain cottonwood are a minor component in all forest districts (<1.9%) except the Robson Valley (2.9%). Aspen is found in 7.4% of stands while birch makes up 4.0% of all stands. The greatest area and percentage of forested area with a hardwood component was in the Arrow Forest District (14.7%).
Age class data for hardwoods show that recent recruitment to hardwood stands is variable between forest districts. The Arrow, Kootenay Lake, Cranbrook, Revelstoke and Robson Valley forest districts show limited recruitment of aspen and birch while the Golden and Invermere areas have substantial areas of young aspen and birch stands. In contrast, cottonwood recruitment is low in the Cranbrook and Invermere forest districts, but is substantial in the other forest districts. Harvesting in conifer stands with a hardwood component has created the disturbance required for hardwood recruitment in many areas, resulting in a substantial hardwood component in many young stands, especially in wet belt areas.