Friday, October 19, 2012

Trees Don't want to Die: A Musing

Trees have been around for millions of years, and they have found ways to survive outside in all kinds of conditions. We as humans try to understand why trees die and fail, and how to predict these events. Along the way our industry has come up with 'rules of thumb' to help us predict this. We hear arborists reference numbers like 30%, 33%, or 70% to quantify chances of tree failure or death, but trees break these rules on a daily basis.

There are 'high risk of failure' trees, condemned years ago, still standing today.  These trees have stood through wind and storms which have toppled other trees.  I've seen mature oaks with more than 50% of their trunk circumference damaged from the ground to 20-ft up the stem that are green and vigorous, and which have had enough energy to form callus tissue around the damage.

My view is becoming not, 'we have this much strength loss,' but instead 'we have this much strength left.'  And the same when it comes to the vascular tissue.  We must realize, of course, that some of this will be species, condition, and site dependent.  But, if a tree has 32% of it's root-flare compromised by decay is it a moderate risk of failure, while at 33% it is a high risk of failure?  Or, is what matters the 2 or 3 root flares not damaged are strong enough to support the weight of the tree even if even if all the other flares were compromised? How long will a tree with 50% of the stem girdled maintain a health canopy? 1yr, 5yrs, or 10yrs?  These are the questions I ask myself, because I don't want to remove a tree until it is necessary  and most of my clients feel the same way.

Notice how this tree has fallen completely over, and yet it continues to put out growth maintained by just a small portion of vascular tissue.

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