Prevent storm damage by training, not shaping, young trees
Winter 2014 probably will go down as a rough one for some trees.
We have seen lots of snow and ice loading lead to broken of branches in large and small trees alike. Just a half-inch of ice on branches can increase loads by 30 times the normal load (or branch weight), and many branches simply break.
Have you ever wondered why some trees or branches break or split in storms (snow, rain or wind) while others survive without dropping a branch? It all comes down to their form and branch structure.
Some species of trees, such as willow, poplar and silver maple, are just weak-wooded because of their fast growth rates and prone to branch breakage, but for most trees, storm damage occurs because they form weak branch unions, or their form, branch architecture and wood strength are impacted by improper pruning practices.
So, what is good form, and how can we train our trees to develop good form?
Remember when your mother told you to eat your vegetables and sit up straight? She was training you to improve your form. Trees are somewhat similar. A tree that has one single, central trunk, or leader, growing up through the tree that does not fork or split into two main branches in the crown will have the strongest form and structure. Smaller lateral branches coming off that central trunk should not be larger than half the diameter of the leader. When the lateral branches are smaller than the trunk, the tree will develop strong branch collars that hold the branch in place, even during strong storms or increased loading.
The weakest branch attachments that often fail in storms are called co-dominant branches because they are almost equal in diameter and form a tight, V-shaped union or attachment. Branch collars (strong attachments) are not formed on these co-dominant branches, and recent research has shown that they split apart when a third of the force or loading is applied to them, compared to branches with branch collars. This is the reason we see so many Callery Pears (especially Bradford Pears) split apart at a certain age. These trees form lots of co-dominant branches.
The other reason branches fail is decay. When branches are heading back (topped) or shortened with indiscriminate cuts that leave stubs, decay fungi will move into that branch and quickly spread down it. As decay progresses in the branch, it becomes weaker and will not hold the increased loads from snow, ice or wind. It is important to learn how to make proper reduction and branch removal cuts.
Treating young trees like hedges and shaping them, or "topping" large trees because we think we are making the tree smaller and safer, just destroys trees' form, disrupting branch structure, promoting the spread of decay in branches and creating trees that will fail in storms.
To learn more about proper pruning and training young trees, contact your local Penn State Extension Office or visit www.patrees.org. To view a Web-based training titled "Pruning Young Trees for Structure and Form," visit http://pacommunityforests.com/webinar/index.htm
VINCENT COTRONE is an Penn State Extension urban forester and an ISA-certified arborist.