Week 4 - Day 18 - Trees
ESSENTIAL READING BEFORE COMMENCEMENT OF COURSE:
Urban
Pest Management in Australia: 2004
Edition, UNSW Press, Sydney
by
J Gerozisis and P Hadlington
- Chapter 17 - Pages 200 to 204.
Chapter 17 – Introduction to Timber & Timber Pests - classification of trees - parts of tree - leaves - trunk and branches - pines and hardwood - roots - growth of a tree - moisture content of timber - fibre saturation point - equilibrium moisture content - inspection of houses and buildings
timber and wood decay fungi
FUNGAL DECAY
Apart from insects, fungi are also important agents of timber
decay. Together they ensure that complex substances
such as cellulose are broken down and returned to the environment. By
this means, the forest floor is cleared of dead trees leaving
room for additional plant growth.
FUNGI
Fungi annually cause enormous amounts of damage to standing
trees, logs in transit and storage and timber after milling,
during seasoning and in service. Here in Australia,
the annual damage bill would run to many millions of dollars.
Sources of nutrition
Fungi differ from green plants inasmuch as they do not contain
chlorophyll. As a consequence of this, they are incapable
of manufacturing their own food by photosynthesis and must
obtain their nutrients by attacking plants and animals, which
are then broken down into a soluble form which can be absorbed
by the fungal cells.
Life cycle
The first stage of the fungus life cycle is a microscopic
round or oval spore. When this germinates, a fine thread
or hypha is produced. The hyphae obtain nutrients for
growth by means of complex chemical reactions. Under
favourable conditions, these hyphae intertwine to form a
dense mat or mycelium. After a certain period of time
the fungi develop fruiting bodies, or sporophores, which
in turn produce vast quantities of spores, sometimes-over
30,000 million per day for six months.
Growth requirements
In order to survive, all fungi need four factors -
i) a
food source (wood);
ii) a suitable
temperature
iii) a supply
of moisture and
iv) a supply of
oxygen.
The single most important factor governing the growth of
fungi is the moisture content of the timber. As the
moisture content increases, so does the likelihood of decay
and below 20% moisture content, timber is too dry for fungi
to become established. Oxygen is also an essential
factor and timber completely immersed in water or waterlogged
soil will not decay. Fungal growth is also temperature dependent. Consequently,
logs from tropical forests may become infected with fungi
before reaching the sawmill. Fungi can be divided into three
main groups according to the type of rot, which they produce. These
groups are - brown rots, white rots and soft rots.
BROWN ROTS
Those fungi responsible for brown rots include some of the
most important decay organisms of wood in service. Brown
rot fungi preferentially attack the cellulose content of
timber. As the decay process continues, the affected
timber becomes darker because a higher percentage of those
materials, which impart a brown, colour to the timber eg.
tannins, remains. With time, the flexible cellulose
framework of the wood cells is destroyed; the wood shrinks,
cracks and ultimately collapses. The decay process also affects
other properties of timber. There is a loss of strength
and the wood becomes more brittle. Decayed timber
is also more susceptible to insect attack. Two groups of
brown rot fungi exist:
Dry Rots
The causative agent of dry rot in Europe is the fungus Serpula
lacrymans and this may be found occurring in buildings
located in the world's temperate regions. In Australia,
only one case in Melbourne has not been recorded. Dry rot
fungi, do not actually grow in dry wood but they have the
capacity to develop special strands, sometimes over 20mm
in diameter, to transport moisture from decaying timber to
dry timber several metres away. These fungi grow best
in situations where there is warmth, poor ventilation and
high humidity. Softwoods with a moisture content of
20 to 40% are the main targets of attack.
Wet Rots
Wet rots are much more common than dry rots. They grow
best on timbers of 40-50% moisture content. They most
often destroy roofing and sub-floor timbers but can be found
in other situations where water leaks or condensation may
occur. These rots will also attack timber in an outdoors
situation. Wood, which has been attacked by these fungi,
may appear quite normal on the surface but there may be considerable
internal decay. Affected timber may be very dark,
almost black, in colour.
WHITE ROTS
The fungi, which cause white rots, utilise all timber constituents
including the brown colouring materials as foodstuffs. Consequently,
as decay proceeds the timber becomes lighter in colour and
fibrous in texture. Some hardwoods are particularly
susceptible to this type of rot. White rots will affect
timber outdoors as well as interior joinery timber. Timber
affected by these fungi rarely shrinks or collapses.
Soft Rots
Soft rot fungi will attack a wider range of timbers than
white or brown rot fungi and hardwoods are particularly susceptible. Wood,
which is exposed to very wet conditions, is likely to suffer
decay by these organisms. Affected timber normally
retains its original shape but the surface becomes discoloured,
soft and possibly eroded.
Moulds & Stains
The fungi responsible for the growth of surface moulds and
stains do not actually attack wood but exist on the carbohydrates
present in the parenchyma cells. Such fungi flourish
in moist, warm, humid climates. Heartwood is less susceptible
than sapwood and softwoods are usually more susceptible than
hardwoods. Sap-stain or blue-stain which actually can be
any colour from blue-black to light grey normally affects
the surface appearance of timber and may reduce its commercial
value. Moulds and stains do not cause significant loss of
strength and affected timber can be used where its appearance
is not important. However, stained wood should be carefully
checked for more serious decay since these fungi also thrive
in conditions conducive to stain and mould growth.
tree
biology – bark – phloem – sapwood – heartwood – pith – medallary
rays
Growth of Trees
Before insect pests of trees can be considered, it is essential
to know the structure of a tree and how it lives and grows. A
basic knowledge of the understanding of nutritive relationships
within a tree is important when dealing with pests of the
conductive tissue of trees and later the finished wood
product. For instance, the heartwood is more resistant
to termites and borers and wood decay because it does not
contain an abundant supply of materials essential to the
growth of these pests. The sapwood, on the other
hand, is well provided with these and is therefore less
durable.
Requirements of various insects
The recognition of timber damage and its cause is important
in all stages of growth and conversion to wood products,
for not all timber destroying insects require the same
type of food from wood and as a result choose different
regions of the tree for their activity. For instance,
longicorn larvae feed in the highly nutritious phloemcambial
region, while powder post beetle larvae feed on the starch
in sapwood.
Uses of timber
Wood is an important component of most buildings and any
threat t-o its permanence is viewed seriously. People usually
have their largest single investment in their house and
much of this investment is in the structural timber framing
and lining. Wood and wood products are the most widely
used constructional materials. Because of its versatility
it is also used decoratively for internal lining and furniture.
Production of wood
A tree, or any green plant, carries out three main functions –
- The intake of water, mineral salts, carbon dioxide and
sunlight;
- The manufacture of these raw materials into sugars, starches,
cellulose etc., and
- The breakdown of these manufactured materials and the
loss of water from the leaves by respiration.
The various tissues in a tree carry out the vital functions
for the tree as a living plant and these tissues persist
after death. Woody plants reach a greater height than other
plants, withstand seasonal changes and storms, structures
carry out the same functions for many years and dead tissues
give structural rigidity. A woody plant grows both in height
and girth. Growth in height occurs at the apical growing
points of main stems and branches and later these tissues
are used for other structures. Growth in girth takes
place between the wood and bark, for, if it occurred in the
centre of the tree, the increase in growth would cause splitting
of the tree.
Sections of the Tree
A tree can be regarded as having 3 main parts leaves, trunk
and branches and the roots.
Leaves
The leaves provide food for the whole plant through the process
of photosynthesis. This process takes place in those
areas, which are green due to the presence of the pigment
chlorophyll. During photosynthesis, carbon dioxide
from the atmosphere and water from the soil are converted
into a sugar (sucrose) and oxygen. Sunlight provides
the energy for this process.
Trunk
The trunk is composed of the following layers: The first
three layers comprise the "outer bark."
- Epidermis or outermost layer. It serves as a protective
layer and prevents, to some degree, losses of moisture.
- Cork - a protective layer of variable thickness depending
on the species of the tree and its age. It protects
against desiccation, injury, etc.
- Cork-cambium. This layer is actively dividing and
produces cork cells as required.
(The next two layers make up the "inner
bark").
- Phloem - this region of the tree is a continuous layer
around the circumference and extends to all branches. It
carries the manufactured foods (sugars) from the leaves
to various parts of the tree. If the bark is cut
to the wood this layer is broken and a tree is said to
be "ringbarked". The passage of elaborated
foods is cut and while the leaves still receive water and
mineral salts, they cannot disburse their manufactured
foods to the areas where they are needed. A tree
which is ringbarked in this way dies more slowly than if
the bark and outer wood areas were cut. In some cases
a tree might survive 12 to 15 months after ringbarking. The
phloem is rich in nutritive foods and many insects favour
this region for their development stages.
- Cambium - the cambium layer is one cell layer in thickness
and therefore is not visible unless viewed under a high
power microscope. It is situated between the bark
and wood and is responsible for the increase in girth of
a tree. The cambium produces phloem cells on the
outside and xylem or living wood cells (sapwood) on the
inside. When insects feed externally in the phloem
region the cambium is severed and, in most cases, a tree
dies as if ringbarked.
The sapwood
This region contains xylem vessels (hardwoods) and tracheids
(softwoods) and is the region of the tree which conveys
water and mineral salts from the soil to the leaves. Food
is also stored in this area in the form of starch and other
carbohydrates. As a tree grows, the sapwood, instead of
becoming more extensive, is relegated in part to form heartwood. When
a tree is felled you will notice that the centre is darker
than the wood at the outside and this is due to the inclusion
of other materials, which are not present or present to
only a slight degree in the sapwood. At one time
this darker heartwood was actively functioning sapwood.
The sapwood, as well as containing tiny pores for conveying
water from the roots, has ray cells, which run from the
phloem into the sapwood. The purpose of these is
to convey sugars to the sapwood where it is converted to
starch and is stored in this form until needed. The sapwood
not only functions as an area of translocation but, together
with the heartwood, gives support to the tree.
Heartwood
This region of the tree is also known as the truewood, but
sapwood is also wood. Both have similar moisture
contents when dried, so that the term 'sapwood' refers to its
place in the living tree not to its water content when in service. Sapwood
is not less strong than heartwood, but by virtue of the food
materials it contains, it is less durable being more susceptible
to insects and wood decay. The main function of heartwood is
to give support to the tree. If the heartwood is partially
destroyed, as sometimes happens, the tree
does not die as no living tissue is destroyed. This type of
damage may be seen in old trees where the heartwood is eaten
out by termites and decay, but the tree continues to lay on
fresh leaves, etc. While a termite 'pipe' affects the
strength of a tree, it does not kill it. The cells in the heartwood
are dead, darker in colour and contain toxic materials. The
lignin content is higher in the heartwood and this is partly
responsible for the darker coloured heartwood seen in most
woods, particularly cypress pine, black bean, etc. The
colour difference is not so marked in pinewoods
Medullary Rays
Medullary rays, the rays of cellular tissue seen in a transverse section of exogenous wood, which pass from the pith to the bark. Area where simple sugars are stored in a living tree.
Pith
The pith is the centre of the tree and is composed of dead
cells, which are frequently eaten by insects. It
represents the earliest growth of the tree.
Roots
Roots serve two basic functions - to obtain water and minerals
from the soil and to anchor the tree in the ground. The
majority of tree roots are found in approximately the top
60cm of soil and it is in this area where they obtain most
of their water and minerals. Minerals are necessary prerequisites
for the manufacture of the actual components of the tree
itself as well as being ingredients for the manufacture of
gums, resins and oils. Water is used in photosynthesis
and transpiration. In the latter, water passes into
the atmosphere via the leaves. The roots also take in oxygen
which they find in the spaces between the soil particles.
Consequently, most trees grow best in porous, well-drained
soil.
Softwoods & Hardwoods
The formation of cells in softwoods and hardwoods to form
wood elements differs considerably. In softwoods
the cells for conduction become elongate and needle-like
and are known as tracheids, which vary in length from 2-12mm. The
walls of these cells may be modified by thickening from
deposition of cellulose, etc. from within the cell. Certain
areas, the pits, are not thickened and it is through these
that conduction from cell to cell of plant foods, etc.
takes place. Tracheids are close together and when viewed
through an X10 hand lens they appear as a honeycomb structure. There
are zones consisting of thin walled tracheids (early wood)
and thick walled tracheids (late wood) and these are visible
to the unassisted eye. A band of thick and thin walled
tracheids usually represents a growth ring. Another cell
known as the wood parenchyma cell is also split off from
the cambium and is associated with food storage. A
group of parenchyma cells is visible to the naked eye,
being in the form of rays. Resin, a gummy, aromatic material,
is produced by many softwoods in the parenchymatous tissue.
It is comparatively easy to differentiate hardwoods from
softwoods. Tracheids in softwoods are more uniformly
distributed than vessels (pores) in hardwoods. The
fibre length, an important economic factor, is smaller
in hardwoods, thus the wood pulp from this group is inferior
to that of softwoods. The conducting region for mineral
salts in hardwoods consists of 'vessels' or 'pores' which
are open-ended pipes or tubes, and are arranged in characteristic
patterns. The strengthening tissue of hardwoods consists
of fibres having pointed ends and very few pits. The
storage tissue of hardwoods is abundant and consists of
wood parenchyma cells and rays. The varied patterns of
the vessels, fibres, parenchyma and rays are of diagnostic
value in timber identification, even to particular timber
species.
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