Understanding Trees Helps to Successfully Use Them for Furniture

I first learned about the Twin Oaks Community while working on “Cut & Dried” with Richard Jones. We needed an index. Members of Twin Oaks, an intentional community in rural central Virginia, make their living, in part, by indexing books. Additional income is generated by making hammocks and furniture and tofu, and seed growing. The Twin Oaks Community, comprised of about 90 adults and 15 children, are income-sharing. Members complete about 42 hours of business and domestic work a week, and in return receive housing, food, healthcare and personal spending money.

Rachel Nishan from Twin Oaks responded to my indexing query, and we agreed to work together. Indexing a technical book such as “Cut & Dried” is a rather monumental task, and just thinking about it made my eye twitch. Yet Rachel approached the project without an air of stress, asking detailed questions about tree types, specificity and British spellings. Throughout our correspondence one sentence has stayed with me, years later: “… a more technically-inclined reader could want to look through the index in a variety of different ways, so I have tried to be pretty redundant, which is the kindest for the user of the index.” 

“Kindest for the user.” I think that’s the heart of bookmaking, no?

Richard and I sent hundreds of emails to each other while working together to turn his years of work into book form. And all of that correspondence, from image selection to epsilon size, was written with Rachel’s not-yet-said phrase in mind: kindest for the user.

I was nervous to begin work on this book. Honestly, I thought the content would be too technical for me to understand. But then I read it. And realized Richard used his genius to transform his scholarly work into easy reading. And Rachel made topics within the text easy to find. And Meghan designed the book to be easy on the eyes. All with kindness in mind.

– Kara Gebhart Uhl

The following is excerpted from “Cut & Dried: A Woodworker’s Guide to Timber Technology,” by Richard Jones.

Many woodworkers are initially reluctant to study trees in detail fearing the subject is dauntingly heavy. Whilst it’s true the subject can be studied with scientific precision it’s really only necessary to get to grips with the main elements to gain a firm basic knowledge. Wood isn’t created with the needs of the woodworker in mind. The creation of wood is necessary for trees’ survival. We simply use what nature provides. Understanding the original function of wood helps woodworkers use it sympathetically and successfully. One example of useful basic knowledge described earlier is to understand the essentials of Latin scientific classification resulting in precision and clarity in any discussion of the subject.

All trees are members of the plant family. Specifically, they are all spermatophytes meaning they are seed-bearing plants. Trees are generally characterised as being perennial seed-bearing vascular woody plants with a root system and (ordinarily) a single trunk supporting a crown of leaf-bearing branches. With exceptions (see mention of the Arctic willow, Salix arctica, earlier) they normally reach a minimum height at maturity of five m (15′) and survive for at least three years.

This basic classification then breaks trees down into two distinctive types – the angiosperms (covered seeds) and the gymnosperms (naked seeds). Alternative names for these two groups are hardwoods, deciduous or broad-leaved trees (angiosperms), and conifers or softwoods (gymnosperms). The terms hardwood and softwood can be misleading as not all hardwoods produce hard wood, e.g., soft balsa wood is the product of a hardwood tree whereas yew is hard and comes from a softwood tree.

Typical of deciduous trees in temperate climates is the loss of leaves during autumn as the tree loses vitality followed by a dormant winter period. As usual there are exceptions where many of the hollies (Ilex spp.) retain their spiky and waxy leaves throughout the year. Spring, with its longer daylight hours and warmer weather, heralds a new period of rapid growth with the emergence of new leaves, flowering and reproduction. This is not true of all hardwoods in all climates. Many equatorial living hardwoods are able to grow all year round and may never lose their leaves en masse. With these trees the cycle is continuous as old leaves reach the end of their useful life to be replaced by new ones.

Angiosperms (deciduous trees) from all climatic conditions have a characteristic growth pattern. Their form is deliquescent or dendritic, meaning there is branching and re-branching of a main trunk.

Gymnosperms (coniferous or evergreen) trees typically retain their leaves throughout the year, with larch being one exception to this trait. Their form is generally excurrent – the main trunk rises singly with lesser sideways branching. Broadleaved trees usually have large, relatively fragile, blade-like leaves and, to prevent dehydration of the tree resulting from their retention, they are lost before winter. Conifers on the other hand typically are able to resist dehydration because of their tough, needle-like waxy leaves, which stay on the tree through all the seasons. As with tropical hardwoods discussed earlier they lose leaves and replace them all year round. However, I’ve noticed even the much-despised fast growing leylandii (Cupressocyparis x leylandii) planted in my back garden by a previous owner loses more leaves in the winter than in the summer. Leylandii are, in truth, a very attractive tree grown where they have space. They grow very swiftly and are really too large in small British gardens – they rapidly exclude light and dominate these small spaces.

In common with hardwood trees living in cool temperate climates, evergreens have a dormant winter period.

Tree growth occurs in just three places. The first two are the tips of the branches and roots, which increases the tree’s height and the spread of the crown along with the range of the roots. The third place where growth occurs is in the girth of the trunk, branches and roots by the addition of an annual growth ring. Meristem or meristematic tissue refers to the growth tissue in trees. The growing tips of twigs and roots is the apical meristem. The lateral meristem is the cambium layer adding girth to the tree’s structure.

The cells produced by meristematic tissue, whether they are leaves, flowers, bark or wood, are largely of cellulose. Cellulose forms strong and stable long chain molecular structures. This, along with the lignin bonded with, or to it, is what gives wood its strength. Lignin is the “glue” holding wood together and is a complex mixture of polymers of phenolic acids. Lignin forms about 25 percent of wood’s composition and becomes elastic when heated. It is lignin’s flexible plastic property allowing wood cells to rearrange themselves that woodworkers use to their advantage during steam-bending wood into new shapes.

The majority of cells making up a tree’s structure are elongated longitudinal cells. Their long axis runs vertically up the trunk (and along the branches and roots). Some of these cells are short and stumpy and others are long and slender. The vascular function of the newly formed longitudinal cells is to conduct liquid raw essentials up the tree to the leaves and processed sugary food down the tree to nourish it. Spread through the wood are rays or medullary rays. These ray cells are also elongated but their long axis radiates from the centre of the tree toward the bark. They are stacked one upon the other throughout the length of the trunk in slender wavy bands.

In many wood species the rays are invisible to the naked eye but in others, such as numerous oaks and maples, they are usually highly visible because the groups of cells are large. Some ray cells – the parenchyma – store carbohydrates for use in cell development. The other primary purpose of the medullary rays is to transport nourishing sap toward the centre of the tree.

3.1 Log Cross Section
From the outside there is the outer bark (see figure 3.6), which is a protective insulating layer against weather, animal, fungal and insect attack. The bark has millions of tiny pores called lenticels through which necessary oxygen passes into the inner living cells beneath. In polluted atmospheres such as cities the lenticels clog with dirt. London plane (Platanus x hispanica) is well suited to city life because it sheds its bark regularly, exposing clear lenticels. The bark of all trees flakes off as the girth gets bigger.

Inside the outer bark is phloem, bast or inner bark. The phloem is produced by the cambium layer and is a soft spongy liquid-conducting vascular tissue that carries processed food – sugary sap – from the leaves to the rest of the tree.

Beneath this layer is cambium – the lateral meristem (growing tissue) that adds girth to the tree. The cambium is a slimy layer only one cell thick. These cells divide constantly when the tree is active. The cambium produces not only phloem towards the outside but, towards the centre, it produces xylem.

Xylem has two major functions. As sapwood it conducts water and minerals from the roots to the leaves. Sapwood contains both live tissue and dead tissue. Dead xylem, the heartwood, is the trees’ structural support. The longitudinal cells described earlier are organised to form water- and nutrient-conducting tracheids in gymnosperms or conifers, although some hardwoods also contain tracheids. In angiosperms (broad-leaved trees) the order is different. Vessels, which are continuous tubular structures, form a pipeline from the root tips to the leaves rather akin to drinking straws bundled and glued together. (Note, though, the comment I made about some hardwoods also containing tracheids.) In oaks, for example (see figure 3.7), the naked eye easily picks out the initial spring-laid vessels or pores. In other tree types magnification is required. Sapwood is often attacked by food-seeking life forms such as fungi, insect and animal life.

As sapwood xylem ages it loses its vitality through the loss of the living protoplasm within the cells and turns into heartwood. In some species the transition between living xylem and heartwood is abrupt and clearly visible as seen in the yew cross section at left. With others it is hard to distinguish between sapwood and heartwood. The sapwood can remain as living protoplasmic cells for several years but this period varies from species to species, and even within trees of the same species. The yew sample at left shows newly laid sapwood that took about 8 or 12 years to convert to heartwood.

Heartwood is the column of xylem supporting the tree. It is dead because it has lost its active protoplasm. Whilst outer layers of the tree are intact – protecting the heartwood nourished by foodstuffs transported to it by the medullary rays – it will not decay. Heartwood is usually, but not always, distinct in colour from sapwood. Extractives cause the colour change. Extractives are trace elements imparting various combinations of characteristics to heartwood, such as colour, fungal- and bacterial-resistance, reduced permeability of the wood tissue, additional density of heartwood, and abrasive deposits.

Tyloses are bubble-like structures that develop in the tubular vessels of many hardwoods during the changeover from sapwood to heartwood. Tyloses block the previously open vessels, preventing free movement of liquid. Red oaks form very few tyloses whereas white oaks produce many and this explains why white oaks are preferred for barrels. It’s possible to blow through a stick of red oak submerged in water and create bubbles. Whisky distillers are well aware of the “Angels’ Share,” which is the part of the spirit, usually about 2 percent, that evaporates through the wood of the oak barrel (Whisky Magazine, 2008).

Growth rings are the result of the cambium layer adding new tissue year upon year. The cambium layer (in temperate climates) becomes active in spring, reacting to chemical signals produced in the tree brought about by warming temperatures and longer daylight hours. During its active period the cambium layer adds open, fast-grown porous tissue to cope with the rush of water and minerals required of the freshly opened leaves. As the summer approaches and the initial high demand for food subsides, the cambium lays down denser, harder latewood, which adds strength to the trunk and branches.

At the centre of the tree cross section is the pith or medulla. The pith is the small core of soft spongy tissue forming the original trunk or branch.

3.2 Gymnosperms & Angiosperms – Differences
3.2.1 Gymnosperms
Gymnosperms (conifers, softwoods) are simpler in structure than angiosperms. Gymnosperms evolved earlier than angiosperms and have some distinct structural characteristics. More than 90 percent of the wood’s volume is made of tracheids. Tracheids are long fibrous cellulosic8 cells approximately 100 times longer than their diameter. They range between about 2 mm and 6 mm (about 1/16″ to 1/4″) in length depending on the species.

The two main functions of tracheids are as structure for the tree and as conductors of sap – nourishment. Tracheids conduct liquid food up the tree after the living protoplasm has left. Water and minerals pass upward to the leaves from one tracheid to the next via osmosis. Osmosis is the process where liquid from a high water (weak) solution passes through a cell wall into a low water (strong) solution. In softwood trees water and minerals move upward from the roots initially through upward root pressure created by soil-borne water migration into the root tracheid cells. Secondly, there is also transpirational pull created by water evaporating from the leaves. This method of conducting foodstuffs is distinctly different to the method used in broad-leaved trees described later.

The cambium layer lays down different forms of tracheids at different times of year. In the spring, the tracheids laid down are thin walled with a large diameter and are lighter in colour. Late-growth tracheids are dark coloured, have thicker walls and a smaller diameter. The early-wood tracheids with their thin walls are better at conducting liquid than the later thick-walled tracheids. Both will conduct water, but a tree needs structure as well as the ability to transport liquid – there is a necessary balance struck between the two functions in tracheid cell structure.

A distinctive characteristic found in some gymnosperms is resin carried in resin canals. Pine, spruce, larch and Douglas fir have resin canals. These timbers have a characteristic scent when worked, and the resin can cause bleeding problems under paint and polishes. One way of setting the resin solid to reduce bleeding problems is to raise the temperature of the wood during kiln drying to 175º F for a sustained period. Genuine gum turpentine is a product of the resin from Southern yellow pine, a tree of the North American continent.

Medullary rays are narrow in conifers and invisible to the naked eye, so to see them it’s necessary to mount thin wood samples on a slide for examination under a microscope.

3.2.2 Angiosperms
Hardwoods are more complex than gymnosperms. There are a number of specialised cells present in angiosperms absent from gymnosperms. For instance, the means of conducting liquid foodstuffs up and down the tree in nearly all cases is through the vascular tubular vessels. This is distinctly different to the liquid-conducting tracheids of conifers. The vessels in angiosperms form a bundle of pipes encircling the tree. The fibrous tracheids of hardwoods are much smaller than they are in conifers and because of their thick walls they are not well suited to conduct liquids. Unlike the softwoods, the rays of deciduous trees are often easily visible, e.g., in oaks, sycamore, maple, beech etc. Resin canals are rare in angiosperms, but some tropical plants such as the rubber tree produce gum and have gum ducts.