For more information about threats and disturbances study texts

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

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.

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

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;

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.

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

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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

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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.