Trees, trees, everywhere. We tie our clotheslines to them, we hang our block heater cord from them, we use them for making a fence. Ancient tree plants buried in the earth's strata are now our coal (and some say oil and gas). Most of the fruits we eat come from trees, a lot of spices come from trees or tree parts (e.g. cinnamon is a bark). Trees are the world's leading O2 producers and do most of the work in atmosphere cleaning. The desk and paper I write on, the air I breathe, the house I live in and the beauty around me all come from trees. Trees, everywhere.
What is a tree? A tree is usually a single-stemmed, woody perennial plant. The variety worldwide is astounding. Nearly any leaf pattern you can imagine, or size or colour of flower is out there somewhere. In the plant kingdom, most trees, as we know them, fall into two major divisions or phyla. Most broad-leafed, perfect-flowering (both stamen and pistil), annual leaf-losing trees are classified in the phyla of angiospermophyta. The angiosperms, the flowering seed-plants, have the embryo contained in an ovary, for example, an apple tree. The next major division, or phylum, is the coniferophyta, the gymnosperms, the naked seed plants. Most evergreens that have leaves (needles) that last more than one year, and that produce some sort of cones and usually have a strongly-controlled natural conical shape, are the conifers; for example, a spruce tree.
How do trees grow? They grow by a process called secondary growth. They use the existing structure from last year as the jumping-off point. Buds formed in the previous growing season at the ends of leaders and laterals are activated, shed their protective bud caps and start to expand. Most broad-leafed trees flower at this time, or even before the juvenile leaves expand. Conifers wait to flower until the shoots (candles) are fully extended. The male cones, which produce pollen, are usually on the lower part of the tree. The female flowers, which will become the mature cones, are usually in the upper section of the tree. Conifers use this strategy to ensure cross-pollination. Many other processes are happening inside the tree and its roots at this time. As the air temperature increases and days get longer the internal alarm clock in the tree goes off. Although the large roots can store a tremendous amount of sugars and starches, the idea of most of the sap rising in the spring needs a readjustment. The main flow of liquids, especially during the growing season, upward in a tree is that of a rich mix of water with essential elements. Once the frost is out of the ground and free water is present in the soil, the woody and non-woody roots start to grow and form their associations with mycorrhiza fungus and can then take up these molecules of water and essential elements.
All substances that enter trees, either gases (O2 and CO2) through the leaves or lenticels (small horizontal lines on the bark) or water or minerals, can enter the tree only as individual molecules. All of the substances move through the tree cell by cell, powered by the process of osmotic pressure. Although in some broad-leafed trees the vessels in the last growth ring and sometimes other growth rings can be as long as two metres, generally the vessel cells in broad leaves are much shorter. In conifers the vessel cells are called tracheids. If all of the sap were being stored in the roots, it would leave the rest of the tree dry. When well watered, a tree is usually nearly full of fluids. A dry branch without sap or internal liquid transport happening is a dead branch. Think of the transition from dormancy to active growth as a major or dynamic shift in the volume of liquid transport, with water and nutrients going up the vessels (tracheids) in the xylem (growth rings), water and sugar-rich solution down through the phloem in the inner layer of the bark, the sugar being input into the system by a downward movement away from leaves via the phloem.
Now to return to the active spring start-up period. Once active liquid transport starts, this water will pick up sugar and other carbohydrates in the radial and axial parenchyma cells. The parenchyma's main job is to store sugars. This sugar-rich energy sap solution can now be transported, and concentrated in terminal shoots and buds where expansion can begin. All of the initial growth from bud-break, flowering through shoot extension and the initial work of starting a new growth ring by the cambium is fuelled by the energy from parenchyma and other cells that store energy, sometimes deep in the trunk. Until the leaves have reached a mature size and are then photosynthetically active, all of the start-up processes are fuelled by stored energy. Severe pruning, which removes a lot of leaf mass, greatly reduces the internal volume of healthy tissue that can store energy.
Another major loss of energy in trees occurs while the tree is defending itself. Like all living organisms, trees have an inner body that is self-regulating and functions as a closed system to the outside environment. When trees are wounded by wind damage, trunk impacts, boring insects, self-wounding branch growth, or pruning, there will be a time at the injury site when the closed system is opened. Trees do not heal; their continued yearly cell-generating growth may be seen as healing, but it is not. When you cut your finger and allow the outside world to enter, some of your tissue, each cell affected, will die. In animals and ourselves, every cell will be replaced in the same spatial position by a new healthy copy of that cell. That is regeneration. In trees, the process is very different. Once a cell has been damaged, either by exposure to the outside environment or by aging process, the cell will die and stay locked in place forever, as long as that tree, or sections of its lumber, exist. The knot in a piece of furniture is a branch core trace that has been compartmentalized. It grew inside the trunk to the depth of growth rings equal to the age of the branch. All wounds and injuries that have been altered and are no longer able to be part of the symplast (the whole living connected tissues of the tree) will be compartmentalized. When trees compartmentalize, they are engaging in a big trade-off, which is a sustained defended inner symplast versus a reduced interior volume to store energy. A wound is not only a volume of injured tissue, but also the volume of tissue compartmentalized to build a barrier between outside and inside.
Dutch elm disease is a study in compartmentalization, because the tree walls off large amounts of tissue to stave off the attacking fungus. Eventually the tree has little or no healthy uncompartmentalized tissue, and thus no room for liquid transport or energy storage and death is near. Compartmentalization works by a series of walls; not solid walls but chemical protection zones that isolate the area and dislocate it from the living symplast. These walls are a concentration of phenols and other chemicals that act like antibiotics to contain the injured area. Such compartmentalized wounds are obvious in longitudinal sections of branches or tree trunks. Longitudinal sections reveal the patterns of Codit (the acronym for compartmentalization of decay in trees). Compartmentalized wood is a darker colour, darker because of a concentration of extractives than the living whiter sapwood and, when tested for the presence of sugars, will come up negative.
In order to prune trees, not only is some idea of tree function needed, but also the 3D thinking of Codit plus a good understanding of the branch-trunk interaction zone. The branch on a tree is an amazing structure. Trees with a branch collar system use good engineering principles to extend a cantilever off a vertical with no support brace underneath. Usually in the spring, as leaf growth is started, the cambial zone that covers the entire last growth ring will be initiated to start forming a new woody section of a new growth ring. As this happens, new xylem wood will be laid upon the existing xylem growth ring from last year. After the new branch wood is formed, the trunk wood is then laid down and wraps around the previous formed teardrop-shaped branch wood. The repeated layering of these two tissues through the years gives the branch its amazing strength and also clearly defines what is trunk structure and what is branch structure. As the trunk collar wraps around the branch collar, a distinctive ring is formed that is much larger than the branch. Literally, it is nearly twice the diameter of the branch, since it contains the trunk collars also. It is exactly at this time when both new collar tissues are still non-lignified and hence unable to set boundaries and defend properly, that green wood removal can be so energy-robbing from the tree. The tree is already in its lowest energy reserve period due to spring start-up and a fungal or pathogenic attack has an advantage. This collar is where nature intended for the branch to be removed, and where most branches will abscise, or shed, on forest trees. The branch base at the collar is one of the tree's strongest protection areas. Since in nature most lower branches are removed through time (decline, die-back, shedding), the branch collar is naturally a place where fungus and pathogens can attack the large reserve of stored energy in the trunk. Research shows that when a branch is pruned properly, only the branch core or trace will be compartmentalized. When the branch collar is injured and the natural protection zone destroyed, a tremendous amount of invasion can occur into the trunk and a much larger volume of living tissue will be compartmentalized. When pruning, always stub cut your branches; that is, cut the end off your branch and leave a two- to three-inch stub. Be careful of bark tear-out on the bottom of the branch. Undercut the bottom first. Now you can concentrate on your target, the trunk branch collar. Look at the collar from both sides and project your imaginary pruning cut through the branch base against the collar. Be careful not to turn your saw in toward the trunk or the bottom of your cut will be too deep. If in doubt, go longer rather than shorter. Trees are much happier to deal with stubs than with flush cuts that remove the collar protection zone.
Dead branches will often have collar protection zones that extend outward. Look for the point just outside the original collar where there is a distinct diameter change in the branch. The branch protection zone has been extended and the tree will try to abscise the branch at this point. Your cut should be just where the smaller diameter dead section of the branch meets the larger diameter extended protection zone. Feel for it; look for a line.
Never cover your pruning cuts with any substance. Research shows that nothing we have made will help the compartmentalization process. Sterilize tools with bleach between trees and between cuts on sick trees.
The gist of this is that a working arborist today needs a 3D or X-ray approach to trees and their branches and roots. There are no extra leaves on trees; they all have important work to do. On a healthy tree we can do some pruning to move ahead our own plans. A 10% reduction of leaf-bearing branches is a lot of pruning for one year. Wait another year, watch the tree's reaction to your work. Trees live a much slower, evenly-paced life compared to our own. On a sick tree, when affected by bacteria, e.g. fire blight, only the deadwood and diseased area should be pruned. The same goes for trees affected by fungi, for example, cytospora. Fungi can usually be dealt with where they are, at that spot, either by removal with a knife or by spraying them with a systemic fungicide (Benomyl).
Trees should never be topped. Plant the right sized tree in the right spot. Always remove deadwood; by doing so, compartmentalization is rapidly sped up. Happy pruning! Hug your trees, and be sure to thank them for all their leaves that we pick up in the fall. As modern urban dwellers, we need as much cardiovascular exercise as we can get.
In preparing this article, Kevin is indebted to Dr. Alex Shigo, who was a U.S. Forest Service research scientist. He spent over thirty years working with trees to bring a true working understanding of tree biology to us. Codit and the branch collar system are his discoveries.