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1997 woods et al 2005 this kind of damage along with

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Unformatted text preview: Clearcut and disease damage, such as mountain pine beetle (Dendroctonus ponderosae Hopkins), Dothistroma needle blight (Dothistroma septosporum (Dorog.) Morelet), and A. ostoyae root disease (Cruickshank et al. 1997; Woods et al. 2005). This kind of damage, along with mortality caused by drought and fire, is expected to increase in southern interior British Columbia with changing climatic conditions (Hansen et al. 2005; Hamann and Wang 2005). Not only is this management pathway expensive, but it may be placing our forests at an elevated risk of future problems in the face of climatic uncertainty. The objectives of this article are to review our understanding of interspecific interactions between conifers and broadleaf trees in the mixed temperate-zone forests of interior British Columbia, and to examine whether management practices aimed at achieving free-growing conifer plantations result in a trade-off between the competitive versus the facilitative effects of broadleaf trees on conifers. Most of the review is focused on our own work because it consitutes a substantial portion of the research conducted in these southern interior forests, but we support and complement it with the results of work done elsewhere in similar forest types. We follow this review with a comparison of management pathways that may be applied across the landscape, ranging from a low-intensity “do nothing” approach to a more intensive management pathway than the current management practices described above. We end with a discussion of new directions for policy and management. Interspecific interactions Our research shows that broadleaf trees are both facilitative and competitive to conifer establishment and growth in mixed forests, and that the relative strength of these interactions varies over time and space and with conifer species (Simard et al. 1997a, 2004b, 2005; DeLong et al. © 2006 NRC Canada Simard and Vyse 2002; Simard and Sachs 2004; Baleshta et al. 2005). During seedling establishment (to 3 years old), soil rhizosphere microbes associated with paper birch facilitate ectomycorrhizal carbon transfer, associative nitrogen fixation, and A. ostoyae antagonism, potentially favoring Douglas-fir survival and growth of seedlings. When the young trees are saplings (3– 15 years old), rapidly growing paper birch overtop Douglasfir, and can reduce light and water availability as well as growth rates of the conifers. Once the stands reach crown closure (at 15–20 years), however, the competitive effects of paper birch diminish with increasing conifer dominance and shading. By 50 years, paper birch has substantially selfthinned from the mixed stands, and inter-conifer competition for soil resources is a much stronger determinant of conifer growth rates than broadleaf competition. Over the long term, ecosystem modeling suggests that maintaining a component of paper birch is necessary for maintaining productivity of Douglas-fir, owing to its nitrogen inputs (Sachs 1996). Stand-establishment stage When stands are establishing, paper birch and conifer root systems are quickly inoculated with mycorrhizal fungi, with greater evenness (Jones et al. 1997), richness (Simard et al. 1997a), and diversity (Simard et al. 1997c) of fungal types on Douglas-fir when it is grown in mixture with paper birch than when it is grown alone. We suggest that this tree species “mixture effect” on the ectomycorrhizal fungi is beneficial to Douglas-fir because mycorrhizal fungi differ in their abilities to break down organic nutrients, transport water, protect against pathogens, and colonize different types of soil, thus conferring increased physiological diversity on the Douglas-fir seedlings. By the time seedlings are 1 year old, more than half of the root tips of paper birch and Douglas-fir are inoculated with the same fungal types (Jones et al. 1997; Simard et al. 1997a), potentially linking the tree species in a common mycorrhizal network (CMN; Simard and Durall 2004). We found that carbon can transfer between paper birch and Douglas-fir through this CMN, with net transfer from birch to fir when seedlings are 2 years old (Simard et al. 1997b). We suggest that carbon is transferred through the CMN in combination with nitrogen as amino acids along a source–sink gradient, with paper birch functioning as a source and Douglas-fir as a sink for carbon and nitrogen. At this age, net photosynthetic rates, fine root carbon allocation, and tissue nitrogen contents of paper birch seedlings are more than double those of Douglas-fir (Wang et al. 1996; Simard et al. 1997b), which agrees with broadleaf conifer comparisons in many other studies (Perry 1994). The high nitrogen content of paper birch may result from associative N2 fixation by the rhizosphere bacteria, with rates of fixation in these forests (estimated up to 25 kg·ha–1·year–1) being higher among fine than among coarse paper birch roots (Blenner-Hassett 1996; Simard 1996). Paper birch grows taller than...
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