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The Wonder of a Tree: All About Wood

Posted: November 14, 2014

Trees have wood. They’re tall, long-lived, woody plants. So what makes wood special and how do trees make it? That’s the wonder of it all.

So what makes a tree different from any other living plant? They all photosynthesize – taking in carbon dioxide and water to make sugars for their growth and maintenance, and in the process give off oxygen for the rest of us. Their roots hold soil and absorb nutrients. Their stems give them height. Their leaves allow them to capture solar energy. They’re all adapted to their location, yet rooted in place. We all know this. So what makes trees special? Simply, trees have wood. They’re tall, long-lived, woody plants. So what makes wood special and how do trees make it? That’s the wonder of it all.

Wood imbues strength, allowing trees to become tall. It is made up of specialized structures that transport water to the leaves from the roots and distribute the photosynthate (the product of photosynthesis) from the leaves throughout the tree (their cellular energy), including to storage in the root system for new growth when needed. Wood is tissue specialized for strength through thick walled-cells. The first erect plants used cellulose to hold their structure. Cellulose is a long-chained, regular molecule. As trees evolved, they added lignin to the mix for strength. Lignin is a highly branched molecule that plasters on to an oriented framework of cellulose, imparting support. Lignin occurs in two places: between individual cells and within the cell walls. Lignin found between the cells acts as a glue to hold the individual cells together. Lignin found in the cell walls provides rigidity to the cell; it makes it “stiffer” so to speak. Wood is usually about two-thirds cellulose and related carbohydrates and one-third lignin.

Wood is created by the cambium. The cambium is a localized cell layer that encases the entire tree producing cells differentiated by use and structure in different directions. Primarily these cells are of several types – xylem that transports water, others that transport photosynthate in the phloem, and new cambial cells to replace those that die and ensure a continuous sheath of productivity around the tree. Xylem and phloem while specialized structures also give the tree strength.  Interestingly, the cambium doesn’t produce new cells equally throughout the tree. In times of stress, such as not enough water or photosynthate, it may remain inactive in certain areas.

Xylem is the wood. The cambium produces xylem towards the interior of the tree. These lignified structures serve different roles, with different orientations. Those vertically oriented serve as strength, waste conduction, and perhaps some storage. In conifers, the xylem is made up of tracheids, small needle-shaped cells a few millimeters long, which provide both transport and support. They strengthen the wood with thick walls and conduct water through their hollow interiors – tracheid to tracheid via pits (holes in the walls). Hardwoods have a division of labor – vessels – tubelike elements lined-up vertically to provide water transport – thick walled fibers specialized for strength, and thin walled parenchyma for storage. Xylem is also made up of horizontally-oriented ray cells (as opposed to vertically for all the other structures), allowing for horizontal transport of materials in the tree or storage. Once wood is formed, the water-transporting cells die rapidly. Some rays (not all) contain ray parenchyma cells that are used for storage. The ray cells may live for decades.

Phloem is produced by the cambium towards the outside of the tree. Phloem is made of up thin, unlignified cells – not giving much biomass, but necessary for translocating sugars produced in the leaves to the rest of the tree. Phloem is often referred to as the inner bark, though technically it is not. Primarily movement in the phloem is downward, but occasionally in the spring of the year the sugars move upward in the phloem, taking energy to growth sites.

Outside the phloem layer is the cork cambium which produces the hard part of the bark – cork to the outside, and only a few thin cells to the inside. Trees with thin, smooth bark have long-lived cork cabium that forms a sheath and expands with tree growth. Trees with rough bark have many areas of cork cambia, each small and relatively short-lived. New cambial layers form under the old. As the tree expands, the bark develops ridges or plates of dead tissue (cork) to the outside, giving different species their different bark characteristics.

Wood varies by tree species:  coniferous (made up of tracheids), and ring-porous and diffuse-porous hardwoods. The hardwoods are differentiated by how they organzie vessels in the xylem. In ring-porous species, there is a ring of large vessels produced at the beginning of each annual ring, and a few smaller vessels during the rest of the growing season – giving a very distinct ring to each year’s growth because the large vessels are in the first-formed earlywood – think about the oaks. In diffuse porous trees, the vessels are scattered throughout the wood – think maple, cherry, or birch – making the rings harder to see. Yet, it is still possible to distinguish growth rings because there are small differences in cell-size and cell-wall thickness despite vessel porosity. In conifers, the first formed tracheids (earlywood) have large diameters and thin walls, and the last formed tracheids (latewood) have small diameters and thick walls. The denser latewood is darker brown because of its higher lignin content. These rings are how we estimate a tree’s age – counting each year’s growth. In times of scarcity, trees may not put on growth, so no rings would be visible – something to think about when you find trees in extreme growing conditions.

As xylem ages, its role changes. Younger wood towards the outside of the tree functions in water transport. The older wood, heartwood, has no function in water transport. Its vessels are filled with air and for water to move against gravity, there can be no air in the chain. The heartwood’s role becomes a chemical dump for unwanted substances. These waste products become extractives and are what gives the heartwood its color.

When I studied tree physiology, my favorite part was always learning about reaction wood. It was fascinating to learn how trees respond to change in orientation respective to gravity. There are two different ways. Imagine a tree struck and tilted by another falling tree. In the tilted tree, the cambium produces extra wood cells to get the tree back to vertical. If this tilted tree is a maple or other hardwood, the tree produces tension wood on the upper side of the stem. Tension wood tries to shrink and pull the tree back to vertical. If this tree is a white pine or other conifer, the tree produces compression wood on the underside of the stem. Compression wood tries to expand and pushes the tree back to vertical. Fascinating! While reaction wood can provide headaches for woodworkers, it’s an amazing tool to keep the tree growing.

So there you have it, your short course in wood. In future articles we’ll talk about structure and function, branching, limits of trees, and reproduction. Trees are amazing!

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208