wood anatomy exam III

biological agents of wood deterioration

fungi (white rot, brown rot, soft rot; mold and stain fungi), bacteria, insects, marine borers

fungi

organisms with cell walls that cannot photosynthesize their own food

decay fungi

degrade and consume the cell walls of wood

mold and stain fungi

do not structurally degrade cell walls

conditions necessary for fungal growth

oxygen, water (MC ~20%), food (wood), temperature

Hyphae

the branching, threadlike tubes that make up the bodies of multicellular fungi; travel through lumens, pits, and boreholes

boreholes

created by hyphae through enzymes secreted through their tips

hyphae eating

break down the cell wall polymers with enzymes into smaller molecules and absorb them

CO2

byproduct of fungal decay

brown rot

attacks cellulose and hemicellulose in cell walls (S2 first), leaves wood brown in color and shrinks it to create cubical cross-checks; common in softwoods

white rot

attacks lignin quicker and holocellulose slower (from the lumen outward), leaves wood white; common in hardwoods

softrot

hyphae tunnel into the S2 layer along the grain, leaves the wood surface soft and decays from outside inward; occurs in softwoods and hardwoods when surface is wet for extended periods

fungal decay prevention

deprive of oxygen, remove water, lower the temperature, make wood less appetizing (extractives); use of chemical wood preservatives

fungal stain

does not degrade cell walls, develops when wood is too wet, stains wood blue or black color, penetrates the wood; common in hardwoods and softwoods

stain hypae

eat simpler carbs (sugars, starches) in the lumens of ray parenchyma in sapwood; usually bluish in color

mold

doesn't degrade wood structurally, grows on surface, develops when wood is too wet, spores may cause discoloration; affects both hardwoods and softwoods

bacteria

can degrade wood cell walls, but very slowly; degrade pit membranes and parenchyma increasing permeability, can cause logs submerged for a long time to stink, many anaerobic

termites

eat wood but some cannot digest cellulose directly, make galleries in the wood, rely on protozoa in get to break down cellulose

carpenter ants

don't eat wood for nourishment, make galleries in wood for nesting

shipworms (mollusks)

burrow into wood as larvae through small hole, grow inside wood; symbiotic bacteria help digest cellulose, calcified shells carve out galleries and honeycomb the wood

wood defects

irregularities in wood that humans may consider unsuitable for a particular purpose

natural defects

irregularities that develop as a normal part of tree life

processing defects

irregularities that develop when processing wood materials as a result of the interaction among wood anatomy, other organisms, and wood physics

natural defects examples

knots, self repair by the tree (bark pockets, pitch pockets, pith fleck, glassworms), insect damage (pinholes and grub holes), resin and gum canals, decay and spalting, mineral streaks, spiral and interlocked grain, shakes, reaction wood

processing defects examples

drying defects (checks, splits, case hardening, reverse case hardening, honeycomb, collapse), fungal staining, warp (bow, crook, twist, cup), wane

knots

cross sections of branches growing perpendicularly or obliquely to the axis of the tree; can reduce mechanical properties

intergrown (live) knots

typically red (in softwoods), tight or sound

encased (dead) knots

typically black, loose (may fall out)

spike knots

knot shape from radial surface

bark pocket

the tree sustains a local injury, the cambium there dies, and the surrounding cambium grows over the gap, encasing the bark

pitch pockets

damage to a softwood tree sustaining a planoconvex cavity within a single growth ring, cavity is often filled with resin and overgrown; pitch pockets can be lines with epithelial cells and contain bark

pith fleck and glassworm tracks

darks car tissue from cambial damage in hardwoods caused by insect larvae; pith fleck resembles pitch but isn't (common in birch and soft maple), glassworm tracks look like dark zigzag marks (common in ash)

pinholes

bored by larvae beetles, surrounding discoloration is caused by fungi; size of a pinhole

grub holes

exceed 1/4 inch, bored by bigger larvae and some adult insects

resin and hum canals

sticky organic material (resin or gum), can cause sanding or finishing problems; resin canals in softwoods, gum pockets in hardwoods

decay

some parts of the tree an start to rot before the tree is felled; some can form dark lines called spalting, which is considered valuable special figure

mineral streaks

caused by accumulation of inorganic minerals in some hardwoods; can be olive to blackish-brown

spiral grain

grain angle oblique to the axis of the tree

interlocked grain

spiral grain that alternates direction every few years

shakes

longitudinal separation along the growth rings at weak spots caused by strong winds; weak spots believed to be caused by bacteria living in the tree

reaction wood

compression wood (softwood), tension wood (hardwood)

drying defects

due to wood not drying equally everywhere all at once, drying stresses occur; can cause the wood to crack, warp, or partially collapse

checks

longitudinal slit-like separations in the grain across the growth rings, occur on the surfaces and ends of boards, often close as the inside dries out; result from wood drying faster on the outside than the inside

splits

complete longitudinal separation of wood from one face to the other, frequently occur on the end of a board; can occur due to drying or growth stresses

case hardening

hardening of the outer surface before the inside, causing honeycomb (inner) checking once the inside has dried

collapse

cell walls in the wet core give way under drying stresses

honeycombing

the core wants to shrink but is constrained by the case-hardened shell resulting in internal cracking

warp

can occur due to reaction wood, juvenile wood, drying stresses, or growth stresses; bow, crook, twist, cup

bow

warp along the length of a board

crook

warp along the length of the edge of the wood

twist

warp resulting from the turning of the edges of a wood piece in opposite directions

cupping

warp across the width of the face of wood, in which the edges are higher or lower than the centre

wane

lack of wood or a bark strip on a piece of lumber; results from the log being round and lumber being rectangle

major factors of density

cell wall thickness, lumen volume, MC, reaction wood, juvenile wood, deterioration

density

oven dry mass / volume @ MC

specific gravity

ratio of the density of wood to the density of water; unitless

earlywood

latewood is denser than

compression wood

denser and more brittle than early wood, much more longitudinal shrinking and swelling, can cause wood to warp and twist, large microfibril angle (less vertical)

tension wood

denser than normal wood, strength problems, fuzzy grain, often silvery appearance, greater longitudinal shrinking and swelling, can cause wood to warp and twist

juvenile wood

less dense than mature wood, greater longitudinal shrinking and swelling, warp and twist common, low strength, unpredictable, less vertical microfibril angle in cell wall

major factors of hygroscopicity

hemicellulose content

wood

hygroscopic; it takes on and gives of moisture

moisture content (MC)

ratio of the weight of water in the wood to its oven-dry weight, expressed as a percentage; MS = ( W1-W0 / W0 ) x 100

adsorption

the process of binding or sticking to a surface; the negatively charges O end of an H2O molecule attaches to the positively charged H end of an available hydroxyl (OH) group on the cellulose, hemicellulose, and/or lignin

major factors of permeability

extractive content, occlusions, pit structure and state, deterioration

permeability

the rate of flow of liquids and gasses through xylem, greater in the longitudinal direction than lateral; heavily dependent on the size of openings between cells (the margo), can be reduced by extractives, occlusions, and aspirated pits

major factors of dimensional change

cell wall thickness, microfibril angle, lumen volume, MC, reaction wood, juvenile wood

shrinkage

loss of bound water

swelling

gain of bound water

cause of swelling and shrinking

water molecules push apart the cellulose or hemicellulose molecules once absorbed, causing dimensional change

cell wall swelling and shrinking

S2 microfibril angle is almost parallel to the axis, so when the cellulose molecules are forced apart, there is swelling in a lateral direction

more

denser species shrink and swell more

shrinkage difference

double in tangential direction than radial

latewood dimensional change

contains more cellulose, so it swells and shrinks more than earlywood

cellulose microfibrils

like steel rebar used to reinforce the tensile strength of concrete

lignin-hemicellulose matrix

like the concrete, high in compression strength