The Science of Trees

The Science of Trees

At our local horticulture group last month we were lucky to have speaker Paul Chaney, an ISA Certified Arborist who enlightened us on the science behind trees. Understanding their structure and how they grow helps us in turn to understand their best care, particularly pruning.

This article is timely since winter through spring are normally the best times for pruning.

Takeaways from his talk:

Fighting off Decay is Pivotal

  • Think of a tree as having two systems 1) the structural support system, composed of the trunk and branches which support the second system, 2) the physiological or green matter that performs the photosynthesis to keep the tree alive.IMG_0671
  • Trees fight off assaults that could cause decay and death – such as insects, bacteria, fungi
  • Compartmentalized system of decay in trees or CODIT: Alex Shigo studied tree decay for years and proposed a model in the late 70s that is now well-accepted — involving the use of four walls that trees develop around wounds. These are 1) the cell wall that resists the spread of decay up and down the vascular system, 2) the wall that becomes the annual ring, slows the inward travel of decay 3) the xylem ray that forms from the point of injury to provide a maze-like protection to decay (Casey said to envision pie slices) – and 4) the reaction wall, the trees acute response to an injury. Example – sometimes this walls off the entire inside of the tree, which is why a hollow tree can survive.
  • If a tree stops growing (stops photosynthesizing), it’s dead!

Don’t Top and the Importance of Branch Structure

  • Avoid co-dominant branches, which are stems or branches that are approximately the same size and arise from the same point. These are weak and are likely to split in a storm.

    Co-dominant branches (Courtesy Pittsburgh Parks)

    Co-dominant branches (Courtesy Pittsburgh Parks)

  • In pruning don’t become a flesh cutter! Remember that branches began as a bud, which grew. During that growth period the tree grew annual rings, which grow over that branch bud, forming a branch core. These branch ‘collars’ become a hard area that forms the knot you see in cut wood. Going back to the defense system, think about a branch breaking off – the tree does not want that to be an avenue for disease to begin. So in pruning it’s important to prune to just outside that branch collar. Chaney said arborists call trimmers who cut flush the tree — which opens the tree for disease — flesh cutters.


    This neighbor’s tree had to be removed after this terrible trim job

  • Branch collars vary among trees so you need to look at the individual tree. For example, elms have branch collars that grow into the interior of the tree so the end of the branch collar is close to the tree, whereas oaks have extensive branch collars.
  • Think about the tree’s needs in other pruning cuts.  Remember the photosynthesis needed for survival. So prune outside branches to let the sun in and to decrease weight. Also preserve the interior, green part of the tree – which also improves resistance during high winds.  Chaney gave the example of deodar cedars that are often incorrectly pruned to remove inside branches while retaining sweeping, branches on the ends. “In 5, 10 or 15 years that tree will be dead when a storm comes through. Instead you want to remove some of the end branch weight,” he said.
  • Remove dead wood/branches in the tree.
  • Don’t top trees. This leaves large wounds open to decay at the top, and also prompts a bunch of weak branches to grow from the top


    Another poor example in our neighborhood

Pruning is Best in Late Spring or Winter

  • Late Spring, right after the flush of growth has begun, which allows the tree to heal the cuts during the remainder of the growing period.
  • Winter is also good, when the tree is dormant (the tree’s carbohydrate is stored in its roots)

Hiring a Tree Trimmer

Here in our area we’ve had numerous examples of tree trimming hack-jobs. (See prior post).

  • The best bet is hire an ISA Certified Arborist. ISA is the International Society of Arboriculture, which not only certifies arborists, but requires them to maintain continuing education credits.

    A good pruning job!



About Linda Richards

My goal is to educate about the science of nature in layperson speak, through my writing, science and education background. I grew up in the Chicago area, loved living in Minneapolis before gravitating to the West, which is now home.


  1. Thanks for sharing your thoughts about tree removal cost. Regards

  2. Silas Knight says:

    There is a lot of good information here about trees. I didn’t realize that co-dominant branches were a bad thing, so that is good to know. So if I see that on my tree, should I just cut one off, or is there something else I should do?

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