40. Pruning – Is a solution that will be discussed in another presentation.
41. Tree structure By Edward F. Gilman Department of Environmental Horticulture University of Florida http://hort.ufl.edu/woody/planting
Notas del editor
What is the problem? This photo should make it obvious. We want large trees in our urban landscapes but when they fall apart they can cause serious damage and even death. Structure has a significant effect on the safety of a tree. This presentation gives and introduction to good structure and discusses some elements of poor structure.
This slide is an outline of what will be covered in this presentation. The goal is to show how trees develop if left to themselves. To demonstrate elements of poor structure vs. good structure – and to show how to recognize them. Finally, to briefly discuss a structurally sound tree.
Many of us don’t think about the space a tree can fill when its grown, or how the trees in the forest acquired the structure they now have. When we put trees in our urban environments, they are essentially “open grown trees” so we need to realize how they are going to develop on their own.
Trees in the forest have to compete with their neighbors for light. As the forest trees grow, lower branches get shaded out, die, and are eventually shed from the tree leaving a clear trunk (no branches).
Notice on these tuliptrees ( Liriodendron sp) how the lower trunks are clear while further up in the tree, there are a number of large branches competing for the light. Branches on these trees start at about 80 feet and the trees are about 120 feet tall.
In contrast, this tree, placed in a buffer strip, without any competition, has large branches developing at about 6 feet because light is abundant everywhere. Pruning to encourage strong structure makes open grown trees better able to remain strong.
This Honeylocust ( Gleditsia sp) in Fall demonstrates the typical development of an open grown, decurrent tree. A decurrent tree grown in an open landscape has a spreading canopy with no main trunk as opposed to an excurrent tree which has a conically developing canopy and dominant, centrally located trunk even in an open landscape (e.g. Baldcypress ( Taxodium ), Sweetgum ( Liquidambar ), Pines ( Pinus ), etc).
This slide shows the development of an open grown decurrent tree and the pruning that it typically receives. Notice how the canopy develops low on the trunk and is limbed up. Also notice that the canopy is usually wider or as wide as it is tall. The tree shown at the far right is a sad, and all too common end, for what could have been a beautiful, long lived, large shade tree. Now it has three huge trunk wounds eight feet from the ground. This can begin a downward health spiral due to lost energy reserves and storage space, cracks and decay.
So what is the problem with large branches developing low on the trunk. This is the first of three slides showing structural problems resulting from large, low branches.
Here you can see the canopy of that tree starts almost right at the ground. The tree has multiple trunks and many large limbs that originate low on those trunks - the negative effect of “party time”. The close up shows the crack where the stems have not been able to grow together. This is because the bark from the adjacent stems is trapped between them. Trees can’t move that bark or break it down so it will be there indefinitely. We’ll see the results of this type of structural flaw shortly.
Here we can see that the crack has not righted itself. As the stems continue to grow radially (in girth), there is no room for growth between them. As a result, they push against each other compounding the problem.
The result of such structure can be a “Fall down go boom tree”. Fortunately in this first slide it looks like the failure didn’t cause any significant economic damage.
The next time the tree failed (two years after the right hand stem broke), the owner was not so lucky. So what leads to failures like these and how can they be avoided? We first need to look at stems and branches and how everything is put together.
This slide is an outline of what will be covered in this presentation. The goal is to show how trees develop if left to themselves. To demonstrate elements of poor structure vs. good structure – and to show how to recognize them. Finally, to briefly discuss a structurally sound tree.
We define large branches that have an upright growth habit as codominant stems. They are easy to recognize because they are usually about the same diameter. Remember, in forest grown trees, codominant stems develop well up into the canopy. In contrast, open grown trees have codominant stems developing very low in the canopy. The difference is the availability of sunlight.
Not all codominant stems have the same structural weakness. The strength of a trees structure is evaluated by how the stems and branches are attached to the rest of the tree. This slide shows two different codominant stems with vastly different structure. Which do you think is typical of the forest grown tree? The open grown tree?
This slide shows probably the two biggest indicators of poor structure in codominant stems – a severely acute angle formed by the two stems, and “elephant ears”. Elephant ears or “hips”, as some call them, are outgrowths from where the two stems are pushing against each other.
Here is the problem that we referred to earlier. Bark, that developed when the stems were smaller and had space between them, is now trapped inside the union between the stems. This doesn’t happen when the angle between stems is large enough to allow the stems to develop independent of each other.
This slide shows the effect of included bark. Notice the branch angle. The top of the elephant ears can be seen as well. Also, notice that the two stems did not connect with each other as they grew together. The decay and discoloration are results of the stems pressing and rubbing against each other as well as microorganisms breaking down the wood. The narrow branch angle is a perfect environment for microorganisms because it is an area that stays moist, warm, and dark.
Here is an example of two codominant stems that grew together forming some connective wood between the two stems. Many trees could never accomplish this. Despite the fact that this tree was able to breach the bark between stems and grow together, there is still bark included in the union. The tree could grow enough that the bark inclusion would become inconsequential, but it would take many years to do so. There is still a weakness at this point on the tree.
Here is that same codominant union. The effect of the narrow branch angle is clearly seen in the development of the stem on the right. It is shaped like a kidney bean! The closure crack seen down the side of the stem is another indicator of included bark.
Here is another example of included bark but it isn’t with a codominant stem! None the less, the two prominent indicators of weak structure are still evident. Notice the branch angle and the early development of elephant ears seen near the closure crack. The next slide is a close up.
This slide shows a close-up of the closure crack. Notice that two ridges are beginning to form on either side of the crack. Remember that as the branch and stem grow radially, they are pushing against each other. The ridges are formed as the tree tries to spread the pressure from that pushing over a larger area. Those ridges are the initial stages of elephant ears.
This slide is an outline of what will be covered in this presentation. The goal is to show how trees develop if left to themselves. To demonstrate elements of poor structure vs. good structure – and to show how to recognize them. Finally, to briefly discuss a structurally sound tree.
What can we do to prevent weak unions? We need to understand how strong unions form and how to recognize them.
Branch wood and stem wood don’t form at the same time! A branch lays down wood earlier in the spring than the trunk. As a result, branch wood and trunk wood overlap where they meet, forming a strong union. The wood at the union between codominant stems does not overlap. For that reason, the union between codominant stems is typically weaker than the union between a branch and the trunk.
This slide shows the formation of the branch union more clearly. In the drawing on the left, the branch wood is brown and forms first each spring growing on top of last years trunk wood. The trunk wood is yellow and forms later in the spring overlapping the recently formed branch wood. The photos on the right show a strong branch union. The trunk has rotted away leaving only branch wood. The ridges are comprised of branch wood and are a result of the alternating growth of branch and trunk wood. The ridges of branch wood act like threads on a screw to secure the branch to the trunk. These ridges only form when the branch is much smaller than the trunk.
One indicator of a good branch attachment is a prominent branch bark ridge. It is formed on the upper side of the branch union. It is the result of new branch wood and new trunk wood pushing up into the union. It is different from the formation of elephant ears because the pushing occurs only at the surface so there is always room to the outside. Unlike a bark inclusion, there is no pressure pushing the branch from the trunk.
Some trees never develop a branch bark ridge. Another indicator of a strong branch union is a swollen area at the base of the branch called the collar. The collar is the result of overlapping branch and trunk wood shown in previous slides.
Some trees have both a collar and a branch bark ridge that are easy to see.
This slide reemphasizes the wood orientation at the union between a branch and the trunk.
We mentioned that codominant stems do not have the same wood orientation seen between branches and stems. The size of the branch in relation to the trunk has an affect on the formation of a collar. Notice that a visible collar is lost when branch diameter is greater than about half the diameter of the trunk it originates from.
The relationship between the diameter of a branch and the diameter of the trunk it originates from has been called the branch “Aspect ratio”. Branch aspect ratio is found by dividing the diameter of the branch by the diameter of the trunk. The most desirable branch aspect ratio is less than 0.5 as was demonstrated in the previous slide.
This slide sums up a number of the elements of good vs. bad structure. Notice the branch angle of the strong union vs. that of the weak union. Notice also the branch bark ridge of the strong union vs. the closure crack of the weak union indicating an bark inclusion. Also notice the aspect ratio of the strong union vs. that of the weak union. Do you think there is included bark in one of these two unions? Can you guess which one?
This slide is an outline of what will be covered in this presentation. The goal is to show how trees develop if left to themselves. To demonstrate elements of poor structure vs. good structure – and to show how to recognize them. Finally, to briefly discuss a structurally sound tree.
Now that we’ve covered branch and stem attachment, we need to briefly mention whole tree structure. Branch spacing, both vertically and radially, is very important to good structure. Live Crown Ratio (L.C.R.) is also important, not only for structure but for the overall health of the tree. We can’t forget that good structure is only one piece of a very complex puzzle that, when solved, provides for large, healthy, and failure resistant trees in our urban environments.
Trees with major branches or stems too close together can fail. The wood in the region of the union can not develop its normal strong connection when many stems are close together.
This presentation did not discuss how to manage a tree to achieve good structure. That is the subject of another presentation that is available in this same format.
