Pruning science

How Science Based Pruning Leads to Stronger, More Storm Resilient, Trees.

“People who prune the old fashioned way should go see an old fashioned dentist” 
-Alex Shigo, the father of modern arboriculture

Arboricultural science has recently made and continues to make massive leaps forward in understanding trees and how to properly care for them. Despite these massive advances in understanding many still cling onto their old ways. This will give you a better understanding of why proper pruning of trees is so crucial and how pruning improperly can make a tree less safe especially over the long term.


There are two primary reasons to prune a tree. The first is to prevent branches from breaking during heavy wind and snow while the tree is leafed out. Trees can break while they are dormant but given Colorados severe and erratic weather it typically happens during the growing season. This is due to increased sail area from the leaves collecting snow or wind and the increased weight from leaves and increased water content in the wood.

The second reason for pruning is to make the tree look better. We will not touch on that aspect in this but one thing to note is that everything we do leads to a more aesthetically pleasing tree
Tip Thinning
Thinning allows more wind and snow to pass through the canopy. When thinning a trees live foliage you shouldn’t remove more than 20% of the live growth in one prune to avoid shock to the tree. When we prune we have to decide where the best place to make our limited amount of cuts. The foliage at the farthest tips of a branch exerts more force on the parent branch than that of the interior foliage due to the fact that the lever arm is longer the further out on the branch it is. Taking weight from very tips reduces leverage on the parent branch and decreases its chances of being ripped off in a storm. Due to this at least half of the cuts we make tend to be in the ends of branches, this leads to a compact shape and mitigates risk of failure.
Lions Tailing
The opposite of tip thinning is called lions tailing where interior branches are cleared out and the exterior of the tree is left untouched. This occurs because unless a tree is large enough to maneuver an aerial lift inside the canopy (and you can access the whole tree) the best way to prune is from the interior by climbing the tree. Climbing out to the tips is hard work so many take the easy way out, clear out some interior branches and call it a day.

 After a tree is pruned the remaining branches have access to more water and sun so they grow quicker. This mean that after lions tailing a branch it causes a flush of new growth in the tips of the branches. This adds more stress to branches that are already regularly subjected to heavy winds and wet heavy snows.

Branch Taper 

The added benefit of leaving interior branches is that they add taper to branches making them stronger. Every twig feeds carbohydrates created in the leaves back towards the roots. As the tree transports nutrients some is used along the way to add on wood to the branches and trunk. Well spaced branches throughout the interior of a tree create branches that gradually get wider (and stronger) towards the base. After pruning the flush of new growth will occur on interior branches making a sturdier branch and a sturdier tree. 




Although not real trees these twisted wire trees give a good idea of how each small wire contributes to the taper of the branch and the overall tree
Structure
In general a tree that will be most able to survive is one that is compact and upright in its growth pattern. Tip thinning takes care of the long overextended parts of the tree and promotes interior growth so that will make it more compact. The next thing we focus on is promoting upright growth. A branch that is completely upright will put less stress on the trunk or ground that it is anchored to. Wood is strongest in compression rather than when it is bent sideways. A pencil can withstand all of the force of you pushing its two ends together but with the smallest force put on it sideways it will break. Branches that grow vertical will collect less snow and are better oriented to withstand the forces created by the added weight of the snow.


Small Cuts
After learning all of this you would think the easiest way to prune a branch would be to remove the last 20% of it and call it good. The main issue with this is that trees must cover up exposed wood to prevent it from rotting. Wounds that are one or two inches in diameter will generally close quick enough that the heartwood will not begin to decay. The issue comes with making larger cuts. Tree bark essentially is an anti rot coating that prevents water and fungi from entering the wood and causing decay. When a cut is made it is a race between wood decay fungus and the tree. The tree needs to cover up the wound or will the fungus colonize and weaken the wood. Small cuts close up much faster so they can prevent a pocket of rot from forming in the branch. 
Included Bark Unions
Proper branching structure is one of the main reasons to prune smaller trees. Trees grow best with one central leader and all branches that come off being half of the diameter of the parent stem. This allows for a strong attachment point for the branch as the wood from the main stem is able to envelop the wood put down by the branch creating overlapping layers that anchor the branch in place. With an included bark union the two stems are growing at similar rates and neither one is able to create enough wood to envelop the other. Rather than the union forming properly a layer of bark forms in between the stems and makes them much more susceptible to breaking. To remedy this we prune one leader more than the other which causes one side to grow faster and over time it will envelop the smaller side and form a proper union.

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