Structural Defects
Nature has engineered wood to be very strong and flexible, but the strength of a tree depends on the arrangement and internal structure of its parts. Weak branch unions, poor pruning, and overextended branches can overwhelm a tree’s ability to withstand wind, ice, and gravity.
Weak Branch Unions
If you’ve ever tried to split wood with a knot in it, you know that the spot where branches meet the main truck can be very, very strong. This is because the wood fibers in these areas are woven together in complicated patterns - important for keeping the branch attached to the tree in high winds or if the branch is loaded with snow.
This picture shows a well-formed, stable branch union. The branches form a U-shape where they meet. The wood underneath is knitted together in whorls, making this high-stress point stronger than straight-grained wood. (Figure from Slater and Ennos, 2016)However, bark can become trapped between the branch and trunk, preventing this normal reinforcement. This is called a bark inclusion. These branch unions are weak and can fail unexpectedly.
To mitigate the risk of failure, the weight of the branch can be reduced, the branch can be cabled to a more stable part of the tree, or the branch may need to be removed entirely
The dark "V" shaped area at the top is included bark that has prevented the wood from fusing between the two branches, contributing to the failure of the branch.Don’t top your trees!
Any time a tree is wounded the exposed wood will decay. Trees can wall-off decay from small cuts fairly easily, but larger cuts will always lead to substantial decay inside the tree.
When you cut the top off of a tree it will rot from the top down. This is big a problem because any branches near the cut will not have a solid anchor and are very likely to fall off the tree. Topping a tree may relieve some anxiety, but it will result in problems down the road.
The tree in the pictures below was topped about 15 years ago. Already there is a large decay cavity. It’s a little surprising that none of the upper branches have fallen off the tree yet!
Not only is this a safety issue, but this tree was dying and the topping cut probably contributed to an early death. Plus, topped trees just look bad. So what can you do about a tall tree near your house? Thinning to reducing the load in the upper portion of the tree can reduce the stress on the tree by up to 20%, which is plenty to make sure a healthy, stable tree doesn't tip over or break in a storm. If the tree isn't healthy or stable, topping won't do much but cause more issues in the future and removal may be necessary.Overextended Branches and Liontailing
Lions-tailing is a poor pruning practice that removes all of the lower or interior branches of a tree. This means all of the weight is concentrated on the end of the tree, and it increases the chance a branch or tree will fail.
Overextended branches are pretty self-explanatory; they are branches that extend beyond the rest of the tree canopy and are thereby exposed to higher wind loads.
The branch in the picture is both lion-tailed and overextended. During a wind or ice storm, this branch is experiencing some pretty strong loads and it’s a matter of time before it breaks. It’s a bit harder to see but the main stem next to it is also lions-tailed, naturally this tree would look much less top-heavy and be much more stable.
