2006 nrc canada simard and vyse baleshta et al 2005

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Unformatted text preview: describe the results of several experiments in which we tested these competition thresholds by manipulating birch densities using a variety of weeding methods. Growth In several experiments we have found that diameter growth of young conifers generally increases with intensity of broadleaf reduction, whereas height is unaffected. © 2006 NRC Canada Simard and Vyse Baleshta et al. (2005) found that 10- to 15-year-old Douglasfir increased in mean diameter increment when broadleaf trees were completely removed or heavily thinned to 400 stems·ha–1, but not where they were thinned to moderate density (1111 stems·ha–1), suggesting that broadleaf densities above 400 stems·ha–1 suppressed Douglas-fir growth potential. In a separate mixed plantation, Simard and Hannam (2000) similarly found that reductions of 8-year-old paper birch from 2500 to 50 stems·ha–1 significantly improved diameter growth of interior spruce. Simard et al. (2005) tested the effect of broadcast weeding over 20 sites using three methods and found that Douglas-fir diameter increased by 37% and 17% following the cut-stump glyphosate and girdling treatments, respectively, when broadleaf density was reduced below 4400 stems·ha–1, or 15% cover. By contrast, it was unaffected by manual cutting, where most broadleaf trees vigorously sprouted back to an average density of 10 812 stems·ha–1, or 25% cover. As predicted by the competition threshold model of Wagner (2000), these studies together show that the greatest average conifer-diameter response occurred where broadleaf trees were reduced to low levels (below 5% of the maximum broadleaf density on the sites), and small responses started at intermediate levels (approximately 20% of the maximum broadleaf density). Our results fall within the competition thresholds of Simard (1990) and Simard et al. (2001), where maximum responses between 0 and 400 stems·ha–1 and minimum responses between 1000 and 4400 stems·ha–1, or ≤15% cover, were predicted (Simard and Heineman 1996; Baleshta et al. 2005; Simard and Hannam 2000). They also agree with the majority of competition studies, which show maximal growth gains between 5% and 10% cover and minimal gains starting below 20% (Wagner 2000). In none of these studies have we found evidence of a competition threshold for height losses, reflecting the relative insensitivity of height to competition (Wagner and Radosevich 1998). Differences in conifer species responses to weeding reflect earlier predictions based on shade-tolerance rankings (Simard and Sachs 2004). Simard et al. (2005) found that lodgepole pine, for example, responded to smaller decreases in paper birch density than interior Douglas-fir. Lodgepole pine is a relatively shade-intolerant species and highly plastic in its response to increasing light availability (Wright et al. 1998), allowing it to respond more readily to moderate increases in light than Douglas-fir. Lodgepole pine also has a more rapid juvenile height growth rate (Klinka et al. 2000), putting it in a better competitive position with respect to broadleaf trees. Other studies have also shown that shadeintolerant species with fast juvenile growth respond more dramatically to release from competition with broadleaf trees than shade-tolerant species do (Frivold and Frank 2002). Survival In none of these studies did reduction of broadleaf density improve conifer survival. This pattern, where conifer survival is restricted at higher competition levels than growth, agrees with the competition-threshold model of Wagner (2000). In stark contrast to this model, however, mortality has consistently increased following manual weeding of broadleaf trees because of increased incidence of A. ostoyae root disease (Woods 1994; Simard and Heineman 1996; 2491 Baleshta et al. 2005; Simard et al. 2005). Here, broadleaf trees appear to facilitate conifer survival through protection against root disease. Mortality responses to weeding of broadleaf trees have varied with treatment method and intensity. While cut-stump glyphosate treatment has had no effect, mortality of Douglas-fir has increased 1.5–7 times following manual cutting and girdling treatments (Woods 1994; Simard and Heineman 1996; Baleshta et al. 2005; Simard et al. 2005). These mortality rates have the potential to reduce stocking levels in young stands and yield at rotation, even though mortality due to A. ostoyae may stabilize with time (Cruickshank et al. 1997). The difference in treatment method effect is likely associated with the competitiveness of A. ostoyae relative to other saprophytic fungi on manually stressed versus chemically killed paper birch roots. When birch is manually cut or girdled, the remaining stumps and roots are stressed, which stimulates infection by A. ostoyae and increases its potential to infect surrounding conifer hosts through rhizomorph spread or root contacts (Morrison et al. 2000, 2001). Increased growth of conifers released from competition wi...
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