A number of studies have examined competition and

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Unformatted text preview: ommunity succession and ecosystem productivity (Perry 1985), there is considerable interest in understanding the interaction mechanisms and regulating factors affecting the balance between inhibition and facilitation. A number of studies have examined competition and facilitation along resource and stress gradients (Goldberg et al. 1999; Maestre et al. 2005; Lortie and Callaway 2006), but few have quantified the net effect of trade-offs between these interactions within sites. While trade-offs in community ecology commonly involve competition versus colonization, or the differential ability of species to acquire and utilize resources (Kneitel and Chase 2004), the concept can also be applied to competitive and facilitative effects on interactions among plant traits, such as trade-offs between survival and growth (Stearnes 1989; Simard et al. 2006). In managed forests, competition from broadleaf trees is commonly manipulated to increase conifer growth (Wagner et al. 2005), but the possible trade-off with reduced facilitation is routinely ignored. In the northern temperate and boreal forests of western Canada, paper birch (Betula papyrifera Marsh.) and trembling aspen (Populus tremuloides Michx.) are commonly removed from plantations to allow high-yield conifer production (Lieffers et al. 1996; Comeau et al. 2000; Wagner et al. 2005). This practice is based on the assumptions that broadleaf trees compete intensively with conifers for resources, that their facilitative effects are of less importance to ecosystem function, and that intensive weeding will result in greater conifer productivity (Wagner et al. 2001, 2005). The practice has been supported in several Canadian provinces by the implementation of free-growing policy to regulate competition from broadleaf trees and other vegetation (Brand and Weetman 1986; Comeau et al. 2000; Lieffers et al. 2002). The ecological and economic value of broadleaf trees to forest ecosystems has received increasing attention in Europe and America, however (Swanson et al. 1997; Lautenschlager 2000; Löfman and Kouki 2001; Comeau et al. 2005), raising questions about the effects of a free-growing policy and resultant practices on ecosystem resilience. Several conifer species commonly associate with broadleaf trees, forming stratified, mixed-species stands in the native temperate-zone forests of western North America (Cameron 1996). Up to 10 species can co-exist on a single site in the Interior Cedar Hemlock (ICH) biogeoclimatic zone of British Columbia (Meidinger and Pojar 1991). For example, shade-intolerant species such as interior Douglasfir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), western larch (Larix occidentalis Nutt.), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Wats.), and western white pine (Pinus monticola Dougl.) can regenerate alongside shade-tolerant western redcedar (Thuja plicata Donn), western hemlock (Tsuga heterophylla (Raf.) Sarg.), interior spruce (Picea glauca (Moench) Voss var. engelmannii (Parry) Boivin), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.), as well as shade-intolerant paper birch, trembling aspen, and black cottonwood (Populus trichocarpa Torr. & Gray) following stand-replacing fires (Johnson et al. 1990) or clear-cutting (Simard and Vyse 2487 1994). The density and proportion of regenerating species vary widely depending on seed source, microsite conditions, and degree of disturbance, with localized broadleaf densities ranging between 1000 and 60 000 stems·ha–1 up to 10 years post disturbance (Simard and Vyse 1994). Species variation in height growth and shade tolerance results in early dominance by broadleaf trees and shade-intolerant conifers, but intense vertical stratification quickly gives rise to mixed conifer – broadleaf stands, with eventual dominance by shade-tolerant conifers such as western redcedar and western hemlock. The structural complexity of these forests is maintained through gap-phase dynamics caused in part by the pathogenic fungus Armillaria ostoyae (Romagn.) Herink, which causes significant mortality (0.1%–0.2%·year–1 in young stands) among susceptible conifer species. Broadleaf species are resistant to this root disease, allowing them to persist in older forests in gaps created by conifer mortality (Morrison et al. 1991). Paper birch and trembling aspen can negatively affect conifer growth and survival early in stand development by competing for light and soil resources (Comeau et al. 2003; Simard and Sachs 2004). Paper birch and trembling aspen are pioneer species, however, and their rapid self-thinning, short life-span, and asymptotic height growth pattern (Klinka et al. 2000; Kabzems and Garcia 2004) result in declining competitive effects on conifers as mixed stands develop (Frivold and Frank 2002; Simard et al. 2004b). Freegrowing policies have focused on early competition from broadleaf trees for site resources (Brand and Weetman 1986; Lieffers et al. 2002), ignoring the possibility that broadleaf trees can have f...
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This document was uploaded on 12/16/2013.

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