Mycorrhizal fungi: Biological/ecological information and effects of
selected types of forest management actions
All information presented here is extracted from the “Annotated Bibliography of
Information Potentially Pertaining to Management of Rare Fungi on the Special Status
Species List for California, Oregon and Washington”. This Bibliography was most
recently appended in June 2010, by Jenifer Ferriel and Katie Grenier. Numbered
references within this document correspond to those within the Bibliography. This
document summarizes the information presented in the Bibliography, focusing on three
types of forest management actions common in federal forests of the Pacific Northwest:
Regeneration harvest, Fire, and Thinning.
Mycorrhizal basics
Basic types of mycorrhizal fungi
Three basic types of mycorrhizal fungi are recognized: ericoid mycorrhizal (ErM),
ectomycorrhizal (EM), and arbuscular mycorrhizal (AM) (79). ErM fungi are strongly
associated with shrubs of the heath family at high latitudes or high elevations while EM
fungi are most commonly associated with the trees of boreal and temperate coniferous
forests (79). AM fungi are best represented in temperate deciduous forests, grasslands,
agricultural ecosystems and tropical forests ((79). ErM and EM fungi are similar in their
association with plant communities with low tree productivity and decay-resistant litter,
and soils with low availability of nutrients (especially N and P) and low pHs (79, 82).
AM fungi are associated with plant communities with readily degradable litter and
nutrient-rich soils with high pHs (79, 82). ErM and EM fungi are the focus of this
review. The dominant plants in communities serviced by ErM and EM fungi are
dependent on the high efficiency of these fungi in collecting scarce organic sources of N
and P in surface soil horizons. Consequently, ErM and EM fungi restrict the availability
of these nutrients to the saprophytic/decomposer (both bacterial and fungal) communities
(116). Mycorrhizal fungi and their associated rhizosphere (root zone) bacteria have
important roles in coniferous ecosystems. Soil bacteria include those that appear to
enhance root colonization and ectomycorrhizal formation by specific fungi (mycorrhizal
helper bacteria) as well as those that promote growth of both ectomycorrhizal and
nonmycorrhizal seedlings (plant growth promoting bacteria) [77].
Functions of EM fungi
EM fungi are known to promote seedling establishment and tree growth by facilitating
water and nutrient uptake, protecting against root pathogens and environmental extremes
and maintaining soil structure and forest food webs (86). Recent investigations in a
variety of environments indicate that mycorrhizal interactions may be important
determinants of plant diversity, ecosystem variability, and productivity (25). EM fungi
may be particularly important at high-elevation sites where the growing season is short
and soil nutrients resources are limiting (20). In conifer forests in Sweden, total
mycorrhizal biomass highest in soils with lowest nutrient availability and lowest tree
productivity (79). Functions of soil bacteria appear to include the promotion of
colonization of roots by EM fungi as well as the promotion of growth of both
ectomycorrhizal and non-mycorrhizal seedlings (77).
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Size/Age
Genet size in fungi is known to range up to150 meters in Tricholomopsis, a litter fungus,
and 635 meters in Armillaria, a root rot fungus (21). Based on sporocarp distribution, a
genet size of 1.5 m is known in Hebeloma and 27 m in Suillus (16). Estimated ages of
EM fungi range from 30-150 years (16). Some genets of root-rot fungi continue mycelial
growth for centuries, with life spans extending across tree generations. Such species
generally can survive extended periods saprobically until roots of new host trees contact
old infected roots and the fungus can resume its expansion (21).
Community composition
Sporocarp surveys appear to poorly reflect the composition of below-ground EM fungal
communities (24). Assessment of total EM community composition requires sampling
for fungi via morphotyping and DNA analysis of soil and wood cores and root tips, as
well as by sporocarp surveys (19, 38, 43). It appears that the majority of EM species as
well as those most commonly found in root tips, form no conspicuous sporocarps (9, 41,
43). A general rule among EM communities appears to be that a few species are
frequently encountered while the majority of species are rare (96, 97).
Environmental gradients
In the Tsuga heterophlla zone of Olympic National Park, EM species richness and
sporocarp standing crop have a positive correlation with mean annual precipitation (49).
Also in Olympic National Park, EM macrofungal composition is correlated with levels of
moisture and available soil nitrogen. The EM genera Cortinarius, Tricholoma,
Hydnellum, Suillus and Sarcodon were dominant at the drier, nitrogen-poor Deer Park
area, while the wetter, higher nitrogen Hoh Valley was characterized by EM genera such
as Inocybe, Russula, Amanita, Boletus, Phaeocollybia and a higher incidence of
saprotrophic fungi (113). Species richness was similar at the two areas but sporocarp
production was much higher at Deer Park. The apparent response to relatively small but
presumably long-term differences in nitrogen abundance suggest that sporocarp
production by macrofungi could be an effective bio-indicator and should be considered in
determination of critical loads for atmospheric nitrogen deposition to temperate and
boreal forests (113). In the forests of eastern Washington, truffle species richness and
biomass increased with cover of coarse woody debris, canopy cover, and transition from
warm-dry aspects to cool-moist aspects.
Succession
The EM suilloid genera Suillus and Rhizopogon are critical to the establishment of Pinus
contorta during primary succession on coastal sand dunes lacking mycelial networks.
The fungi appear to be dispersed as spores within the pellets of deer (56).
Field experiments conclude that certain ectomycorrhizal fungi appear early in the
successional sequence, and are a major component of disturbed systems. Others dominate
later stages of succession. Early colonizing species typically colonize by spores, and
appear to have small non-persistent genets. Later successional species colonize initially
by spore but then spread primarily from hyphal networks (36). While some authors
appear to see evidence for "true" succession (successive species replacing earlier species
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over time) within EM fungal communities as forest stands age (73), other authors cite
evidence that change in community composition over time is essentially an accumulative
process, with species adding in to the community as stand conditions (physical, chemical,
biological) change with stand age (7,74).
Genets
It is most appropriate to consider the genet as a mobile unit, which may fluctuate in size
and location. At the local level, delimitation of a population may be risky, since natural
boundaries are often uncertain. Most species have patchy distribution, since the area over
which conditions are favorable for mycelial growth can vary greatly (16).
Soil compaction
Soil compaction has been considered a principal form of damage associated with logging
(48). Soil compaction degrades soil structure and restricts movement of oxygen and
water through soil and reduces pore space for root penetration and production of feeder
rootlets where mycorrhizae form (1). At 6 field locations including a range of dry and
wet forest types, 62% of the compaction in the top 10 cm of soil occurred after only one
pass of a laden logging machine, while 80-95% of final compaction to 30 cm was
observed after only three passes. Compaction was strongly related to the original soil
bulk density, forest type and soil parent material (48). Susceptibility of soil to
compaction strongly depends on soil moisture content, soil organic matter, soil type,
number of machine passes, the load applied, and machine characteristics.
Recommendations to reduce soil compaction include use of defined skid trail systems,
confining traffic to a minimum proportion of a logged area, and operating during drier
soil conditions (48). Retaining woody residue and surface organic matter helps protect
mineral soil from detrimental compaction; is also reduces erosion, maintains soil nutrition
and soil microbe populations (1).
Regeneration timber harvest
Overview
Numerous studies in a variety of coniferous forest types indicate directly or indirectly,
whether based on sporocarp surveys, soil cores or root tip sampling, that clear-cut timber
harvest treatments significantly change the composition, and reduce the diversity of the
EM fungal community (7, 13, 14, 20, 50, 53, 54, 74, 75, 76, 80, 89, 90, 94, 104, 122, 133,
140). A thorough examination of site preparation treatments suggests that the changes in
fungal species composition are driven by changes in the biology and chemistry of the soil
environment after clear-cutting as much as they are by loss or change in fungal inoculum
(99). Means of ameliorating the negative effects of clear-cutting treatments on EM
fungal communities appear to principally include retention of coarse woody debris
(CWD) and green tree retention (GTR).
Coarse woody debris
In general, and particularly in drier habitat types, CWD is important in reducing loss of
EM development and sporocarp production following clear-cutting (14,22). In forests of
western Montana, preliminary estimates indicate that approximately 25-37 tons/hectare,
14cm in diameter or larger, of fresh residues should be left after harvesting and
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prescribed burning to provide parent materials for the decayed soil wood (22).
Cut block size and green tree retention
“Edge effect” and its relative, “gap size”, are important concepts in efforts to reduce the
negative impacts to EM communities associated with clear-cutting treatments. Studies in
a variety of forest types indicate that EM diversity and sporocarp production decline with
distance from forest edge (19, 20, 50). This observation implies that smaller cut blocks
(gaps) will retain higher diversity of EM fungi because they have a greater perimeter to
area ratio than larger cut blocks. Creation of above-ground gaps through tree harvesting
can create below-ground gaps in the EM hyphal network. Studies in interior cedarhemlock forests of NW British Columbia indicate that a significant gap in the belowground EM hyphal network occurs when above-ground gaps of 900 square meters or
greater are created (19). Along with smaller cut block size, GTR within cut blocks has
been shown to be an effective means of reducing EM fungal loss within clear-cuts. A
study of regeneration harvest in Douglas fir forest using evenly dispersed GTR
accounting for 15% basal area found a 32% decline (relative to pre-treatment) in total
EM-type richness at the outside drip line of sampled retention trees. This is in contrast to
a greater than 50% reduction in richness in soil cores taken away from the retention trees
(76). Likewise, seedlings planted within 6 m of Douglas fir retention trees were found to
have higher species richness and diversity of EM fungal communities compared with
seedlings either planted beyond 16 m from retention trees or planted in soil collected
from under retention trees (89). Literature review supports the understanding that
dispersed GTR in combination with aggregated retention is an effective means of further
reducing negative impact of clear-cutting on sporocarp production (75). Edge effects of
retention patches have been documented to extend nearly 10 m in coastal western
hemlock (122) and at least 15-20 m in Douglas fir forest (80).
Refuge species
There is evidence that shrubs such as bear-berry (Arctostaphylos uva-ursi) can help retain
the EM fungal richness of a pre-treatment interior Douglas fir community following
clear-cutting (70).
Fire
In assessing the effects of fire on mycorrhizal fungal communities, fire intensity, whether
associated with prescribed burning or wildfires, appears to be a key factor. High soil
temperatures associated with fire tend to be lethal to mycorrhizal fungi, and generally,
fire intensity is directly related to the soil depth to which lethal temperatures extend. The
biomass and/or diversity of ectomycorrhizal fungal communities (EMC) are often
markedly reduced by fire (9, 10, 40, 45, 65, 81, 84, 112). Many ectomycorrhizal (EM)
fungal species are decomposers (mineralizers) of organic matter and accordingly, EMC
biomass tends to be high in the litter and upper organic soil horizons (10, 40, 81, 84),
although EM species richness may be highest in the deeper soil layers (10). Depending
on fire intensity, lethal temperatures may extend through the organic soil horizon, but
generally not deeper than the upper 5 cm (~2 in) of mineral soil (3, 45). There is reason to
suspect that immediate post-fire changes in EMC species composition and structure, as
well as post-fire “recovery” that is generally witnessed in a local EMC is ultimately
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driven by changes in soil chemistry (67, 84). Repeated burning (two year intervals) has
been observed to reduce the presence of fine roots, mycorrhizal root biomass, and soil
nitrogen and phosphorous levels for the duration of the burning schedule (95). Post-fire
re-colonization by EM fungi to pre-fire levels can require up to 15 years in a boreal forest
community (84). Residual propagules as well as spore dispersal (by wind, or
mycophagous animals) from adjacent unburned communities are likely to contribute to
re-establishment of pre-fire EMCs (3, 8, 45). The roots of plants capable of re-sprouting
after fire (e.g., Arctostaphylos) may possibly act as refugia for a variety of EM fungi that
can contribute to post-fire re-establishment of pre-fire EMCs (45). In dry ponderosa pine
communities, spring burning, relative to fall burning, has been shown to significantly
reduce short-term (2 years) impacts to duff depth, live root biomass and EM species
richness (81). In communities where fire is to be re-introduced following a long
exclusion, reduction of fire intensity by an initial mechanical reduction of fuel loads is
recommended as a means of reducing impacts to below-ground biomass pools and
nutrient cycling processes (95). Timing burns to reduce fire intensity and leaving
unburned patches of trees and shrubs may facilitate the re-establishment of the pre-fire
EMC (112).
Thinning
As a generality, it appears that thinning reduces the species richness of the EM fungal
community within the forest/stand being thinned. Further, thinning effects to the EM
community appear to relate relatively directly to thinning intensity. Examples follow.
Four years of data obtained from concurrent studies researching species richness of
western North American Douglas-fir ectomycorrhizal epigeous basidiomycete (EEB)
communities in two different Oregon Coast Range forests indicate that after timber
removal, species richness post-/pretreatment ratios were significantly depressed in the
two most heavily thinned stands, but light to moderate forest thinning did not appear to
have much effect on EEB species diversity (125). In a study of wood-inhabiting fungi in
two stands of Norway spruce forest (old-growth vs. selective logging 60-80 years prior) it
was determined that forest management had a negative impact on species diversity.
Newly fallen and weakly decayed logs in the old growth stand had a higher species
richness, more red-listed species and more indicator species compared to similar logs in
the managed stand (29). A 3-year study in a 55-65 year-old Douglas fir forest indicated
that, overall, standing crop biomass of truffles was significantly lower in stands subjected
to variable density thinning (VDT) than in control stands. Notably, however, for some
taxa (Melanogaster), species diversity and productivity were highest in the VDT stands,
and numerous truffle species were found only in the thinned stands (62). In the Sierra
Nevada of California, short-term truffle frequency, biomass and species richness were
lower in thinned or burned plots than in controls. Truffle abundance was inversely
related to treatment intensity (106). In another study of hypogeous fungi, while total
relative frequency and biomass of sporocarps did not differ significantly among thinning
levels at two sites, there was significant association between thinning level and
frequencies of the most common genera (i.e., thinning level significantly affected the
composition of the hypogeous fungal community) [85]. When thinning in a western
hemlock-western redcedar forest, partial cutting that favors retention of a diverse mix of
ectomycorrhizal tree species over western redcedar (a non-mycorrhizal species) may
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benefit ectomycorrhizal mushroom richness. Partial cutting systems could allow some
timber removal without necessarily reducing ectomycorrhizal mushroom communities.
Timber harvest prescriptions that can retain some stand basal area with good tree vigor
would be one option to accommodate commercial timber and mushroom resources (27).
Preliminary results indicate that populations of arbuscular mycorrhizal fungi can rapidly
increase following restoration thinning in Northern Arizona ponderosa pine forests. This
may have important implications for restoring herbaceous understory of these forests
because most plants depend upon arbuscular mycorrhizal associations for normal growth.
Two main processes control population densities of mycorrhizal fungi following
disturbance: immigration of new propagules from nearby areas and survival and spread of
residual propagules (25).
Compiled by Rick Dewey
April 2012
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