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FACTORSAFFECTINGECTOMYCO~ZAE
AND FOREST REGENERATION FOLLOWING
DISTURBANCE IN THE PACIFIC
NORTHWEST
Michael P. Amaranthus
influenced by harvesting and site preparation. Clearing
vegetation and disturbing the forest floor remove nutrients and reallocate them within the ecosystem. Harvesting host trees eliminates the photosynthate source for
dependent ectomycorrhizal fungi and associated microbes.
Converting a mature forest to a clearcut increases soil
temperatures by removing the protective canopy. Prescribed broadcast burning increases soil pH, creates a
nutrient flush, and can reduce litter and dufflevels
(Amaranthus and McNabb 1984). Soil organic matter,
humified material, and decaying wood are centers ofmicrobial activity and are substantially diminished as a
result of intense fire. Changes in aboveground community composition alter the quality and quantity of root
exudates and litter leachates.
Wright and Tarrant (1958) found fewer ectomycorrhizae
on Douglas-fir seedlings growing in burned, compared to
unburned, clearcuts. The greatest reductions were associated with the hottest burns. Thus, not only the type of
activity, but its severity, is critical. Parke and others
(1984) compared mycorrhiza formation in soils from
burned and unburned clearcuts of 36 "difficult to regenerate" sites in northwestern California and southwestern
Oregon. Douglas-fir (Pseudotsuga menziesii [Mirb.]
Franco) and ponderosa pine (Pinus ponderosa Laws.)
seedlings grown in soils from the burned clearcuts formed
40 percent fewer ectomycorrhizae and seedlings grown in
soils from the unburned clearcuts 20 percent fewer
ectomycorrhizae than seedlings grown in undisturbed
forest soil. Amaranthus and others (1987) found 90 percent less native mycorrhizae and 44 percent less basal
area growth on Douglas-fir seedlings grown in clearcut
and severely burned soils compared to undisturbed forest
soil. However, all studies do not report mycorrhizal reductions following clearcutting and prescribed fire (Pilz
and Perry 1984; Schoenberger and Perry 1982). It is difficult to generalize about effects of burning on ectomycorrhizal populations because they are highly dependent on duration and intensity offire as well as soil and
site conditions (Perry and Rose 1983).
Because ectomycorrhizae predominate in the organic
layers of the soil (Harvey and others 1976, 1979; Trappe
and Fogel 1977), the degree of organic matter lost from
a site can influence mycorrhiza populations. Harvey and
others (1976) found up to 95 percent of the active
ectomycorrhizal fungus types in humus and decaying
wood in a mature Douglas-fir/western larch (Larix
occidentalis Nutt.) forest. The importance of decaying
wood to support ectomycorrhizal populations may be most
ABSTRACT
Studies from the Pacific Northwest indicate that disturbance can reduce ectomycorrhizal formation and forest
regeneration. However, the degrees of reduction and impact on forest regeneration vary widely and depend on
many factors. Among these are the type and severity of
disturbance, ectomycorrhizal diversity, climatic conditions, biotic conditions, and the effects of non hos ts over
time. Mycorrhizal formation and regeneration are most
greatly impacted on severely disturbed and environmentally limited sites. The rapid occupation of such sites by
ectomycorrhizal host plants following disturbance is critical to stabilizing native mycorrhizal populations that may
aid forest regeneration.
INTRODUCTION
Much of the research on effects of forest disturbance on
soil biology has focused on ectomycorrhizae. Because
most forest-tree species in the Pacific Northwest require
ectomycorrhizae for nutrient and water uptake, the importance of understanding the relationship between disturbance, site conditions, and mycorrhizae cannot be
overstated. Numerous authors have reported reductions
in mycorrhiza populations due to forest disturbance
(Amaranthus and others 1987; Harvey and others 1976,
1980; Parke 1984; Perry and others 1982). However, the
degree of reduction and its impact on forest regeneration
varies widely and depends on many factors.
TYPE AND SEVERITY OF
DISTURBANCE
Timber harvest and site preparation are the most widespread forest activities in the Pacific Northwest that alter
both the aboveground and belowground environments and
therefore potentially impact ectomycorrhizae. In the
Pacific Northwest, clearcutting and prescribed burning
are the common harvesting and site preparation practices. Soil nutrient status, moisture, temperature, pH,
and organic matter contents, litter inputs, and species
composition affect the community structure of soil organisms (Harvey and others 1980)-and all of these are
Paper presented at the Symposium on Management and Productivity
of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Michael P. Amaranthus is Soil Scientist, Pacific Northwest Research
Station, Forest Service, U.S. Department of Agriculture, 3200 Jefferson
Way, Corvallis, OR 97331.
205
critical following disturbance such as wildfire. Immediately following the 1987 wildfires in southern Oregon and
northern California, decaying wood contained 25 times
more moisture compared to mineral soil and was a center
of ectomycorrhizal activity for recovering vegetation
(Amaranthus and others 1989). Decaying wood also acts
as habitat for small mammals that are important in distributing fungal spores of several belowground mycorrhizal fungi (Maser and others 1978).
BIOTIC CONDITIONS
The importance of aboveground community structure
to belowground biological functioning is unclear. It is
increasingly apparent, however, that an ectomycorrhizal
fungus can link some plant species together through
shared fungal mycelia (Bjorkman 1970; Finlay and Read
1986; Read and others 1985). In the natural forest environment, ectomycorrhizal fungi, supported by nonconiferous hosts, can actively colonize conifer seedlings.
Root-chamber analysis of the development of ectomycorrhizal mycelium has shown that expanding hyphal
fans not only act as nutrient-absorbing structures but also
can colonize nonmycorrhizal feeder roots in host-plant
combinations within and among host species. Using radioactive labeling, Finlay and Read (1986) have demonstrated the movement of carbon among plants connected
by mycorrhizal mycelia. Clearly, the existence of "pipelines" for distributing materials among plant species has
important implications for forest regeneration following
disturbance.
Little is known of the persistence and distribution of
ectomycorrhizae in the absence of living hosts. It has
been suggested (Hacskaylo 1973) that ectomycorrhizal
fungi do not persist long in the absence of host-supplied
substrate. In the Klamath Mountains of northwestern
California and southwestern Oregon, sites that have been
logged and burned are often rapidly invaded by woody
shrubs (Gratkowski 1961). These shrubs, members of the
Ericacae and Fagaceae, form mycorrhizae with many of
the same fungi as do members of the Pinaceae (Molina
and Trappe 1982). These shrubs may preserve mycorrhiza diversity during periods of rapidly changing aboveground community structure.
Amaranthus and Perry (1989b) planted Douglas-fir
seedlings at two locales: (1) a site cleared of whiteleaf
manzanita (Arctostaphylos viscidi Parry); and (2) a
meadow cleared of annual grasses. In the first year, there
was more dramatically improved seedling survival and
growth on the manzanita site than the adjacent meadow
with similar moisture and temperature conditions. By
the second year, ifbasal area growth of surviving individuals is taken into account, Douglas-fir at the manzanita site, on an area basis, was nearly 10 times that of
seedlings in the cleared meadow. Douglas-fir seedlings,
outplanted at the manzanita site, formed mycorrhizae
more rapidly than those seedlings outplanted at the
meadow. There were also dramatic shifts in the types
ofmycorrhizae found on seedlings grown at the two sites.
Douglas-fir seedlings, at the manzanita plots, contained
significantly higher proportions of Rhizopogon sp. mycorrhizae. Certain Rhizopogon species have been demonstrated to decrease seedling moisture stress and improve
seedling outplanted performance. Rapid mycorrhiza formation, with fungi well adapted to site conditions, is key
to seedling establishment on sites difficult to regenerate.
Some woody shrub species may act as biological reservoirs, not only of mycorrhizal fungi but of other microflora
as well. Significantly higher rates of nitrogen fixationand increased seedling survival and growth-were found
ECTOMYCORRHIZAL DIVERSITY
Ectomycorrhizal diversity within the soil may buffer the
impact of disturbance on forest sites. Disturbance did not
reduce ectomycorrhiza formation on Douglas-fir seedlings
grown in soil from extremely productive sites in the
Oregon Cascades, where diversity of ectomycorrhizal
fungi is high (Pilz and Perry 1984; Schoenberger and
Perry 1982). However, the proportion of each mycorrhizal
fungus type shifted significantly with both soil disturbance and plant community; different environments reduced ectomycorrhiza formation by some fungus types
and apparently stimulated formation for others. In contrast, clearcutting significantly lowered ectomycorrhiza
formation on Douglas-fir seedlings grown in soil from a
harsh, less productive site in southwestern Oregon,
where soil contained relatively few ectomycorrhiza types
(Amaranthus and others 1987). Reduced ectomycorrhiza
formation correlated positively with decreased basal area
Douglas-fir growth after outplanting. The southwestern
Oregon soil, with low fungus diversity, was poorly buffered against disturbance, compared to the soil from the
Oregon Cascades.
CLIMATIC CONDITIONS
Climate influences seedling growth and ectomycorrhiza
formation (Harvey and others 1980; Pilz and Perry 1984).
The importance of early mycorrhiza formation in dry
areas has been emphasized (Mikola 1970; Parry 1953).
Dry climates may limit the activity of mycorrhizal fungi
by decreasing the length of time for spore production,
germination, and optimal mycelial growth, which in turn
can decrease the chances for planted seedlings to become
colonized (Amaranthus and Perry 1987). Seedlings in
moist climates may be able to survive longer without
mycorrhizae than those in dry climates, increasing their
chances of becoming colonized. Moisture content also
affects uptake of certain nutrients by mycorrhizae (Gadgil
1972).
Seedlings growing in cold climates may also require
rapid, early mycorrhizal colonization to take advantage
of the short growing season and obtain the necessary
nutrients and water to survive the long cold season and
early frosts. In studies in the Klamath Mountains of
northwestern California and southwestern Oregon,
Amaranthus and Perry (1987, 1989a, 1989b) found that
mycorrhiza formation most strongly influences seedling
survival and growth on sites limited by both moisture and
tern perature.
206
in association with the mycorrhizae of Douglas-fir seedlings in a stand cleared of whi tel eaf manzanita than in a
meadow cleared of annual grass (Amaranthus and others
1990). Azospirillium, a nitrogen-fixing bacterium, was
isolated with Douglas-fir mycorrhizae at the manzanita
site. Whiteleafmanzanita occupies particularly hot, dry
sites where fire is frequent. Because high nitrogen losses
can accompany intense fire, natural mechanisms by which
nitrogen is returned to the soil can be important to forest
regeneration.
REFERENCES
Amaranthus, M. P.; McNabb, D. H. 1984. Bare soil exposure following logging and prescribed burning in southwest Oregon. In: New forests for a changing world:
Proceedings; 1983;Portland, OR. Bethesda, MD:
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Amaranthus, M. P.; Perry, D. A. 1987. Effect of soil transfer on ectomycorrhiza formation and the survival and
growth of conifer seedlings in disturbed forest sites.
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Amaranthus, M. P.; Perry, D. A.; Borchers, S. L. 1987.
Reduction of native mycorrhizae reduces growth of
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Amaranthus, M. P.; Perry, D. A. 1989a. Interaction effects
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Amaranthus, M. P.; Perry, D. A. 1989b. Rapid root tip
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EFFECT OF NONHOSTS OVER TIME
Many grass and shrub species, such as Ceanothus and
Rubus (for example, blackberry, salmonberry), form vesicular-arbuscular mycorrhizae (VAM) with fungi incompatible with members of the Pinaceae (Rose and
Youngberg 1981; J. M. Trappe, unpublished data, USDA
Forest Service, Pacific Northwest Research Station). On
sites long dominated by VAM species, the ectomycorrhizal
fungi needed by members of the Pinaceae will gradually
diminish, and the soil microbial complex associated with
ectomycorrhizae can be reduced. Invasion of sites by
nonectomycorrhizal plants over years can seriously affect
reforestation (Amaranthus and Perry 1987), particularly
in the case of ectomycorrhizal tree species growing on
difficult sites where seedlings must establish ectomycorrhizae early to survive.
How long soils retain their mycorrhizal colonization
potential in the absence ofliving hosts is unknown.
Ectomycorrhizal spores and hyphal fragments have remained metabolically active after 2 years in Scandinavian
forests, though the number of active fragments dropped
dramatically over that period (Ferrier and Alexander
1985; Persson 1982). In the Pacific Northwest, mycorrhiza formation generally decreases as the length of time
between disturbance and reforestation increases (Perry
and others 1989; Pilz and Perry 1984).
CONCLUSIONS
Studies from the Pacific Northwest indicate that reductions in mycorrhizal formation may affect the performance of outplanted seedlings, particularly on severely
disturbed or environmentally limited sites where rapid
early growth is important. On these sites, planting seedlings with well-developed mycorrhizae and protecting
native populations of mycorrhizal fungi aid regeneration.
Why mycorrhizal formation is reduced following disturbance in some areas and not others is not understood.
Great variability in the physical and biotic environment
and disturbance history and intensity are likely contributing factors. One factor that appears to be important is the
presence of ectomycorrhizal host plants. On many sites,
noncommercial plants may serve as reservoirs of ectomycorrhizal fungal inocula while conifers are becoming
established.
207
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