Wood Anatomy.

Wood is a complex system of many specialized cells and fibers that work together in order to not only support the tree but also act as a transport system for essential elements. As we know, wood also has the ability to compartmentalize decay. As arborists it is important to understand wood, how it's built and how it can keep us safe.
There are certain thresholds of imperfections in wood that may deem a tree unsafe to climb. But imperfections don't necessarily mean that an extremely dangerous situation is at hand either, or that a tree should be removed. Trees are amazing systems and there support system is just as fascinating. A pre-climb inspection is a great opportunity to apply our knowledge of wood, how it's built and how it functions as a support system even with imperfections and decay present.
Shigo tells us to "touch trees" in order to learn about them on a deeper level. Inspecting a tree may include things like looking high up into the crown with binoculars in order to spot included bark in major branch unions, hollows high up on the stem, conks or fruiting bodies that may indicate wood decay and weak structure, sounding low on the stem to listen for absent heartwood and decay pockets in which we can't see behind an otherwise healthy looking outer bark layer. Feel the tree, is it soft? Is it spongy, does the bark flake off clean from an inactive cambial zone behind?
Wood can determine our operations as arborists as well. Consider a cable installation over a weak branch union. In this particular case, the annual growth of wood in a narrow crotch can actually be working against the tree in terms of stability. Each year the wood grows in girth ( a meristematic process) and continues to weaken that crotch as the inclusion of bark grows. As we mitigate that inclusion with a cable or brace rod, hopefully we don't find more decay or unsound wood where we are drilling. Now this is a rare case, but think about how many situations can potentially occur just on the condition of the tree's wood alone! Wood constantly affects the decisions we make as arborists.
So back to the question, what's the threshold of safety when analyzing the condition of wood, the extent of decay, the absence of heartwood? It is so situational, I do understand that, and of course we must consider the value of a tree, it's location, it's history and the list goes on and on. But let us focus only on the tree's wood. Like a basket that is weaved many times over, and on many different planes, internally it seems almost as if the wood of a tree in infinite in it's internal dimensions.
We sound a tree and find that it is indeed 'hollow'. Now what? Does the tree automatically come down? A good application of a resistograph, but let's get real here, is the average homeowner going to pay for a resistograph reading? Maybe in your neighborhood, definitely not in mine. Well, maybe that's for another article. Maybe there is an opening in the trunk in the middle of some callous wood that will allow us to probe and to see the extent of the missing heartwood. Sapwood itself has many structural benefits and can be astoundingly supportive, even in the absence of the static mass that heartwood provides. But honestly, I'm not sure, as a climbing arborist, just what I'm comfortable with.
Of course, we must always error on the side of safety for our loved ones, our friends and our industry as a whole. But when the question arises, "how far are you willing to take it, how far are you willing to push keeping a tree around based on it's structure?, how much higher are you willing to climb above a defect?", that conversation will be about wood and it's ability to carry the load.
Supplemental support, tree injections, maple syrup harvesting, old wounds to the trunk, trees and construction; all these things affect wood and how it functions and how it's forced to respond and still maintain it's integrity. Wood is a fascinating thing yet I always find myself taking it for granted.