Wood Biology (Summary Notes)

What types of timber are associated with angiosperms and gymnosperms?

Gymnosperms:

- Contains pinales (conifers)

- Referred to as softwoods

Angiosperms:

- Referred to as Hardwoods

How does primary and secondary growth in plants differ?

Primary Growth:

- Ensures axial expansion at shoot tips and roots

- Present in all plants

- Forms primary xylem and phloem

Secondary Growth:

- Only in some plants (lateral growth)

- Meresmatic cells in vascular bundles between xylem and phloem (cambium)

- Cambium forms secondary phloem (bark) to outer xylem (wood)

What tissue in plants is commonly referred to as wood?

Secondary xylem

What is the function of phloem and xylem in plants? Which are specialised cells for those functions?

Xylem:

- transport water & nutrients

- support leaves to sunlight

Phloem:

- Transports photosynthates from leaves to cambium/roots

- Forms bark

What is the function of rays?

Rays are responsible for radial transport in a tree (often also fulfil the function of storage and wound response)

What causes the length of tracheids to increase with cambial age?

1) More cambial initials and less radial growth reduces the frequency of divisions and therefore allows more time for tip-growth after pseudotransverse division.

2) Longer initials tend to stay in the cambium as they have statistically more contact to rays and consequently resources. This results in a selection for length.

Describe apical dominance and why it is important for forestry?

- Ensures single stem (ideal for sawmilling)

- Apical (top most) bud produces auxin (growth hormone) which flows down stem & supresses auxiliary buds

- Auxiliary buds take over if leading bud is lost to ensure survival

- Reason for why most solid wood processing is based on softwoods rather hardwoods as conversion rates of log volume into sawn timber are lower for trees w large crowns/branches (hardwoods)

What is the difference between cambial age and tree age?

- Tree age is the time from germination to date (corresponding to the number of annual rings at the base of a tree).

- Cambial age is the age of the cambial initials at a certain height of a tree (i.e. tree age at the bottom and 0 at the top).

- important as wood properties change w cambial age (radially from pith to bark)

What types of knots can be found in timber?

- knots are branch bases within stem

- live knots are formed by live branches and connected to stem

- black knots are formed when dead branches are still attached to stem and overgrown by the stem (devalues wood more)

Which part of a plant cell is 'wood'?

Mainly the cell wall skeleton of xylem after cell death, predominately the secondary cell wall.

Describe the formation of the plant cell wall.

- Mitosis of 2 sets of chromosomes

- cell plate forms between daughter nuclei (cytokinesis)

- formed by golgi vesicles which transport pectins to growing cell plate (guided by phragmoplast)

- Golgi vesicles fuse and cell plate grows to existing cell walls

- Primary walls deposited

- secondary wall deposited next for mechanical support

- Most cells now die to be used for water transport

How do cell walls expand?

- Only cells with primary walls expand.

- The force for expansion is turgor pressure which results from osmosis (vacuole/tonoplast)

- The main volumetric expansion is increase of the vacuole

- Expansion is tightly controlled by enzymes which modify the plasticity of the cell wall matrix.

How does a plant cell control its shape during growth?

- Primary wall is a complex composite of stiff cellulose fibres in a plastic matrix

- orientation of the cellulose fibrils expansion occurs perp to their direction.

- cell wall matrix can be locally weakened/strengthened to allow local expansion

What is the difference of primary and secondary cell wall formation?

- Primary cell walls are deposited when cells grow (stretchable)

- Secondary cell walls are much thicker, stiff and non-stretchable and deposited after growth has finished for mechanical support

What are pits?

- provide pathways b/w cells for transport

- bordered pits in radial walls b/w tracheids (secondary cell wall structure that can close)

- simple pits b/w parenchyma cells

- half/half b/w tracheids and parenchyma cells

What is understood by pit aspiration?

- bordered pits can open/close in case of gas/air bubbles

- surface tension of retreating water pulls in the pit membrane into chamber

- torus acts as a plug and permanently seals.

- Ensures water flow in stem but can complicate wood processing

Which are the principal directions in wood anatomy?

1) Longitudinal (axial)

2) Radial

3) Tangential (perp to radial)

What implications do the 3 principal directions have on wood properties?

- Wood properties differ in the 3 directions, therefore it is an anisotropic material.

- Main reason is the cell anatomy, i.e. the shape of the cells.

What cell types are found in the axial cell system of conifers and what is their function?

1) Tracheids: water transport EW, mechanical support LW, 90% of wood volume

2) Axial parenchyma: wound response, storage (starch)

3) resin canals/epithelial cells, wound response

4) Strand tracheids: function unclear but w RCs

What cell types comprise the radial cell system in conifers?

1) ray parenchyma: 10% wood volume, storage, wound response, heartwood formation, radial transport

2) ray tracheids: function unclear

3) ray resin canals / epithial cells, wound response

How does a tree control water transport?

- Bordered pits allow transport in longitudinal and tangential (i.e. pits only in radial walls) direction and provide a safety system against embolism (pit aspiration).

- Radial transport is provided mostly by living ray cells (or apoplasitc).

Describe a softwood tracheid.

- long (2-5 mm) & elongated (1:100) tracheids

- cell wall skeletons made of (thick) secondary walls (multi layered)

- bordered pits in radial walls

- large cross fields (pits b/w rays parenchyma & axial tracheids)

- overlapping of tips aid w mechanical entanglement and efficient water transport

- walls thicker in latewood

cell lumens larger in early wood

Describe resin canals and their function.

- form a 3D network by epithelial cells which synthesise resin

- defence mechansim

- arranged in tangential bands and can agglomerate to resin pockets

- devalues wood (dissolves paint)

Name some wood anatomy features which are of taxonomic value.

- cross fields (shape)

- resin canals (yes/no)

- cell wall of epithelial cells (lignified/undignified)

- helical thickenings

- number of bordered pits in a row

- axial parenchyma (yes/no)

- strand tracheids (yes/no)

Are softwood always soft and hardwoods always hard?

This terminology originates from the medieval timber trade in England where the phylogenic and physical properties (largely) match. Globally, however, there are many soft hardwoods and also hard softwoods. This ambiguity does not exist in other languages.

What cell types are found in hardwoods and what is their function?

- vessel elements (form vessels, water transport)

- fibres (mechanical support)

- tracheids (optional, mech supp & water transport)

- axial parenchyma (storage, defence, thylosis)

- ray parenchyma (storage, defence, radial support, thylosis)

- epithelial cells (gum ducts, defence, sealing vessels)

How is the flow of water controlled between vessel elements and between vessels?

- perforation plates form b/w vessel elements to form vessels by lysis of the hemicelluloses in primary wall.

- water flow b/w vessels need to pass through inter-vessel pit-fields where pir membranes ensure a safety mechanism against embolisms.

What is understood by thylosis?

Parenchyma cells neighbouring vessels can grow through pits into the vessel forming thyloses which seal the vessels. This happens during heartwood formation to protect the older part of the stem against biological attack (and to protect the water transport system).

What arrangements of vessels can be found in hardwoods? Do they contribute to the figure in wood?

Diffuse porous, ring porous; Vessels solitary or in radial/tangential clusters. Large vessels in ring porous arrangement or wide tangential vessel bands can be seen with the naked eye and give rise to figure in wood.

What arrangements of axial parenchyma can be found in hardwoods?

Apotracheal (not in contact with vessels) or paratracheal (in contact with vessels); arranged in tangential bands or diffuse etc.; can give rise to figure in wood.

Comment in the structure of rays in hardwoods? How do they affect figure in wood?

- rays made from parenchyma cells

- usually multiseriate (larger than those in softwoods)

- large rays can be seen w naked eye

- give rise to mirror like structures in radial planes

- spots in tangential planes

- small uniseriate rays also present

How do resin canals of softwoods and gum ducts in hardwoods compare?

Gum ducts and resin canals have the same function i.e. defence/sealing of wounds. Gum and resin is made of different chemical compounds.

What is the difference between warp caused by growth stresses and warp caused by changes in moisture content?

- warp during sawing cused by internal growth stresses present in living tree (green log)

- when timber dries, inhomogeneous dimensional changes can also cause distortion

How do growth stresses affect the utilisation of wood?

- usually only in hardwoods, tangential stresses can cause star checks of logs and jam saws

- steep longitudinal growth stress gradients can cause bending of boards (smaller D logs)

- extreme cases longitudinal growth stresses can cause logs to split after felling

- large axial compressive stresses in centre of big logs cause brittle heart (compression failures in centre)

Why do Australian hardwood mills traditionally demand log diameters greater than 60cm?

- stress gradients leading to deform/warp boards when sawn

- max growth stress is constant on the periphery of the log and a larger D results in flatter gradients (warp less)

What could be the mechanism generating growth stresses in trees?

- swelling of the cell wall during lignification (lignin swelling) and/or contraction of the cellulose fibrils during cell differentiation (cellulose contraction) in conjunction w an appropriate MFA

What Is the internal stress profile in trees and how do trees benefit from it?

- axial tensile stresses on the outside improve compression strength in bending

- axial compressive stresses in the centre can cause compression failure but the centre is not needed for bending strength

- peripheral tangential compressive forces help closing longitudinal cracks developing at rays

- tensile tangential stresses in the centre can cause checks (devalue logs) but axial strength is not affected

How does longitudinal growth stress impact on the longitudinal stiffness and strength of stems?

stiffness is unaffected by pre tensioning, while bending strength is increased.

What is reaction wood?

a special tissue formed by trees allowing to control their orientation in space by exerting asymmetrical growth stresses.

Different forms of reaction wood have evolved in softwoods and hardwoods. What is their mechanism?

- reaction wood of softwoods called compression wood (expansive force - pushing stem)

- reaction wood in hardwoods tend to contract, pulling the stem.

What properties of reaction wood impact on the utilisation of wood?

- excessive longitudinal shrinkage causes warp

- thick cell walls reduce paper quality

- high lignin content of compression wood make it more difficult to pulp

- low relative stiffness, brash and catastrophic failures are safety concerns for compression wood

- tension wood is difficult to machine

Why is reaction wood a problem in commercial logs?

- Only predicted in extreme cases (bend stems)

- 15-30% of straight trees

- highly variable, unpredictable pattern and variable severity, therefore unfavourable

What are the anatomical/molecular characteristics of compression wood and tension wood?

Compression Wood:

- high MFA, thick walled round cells, no S3, S2 (L) layer composed of lignin, 1,4-beta-D-galactan free of cellulose

Tension Wood:

- low MFA in G layer, no lignin in G layer, high MFA in S2 of tension fibres

Comment on the 'lignin swelling' in relation to force generation in reaction wood

Possible for compression wood (high MFA and lignin content) unlikely to be the mechanism in tension wood w G fibres as there is no lignin

What is the difference between the grain angle and the MFA?

- MFA is the angle b/w cellulose fibrils and cell axis

- grain angle is angle b/w cell axis and stem/board axis

What anatomical structure is best suited to judge the grain angle on boards?

- resin canals on surface run along grain and indicates its direction

- Annual ring boundaries have no relationship to the grain in the boards

How does the grain impact on wood quality?

- some grain deviation result in highly sought after appearance grades (wavy/curly grain)

- one-sided grain in timber causes twist (wood defect)

What tends to be the part of the steam which has the highest grain angle?

- corewood of young softwoods

- outerwood of older hardwood trees

- radiata top has higher grain angle (large variation w/in one tree)

How could the grain angle be controlled in plantation forestry?

- genetic selection

- selective thinning

Where is heartwood found in trees?

central cone of a stem which increases w tree age

Where is heartwood formed?

- reserve materials (starch) are mobilised and together w sucrose from the sap transported to transition zone

- parenchyma cells synthesise extractives and finally undergo programmed cell death

What are the characteristics of heartwood?

- low MFA and permeability

- no living cells

- no reserve materials

- high extractive content (naturally durable and coloured)

How can the quality and quantity of heartwood be improved?

- planting genetically improved trees

- longer rotation length increase heartwood % (species specific)

How does heartwood determine wood quality?

- negative effect on processing, pulping (more chemicals) and preservation and drying (less permeable)

- positive: colour, durability dimensional stability

Considering heartwood: why can it be argued that, from a processing POV, radiata is preferable to Sitka spruce?

- both species do not provide any colour or natural durability

- radiata has a permeable heartwood (thin walled resin canals are easy to collapse)

- Sitka heartwood is highly impermeable (thick walled resin canals)

In what ways can materials degrade and where is wood superior to other materials?

- photodegradation

- chemical degradation

- Mechanical wear

- fire cases

Is biodegradability a disadvantage?

No, it is an advantage considering waste disposal. It is only a problem if decay occurs premature/in-service.

Why are logs often stored under water sprinklers or in lakes?

Full water saturation excludes oxygen and therefore prevents fungal decay as well as prevents logs from cracking due to drying.

What do wood decaying organisms need to colonise wood?

- Nutrients

- Moisture

- Oxygen

- Ambient temp

What protects wood best against fungal decay?

- dry below 20% MC

- This does not protect against dry wood borers

Why do fungi need free water?

Because they release exo-enzymes as well as take up the degradation products sugar monomers. Both only works in solution

What types of fungal decay can be differentiated and when do they impede with the utilisation of wood?

- Sapstain (no mechanical degrade but colour change and hygienic issue) - sapwood only.

- Brown-rot (mainly softwoods),

- white-rot (softwoods and hardwoods)

- soft-rot all cause severe decline of mechanical properties.

Why are fungi often specific to hardwoods or softwoods?

Because different enzymes are needed to degrade the hardwood and softwood specific hemicelluloses.

Why is brown-rot more common to softwoods?

Because the lignin is more condensed and therefore more resistant against enzymatic attack

How does the damage to wood differ between insects and fungi?

- Insects cause damage by mechanical action while fungi cause damage by chemical/enzymatic action.

- While wood decaying fungi feed on the cell walls, insect typically only feed on the reserve carbohydrates (e.g. starch) present in sapwood.

- They can live in symbiosis (ambrosia beetles)

Which insects pose the biggest thread to timber constructions?

- Insects that attack dry wood

- dry wood safe from fungal decay

How can insect infested buildings be remediated?

- heating the timber (>55 deg, > 4hrs)

What do you know about wood degrading termites and ants?

- Termites feed on wood (with the help of symbiotic bacteria).

- Problem in tropical countries.

- Very view timbers are termite resistant.

- Damage invisible from the outside.

What are the dangers of using wood in maritime conditions?

Molluscs (Teredo, Pholadidae) and Limnoria (Gribbles). These organisms need salt water conditions and not many preservatives/wood species are able to protect wood against their attack