That seems a silly name for essays dealing with trees, but maybe not. In my mind the name reminds me of a young family with their puppy, another lifeform, not human but definitely alive. How best to take good care of it so that your new friend grows, matures and lives the fullest life possible? As far as care goes, right from the purchase, planting, watering, and sometimes a little pruning, there are many good care practices that will keep your young tree healthy. Understanding the phases of its life are important. Much of this choosing, planting and pruning has been well covered in my book, “Your Trees, What They Want You to Know”. Here we will be taking a more detailed look at tree biology, all of the tree’s tissues, how they grow, interact and change through the seasons and years. This will be an expansion of the section of my book called, “A Year in the Life of Your Tree”. This is part of my ongoing work as a Calgary arborist and Calgary tree doctor. I supply this for the greater good of our urban forest, which is one of the most challenging created biomes we have.
Trees are a very difficult life form to relate to, creature to creature. They are so different from us that it might seem that they aren’t alive; they just stand there through the seasons. But life on earth, for all its tremendous diversity, shares many of the same cellular processes and structures. Once nature creates a system that works, it is used again and again in many diverse ways. Almost all living things share the process of respiration to supply the energy the cells need. And this happens every second for all of us, through the heat and the cold, summer and winter. Trees are alive every second. Are you alive when you sleep? So is a tree alive, all through the dormant season, without leaves. All living things except the bacteria have similar cellular life functions. One important cellular exception is the reinforced walls of plant cells, which in their trillions allow a tall tree to free stand without any bones (really, the trunk is the bones).
At a basic chemical bonding and breaking level, the chemistry of life is an exchange of energy, electrical energy converted into chemical energy. This is part of the force of life we all share. Approach a healthy tree, walk up with an open mind, a questioning mind frame, not knowing for sure. Seriously, this won’t take long; just forget about yourself and all your problems and dreams and be in company with the tree for one minute. Now find a dead tree, walk up and repeat the same experience. Most of us, honestly experiencing this experiment, can feel the difference. You could try the same thing with two different types of humans; one is friendly, one is not. In each case the closer you get, the forces of attraction or repulsion increase.
This is not going to be another study of metaphysics. My goal is to explain the complexity and marvel of the lives of trees. If the magic of life isn’t readily clear to you, then please spend some more time in nature, and getting to know the vast ancient non-human world, the one we rely on every second for our continued life support. Earth is a giant spaceship. Everything we need is on board. My wife and I thought about our camper van that way when we went to fierce landscapes like the Mojave desert. We had everything we needed to live for several days, but when the water jugs got low, it was back to base for a refill.
Lets get back to the trees, all plants, trees included, have three main organs. Roots, stems leaves, all three in balance, growing in sync, working in sync, essential for life. No waste, no fat, nothing extra. Let’s start at ground level, in the soil with the roots. Almost all soils usually have some available water and nutrients. The tree constantly needs both to live. What better medium than a semi solid, like a tight sponge, made of a variety of rocky particle sizes, mixed with bits of organic material and many types of life forms, all designed to live underground. There are the fungi, what could be mushrooms, that digest organic material, mainly cellulose and lignin, dead plant parts, and recycle the mineral nutrients that were held in their bodies. There are the trillions of single celled bacteria, also living in the soil, tirelessly keeping things moving and recycling, as they live and die. Then there are the insects and other animals, worms, beetles, both consuming, recycling, dying, and being themselves recycled. This is the most basic picture of the community we call soil. In an old forest nothing ever leaves, all rises, lives, declines and dies to be broken up into what it was made from, giving back their bodies to the greater ongoing forest above.
Next is the stem or trunk. Its two main jobs are physical support of the branched leaf mass above and the conduction of fluids, water mostly upwards, and the photosynthate, sugary sap produced in leaves, which is mostly transported downwards. There is a separate plumbing system for each, water in the xylem and the photosynthate, sugar, in the phloem.
Last are the leaves. When present on the twigs, leaves work whenever sunlight and enough water and CO2 gas are available. The minimum temperature for photosynthesis is 3 degrees Celsius, and the most productive temperature is 25 degrees Celsius. Above that temperature too much evaporative water loss affects the rate of photosynthesis. Photosynthesis is a chemical process that occurs inside leaf cells called chloroplasts. The chloro part of the name is the same as in chlorophyll, the green pigment that gives leaves their dominant color. Light energy from the sun reacts with water and CO2, producing sugar, water vapor and oxygen. These sugars are then downloaded into the phloem plumbing system and delivered whenever energy is required. The leaves produce enough sugar to feed themselves plus an abundance that is delivered to all the other parts of the tree that don’t produce their own energy.
This is the basic three-organ outline. Later I will get into more detail about these main parts and their constituent sub-parts. Think of the three organs all growing, living, working in sync; they need to be balanced. Think of them as units of one, one and one. We need one root system to supply one set of leaves, all of the conduction of fluids done by one stem. This balance is very important, they always match, as new leaves begin to grow in the spring, so it is with the equal amount of new non-woody root growth to match the needs of the leaves. Newly formed cells in the xylem and phloem are laid down from the cambium to keep up with the increasing demand for fluids, above and below. Understanding the beautiful balance of the three-organ system leaves us with one powerful lesson when it comes to pruning: there are no extra or unnecessary leaves on any plant or tree. Go easy with the pruning of live branches.
There is a lot of ground to cover, so here is a brief outline about most of the major subjects I am going to consider. The seedling tree, let’s start at the start, the tree’s first year, what has happened so far, where the energy came from, organ differentiation; here we will be dealing with primary growth, root and shoot extensions, that at root and shoot tips occurs every year.
The older tree:, by this I mean any tree that has at least one year’s growth and is into its second season or, for that matter, its 22nd season. In the second year the tree will experience secondary growth for the first time. From here on every year it will experience primary growth at root and shoot tips and secondary growth in the branches, trunk and major roots. Secondary growth is what adds the latest growth ring to these organs.
Cells, cell walls and cell types. In this essay I will cover the basics of plant cells, how when needed their walls are singly and doubly reinforced by cellulose and lignin, which is what wood is made of. And how cells develop when in positions in different tissues and form into the correct cells for the application in the organ.
Roots: here I will cover the amazing size and complexity of the tree’s most misunderstood organ. Much larger and more spreading than most people think, the roots perform the essential jobs of anchorage, water and mineral absorption, and also hormone production. Thinking of roots includes their version of primary and secondary growth, woody versus non-woody roots and their great expanse into the soil under the tree.
Stem or trunk: this is where we get to study the cambium, a miracle bifacial layer of meristematic, tissue-generating cells that ceaselessly throughout the growth season supplies the new cells for the growth ring, the constant renewal of the phloem, and outwards the growth of the outer bark. It is the cambium that grows the trunk in its yearly cycle of expansion. Most of the tree’s live storage tissue, its major plumbing for liquid transport up and down, and the creation of the bark are products of the annual growth of the cambium and the cork cambium.
Leaves: from their miniature beginnings inside the overwintering bud, through the summer and their final failure and downloading of valuable molecules as they lose their chlorophyll and change color in the fall, leaves are the tree’s tireless worker bees. Supplying the lion’s share of the energy made by the tree that is used for all of its life process, tough yet delicate, their outer cuticle needs to be weatherproof to deter insects and other animals and yet be transparent enough to allow the free penetration of sunlight into the chloroplasts. Their inherent and necessary flexibility means they can never be built of the strongest tree cells, but make their way using the principle of bending so as not to break.
Soils: the soil the tree is asked to live in is of the greatest importance to its overall health. Soil/water relations can not be separated. The depth, quality, and physical makeup of urban soils and what is more important, your watering practises, will decide the fate of your tree. Here we will deal with what urban soils are made of, their recipe , how thick they should be, and the interaction of the roots within.
Water relations: using the information in the soils article we will move on to the greater subject of water relations within the whole tree. We start with the roots in the soil, and their interactions, then move up into the trunk. The vessels that move the water upward in the tree are tiny; some of the largest are 1/20 of a mm in diameter and 3 mm long, yes, they are tiny. The human hair is a common example we are all familiar with. To get a glimpse inside the trunk, think of its tissue inside as a giant thick pony tail of little pipes the size of human hair. From the trunk the water moves into the branches and leaves. This is called the cohesion/tension model of the ascent of sap and is one of life’s miracles. Using the simplest of materials and natural forces, trees can supply their leaves with all the water they need, sometimes over 400 feet from the ground.
Sugar relations: another essential aspect of tree life. Every second every cell needs water, oxygen and sugar. Where does it come from? How is it made? And how do we get it to every living cell, no matter how far, deep or hidden. Here we will look at how the sugar is produced in leaves, by the process of photosynthesis, then how these sugars are moved through the entire living sections of the tree’s body by the phloem and its connection with groups of horizontal transport cells called rays. The more active a tissue, the greater its need for energy. It is not by chance that the phloem lies right outside the cambium, which grows the whole trunk of the tree yearly, ring by ring.
Architecture: this is about the form of the tree, how the major branches and the trunk or trunks interact. It creates the growth patterns that give us a very strong tree, able to withstand any storm, or the weak tree that splits apart with a light coating of snow on its leaves. The interaction sites between branches and trunks are the focus here, and we will discuss the branch collar, the branch bark ridge, codominant trunk growth and included bark.
Compartmentalization: being able to wrap your problems up in containers where they can’t spread and make things worse is a miracle function of trees. Starting with the individual cell and its walls and, on the greater scale, volumes of tissue can be separated in the same way through different processes, usually the production and allocation of secondary metabolites, phenols and terpines. Here we discuss CODIT, Dr Shigo’s great insight into the powerful defence mechanisms trees have to developed to deal with their lifelong companion/enemy fungus. Sometimes beneficial like mycorrhizae, other times a real problem as inner trunk tissues rot, fungus travels with a tree through its whole life and will eventually be the force that weakens the trunk to the point where it can no longer stand, back down to the forest floor where recycling can occur
Conifers, the evergreen or cone bearing trees: very different from their younger cousins, the broadleaf trees, conifers are the original pattern, the first real trees that evolved about 350 million years ago. Still highly successful in their chosen environments, they have physical differences from deciduous trees that can be very advantageous. An array of differences, from leaf design, needles instead of leaves and thick waxy outer coatings on the needles, are very effective at water conservation. Each of the major patterns, leaves and needles, has distinct advantages and disadvantages. For example, growing a new set of leaves is expensive, but it is also expensive keeping your needles through the worst of summer heat and the subsequent evaporative water loss; more of this to come.
Seasonal activity: at different times of the year trees behave differently. Sometimes everything is happening at full speed, other times are quiet and life becomes day to day maintenance, subsistence, dormancy. The yearly cycle can be compared to one of our own full days. In this essay I will chart the tree’s yearly process from dormancy, sleep, to first wakings, first coffee, then getting dressed, and beginnings of the day’s hard work, then later in the day slowing down, and finally rest and sleep. During all of these times of activity different things are happening in different tissues in the tree. I am going to walk you through the whole year and what is happening where and with what tissue. I have seen all this for years as a Calgary arborist and Calgary tree doctor and I am happy to share it. Please enjoy it; if you want to go into it deeply on your own property with your own trees, have me over for a consultation.
