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Unformatted text preview: th broadleaf trees have also been associated
with increased infection rates (Rosso and Hansen 1998;
Baleshta et al. 2005), and the faster Douglas-fir growth following girdling than manual cutting reported by Simard et
al. (2005) corresponded to slightly higher mortality rates.
Cut-stump glyphosate treatment, by contrast, has not increased mortality of Douglas-fir due to A. ostoyae (Simard
and Heineman 1996; Simard et al. 2005), probably because
the root systems are rapidly killed, thus providing a ready
food source for saprophytes that are more aggressive than
A. ostoyae (Morrison and Pellow 2002).
Where manual cutting of paper birch was used, mortality
of Douglas-fir caused by A. ostoyae increased with cutting
intensity (Baleshta et al. 2005). Any amount of birch cutting
stimulated disease spread, but low-intensity cutting kept
mortality rates relatively low. This may be explained by the
greater inoculum load remaining on the more intensively cut
sites, or by the faster growth of residual Douglas-fir. Vegetation-management treatments aimed at releasing conifers
from competition with paper birch ought to balance growth
gains against mortality losses in stands where A. ostoyae
root disease is present. To achieve this balance on diseased
sites, we suggest that paper birch be manually cut on a selective basis, targeting only individual Douglas-fir whose
productivity is clearly compromised by competition. Effects of weeding on plant community
The broadcast weeding treatments in these studies reduced overstory broadleaf trees, affecting resource availability to the understory, but they did not expose mineral soil,
and therefore had no effect on species richness or susceptibility to invasion of the plant community. However, according to Simard et al. (2005), structural diversity increased
following cut-stump glyphosate and manual-cutting treatments, a result consistent with vegetation-management experiments carried out in other forest regions (Haeussler et al.
1999; Boateng et al. 2000). This resulted from removal of
the broadleaf overstory, which was followed by increased
© 2006 NRC Canada 2492 abundance of understory plant species and sprouting of cut
paper birch stumps, which grew into shorter but more diverse structural layers. Here, the dominant broadleaf trees
lost their controlling effects on the subordinate plant layers
(Grime 1998). The mass-ratio theory of Grime (1998) predicts that dominant plants comprising most of the total plant
biomass, such as broadleaf trees in our ecosystems, have an
overwhelming influence on ecosystem function compared
with subordinate or transient plants. The results of our study
support this theory because Simard et al. (2005) and
Baleshta et al. (2005) report that manual removal of
broadleaf trees increased mortality due to A. ostoyae root
disease. It is also predicted that removal of large paper birch
trees will negatively affect cavity-nesting birds (Aitken et al.
2002) and songbirds (de Bellefeuille et al. 2001). Future management pathways
Our review has shown that there are biological risks and
considerable uncertainty associated with the expected outcomes of current management practices aimed at producing
coniferous sawlogs in the mixed forest types of the ICH in
British Columbia. Concerns about conifer regimes have been
expressed by environmental groups, and the province seems
likely to experience pressure for a more natural forestry
(Hammond 1991), including mixed conifer and broadleaf
management, to meet visual and biodiversity objectives, following trends in Germany and Scandinavia (Kenk 1992;
Puttonen 1996) and elsewhere in Canada (Comeau et al.
2005). Less prominent are concerns about the economic wisdom of a conifer strategy. Broadleaf trees were once regarded as low-value species in Europe, in much the same
way that they have been regarded in British Columbia in recent times. In Finland, prices for birch products rose rapidly
in the 1980s (Puttonen 1996). Haight (1993) demonstrated
the importance of stochastic price trends in his analysis of
the economics of Douglas-fir and red alder (Alnus rubra
Bong.) management in the Pacific North West. When investments have a long gestation period and prices are very uncertain, portfolio diversification is usually recommended.
What might be done to minimize biological, economic,
and social risks in future forest management? In principle,
we suggest implementation of a deliberate program of mixing conifer and broadleaf species both spatially and temporally across the landscape in much the same proportions in
which they have occurred in recent history (Sachs et al.
1998). This would be preferable to the present approach,
which we suggest is leading to forest simplification. For example, Simard et al. (2001) found that all ICH forest types
in southern interior British Columbia were composed of a
mixture of species prior to the mid-1990s, but since then, the
current combined program of planting, brushing, and spacing has converted 39% of new forests to types composed
predominantly of either Douglas-fir or lodgepole pine.
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This document was uploaded on 12/16/2013.
- Fall '13
- The Land