There are a number of incentives to retain broadleaf tree species, such as paper birch (Betula papyrifera Marsh.), following logging in the Interior Cedar-Hemlock biogeoclimatic zone. For example, the roots of paper birch are intimately associated with numerous species of ectomycorrhizal fungi. The retention in plantations of paper birch may, therefore, contribute to below-ground biodiversity by hosting a variety of mycorrhizal fungi. Furthermore, the retention of paper birch in young mixed plantations appears to reduce the incidence of Armillaria ostoyae among Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) (Morrison et al. 1988; Simard 1997) and improve both the nitrogen and pH status of soils. Finally, the presence of paper birch increases the diversity of tree species and contributes to the structural diversity of a stand (Simard and Vyse 1994).
Ectomycorrhizae are the nutrient-and water-absorbing organs of most woody plants and, as such, mycorrhization of planted seedlings is thought to be critical for adequate growth and survival. However, ectomycorrhizal fungi do not persist in the absence of a plant host and therefore the quantity and diversity of fungal inoculum often decreases after logging. Ectomycorrhizal species, such as paper birch, may act as reservoirs (refuge plants) for ectomycorrhizal fungi on a site, thereby maintaining a diversity of fungi that may eventually associate with planted conifers.
Interior Cedar-Hemlock forests are often considered rich in tree species; however, as with most temperate forests, the diversity of plants is small compared to the enormous diversity of soil organisms. For example, there are an estimated 5000 species of fungi capable of forming ectomycorrhizae. Douglas-fir alone can associate with an estimated 2000 species of ectomycorrhizal fungi, and there can be as many as 100 different species in a typical Douglas-fir stand. Among the many species of ectomycorrhizal fungi, considerable differences exist with respect to their physiology and ecology. Ectomycorrhizal fungi differ in their ability to take up various forms and types of nutrients, in their tolerance to water stress and temperature extremes, and in their resistance to pathogens. Therefore, it is hypothesized that seedlings associated with a diverse array of ectomycorrhizal fungi will have an increased capability to fully exploit the heterogeneous soil through which the roots are growing.
The objectives of this study were to investigate the effects of planting mixtures of paper birch and Douglas-fir at various densities and proportions on the diversity of ectomycorrhizal fungi. This study was carried out at three different sites over three years (1992-1994) in the Interior Cedar-Hemlock biogeoclimatic zone (ICH) of the southern Interior of British Columbia. Two of the three sites, Adams Lake and Malakwa Lake, are located in the ICHmw3 variant, whereas the site at Hidden Lake is located in the ICHmw2 variant. All sites were harvested prior to 1988 and planted with Douglas-fir. In the fall of 1991, the sites were de-stumped to reduce the inoculum load of A. ostoyae and, as a result, the previously planted seedlings were also removed.
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Updated April 17, 2007