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Threats and Physiology of Wood Destroying Fungi from the Supplement.
Project “Threats and Disturbances”
Divide into teams of 4 or 5 students. Choose any aspect of the problem
of the forest disturbances. Jointly arrange a presentation of the materials
collected in form of PowerPoint demonstration. Try to illustrate your
points of presentation.
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SUPPLEMENTARY TEXTS
THE TREE
A tree is a woody plant with a single stem more or less branched and
taking on what is commonly known as the tree form.
The most evident parts of a tree are roots, stem, or trunk, branches,
buds, leaves, flowers, fruit, and seed.
The stem, branches, and roots are made up of inner bark, outer bark,
sapwood, and heartwood. The outer bark, sapwood and heart-wood are
made up of concentric circles termed annual rings. During each period of
growth two new rings are formed — one on the outside of the sapwood
and another on the inside of the outer bark, and as we seldom have more
than one season of growth each year but one ring is formed on the wood in
a year; so that by counting the rings of wood in the stem we can determine
very closely the age of trees. In very rare cases we have two periods of
growth and two rings of wood in one year, as in 1894, when the draught of
midsummer ripened up the wood of the trees by the first of August and the
rains of autumn started a new growth and caused some trees and shrubs to
flower in October, but such occurrences are very uncommon and the extra
rings formed are readily detected by their being smaller than adjoining
rings and less distinctly defined. The age of trees could be told by the rings
of the outer bark nearly as well as by those of the wood were it not for the
fact that the outer layers of bark fall off as the tree grows older.
Wood once hardened never changes, and the branches are practically
always at the same height from the ground. They might be raised a little by
the thickening of the main roots. In some experiments the bark of rapidly
growing branches was peeled back in the spring for a few inches, the wood
covered with tin-foil and the bark replaced.
At the end of the season there was found a ring of wood outside of the
tin-foil, thus showing where the annual growth of the tree was made.
The bark covers the whole exterior surface of the trunk, branches, and
roots and serves as a protection. It is made up of two parts, the outer or
corky layer which is dead bark and the inner or live bark. These vary much
in appearance and thickness on different kinds of trees. For instance, on
the White Pine it is very dark brown and often an inch or more in thickness
and quite brittle.
The sapwood is the portion of the wood next to the bark. It varies
much in thickness in different species and in trees of the same species, the
most rapidly grown trees contain the largest amount. It is the most active
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portion of the wood in the growing tree, and contains considerable plantfood
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and more water than the heartwood.
The heartwood is the wood in the centre of the trunk and is generally
distinguished from the sapwood. It is also harder and more valuable for
fuel, shrinks less in drying, and is more durable in contact with the soil
than the sapwood. There is very little movement of the sap in the heartwood.
The roots furnish water and nourishment that the plant receives from
the soil, but only the young roots have the power of taking up the soil water;
the elder roots are most useful in holding the tree in place. It is common
to classify roots into surface roots and tap-roots, depending on the
shape and the depth they go in the ground. Some trees have nearly all surface
roots, as the Birch and Spruce, others have nearly all tap-roots, which
often go to a great depth on dry land, as these of the Bur Oak, Black Walnut,
and Butternut. Most of our trees have a combination of the two kinds,
as the Maple, Hackberry, and Ash. Seeding trees of most kinds have a decided
tap-root when young, but in many species it ceases to grow downward
when a few years old. This, is true of the Red and Scarlet Oaks
which often have a tap-root extending four feet in depth before the tree has
attained a corresponding height above ground, but after about five large
lateral roots develop and the growth of the tap-root nearly ceases.
Root-growth is relatively less to the extent of ground occupied in
moist and fertile soil than in a dry and poor soil, but the roots are proportionately
more branched. In wet seasons the root development is less for a
given plant than in dry seasons, because the roots may get their needed
food and water from small area. Nursery trees grown on moist rich land
have a more compact root system than those grown on poor land.
Buds are placed regularly on the young branches and are said to be either
alternative or opposite. When they occur on the stump or on roots they
are not arranged in any regular order. There are two kinds of buds—
flower-buds, which develop into flowers and fruit; and leaf-buds, which
develop into leaves and branches. These can generally be distinguished
from each other by their shape and size and by cutting through them and
noting their construction. Flower-buds are generally more liable to injury
from climatic changes than leaf-buds.
The leaves of our trees vary much in size and shape. They are simple
when composed of but one piece, as the leaves of the Oak Maple, and
Birch, and compound when composed of more than one piece, as the
leaves of the Locust, Ash, and Black Walnut. Leaves are made up of a
framework filled in with cellular tissue and cove red with a thin skin. This
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skin has very many small pores called stomata, through which the plant
takes in carbon dioxide "from the air and gives off oxygen and water.
All our trees shed at least a part of their leaves each year. Al the
broad-leaved trees and the Tamarack shed their entire foliage yearly, while
our so-called evergreen trees lose a part of their leaves each year. The
length of time leaves remain on this latter class of trees varies from two or
three years, in the case of White Pine growing in very severe locations, to
perhaps eight years, in the case of Red Cedar favorably located. The time
that leaves remain on the branches of evergreens depends to some extent
or the location and age of the individual tree.
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Flowers are parts of the plant especially modified for the reproduction
of the plant by seed. Both sexual organs may be located together in the
same flower, as those of the Basswood, Mountair Ash, and Cherry; or in
separate flowers on the same plant as those of the Birch, Oak, and Black
Walnut; or they may be separate or entirely different plants, as in the Willow,
Poplar, Box-elder, and Ash.
The Fruit, botanically defined, is the seed-containing area, derived
from a single flower. As used in nursery practice the term is generally applied
to seeds having a fleshy covering or an adjoining fleshy part.
The Seed, botanically defined, is the ripened ovule, but as the term is
used in nursery practice it often includes the ovary and other parts that
may be attached to it. What is commonly called the seed of Maple, Ash,
Elm, Walnut, and Basswood is really the fruit.
Distribution of Seeds. The seeds of plants are distributed in various
ways, the most common of which are (1) by means oi floats or wings
which buoy the seeds up in the air or 'water, and (2) by animals. The seeds
of Ash, Box-elder, Elm, Maple, Pine, and Spruce have wings which allow
them to be blown great distances by the wind, especially when they break
loose from the upper branches of high trees during severe winds. The
seeds of the Honey Locust are not shed from the pod until after it has
fallen, and as the pod is ten inches or more long and spirally twisted it may
be blown long distances on level ground or snow crust. The seeds of the
poplars and willows have a cottony float attachment which buoys them up
in the air. In the case of the Basswood, the parachute-like bract attached to
the seed-cluster aids in spreading the seeds by carrying them through the
air along the snow crust. The seeds of Mountain Ash, Wild Black Cherry,
Hawthorn, and others are largely distributed by wild animals which eat the
fruit and allow the seeds to pass through the alimentary canal uninjured or
carry off the fruit and spit out the seeds. Many seeds or seed-vessels have
bur-like or sticky coats by which they adhere to animals and are thus car 115
ried considerable distances. Very often bodies of water aid in the distribution
of seeds, since all that are spread by the agency of the wind and most
of those that have fleshy coverings will float on the surface of the water
and may in this way be scattered.
Shapes of Trees. Different species of trees naturally develop different
shapes. Some, like Spruces, have a decided tendency to form a strong stem
and to take on a conical form in preference to the development of a crown
or head; while others, like the Basswood, Oaks, Maples, and Box-elder,
develop their crown in preference to their stern. The actual shape of trees
depends on the space they have to grow in, on the soil, situation, and on
the age of the trees. Where trees have plenty of room to grow, and their
natural development is not interfered with, their individual characteristics
are most apparent.
Cambium Layer. A visual examination of the end section of a softwood
log, such as Douglas fir, will reveal a series of light and dark colored
bands or rings of wood with the pith as the common centre. The lightcolored
rings are known as Springwood and the dark ones as Summerwood.
One band of each, together, constitutes one season's growth of the
tree in diameter and is called an annual ring.
In temperate regions, the cambium layer cuts off new sapwood cells
very rapidly in the spring months when the tree is making a quick upsurge
of growth demanding and drawing large quantities of moisture from the
soil.
These springwood cells are well adapted to the upward passage of sap
because they are formed with thin walls and large central cavities. In the
summer and autumn, tree growth slows up and less moisture is needed.
Consequently, we find the cells cut off by the cambium during that part of
the growing season have thicker cell walls and smaller central cavities.
Summerwood, therefore, is more adapted to giving strength and hardness
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to the wood than for passing moisture. In fact, the strength of wood, to a
great extent, depends on the amount of summerwood present in the annual
rings compared to the springwood. During the normal winter the cambium
lies dormant and growth ceases. The following spring, thin-walled springwood
cells are again cut off by the cambium followed by the summerwood
cells, and so on throughout the life of the tree.
The cells cut off by the cambium layer are very tiny, numbering many
thousands to the cubic inch. Seen under the microscope, they are much
longer than wide, hollow with thin or thick walls as explained above and
have tapering closed ends. The passage of moisture from cell is made pos 116
sible by the presence of tiny openings in the sell walls, called pits.
Most of these tiny cells are positioned in fairly straight rows with their
long axis parallel to the length of the trunk or branch. About seven to ten
per cent of the total volume of the wood is made up of cells placed in narrow
bands which run radially in the tree, i. e., in a direction from pith to
bark. These bands of cells, called wood rays, serve the tree in storing and
distributing food materials horizontally. They are of interest to lumbermen
because of the important part they play in the shrinking and swelling of
lumber which undergoes changes in moisture content.
Wood, as a construction material, has several advantages over other
building materials, mostly due to its characteristic cellular structure. For
example, it is comparatively light in weight, combined with great strength:
the cell walls give in, allowing nails and screws to be driven in, and the
displaced fibres tend to recover thus giving great nail holding power, the
porous surface takes and holds stains and paints and also makes strong
glued joints; it is easily worked into shapes and smooth surfaces, insulation
value is high because of the many dead-air spaces in the cell cavities if irregularities
are present they are usually on the surface, making it easier to
grade and reject poor pieces; the salvage value of wood is high, in many
cases being perfectly sound and suitable for re-use after 30 years or more.
THE EARTH'S BIOSPHERE
Biosphere is the thin layer of life that covers the Earth. Biosphere is
the earth's relatively thin zone of air, soil, and water that is capable of supporting
life, ranging from about 10 km into the atmosphere to the deepest
ocean floor. Life in this zone depends on the sun's energy and on the circulation
of heat and essential nutrients. The biosphere remained sufficiently
stable for hundreds of millions of years to keep going the evolution of today's
life forms. Major divisions of the biosphere into regions of different
growth patterns are called plant formations, or biomes.
Biomes or Plant Formations
The broad units of vegetation are called plant formations (фитоценоз,
растительное сообщество) by European ecologists and biomes by North
American ecologists. The major difference between the two terms is that
biomes include associated animal life. Major biomes, however, go by the
name of the dominant forms of plant life. Biome is a large ecosystem characterized
by similar vegetation, animals, and climate. Abiome's abiotic
(non-living) factors, such as light intensity, wind, soil quality, amount of
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rainfall, temperature, and nutrients, determine what plants and animals inhabit
the zone.
There are various terrestrial and two aquatic (freshwater and saltwater)
biomes. While scientists do not agree on the number of land-based biomes,
the six most widely accepted biomes are:
1) tundra;
2) taiga;
3) grassland;
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4) deciduous forest;
5) desert;
6) tropical rain forest.
Influenced by latitude, elevation, and associated moisture and temperature
regimes, terrestrial biomes vary geographically from the Tropics
to the Arctic and include various types of forest, grassland, shrub land, and
desert. These biomes also include their associated freshwater communities:
streams, lakes, ponds, and wetlands. Marine environments, also considered
biomes by some ecologists, comprise the open ocean, littoral (shallow water)
regions, benthic (bottom) regions, rocky shores, sandy shores, estuaries,
and associated tidal marshes.
Ecosystems
A more useful way of looking at the terrestrial and aquatic landscapes
is to view them as ecosystems, a word coined in 1935 by the British plant
ecologist Sir Arthur George Tansley to stress the concept of each locale or
habitat as an integrated whole. A system is a collection of interdependent
parts that function as a unit and involve inputs and outputs. The major
parts of an ecosystem; are the producers (green plants), the consumers
(herbivores and carnivores), the decomposers (fungi and bacteria), and the
non-living, or abiotic, component, consisting of dead organic matter and
nutrients in the soil and water. Inputs into the ecosystem are solar energy,
water, oxygen, carbon dioxide, nitrogen, and other elements and compounds.
Outputs from the ecosystem include heat of respiration, water,
oxygen, carbon dioxide, and nutrient, losses. The major driving force is solar
energy.
Energy and Nutrients
Ecosystems function with energy flowing in one direction from the
sun, and through nutrients, which are continuously recycled. Light energy
is used by plants, which, by the process of photosynthesis, convert it to
chemical energy in the form of carbohydrates and other carbon compounds.
This energy is then transferred through the ecosystem by a series
of steps that involve eating and being eaten or what is called a food web.
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Each step in the transfer of energy involves several trophic (trophism —
питание), or feeding, levels: plants, herbivores (plant eaters), two or three
levels of carnivores (meat eaters), and decomposers (редуценты —
организмы, минерализующие органические вещества, напр. бактерии,
грибы; сапрофиты). Only a fraction of the energy fixed by plants follows
this pathway, known as the grazing food web. Plant and animal matter not
used in the grazing food chain, such as fallen leaves, twigs, roots, tree
trunks, and the dead bodies of animals, support the decomposer food web.
Bacteria, fungi, and animals that feed on dead material become the energy
source for higher trophic levels that tie into the grazing food web. In this
way nature makes maximum use of energy originally fixed by plants.
The number of trophic levels is limited in both types of food web, because
at each transfer a great deal of energy is lost (such as heat of respiration)
and is no longer usable or transferable to the next trophic level. Thus,
each trophic level contains less energy than the trophic level supporting it.
For this reason, there are more deer or caribou (which are herbivores) than
wolves (carnivores).
Energy flow fuels the biogeochemical cycles, or nutrient cycles. The
cycling of nutrients begins with their release from organic matter by
weathering and decomposition in a form that can be picked up by plants.
Plants incorporate nutrients available in soil and water and store them in
their tissues. The nutrients are transferred from one trophic level to another
through the food web. Because most plants and animals go uneaten, nutrients
contained in their tissues, after passing through the decomposer food
web, are ultimately released by bacterial and fungal decomposition, a process
that reduces complex organic compounds into simple inorganic compounds
available for re-use by plants.
Imbalances
Within an ecosystem nutrients are cycles internally. But there are
leakages or outputs, and these must be balanced by inputs, or the ecosystem
will fail to function. Nutrient inputs to the system come from weathering
the rocks, from windblown dust and from precipitation, which can
carry material great distances. Varying quantities of nutrients are carried
from terrestrial ecosystems by the movement of water and deposited in
aquatic ecosystems and associated lowlands. Erosion and the harvesting of
timber and crops remove considerable quantities of nutrients that must be
replaced. The failure to do so results in an impoverishment of the ecosystem.
This is why agricultural lands must be fertilized.
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If inputs of any nutrient greatly exceed outputs, the nutrient cycle in
the ecosystem becomes stressed or overloaded, resulting in pollution. Pollution
can be considered an input of nutrients exceeding the capability of
119
the ecosystem to process them. Nutrients from agricultural lands, along
with sewage and industrial wastes accumulated from urban areas, all drain
into streams, rivers, lakes, and estuaries. These pollutants destroy plants
and animals that cannot tolerate them or the changed environmental conditions
caused by them; at the same time they favour a few organisms more
tolerant to changed conditions. Thus, precipitation filled with sulphur dioxide
and oxides of nitrogen from industrial areas converts to weak sulphuric
and nitric acids, known as acid rain, and falls on large areas of terrestrial
and aquatic ecosystems. This upsets acid-base relations in some
ecosystems, killing fish and aquatic invertebrates, and increasing soil acidity,
which reduces forest growth in northern and other ecosystems that lack
limestone to neutralize the acid.
Populations and Communities
The functional units of an ecosystem are the populations of organisms
through which energy and nutrients move. A population is a group of interbreeding
organisms of the same kind (a species) living in the same place
at the same time. Groups of populations within an ecosystem interact in
various ways. These interdependent populations of plants and animals
make up the community сообщество, which encompasses the biotic portion
of the ecosystem.
Diversity
The community has certain attributes, among them dominance and
species diversity. Dominance results when one or several-species control
the environmental conditions that influence associated species. In a forest,
for example, the dominant species may be one or more species of tree,
such as oak or spruce; in a marine community the dominant organisms are
frequently animals such as mussels or oysters. Dominance can influence
diversity of species in a community because diversity involves not only the
number of species in a community, but also how numbers of individual
species are apportioned.
The physical nature of a community is evidenced by layering, or
stratification. In terrestrial communities, stratification is influenced by the
growth form of the plants. Simple communities such as grasslands, with
little vertical stratification, usually consist of two layers, the ground layer
and the herbaceous layer. A forest has up to six layers: ground, herbaceous,
low shrub, low tree and high shrub, lower canopy, and upper canopy.
These strata influence the physical environment and diversity of habitats
for wildlife. Vertical stratification of life in aquatic communities, by
contrast, is influenced mostly by physical conditions: depth, light,
temperature, pressure, salinity, oxygen, and carbon dioxide.
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Habitat and Niche
The community provides the habitat (естественная среда обитания)
— the place where particular plants or animals live. Within the habitat, organisms
occupy different niches. A niche is the functional role of a species
in a community — that is, its occupation, or how it «earns its living». The
more a community is stratified, the more finely the habitat is divided into
additional niches.
Population Growth Rates
Populations have a birth rate (the number of young produced per unit
of population per unit of time), a death rate (the number of deaths per unit
of time), and a growth rate. The major agent of population growth is births,
and the major agent of population loss is deaths. When births exceed
deaths, a population increases; and when deaths exceed additions to a
population, it decreases. When births equal deaths in a given population,
its size remains the same, and it is said to have zero population growth.
When introduced into a favourable environment with an abundance of
resources, a small population may undergo geometric, or exponential
growth, in the manner of compound interest. Many populations experience
exponential growth in the early stages of colonizing a habitat because they
take over an underexploited niche or drive other populations out of a profitable
one. Those populations that continue to grow exponentially, however,
eventually reach the upper limits of the resources; they then decline
sharply because of some catastrophic event such as starvation, disease, or
competition from other species. In a general way, populations of plants
and animals that characteristically experience cycles of exponential growth
are species that produce numerous young, provide little in the way of parental
care, or produce an abundance of seeds having little food reserves.
These species, usually shortlived, disperse rapidly and are able to colonize
harsh or disturbed environments. Such organisms are often called opportunistic
species.
Other populations tend to grow exponentially at first, and then logistically
— that is, their growth slows as the population increases, then levels
off as the limits of their environment or carrying capacity are reached.
Through various regulatory mechanisms, such populations maintain something
of an equilibrium between their numbers and available resources.
Animals exhibiting such population growth tend to produce fewer young
but do provide them with parental care; the plants produce large seeds with
considerable food reserves. These organisms are long-lived, have low dispersal
rates, and are poor colonizers of disturbed habitats. They tend to respond
to changes in population density (the number of organisms per unit
121
of area) through changes in birth and death rates rather than through dispersal.
As the population approaches the limit of resources, birth rates decline,
and mortality of young and adults increases.
COMMUNITY INTERACTIONS
Major influences on population growth involve various population interactions
that tie the community together. These include competition, both
within a species and among species; predation, including parasitism; and
co-evolution, or adaptation.
Competition
When a shared resource is in short supply, organisms compete, and
those that are more successful survive. Within some plant and animal
populations, all individuals may share the resources in such a way that
none obtains sufficient quantities to survive as adults or to reproduce.
Among other plant and animal populations, dominant individuals claim
access to the scarce resources and others are excluded. Individual plants
tend to claim and hold on to a site until they lose vigour or die. These prevent
other individuals from surviving by controlling light, moisture, and
nutrients in their immediate areas.
Many animals have a highly developed social organization through
which resources such as space, food, and mates are apportioned among
dominant members of the population. Such competitive interactions may
involve social dominance, in which the dominant individuals exclude subdominant
individuals from the resource; or they may involve territoriality,
in which the dominant individuals divide space into exclusive areas, which
they defend. Subdominant or excluded individuals are forced to live in
poorer habitats, do without the resource, or leave the area. Many of these
animals succumb to starvation, exposure, and predation.
Competition among members of different species results in the division
of resources in a community. Certain plants, for example, have roots
that grow to different depths in the soil. Some have shallow roots that
permit them to use moisture and nutrients near the surface. Others growing
in the same place have deep roots that are able to exploit moisture and nutrients
not available to shallow-rooted plants.
Predation
One of the fundamental interactions is predation, or the consumption
of one living organism, plant or animal, by another. While it serves to
move energy and nutrients through the ecosystem, predation may also
regulate population and promote natural selection by weeding the unfit
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from a population. Thus, a rabbit is a predator on grass, just as the fox is a
predator on the rabbit. Predation on plants involves defoliation by grazers
and the consumption of seeds and fruits. The abundance of plant predators,
or herbivores, directly influences the growth and survival of the carnivores.
Thus, predator-prey interactions at one feeding level influence the
predator-prey relations at the next feeding level. In some communities,
predators may so reduce populations of prey species that a number of competing
species can co-exist in the same area because none is abundant
enough to control the resource. When predators are reduced or removed,
however, the dominant species tend to crowd out other competitors, thereby
reducing species diversity.
Parasitism
Closely related to predation is parasitism, wherein two organisms live
together, one drawing its nourishment at the expense of the other. Parasites,
which are smaller than their hosts, include many viruses and bacteria.
Because of this dependency relationship, parasites normally do not kill
their hosts the way predators do. As a result, hosts and parasites generally
co-evolve a mutual tolerance, although parasites may regulate some host
populations, lower their reproductive success, and modify behaviour.
Co-Evolution
Co-evolution is the joint evolution of two unrelated species that have a
close ecological relationship — that is, the evolution of one species depends
in part on the evolution of the other. Co-evolution is also involved in
predator-prey relations. Over time, as predators evolve more efficient ways
of capturing or consuming prey, the prey evolves ways to escape predation.
Plants have acquired such defensive mechanisms as thorns, spines,
hard seed-coats, and poisonous or ill-tasting sap that deter would-be consumers.
Some herbivores are able to breach these defences and attack the
plant. Certain insects, such as the monarch butterfly, can incorporate poisonous
substances found in food plants into their own tissues and use them
as a defence against their own predators. Other related, similar organisms
such as the viceroy butterfly may acquire through natural selection a colour
pattern or shape that mimics the inedible species. Because they look
like the distasteful model, mimics thus avoid predation. Other animals
avoid predators by assuming an appearance that blends them into the
background or makes them appear part of the surroundings. The chameleon
is a well-known example of this interaction. Some animals possessing
unbearable odours or poisons as a defence also have warning colorations,
usually bright colours or patterns, that act as further warning signals to potential
predators.
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Another co-evolutionary relationship is mutualism, in which two or
more species depend on one another and cannot live outside such an association.
An example of mutualism is mycorrhizae, an obligatory relationship
between fungi and certain plant roots. In one group, called ectomycorrhizae,
the fungi form a cap or mantle about the rootlets. The fungal
hyphae (threads) (гифы) invade the rootlet (корешок) and grow between
the cell walls as well as extending outward into the soil from the rootlet.
The fungi, which include several common woodland mushrooms, depend
on the tree for their energy source. In return the fungi aid the tree in obtaining
nutrients from the soil and protect the rootlets of the tree from certain
diseases. Without the mycorrhizae (грибница) some groups of trees,
such as conifers and oaks, cannot survive and grow. Conversely, the fungi
cannot exist without the trees.
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