Friday, August 10, 2012

Structural Pruning

I went to the NC Urban Forestry Council's annual conference yesterday, and had the opportunity to here Dr. Ed Gilman speak about one of my favorite topics... Structural pruning.  Now I must admit, while I enjoy listening to Dr. Gilman and I love the topic, I felt like sitting through another lecture on structural pruning was a bit below me. I was pleasantly proven wrong.  Here are some of the highlights:

- Pictures presented displayed a tree just before being thinned, just after being thinned, and the same tree 12 months later.  Twelve months after thinning the tree looked almost identical to just before it was thinned.  The conclusion is, thinning trees may not be the best practice when attempting to abate the risk of branch/whole tree failure.

- Don't be afraid to remove over 50% of a tree's canopy when structural pruning at planting to promote a "hyper" central lead.  Young trees recover fast from pruning.  Removing or subordinating all competing branches at planting will guide good tree structure for some time.  Competing branches can actually shade out the central leader, weakening it, and this may contribute to branch failure in the future.

- Don't be afraid to make big cuts to remove or subordinate competing branches on medium size trees.  Again, Dr. Gilman presented pictures of aggressive cuts, and how the tree recovered with much improved structure a few years down the road.

- Dr. Gilman introduced the idea of aspect ratio when it comes to branch size.  Lateral/secondary branches should be smaller in diameter (smaller in aspect ratio) then their parent stem, and the smaller the better.  Branches with larger aspect ratios are at greater chance of failure.  Ideal aspect ratio was not specified, but smaller the better was the conclusion.

- Finally, we where shown video of a tree before canopy reduction being blown by a dynamic wind load machine.  There was about 12 inches of play at about 1/2 way up the stem.  The same tree was then pruned to reduce about 30% of the branches from the crown.  The tree then had about 3-4 inches of play at about 1/2 way up the stem.  We may relate this data to individual large branches in trees with stem and root defects.  If we reduced all large branches by 30% on tall trees with defects that may predispose the tree to failure, then we may be able to retain more trees vs. removing them.  Awesome!

As with everything, we must take in to consideration the tree's species, condition, and the site when applying this data in the field.  Never the less, a great presentation, and I can't wait to get out there to get my structural prune on.