Architecture for the Tropics
revises by Cosmo; book-based by Alois Payer – Read original (DEU) article on his website
Wood as a Material
- 1. Introduction
- 2. Broadleaf – Conifers
- 2.1. Softwood – Hardwood
- 3. Macroscopic Cross Section
- 3.1. Bark (outer bark)
- 3.2. Bast (inner bark)
- 3.3. Cambium
- 3.4. Sapwood
- 3.5. Heartwood
- 3.6. Growth rings
- 3.7. Pith
- 3.8. Rays
- 3.9 Pore
Wood evolved as a functional tissue of plants and not as a material designed to satisfy the needs of woodworkers. Hoadley, R. Bruce: Understanding wood : a craftsman’s guide to wood technology.
2. Broadleaf – Conifers
Wood stems from trees. Unlike Europe, where most of the wood used for construction is softwood, in the tropics it is hardwood. Hardwood is phylogenetically younger than softwood. Its structure is more complicated than that of softwood. Above all, the absorption of water is fundamentally different. In addition to hard- and softwood timber, there are also some monocotyledonous plants (palm trees).
2.1. Softwood – Hardwood
“Hardwood” and ”softwood” is only indirectly related to the hardness of the wood. They are identifiers for the origin of evergreen (conifer) and deciduous trees (broadleaf). Many of the so called “softwood” trees are harder than some of the “hardwood” trees. Softwood – Conifers Hardwood – Broadleaf
3. Macroscopic Cross Section
The macroscopic structure is visible to the naked healthy eye; it is the wood structure visible when the wood is cut. Cutting planes: A – Transverse or horizontal sectional surface, transverse section: the vertical stem axis B – Radial section surface, radial section: parallel to the stem axis in the direction of the radius, almost parallel to the medullary rays C – Tangential section surface, tangential section: parallel to the stem axis, tangential to the annual growth rings or zones, transverse to the medullary rays
Cutting edges:Red = Transverse Sections Blue = Radial Sections Green = Tangential Sections
1 = Pith 2 = Annual ring 3 = Primary wood ray (pith ray) 4 = Secondary wood ray (from outside in) 5 = Cambium 6 = Inner bark 7 = Outer bark red dot = Horizontal section blue dot = Radial section green dot = Tangential section
Structural components of wood are:
|Bark – Bark (outer bark) – Bast (inner bark)||Cambium Body of wood – Sapwood – Heartwood||Pith Ray Pore Resin canal|
Timber of a 4-year old pine cut in winter
Annual rings of the wood body Cross section of a 4-year old branch of a linden tree
- Blue Radial section
- Green Tangential section
- B Bast
- BR Bark
- E Earlywood
- L Latewood
- P Pith
- PC Primary components in timber
- R Resin canal R
- H Ray in horizontal section
- RR Ray in radial section
- RRBRay in radial section in bast
- RT Ray in tangential section
- Y Year transition/border 1, 2, 3, 4
- SR Secondary medullary ray
- B Bast area
- PR Primary medullary ray
- Y Year transition/border
- PRO Outer, by dilatation extended end of a primary medullary ray
3.1. Bark (outer bark)
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm. The outer bark in older stems, includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees is also called the rhytidome. The bark protects the underlying layers against physical influences such as temperature, rain, wind, sun, fire or mechanical influences and serves as a defense against pests and infections.
3.2. Bast (inner bark)
Flax stem cross-section, showing locations of underlying tissues:
The bast is the living tissue under the bark of trees and other woody plants. This tissue transports in water dissolved nutrients (sucrose (as a transport form of glucose), ions, secondary plant substance) from the crown to the roots – the transport from the roots to the crown takes place in the sapwood. The bast consists of sieve-tube cells (which in turn form the sieve tubes), companion cells, bast fibers and storage cells. Therefore, the bast tissue of a living tree is moist and in relation to the wood and bark it is very soft, but always very tough and resilient. Bast fibers are flexible long cells, onto which softer fibers build up (such as flax or hemp). The corked bast forms the protective layer for stem and roots of plants.
Cork cambium (pl. cambia or cambiums) is a tissue found in many vascular plants as part of the periderm. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems. It is found in woody and many herbaceous dicots, gymnosperms and some monocots, which usually lack secondary growth. Cork cambium is one of the plant’s meristems – the series of tissues consisting of embryonic (incompletely differentiated) cells from which the plant grows. It is one of the many layers of bark, between the cork and primary phloem. The function of cork cambium is to produce the cork, a tough protective material. Synonyms for cork cambium are bark cambium, pericambium or phellogen. Phellogen is defined as the meristematic cell layer responsible for the development of the periderm. Cells that grow inwards from the phellogen are termed phelloderm, and cells that develops outwards are termed phellem or cork (note similarity with vascular cambium). The periderm thus consists of three different layers:
- phellogen (cork cambium) and
Growth and development of cork cambium is very variable between different species, and also highly dependent on age, growth conditions etc. as can be observed from the different surfaces of bark; smooth, fissured, tesselated, scaly, flaking off, etc.
A section of a Yew branch showing 27 annual growth rings, pale sapwood and dark heartwood, and pith (centre dark spot). The dark radial lines are small knots. Sapwood (or alburnum) is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves. However, by the time they become competent to conduct water, all xylem tracheids and vessels have lost their cytoplasm and the cells are therefore functionally dead. All wood in a tree is first formed as sapwood.
The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required. Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Sometimes trees (of species that do form heartwood) grown in the open may become of considerable size, 30 cm or more in diameter, before any heartwood begins to form, for example, in second-growth hickory, or open-grown pines. Thin sapwood is characteristic of such species as chestnut, black locust, mulberry, osage-orange, and sassafras, while in maple, ash, hickory, hackberry, beech, and pine, thick sapwood is the rule. Others never form heartwood. No definite relation exists between the annual rings of growth and the amount of sapwood. Within the same species the cross-sectional area of the sapwood is very roughly proportional to the size of the crown of the tree. If the rings are narrow, more of them are required than where they are wide. As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume. Sapwood is thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less. When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and are either broken off or fall off. Subsequent growth of wood may completely conceal the stubs which will however remain as knots. No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle. Consequently the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the inner heartwood. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that a given piece of sapwood, because of its position in the tree, may well be stronger than a piece of heartwood from the same tree.
Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay. Heartwood formation occurs spontaneously (it is a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once. Usually heartwood looks different; in that case it can be seen on a cross-section, usually following the growth rings in shape. Heartwood may (or may not) be much darker than living wood. It may (or may not) be sharply distinct from the sapwood. However, other processes, such as decay, can discolor wood, even in woody plants that do not form heartwood, with a similar color difference, which may lead to confusion.
It is remarkable that the inner heartwood of old trees remains as sound as it usually does, since in many cases it is hundreds, and in a few instances thousands, of years old. Every broken limb or root, or deep wound from fire, insects, or falling timber, may afford an entrance for decay, which, once started, may penetrate to all parts of the trunk. The larvae of many insects bore into the trees and their tunnels remain indefinitely as sources of weakness. Whatever advantages, however, that sapwood may have in this connection are due solely to its relative age and position. Different pieces of wood cut from a large tree may differ decidedly, particularly if the tree is big and mature. In some trees, the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier, but in other trees, the reverse applies. This may or may not correspond to heartwood and sapwood. In a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log. In a smaller tree, the reverse may be true.
3.6. Growth rings
Growth rings, also referred to as tree rings or annual rings, can be seen in a horizontal cross section cut through the trunk of a tree. Growth rings are the result of new growth in the vascular cambium, a lateral meristem, and are synonymous with secondary growth. Visible rings result from the change in growth speed through the seasons of the year, thus one ring usually marks the passage of one year in the life of the tree. The rings are more visible in temperate zones, where the seasons differ more markedly.
The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as “early wood” or “spring wood” or “late-spring wood”. The outer portion is the “late wood” (and has sometimes been termed “summer wood”, often being produced in the summer, though sometimes in the autumn) and is denser. “Early wood” is used in preference to “spring wood”, as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer (e.g. Canada) or in autumn, as in some Mediterranean species. Many trees in temperate zones make one growth ring each year, with the newest adjacent to the bark. For the entire period of a tree’s life, a year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring. A drought year may result in a very narrow one. Alternating poor and favorable conditions, such as midsummer droughts, can result in several rings forming in a given year. Missing rings are rare in oak and elm trees—the only recorded instance of a missing ring in oak trees occurred in the year 1816, also known as the Year Without a Summer. Trees from the same region will tend to develop the same patterns of ring widths for a given period. These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions. Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions, and for sub-regions of the world. Thus wood from ancient structures can be matched to known chronologies (a technique called cross-dating) and the age of the wood determined precisely. Cross-dating was originally done by visual inspection, until computers were harnessed to do the statistical matching. To eliminate individual variations in tree ring growth, dendrochronologists take the smoothed average of the tree ring widths of multiple tree samples to build up a ring history. This process is termed replication. A tree ring history whose beginning and end dates are not known is called a floating chronology. It can be anchored by cross-matching a section against another chronology (tree ring history) whose dates are known. Fully anchored chronologies which extend back more than 11,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers) and pine from Northern Ireland. Furthermore, the mutual consistency of these two independent dendrochronological sequences has been confirmed by comparing their radiocarbon and dendrochronological ages. Another fully anchored chronology which extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California). In 2004 a new calibration curve INTCAL04 was internationally ratified for calibrated dates back to 26,000 Before Present (BP) based on an agreed worldwide data set of trees and marine sediments.
Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant. In eudicots, pith is located in the center of the stem. In monocots, it extends also into flowering stems and roots. The pith is encircled by a ring of xylem; outside that, a ring of phloem.
While new pith growth is usually white or pale in color, as the tissue ages it commonly darkens to a deeper brown color. In trees pith is generally present in young growth, but in the trunk and older branches the pith often gets replaced – in great part – by xylem.
In some plants, the pith in the middle of the stem may dry out and disintegrate, resulting in a hollow stem. A few plants, such as walnuts, have distinctive chambered pith with numerous short cavities (See image in section 3.4 Sapwood). The cells in the peripheral parts of the pith may, in some plants, develop to be different from cells in the rest of the pith. This layer of cells is then called the perimedullary region of the pithamus. An example of this can be observed in Hedera helix, a species of ivy.
Rays are part of the wood, passing through the xylem from the center of the timber body out to the cambium. They radially supply the timber with water and nutrients. If a ray starts at the marrow of the wood, the pith, leading outward, it is called a primary or medullary ray. If the ray begins in the xylem and not in the pith, it is referred to as secondary ray. If the rays extend across the cambium and to the phloem they are called bast rays. Cellular structure
Deciduous Trees (hardwood)
In hardwood trees the rays are composed solely of storage cells. Depending on the type of wood, they line up in single or multiple rows. Only in a few tropical species the rays form a regular pattern. Also, resin canals, which are surrounded by epithelial cells, are found only in the rays of some tropical timber and are often filled with a white or dark substance. In hardwood trees rays have a volume fraction of 8 – 33 percent.
The construction of rays in coniferous timber is much more distinct than in wood from deciduous trees. In wood such as pine, spruce and larch resin canals may occur, but they are always surrounded by epithelial cells. The structure is either homocellular, composed of parenchyma cells alone, or heterocellular, containing both parenchyma cells and elongated tracheids. At the contact surfaces of the radial spreading rays cells and the axial running tracheids there are pits that allow for water and nutrient to be transported and (unlike the pits in hardwoods) indicate the wood type. The volume fraction of the rays in softwood amounts to one percent. Purpose and functionality If the rays do not contain resin canals, they themselves are responsible to transport water and nutrients and store reserve materials. At the same time the rays increase both, the strength and the stiffness of the wood: trees with many thick rays get less often longitudinal cracks. Special features Conifers such as pine or yew trees, which have no resin canals, may form traumatic resin ducts in the event of a injury. This is caused by the parenchyma cells, which, under high pressure, excrete resin into the rays. Hence parenchyma cells are often also called pithelial or excretion cells. Another interesting feature of the rays, especially in conifers, is the appearance of the pits between the elongated tracheids and the ray cells. They can be used to determine the kind of wood one is dealing with.
A distinction is
- ring-porous hardwoods
- diffuse-porous hardwoods: no demarcation between early wood and late wood (most tropical woods)
Characteristic of hardwoods are not present in softwoods vessels.
They are often visible to the naked eye as small pores in the wood section and grooves than the tangential. A distinction is made, depending on the arrangement of the tracheae:
Ring-porous wood (eg oak, chestnut, ash, locust, elm): these species form in the early wood vessels with a wide lumen, in contrast, mainly narrow-bore latewood tracheids and wood fibers.
Semi-ring porous wood (eg walnut, cherry)
Diffuse-porous wood (eg birch, alder, linden, poplar, beech, willow): These species form during the whole vegetation period on vessels with about the same lumens.
The growth zones (tree-ring pattern) and the species-specific arrangement of pores and parenchyma results in the characteristic grain of the wood.