1988 gerlach et al 1997 reduce the risk of conifer

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Unformatted text preview: acilitative effects on conifer regeneration, and that competitive effects can vary over time and space. It has previously been shown, for example, that broadleaf trees help limit the spread of root disease among conifers (Morrison et al. 1988; Gerlach et al. 1997), reduce the risk of conifer attack by weevils and spruce budworm (Taylor et al. 1994; McIntosh et al. 1996), protect understory conifers against frost (Andersson 1985; Pritchard and Comeau 2004), provide habitat for ungulates, small mammals, and birds (Peterson et al. 1997; Aitken et al. 2002), and contribute to soil productivity through nutrient-rich litter inputs (Brockley and Sanborn 2003). In keeping with this last point, total yield (conifers plus broadleaf trees) in mixed forests has sometimes been higher than in pure coniferous forests because of niche separation (Kenk 1992; Mielikainen 1996; Frivold and Kolström 1999; Man and Lieffers 1999; Valkonen and Valsta 2001). The typical management pathway for achieving freegrowing high-yield conifer stands in temperate-zone mixed forests over the last 20 years can be described as intensive, and is aimed at increasing conifer survival and growth rates through reduction in competition from broadleaf trees (Table 1). In British Columbia, standards governing freegrowing plantations were recently modified (British Columbia Ministry of Forests 2000) to allow a conifer two to five neighboring broadleaf trees (the number depending on the conifer species) within a 3.99-m radius, provided that they are shorter than the conifer in three of four quadrants (British Columbia Ministry of Forests 2000). Broadleaf trees can be included as a component of acceptable stands only where the effect on crop-tree growth is minimal. This condition is © 2006 NRC Canada 2488 Can. J. For. Res. Vol. 36, 2006 Table 1. The current management pathway and three alternative pathways for managing mixtures across the landscape. Management action Management pathway Site preparation Stumping for root-disease management Current pathway: conifer sawlog production Mechanical Yes First pathway: conifer enrichment Mechanical or none None Second pathway: semi-natural management None None Third pathway: high-intensity management Mechanical Yes Management aim Vegetation control Regeneration Planting at moderate density; two or three early-successional conifer species Planting two or three early- and latesuccessional conifer species at low density No planting Planting single earlysuccessional species at moderate to high density rarely encountered because trembling aspen and paper birch grow more rapidly than most conifers. Thus, almost none of the conifer plantations in the interior cedar–hemlock forests meet the requirements for free-growing plantations without a brushing treatment to remove the broadleaf trees. The entire management pathway for achieving free-growing plantations is as follows. Following harvesting of root-disease-infested sites, infected stumps are usually extracted to reduce rhizomorph production and inoculum loads of A. ostoyae (Morrison et al. 1991). This is rapidly followed by planting of large container-stock conifer seedlings of one or two early-succession species, usually Douglas-fir or lodgepole pine. These species are chosen because of their current commercial value, rapid juvenile height growth allowing early attainment of free-growing status, and broad edaphic amplitude (British Columbia Ministry of Forests 2000; Klinka et al. 2000). Prior to the assessment of free-growing status, the plantations are weeded of broadleaf trees and other encroaching shrub species, using herbicides or manual cutting. Spacing to reduce conifer densities is commonly applied either with the weeding operation or a few years later, favoring the early-succession conifer species. Even with the recent modifications allowing some broadleaf trees, the general intent remains one of producing early seral conifer sawlogs. Based on international standards this management pathway is expensive (Simard and Vyse 1994), costing in 2005 up to $3000/ha where repeated manual treatments were necessary. It follows the basic principles of management for high-yield conifer production practiced elsewhere in the world (Wagner et al. 2005). Extensive application of similar intensive-management pathways is raising concerns about the diversity, health, and resilience of western North American forests (Lieffers et al. 2002; Woods et al. 2005). In addition to their lower structural diversity (Haeussler and Bergeron 2004), these conifer plantations have been subject to widespread insect Stand Landscape Manual brushing to meet free-growing regulation Selective manual brushing Mixed conifer and minor broadleaf trees, even-aged, single storey Stratified mixture of conifers and broadleaf trees Clearcut No brushing Uncontrolled stratified mixture of conifers and broadleaf trees Single species, conifer or broadleaf trees, even-aged, single storey Continuous cover Broadcast chemical brushing Continuous cover...
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