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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.
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).
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
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This document was uploaded on 12/16/2013.
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