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Wounding and decay in trees


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Wounding and decay in trees


Have you ever cut up a tree or branch and observed colours, hollows, wedge-shaped discolouration, rings of gum or kino, ring shakes or ray shakes? If you haven’t, then perhaps you need to look more closely at trees and timber. The history of trees is locked up inside them, and phenomena such as the above tell us about it.

Trees, during their often long lives, may be wounded many times both ‘naturally’ (branch-shedding, storm damage, insect attack) or by people (vandalism, mechanical injury, pruning). Trees and animals differ radically both in their anatomy and in their response to wounding. Animals heal: that is, they repair or replace cells. Trees don’t heal by repairing cells; they defend themselves from the effects of injury and infection by walling off the damage. This section of the unit looks closely at the tree’s response to wounding and the practical implications for tree care procedures. A principal researcher into this area has been Dr Alex Shigo1, who has spent over 25 years wounding, dissecting and analysing more than 15 000 trees. His findings and the work of many other researchers have been included in this section.


The tree’s response to wounding


When a tree is wounded, two things occur:

  • the tree responds; and

  • many micro-organisms colonise the wound surface.

As mentioned above, trees do not repair wounded cells but rather they wall off the damaged area from the rest of the tree: that is, they compartmentalise the wounded tissues.

Compartmentalisation in trees is a defence mechanism wherein boundaries form to contain injuries to tissues and to resist the spread of pathogens. The process will be discussed in detail in the ‘CODIT’ topic later in this section.

The capacity of a tree to compartmentalise appears to be under strong genetic control: that is, some species react more readily and more successfully than other species. Sometimes the defence response is so successful that too much infected tissue can be walled off and the tree starves; this is the case in Elms resisting Dutch Elm disease.

The whole process of the tree’s response to wounding is an interaction between the tree and the micro-organisms and pathogens that may have invaded it. Survival of the tree after injury and infection depends greatly on its ability to compartmentalise pathogens. This process requires energy. Survival of the pathogens after injury and infection depends greatly on their ability to occupy as much tissue as possible before they are compartmentalised. If the tree’s responses are stronger than those of the pathogen, then the spread of the pathogen is resisted. If the pathogens are very aggressive, they may spread rapidly and the tree may decay.

The genetic make-up of the host and pathogens play an important role in the interactions, as do environmental conditions at the time of wounding. The tree’s history of wounding; the tree; age, health and vigour of the tree; and the amount of energy reserves.

Stems and trunks


The stem or trunk of a tree is composed of many tissues (groups of cells with specialised functions). The main tissues and their functions are listed below.

See page 3 for diagram.



Outer bark (periderm)


This consists of three layers of cell types: cork (phellem) on the outside, cork cambium (phellogen) in the middle and phelloderm on the inside. Cork which contains suberin and is non-living, is an important protective tissue. It prevents loss of water from the stem; it insulates internal tissues against extreme temperature fluctuations; and it is very resistant to hydrolysis by fungal enzymes. Some trees have persistent bark (eg Eucalyptus sideroxylon and Melaleuca quinquenervia) while others, such as the smooth-barked gums, shed their outer bark regularly (eg Eucalyptus haemastoma and Eucalyptus citriodora).

Phloem (inner bark)


This is a complex, vascular tissue consisting of various cell types, all living, including sieve elements, companion cells, fibres and parenchyma. The function of phloem is the movement of food (sugars) from the leaves to other plant parts.

Vascular cambium


These are meristematic cells which lead to an increase in girth of the tree by producing xylem (wood) cells on the inside and phloem on the outside. The vascular cambium also produces ray cells. The cambial zone includes the cambium and a zone of cells on either side which have not yet differentiated into phloem or xylem.

Xylem


Xylem is otherwise called wood: that is, an orderly arrangement of living, dying and dead cells that have walls of mostly cellulose and lignin. Wood that is living is called sapwood, and wood without living cells is called heartwood. The structure of wood varies between conifers (softwoods) and dicotyledons (hardwoods).

Sapwood


Sapwood has four functions:

  • transport of water and nutrients from the roots

  • storage of energy reserves and other materials for maintaining life

  • mechanical support

  • protection and defence (really a combination of the other factors).

Together, the phloem and the xylem form a continuous system of vascular tissue extending throughout the plant body.

Heartwood


Heartwood is wood which has been altered according to normal ageing. The wood contains no cells with living contents, and there is no active transport. It does maintain a protection and defence system and continues to provide mechanical support. Heartwood is usually a different colour to the sapwood because, as the nutrients leave the cells, the nuclei in the living cells disappear, the cell walls are altered and extractives such as gums, oils, resin and tannin may impregnate and accumulate. However, not all darker-coloured wood in trees is heartwood. Discoloured wood can also develop as a result of injury and the processes which follow wounding. When heartwood is injured it will also discolour. There is often no clear distinction between heartwood and sapwood.




Comparison of dicotyledon wood and conifer wood


If you looked closely at the wood of a conifer, you would find that it differs slightly from the wood of a dicotyledon. Conifer wood contains tracheids rather than vessels, and small amounts of parenchyma. Resin ducts occur in the wood as well as in the rays. Resin appears to protect the plant from attack by certain fungi and bark beetles. Conifers are also known as ‘softwoods’ although the wood is not always soft.

To go back to dicotyledon woods, the main cells are:



  • vessels—dead, lignified, pitted water-conducting elements

  • tracheids—which are similar to vessels

  • parenchyma—living storage cells

  • fibers—often dead, lignified support cells.

As in conifers, dicots may contain protective substances such as kino or gum. Kino is a dark red to brown resin-like substance produced by Eucalyptus spp. Large kino veins may form in some trees in response to injury and infection. Kino is made of phenol-based substances. It also forms in the barrier zones, which we will discuss further shortly. Dicots are often called ‘hardwoods’ although the wood is not always hard; for example, balsa is a tropical dicot and is extremely soft.

Rays


These are produced by the vascular cambium, are variable in length and are composed mostly of parenchyma cells. Nutrients and sugars are transported from the phloem through the vascular cambium to the living xylem cells. They also transfer water from the xylem to the vascular cambium and phloem. The rays serve as storage centres for starch and lipids.

Lenticels


These are small patches of loosely packed corky cells on the surface of stems and which allow gaseous exchange. Some trees have very distinctive lenticels.

Palms


Palms are monocots which sometimes reach the proportions of a tall ‘tree’. Monocots differ from dicots in stem anatomy in that the vascular tissue is scattered throughout the stem, and in the absence of true secondary growth with a vascular cambium. Increase in girth is generally brought about by sustained primary growth, including cell enlargement and cell division. This increase in girth ceases when the crown attains maximum diameter.

Roots


There are several functions of roots:

  • absorption of water and nutrients

  • anchorage

  • storage of food reserves

  • synthesis of some growth regulators required by shoots

  • conversion of inorganic nitrogen to organic amides and amino adds.

The importance of tree roots is easily overlooked because they are not visible—’out of sight, out of mind’. Damage to the root system (for example, as a result of construction, soil level changes, flooding or compaction) is a common cause of tree decline and death.

Root growth is opportunistic and takes place wherever the environment is favourable. The root system of trees consists of several ‘types’ of roots found in different parts of the soil, each with its own functions. The first root (or radicle) to emerge from germinating seeds sometimes persists and becomes a tap root. Large, woody, perennial, lateral roots develop from the tap root and at the rootstem base. These rapidly taper to a small diameter a short distance from the base of the tree. They are predominantly found in the top 300 mm of soil and do not normally extend to depths greater than one to two metres (99% of all roots are found in the top metre of soil).

From these basal woody roots, second-order woody roots arise. They often extend outward from the trunk to occupy a considerable, irregularly shaped area, four to seven times larger than the crown area (or drip zone) and having an average diameter equivalent to one, two or more times the height of the tree. Sometimes ‘sinker’ or ‘striker’ roots form at intervals along the woody framework. These grow downwards and may divide to form deeper layers of horizontal roots. The woody roots anchor the tree, are important for lateral and upward transport, and store large amounts of food reserves (mostly starch). Root grafts can be common among trees of the same species.

A fine, non-woody root system develops from the growing root tips of the long woody roots. These roots branch and re-branch as many as five times and fill the soil spaces between the woody roots. Typically, these roots grow upward into the leaf litter and the top few millimetres of soil. They constitute the major part of the surface of the root system of a tree.

They are often referred to as ‘feeder’ roots as their multiple tips are the primary site of absorption of water and nutrients. They also store significant amounts of carbohydrate, produce amino acids and growth regulators, and form mycorrhizal associations, which we will deal with shortly. This delicate, non-woody system, because of its proximity to the soil surface, is very vulnerable to injury and can therefore be short-lived. New roots can form rapidly after injuries so the population of the roots tends to be more dynamic than the crown. However, this growth depends on the presence of stored carbohydrates and so is closely related to shoot growth.

The rhizosphere effect


This is the concentration of organism activity in the root zone of plants as a result of organic materials exuded from or rubbed off plant roots. Recent research has shown that as much as two tonnes per hectare can enter soil each year from this source. Another 300 kg/ha may reach the soil after being washed from plant leaves.

Mycorrhizal association


A mycorrhizal association is a symbiosis between the fine feeding roots of plants and various fungi. There are several types of mycorrhizal fungi with varying methods of host infection. For example, ectomycorrhizas occur in many tree species from eight or nine families, including Eucalyptus and Casuarina. In this case, the fungus forms a sheath around short lateral roots, and the fungal hyphae grow into root tissues between cortical cells. Penetration of the root is not always necessary as mycorrhizal stimulation of growth has been recorded in situations where the growth of the fungi was restricted to the rhizosphere.

Some fungi infect a wide range of tree species and others only a very narrow range.

The major function of mycorrhizal fungi is to increase the root surface area for nutrient uptake. The fungal hyphae absorb ions from the soil and transport them to the higher plant. Mycorrhizal fungi enhance the uptake of phosphate and other less mobile ions such as zinc and possibly ammonium.

There is no good evidence to suggest that mycorrhizas use phosphate sources not normally used by higher plants; they exploit the same sources but do so more effectively than non-mycorrhizal roots. Some fungi are able to store considerable amounts of polyphosphates when phosphate is abundant, as might occur with flushes of decomposition. Mycorrhizal infection is probably indirectly important to the sustained production of natural ecosystems involving nitrogen-fixing plants. This is because uptake of phosphate, sulphate, zinc and probably cobalt, molybdenum, copper and iron by mycorrhizas might stimulate growth and nitrogen fixation.

There is no reason why thinking on mycorrhizas should be restricted to the roles that they obviously play in the absorption of nutrients from nutrient-poor soils. The mycorrhizal root could be considered as an alternative strategy for ‘root growth’ with other possible functions, including the following:


  • It promotes more rapid transport of water to the plant than through the soil. This is likely to be especially so under low soil moistures or in plants with low rooting densities.

  • It counteracts soil toxicities to plant roots such as high acidity, high aluminium, salty, high soil temperatures and root disease. (If the fungus is not as susceptible as the root to the deleterious factor, then a plant may be able to compensate for loss of roots via the fungal hyphae of mycorrhizas.)

  • It improves soil structure.

  • It enhances nutrient conservation. In natural communities, even in the high-rainfall areas of the tropics, losses of nutrients from soils are very low. It is probable that mycorrhizal associations play an important part in the conservation of nutrients and in nutrient cycling. This is because fungal hyphae readily penetrate litter and decomposing organic matter and there compete with other soil micro-organisms for a range of organic and inorganic nutrients more effectively than non-mycorrhizal roots.

Proteoid roots


These are roots developed by large numbers of species of the Proteaceae Family. This form of root is characterised by intense, local lateral root production. Proteoid roots tend to form as quite dense mats in the surface soil, and though particularly well developed in pockets of humus-rich soil they are not necessarily restricted to such regions. Similar structures are now known to occur in other families; for example, in some species of legumes and also in Casuarina equisetifolia. (Future study may show them to be present in a wide range of other plants.)

Proteoid roots are caused by the presence of particular rhizosphere micro-organisms. Proteoid roots have a greater phosphate uptake than unmodified roots, may possibly store phosphates, have assimilates preferentially translocated to them, and are very susceptible to the soil-borne plant pathogen Phytophthora cinnamomi. (In this case, their critical roles in nutrient and possibly water uptake are indicated by the rapid collapse of proteaceous plants in the understorey following infection with this disease.)


Growth rings


As has already been discussed, the growth in girth of a woody dicot or gymnosperm is produced by the vascular cambium. In temperate zones, the periodic activity of the cambium produces growth increments or growth rings. If a growth layer represents one season’s growth it is called an annual ring. Sudden changes in available water, other environmental factors, or damage to the tree may be responsible for disrupting growth rings. The width of a growth ring is a good indicator of the prevailing rainfall of the time and can also be used to date a trauma in the life of the plant.

The structural basis for the visibility of growth rings in wood is the difference in density of the wood cells produced during the growing season. Early in the season large cells with relatively thinner walls are produced (ie early season’s wood is less dense). Late wood has narrower cells with thicker walls.


Shoot growth

Buds


These are compact, undeveloped shoots consisting of a short stem bearing crowded, overlapping, immature leaves; they may be terminal, axillary or adventitious; leaf, flower or mixed, and are connected to the cambium either at the tip of the branch or laterally.

The growing points of trees are the apical or terminal meristems of the main trunks and branches and the axillary buds. The apical meristem increases the height of the tree while the axillary or lateral buds produce branches. Generally, the growing terminal bud inhibits the development of lateral buds, a phenomenon known as apical dominance. As the distance between shoot tip and lateral buds increases, the inhibiting effect of the terminal bud decreases and the lateral buds develop. Apical dominance is due to the hormone auxin which is produced by the terminal bud and which moves down and inhibits the lateral growth. If the terminal bud is removed, the lateral buds develop.

Have a close look at a terminal section of a branch of a deciduous tree. You may be able to discern a change in bark colour and diameter associated with segments separated by a scar around the stem. This scar is known as the terminal bud scar and the distance between scars indicates annual growth. As with growth rings within the trunk, the extension growth of the shoot can be an indicator of tree vigour, environmental factors or stress. Compressed scars (ie minimal extension growth) may indicate a poor growing season or stress. Species and age factors will also affect extension growth.


Epicormic buds


These are present along the trunk and main branches of some trees, especially Eucalyptus spp, but also in other plants. There are two types of epicormic buds: those that are dormant within the cambial zone, and those which newly arise from the cambial zone. When the stem is injured or when localised or general depletions occur in energy reserves, the buds begin to form. The ‘injury’ may be pruning, and energy losses may be due to root damage. The production of epicormic shoots is a survival mechanism and indicates stress. In a Eucalypt, particularly during the mature and later stages of its life, the crown is maintained by a continuous process of extension and dieback in which epicormic buds play a major role in replacing dead shoots and reshaping the crown after loss of branches. Epicormic shoots are an integral part of the adaptation of Eucalypts to nutrient-poor sites. They are not necessarily a direct evolutionary response to fire but certainly are essential in recovery after fire.

The epicormic branches produced after lopping are always more weakly attached than natural branches and will always be a weak spot in the tree, because they arise from the cambial layer on the outside of the trunk or branch.


Shedding parts


As trees grow, various parts of them will be shed. These include leaves/needles, fine absorbing root hairs and roots, reproductive parts (flowers, seeds, fruit), bark and branches. The tree has specialised areas which allow parts to be shed with the minimum of damage to the tree; for example, leaf abscission zones and the branch collar.

Occasionally, trees unexpectedly drop large, healthy limbs with no external sign of damage or decay. This phenomenon, known as summer branch drop, has been reported in several countries including Australia. It usually occurs on calm, clear, warm days and often on mature trees. Several species of Eucalypts, Elms, Pines, Plane trees, Olives and figs have been reported to drop limbs in this way. There is no explanation for this phenomenon but it appears to be related to internal and external water movements.





1 Reference for Shigo: www.chesco.com/~treeman/SHIGO/


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