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Unformatted text preview: Douglas-fir at this early age, but the more
Douglas-fir is shaded by paper birch, the more carbon (and
potentially nitrogen) it receives from paper birch through the
CMN, representing approximately 10% of total carbon fixation (Simard et al. 1997b). We found that 2- and 3-year-old
Douglas-fir linked into a CMN with either same-age or mature (80 years old) paper birch grew significantly taller and
had higher foliar nitrogen concentrations than those that 2489 were isolated from paper birch by means of trenching
(Simard et al. 1997c; Baleshta 1998).
When conifer seedlings in this forest type are regenerating
following harvest, they can also suffer significant levels of
A. ostoyae infection and mortality when contacting inoculated root systems remaining in the soil (Morrison et al.
1991). Any subsequent silviculture treatment that increases
the pathogen inoculum load, such as brushing, spacing, or
thinning, can cause additional mortality among susceptible
seedlings (Cruickshank et al. 1997; Morrison et al. 2001;
Simard et al. 2005). While root extraction, or stumping, has
been the treatment of choice for mitigating A. ostoyae root
disease in young conifer plantations in British Columbia
(Morrison et al. 1991), retaining resistant paper birch in intimate mixture also appears to reduce disease incidence
among the neighboring susceptible conifers (Morrison et al.
1988). Several mechanisms have been proposed to explain
this reduced conifer mortality, including reduced probability
of attack, discontinuity of available inoculum, and birchinduced changes in soil microbiology. We have found evidence for the latter — young paper birch stands contained a
population of Pseudomonas fluorescens Migula, 1895 which
was 4 times that of young Douglas-fir stands — and that
most isolates reduced growth of A. ostoyae in dual culture
tests (DeLong et al. 2002). Paper birch appears to provide a
more favorable environment for P. fluorescens than Douglasfir, which suggests a mechanism by which paper birch positively influences the susceptibility of regenerating conifers
to A. ostoyae root disease (DeLong et al. 2002).
Once healthy seedlings are established and growing, paper
birch gains a height advantage over Douglas-fir, as well as
all other neighboring conifer species, except for rapidly
growing individuals of western larch or lodgepole pine.
Three years after establishment, we found that height growth
and net photosynthetic rates of Douglas-fir in root contact
with paper birch (untrenched; Baleshta 1998) lagged behind
those that were isolated from paper birch (trenched), suggesting that the competitive effects of paper birch on
Douglas-fir had become more important than its facilitative
effects. At this age, the taller paper birch compete with
Douglas-fir for available light, but also have larger root systems, which reduce soil water content, as is shown in the
study of an untrenched compared with a trenched treatment
by Baleshta (1998) (see also Simard and Durall 2004). By
the time single-cohort plantations are 10 years old, overtopping paper birch trees strongly attenuate light through their
canopies, with transmittance (photosynthetic photon flux
density (PPFD) as a percentage of full sunlight) at 1 m
height ranging from as low as 10% with 10 000 overtopping
stems·ha–1 to 60% with 1500 stems·ha–1 and almost 100% in
the absence of broadleaf trees (Simard and Sachs 2004).
Conversely, Baleshta et al. (2005) found that reducing
broadleaf density increases %PPFD from 40% at 7000
broadleaf stems·ha–1 to 62% at 1111 stems·ha–1, 70% at
400 stems·ha–1, and 82% at 0 stem·ha–1 (not 100% because
of shading by neighboring conifers). Baleshta et al. (2005)
also found that soil water content in August increased linearly with declining broadleaf density (from 9% to 15%
with complete removal of broadleaf trees), suggesting that
© 2006 NRC Canada 2490 paper birch competes with Douglas-fir for soil water. The
light-attenuation effects measured by Simard and Sachs
(2004) and Baleshta et al. (2005) agree with Comeau et al.
(1998), who found that growing-season PPFD declined
logarithmically with increasing paper birch density in 35year-old stands.
Light availability strongly affects understory conifer
growth rates, and several studies show that the magnitude of
this effect varies with conifer shade tolerance (Wright et al.
1998; Drever and Lertzman 2001). Shade-tolerant western
redcedar, western hemlock, and subalpine fir, for example,
reach 50% of their maximum height growth at 25% full sunlight (Wright et al. 1998), while shade-intolerant Douglas-fir
and lodgepole pine reach 60%–90% of their maximum at
60% light (Wright et al. 1998; Drever and Lertzman 2001).
In keeping with this, Simard and Sachs (2004) found that in
10-year-old mixed plantations, diameter growth of shadeintolerant western larch that had become overtopped by
broadleaf trees was reduced more severely with light attenuation than was shade-tolerant western redcedar. Diameter
growth of western redced...
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This document was uploaded on 12/16/2013.
- Fall '13
- The Land