Overstory #132 - How Trees Survive
Trees are the tallest, most massive, longest-living organisms ever to grow on earth.
Trees, like other plants, cannot move. However, trees, unlike other plants are big, woody and perennial, which means they are easy targets for living and nonliving agents that could cause injuries. Trees cannot move away from potentially destructive conditions. Wounding agents and destructive conditions do destroy trees, but somehow, trees have grown in ways that give them super survival powers.
The big question is, how do trees do it?
The answer lies in concepts of biology and mechanical engineering.
This article examines the question of tree survival power more from the concepts of biology, but also to be aware of concepts of mechanical engineering. Details on all subjects given here are in my books.
Because different disciplines often use similar terms that have different meanings for their work, it is important to start with some definitions of terms I will use. You may not accept my definitions, but you will know what I mean when I use a term. I believe if a person cannot define a term in 25 words or less, they should not use it because they probably do not understand it.
Keyword definitions of terms
Capacity - What you have as a result of your genetic code; a potential source for some future action or product.
Ability - What you are doing with what you have; a dynamic or kinetic process.
System - A highly ordered connection of parts and processes that have a predetermined end point - product, service.
Stress - A condition where a system, or its parts, begins to operate near the limits for what it was designed.
Strain - Disorder and disruption of a system due to operation beyond the limits of stress.
Vigor - The capacity to resist strain; a genetic factor, a potential force against any threats to survival.
Vitality - The ability to grow under the conditions present; dynamic action.
Health - The ability to resist strain.
Disease - A process that decreases the order and energy of a living system to the point of strain.
Survival - The ability to remain alive or functional under conditions that have the potential to cause strain.
Generating system - New parts and processes form in new spatial positions; plants.
Regenerating system - New parts and processes form in old, or preoccupy, spatial positions; animals.
Wood - A highly ordered connection of living, dying and dead cells that have walls of cellulose, hemicellulose and lignin.
Symplast - The highly ordered connection of living axial and radial parenchyma in wood and bark.
Apoplast - The highly ordered connection of dead cells and cell parts that make up the framework that holds the symplast.
Quality - The characteristics that define a product, service or performance; quality can be low or high.
Hypothesis for survival
Because trees cannot move away from potentially destructive agents and conditions, they have grown in ways that give them the capacity to adjust rapidly after being threatened by agents or conditions that could cause strain or death.
The capacity to adjust is a genetic feature called vigor. The program of vigor of an organism is defined by the limits of factors essential for survival. For example, one tree may have broad limits for water utilization. When drought occurs, it will still survive. Another tree may have very narrow limits for water utilization. Even the slightest disruption in availability of water would lead to strain or even death.
A vigor code then determines the limits for such essential factors as space, water, elements, temperature and soil pH.
The vigor of an organism cannot be measured until a life threatening stimulus contacts the organism.
When any potentially destructive stimulus occurs, the ability of the tree to adjust will be due not only to its vigor, or genetic code, but to its vitality. A tree that is very vigorous by nature of its genetic code may be growing on a rock. It would not be very vital. What this means is that for survival, both the vigor and vitality of a tree must be optimized.
Forest tree, city tree
Trees became tall, massive and long-living plants as they grew in groups. Trees not only connect with other trees by way of root grafts but also by way of the fungi that are associated with non-woody roots; the organs are called mycorrhizae. Trees also connected with many other organisms, very large to very small, in ways that benefited the trees and their associates. Synergistic associations are important parts of the tree system.
A forest is a system where trees and many associates are connected in ways that ensure survival of all members.
It is important to remember that the genetic codes for survival, or vigor, came from trees growing in forests.
When the forest-coded tree is brought into the city, the factors that affect vitality become extremely important. The architecture of most city trees as they grow as individuals is different from most of their relatives in the forest where trees grow in groups. Forest trees have group protection and group defense. The individual tree has neither.
The good news is that most of our city trees have strong vigor codes that have made them super survivors for hundreds of millions of years.
The bad news is that many human actions and mistreatments affect vitality and undo all the benefits of wondrous vigor code. It is only because most trees have such strong vigor codes that they still survive in cities.
There is no doubt in my mind that the greatest threat to survival faced by city trees are mistreatments by humans. Many trees tolerate mistreatments. Too often their tolerance is perceived as justifications for the mistreatments. I have heard it said many times that the tree did not die, so therefore the treatments must have been correct.
How do trees adjust?
Reaction zones and barrier zones
After injuries, boundaries form that resist spread of infections. By resisting spread of infections, the boundaries protect and preserve the water, air and mechanical support systems of the tree. Two types of boundaries form: reaction zones and barrier zones. The reaction zone is a chemically altered boundary that forms within the wood present at the time of wounding. The barrier zone is an anatomical and chemical boundary that forms after wounding. The barrier zone separates the infected wood from the new healthy wood that continues to form in new spatial positions. The tree is a generating system. The tree has no mechanism to form new, healthy cells in the same positions as those that are infected. Regenerating systems in animals do restore, repair, replace and regenerate parts in the same spatial positions. Animals have a process call apoptosis, which means programmed cell death followed by lysis, and new cells forming again in the same positions of those that died, lysed, and were eliminated. This normal process of apoptosis accelerates after animals are injured and infected. This accelerated restoration process is then called healing. In this sense, trees have no healing process.
Trees are highly compartmented, woody, shedding, perennial plants. Trees are generating systems. Every growth period, trees form new compartments over older ones. Trees grow as their apical and vascular meristems produce cells that differentiate to form all parts of the tree. The important part to remember is that trees grow as new parts form in new spatial positions.
Trees cannot "go back" to restore, repair, replace or regenerate parts. You do not restore a church by building a new one next to it. All words in English that start with "re" mean that new parts will go back in previously occupied positions or back to an original state. These words have no meaning for trees. These words have been the basis for great amounts of confusion. A tree cannot function in the same ways as animals do after injuries or threats to their survival. The continuing use of such meaningless words for trees is a strong indication why tree basics should be understood by people who work with trees.
Reaction Wood and Wound Wood
Now for the second adjustment feature of trees. After wounds or threats to their survival, trees also grow in ways that will maintain their mechanical structures. Now we come to the mix of biology and mechanical engineering.
There are two basic ways trees adjust to maintain and strengthen their structural stability: reaction wood and woundwood.
Reaction wood can be of two types. Compression wood forms on the down side of leaning trunks and tension wood forms on the upper sides. Compression wood is common in conifers and can be seen on a transverse dissection as dark bands in the wood, usually resin soaked. Or the growth increments could be larger in width and still be dark and resin soaked.
It is not possible to see tension wood because the changes take place in the cell walls. A gelatinous layer forms in the cell walls, and this layer can only be seen when properly stained and viewed under a microscope. The important part here is to know that these altered cell forms occur and that they occur after a stimulus that threatens survival mainly because of a lean in the stem that could lead to a fracture.
Woundwood is altered wood that forms about the margins of wounds. When wounds release the pressure of the bark, some of the still living parenchyma in the symplast begin to divide and produce new cells in new positions. These new cells no longer are held in place by the pressures of the bark or of the apoplast. The new cells become rounded and have a thin, primary cell wall. The cells exercise their ability (now) to divide and divide and divide. Because they are thin-walled, dividing cells, and because they contain the genetic codes for forming all parts of the tree, some of the cells begin to differentiate to form sprouts, prop roots, roots or flowers. This capacity for division and differentiation is called meristematic.
Callus is the name given to the thin walled, mostly round, meristematic cells that first form after wounding about the edges of the wound. Callus has very little lignin, the tough "natural cement" that gives cell walls great strength.
Within a few weeks to a few months after wounding during the growth period, callus formation begins to diminish and woundwood formation begins.
Woundwood has fewer vessels than "normal" wood. The cell walls are usually thicker than normal and usually contain more lignin. The woundwood cells cease to be meristematic. A new vascular cambium forms and continues to form woundwood. These woundwood tissues are seen as ribs about the margins of wounds. The woundwood ribs also add new strength to the weakened side of a stem, branch or woody root.
When woundwood closes wounds, then normal wood continues to form. The internal boundary-forming processes of compartmentalization are separate from the processes of callus and woundwood formation.
What can go wrong?
It appears that trees could live forever. Of course, that is not so because the tree system, like all systems, must obey natural laws. And, again, the laws bring together biology and mechanical engineering.
Because a tree is a generating system, it is bound by its genetic codes to increase constantly in mass. The second law of energy flow begins to take its toll. The second law states that no system can remain in an orderly condition without a continuous supply of energy. As the tree system begins to increase in mass, the demands for energy to maintain order in the system begin to increase at exponential rates.
The tree still has ways of living within the limits of this law. The tree is a shedding organism. It uses and sheds non-woody and woody parts as they die. Decayed wood that develops within boundaries is even a form of shedding. Also, as trees age, the percentage of the entire tree that is symplast begins to change. The ratio of apoplast to symplast increases. So, the tree has both dynamic mass - symplast - and apoplast.
As the inner cells in the symplast die, the inner apoplast that now has all dead cells is called protection wood.
Protection is a static feature. Defense is a dynamic action. Protection wood is more protective than the sapwood because protection wood often contains substances called extractives that resist decay. Protection wood may also be so altered that its water, pH and available elements may not support growth of microorganisms.
Sapwood has a symplast. When sapwood is injured and infected, dynamic processes take place. There are two types of sapwood: sapwood that conducts free water, and sapwood that has its vessels plugged and does not conduct free water.
When protection wood is injured and infected, the intrinsic characteristics of the wood resist spread of infections. There are four types of protection wood: heartwood, false heartwood, discolored wood in early stages and wetwood (Shigo 1994).
The biology of fractures
Trees, like all organisms, die in three basic ways: depletion, dysfunction and disruption.
Depletion means that energy decreases to the point where disorder increases and the survival of the system is threatened. Examples are infections and starvation.
Dysfunction means that highly ordered parts and processes begin to become disordered to the point where survival is threatened. Some examples are genetic problems and toxins.
Disruption means that the highly ordered structure of a system is disordered to the point where survival is threatened. Some examples are storm injuries and wounds inflicted by large machines.
Limits for survival
There are no absolutes. There are no perpetual motion machines. Every system has its limits for survival. The tree system also has its limits for survival. As it increases in mass and gets older, the likelihood for injuries increases. A mature, healthy tree may have thousands of compartmentalized infections. Yet, there comes a time when even the limits of a super survivor begin to be approached. There are no absolutes.
When trees are young, depletion and dysfunction are the major causes of death. As trees get older and have survived thousands of injuries and infections, disruption becomes the greatest threat for high-quality survival. When a branch fractures and falls, it dies. When a trunk splits and falls to the ground, it dies. And, as larger and larger wounds result from such fractures, the likelihood of more fractures increases greatly.
When the pattern of fractures begins in city trees, not only are the trees in potential trouble, but so is the property near the trees. Also, people who go near the trees could have problems if trees or their parts fracture and fall.
Trees are living systems. They are unique living systems because they have the capacity to add strength to their structure at exactly the most effective places. This capacity is built into their genetic code. As generating systems, they are always building in front of themselves. When any part of the structural framework is weakened to the point where survival is threatened, the new parts that form in new positions form in ways that add strength to the weakened place.
Having the capacity to respond effectively to survive is dependent on having the energy, conditions and other ingredients necessary to turn the capacity into an ability
Both capacity as a vigor ingredient and ability as a vitality ingredient are necessary for long-term, high-quality survival. Vigor without vitality, or vitality without vigor will not support long-term, high quality survival.
The vigor codes for trees have met the test of time in forests. Many trees in many cities of the world are having great difficulties in expressing their vigor codes because human activities and treatments have affected their vitality.
There are no absolutes. No system, or its parts, will survive when stress goes to strain.
It is time to reexamine the tree system.
It is time to start basing tree treatments on tree biology.
It is time for modem arboriculture!
Shigo, A.L. 1994. Tree Anatomy. Shigo and Tree Associates, Durham, New Hampshire, USA.
This excerpt was reprinted with the kind permission of the author from:
Shigo, A.L. 1996. "How Trees Survive". In: Tree Care Industry, Volume VII, Number 2.
The full text of this article including many excellent full color images can be purchased as part of a compilation of articles entitled Shigo on Trees, from Shigo and Trees, Associates, P.O. Box 769, Durham, NH 03824, USA. Phone: 603-868-7459; Fax: 603-868-1045.
About the author
Alex L. Shigo is retired chief scientist with the US Forest Service. He is now an internationally recognized researcher credited with the development of expanded interpretations of decay based on compartmentalization and microbial succession. His research includes over 15,000 longitudinal tree dissections with a chainsaw. He has published over 15 textbooks used in many universities worldwide. Contact: Shigo And Trees, Associates, P.O. Box 769, Durham, NH 03824, USA; Tel: 603-868-7459; Fax: 603-868-1045.
Related editions to The Overstory
- The Overstory #126--Trees for Urban Planting
- The Overstory #92--Trees and Their Energy Transactions
- The Overstory #70--Rhizosphere
- The Overstory #69--Some Tree Basics
- Overstory #68--Twelve Tree Myths