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I have a very nice oak tree in my backyard that is about 100-150 years old. The tree is in a good visual shape and we trim it about every two years.

However, there is a wound on tree's stem, about 25 feet up from the ground. We hired an arborist who performed "resistograph" testing and says there is over 35% decay at this place and it needs to be removed.

Resistograph results are attached in the picture. Does it really need to be removed? Or should we get a second opinion?

(click on the image to get the fullres version for readability)

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    What is the total hight of the tree? Is it dangerous enough (where does it fall on) to do these costs? (35% sounds reasonable to me, I have a 100ft popular up to 50% standing and I have had apple trees up to 90%, just don't go in the backyard when there is a storm.) – LaurensP Apr 27 '14 at 6:38
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As it's such an old tree, I'd be inclined to get someone to carry out a Picus or sonic tomograph test as well - if the Resistograph results are echoed by the Picus, then you might need to do something, even if that's removing the tree above the point where it's weak. It's probably sensible to get another person to do more testing, then you get a second opinion as well, and in cases like this, with such an ancient tree, both tests should really be carried out to give a more complete picture.

If the wound you mention is weeping at all, it's likely the tree will need to have something done to it, and that might mean removal.

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A picture would be helpful.

If the cavity is less than 25% of the diameter of the tree, it may be able to survive. If the arborist is saying the decay is 35% of the way through, that will usually be fatal depending how wide the wound is. If it is narrow, like less than 5" wide it might be ok.

With any chronic rot you should chip it out. Obviously the incision should be as narrow (up and down) as possible. The wound must be kept dry. This can be done by melting beeswax in an open-topped tea kettle and basting the wound with it. Do not use any kind of wax except pure beeswax. You can also hang a flap over the wound to keep the rain out. Do not seal it off with pitch, like some repairers do, because that will cut off the air supply, which is bad.

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