Tree Science of Northcoast Tree Care - Tree Biology


We use the term biology to include both the anatomical and physiological aspects of trees.  As we state in our Mission, we do our best to base our treatments on a modern understanding of tree biology.

Tree literature is filled with information about trees and how to treat them.  This information is not completely consistent nor always based on scientific research.  Our trade, like most others, has had its share of “snake oil” salesmen.

This is not to say that all misinformation has had an ulterior motive.  Some people just didn’t look in the right places or make the most accurate observations.  Part of the problem was that people looked at trees primarily from the outside.  Those that did open trees up made cross-sectional observations (for the most part).

All of this began to change in the late 1950’s when Dr. Alex Shigo was given the assignment by the USDA Forest Service to lead the pioneering project on the discoloration and decay of trees.  He and his co-workers dissected over 15,000 trees, mostly with a chainsaw.  From these, they saw patterns that revealed new ways of looking at trees and how they function.  By the 1970’s, this information was being translated into practical applications.

Some have criticized Shigo’s work as being observational, rather than adhering to the scientific process.  To some degree, this may be true.  However, he published over 200 papers, a fair number of which were in highly esteemed journals (a complete listing of these is given in the appendix of both Modern Tree Biology and Modern Arboriculture shown in our Library).

It is some testament to the value of Shigo’s basic work that it has been adopted into the major texts and standards of practice.  Some of this work is exhibited in the photos that follow.

Shigo is most known for looking inside of trees.






Historically, when trees were opened up, they were viewed as cross-cut sections.  This is useful for certain aspects.  In this picture, the numbers refer to “walls” of the model “CODIT”.  This acronym stands for the “compartmentalization of decay in trees”.  Shigo’s team “discovered” that trees do not repair or replace injured tissue.  Instead, they build walls around the injury to contain it.  These “walls” resist the spread of discoloration, and ultimately decay,  continuing to add new tissue in new spaces as the tree grows.  As long as the rate of new tissue production exceeds the advancement of decay, the tree has a chance to stay alive and intact.

To fully understand compartmentalization, Shigo had to dissect trees in a vertical or longitudinal manner.  It also led to a better understanding of discolored wood.  All of the features in this photo would not be clear without this kind of dissection. (This and the following five photos are courtesy of Shigo and Trees, Associates).


Another landmark in Shigo’s work was explaining how branches are attached to trunks.  Again, it was only through his longitudinal dissections that this picture became apparent.  The darker tissue represents the branch and the yellow is trunk tissue.  This drawing represents three years of growth, where the trunk tissue envelopes the branch in a succession of dowel and socket arrangement.   It is very strong. The work that revealed this phenomena was published in a journal of the highest quality.  As far as we know, it has not been refuted.  It was to become the basis for pruning cut recommendations.

This photo of a dissected stem shows the place to make a correct pruning cut (the black line to the right) and a poor one (the black line to the left or below the other line).  This photo also shows the branch protection zone (the discolored portion below the rotted branch stub).  The BPZ was another of Shigo’s revelations.


A correct pruning cut.




The red line would be a correct cut.  The saw is placed just behind the branch bark ridge.  Such a cut would be far more injurious.


These dissections show the internal results of a correct cut (left) and a poor cut (right).  The latter is called a “flush” cut because it cut through the branch collar and behind the branch bark ridge.  It takes a keen eye to see the internal differences and is beyond the scope here to try and explain.  Suffice it to say that the flush cut produces far more defect.

Trees with codominant trunks or stems do not have the same anatomy as those where one is smaller in diameter relative to the other one.  It is common to see a crack or seam develop between stems like this.  This can produce a weak structure called a “bark inclusion”.


This split trunk broke at a bark inclusion point.  The dark tissue color in the broken area is an indication that there was this bark inclusion.  Essentially, the tree has wounded itself.



This cross section shows both wood checks and wood cracks.  One of the checks is at 9 o’clock and is v-shaped with the opening on the outside of the tree.  These occur when wood dries.  Cracks on the other hand, develop mostly from the inside out.  They commonly originate from wounds.  They are a hidden and highly prevalent cause of branch and trunk failure.  Shigo speculated that cracks cause more failures than decay.


Every aspect in the tree has a relationship at some point to energy.  Energy is trapped in the photosynthetic parts of the tree, mostly in leaves.  Any and all treatments to trees will be most successful if the use of energy and where it comes from is understood.



Would you hire a tree guy who didn’t understand tree biology?  Cartoon by Carl.