The Life of a Branch
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
NOTE: This Naturalist Note is the result of a curious reader asking me a question about a previous Naturalist Note. If you have any suggestions for, or questions about, a Naturalist Note, let me know, and if possible, I’ll cover your question in an upcoming Naturalist Note. Warning: Based on the nature of the question, and the time of year, the response might not be fast. – Bruce Rottink
As you walk through the forest at Tryon Creek State Natural Area (TCSNA) you see trees with lots of branches, some dead and some alive. Each branch has a life cycle, and knowing that cycle will help you understand what you see in the forest. Let’s follow the life cycle of a Douglas-fir (Pseudotsuga menziesii) branch. There are minor variations between tree species on how their branches grow and develop, but Douglas-fir is typical of many trees.
In the beginning…
All branches begin life as a bud. The buds start out very tiny, and complete their development in the first growing season. In Douglas-fir, by autumn every needle that will appear the following year is already present as a small mound of tissue inside the bud. Most of the needles have been removed from the twig pictured below so the buds (the brown pointy things) are more visible.
Buds on a Douglas-fir branch
The bud “rests” all winter, with no obvious growth. However, the cool winter temperatures from approximately 32° to 40° F (“chilling”) facilitate chemical changes in the bud which allow it to start growing in the spring. Temperatures below freezing are not effective in the chilling process. If there is not enough cool weather in the winter, the bud will be very slow to start growing come spring.
Year Two – The Bud Bursts
After the appropriate chilling, warmer temperatures in the spring cause the bud to burst open. Both the branch and the needles then elongate to their final mature length that summer. The bud at the tip of the branch (the terminal bud) will grow the longest, and extend the branch more or less straight out from the end. The buds further back from the tip (the lateral buds) will create side branches at roughly a 45 to 75 degree angle to the main branch.
This cycle of bud formation and the bursting of the bud to form a new branch is repeated year after year, and eventually, the branch of a typical Douglas-fir will look something like the one pictured below, which has been stripped of needles. In the picture below, the branches have been painted to indicate which year they elongated:
Pink = elongated in 2015
Green = elongated in 2014
Yellow = elongated in 2013
Blue = elongated in 2012 (only part of 2012 growth is included in this picture)
Branch of Douglas-fir stripped of needles.
The Beginning of the End
As seen in the picture above, the green branches labeled as “dying branch” did not elongate in 2015, hence they don’t have a pink segment at the end. These branches still had needles in 2015 as Douglas-fir needles tend to persist 5 or 6 years. At this point these branches are being heavily shaded by the branches above it on the tree, and thus receive minimal sunlight for photosynthesis. Sometime in the next several years these dying branches will lose the last of their needles, and the twig will be completely dead.
Getting Rid of Dead Branches
The process for getting rid of dead branches is dramatically different than the falling of leaves. First of all, the leaves have a layer of cells (the abscission layer) at the base of the leaf. These cells weaken at the appropriate time so they are no longer able to hold the leaf onto the plant. Branches have no such abscission layer. Let’s take a look at how the branch is actually attached to the trunk of the tree. Below is a typical cross section of a tree trunk. Note how I have cut through the trunk vertically.
Note the edge of the vertical cut through this trunk
The photo below shows the surface of the vertical cut through the center of the trunk of a Douglas-fir. I cut the branch off about 1-1/2 inches from the trunk.
Vertical cut through a young Douglas-fir
The brown pith is relatively “soft” tissue that is formed at the center of every branch the very first year that branch elongates. The important thing to notice here is that the branch is very well integrated into the stem of the tree. There is no obvious place where the branch can easily separate from the tree trunk.
With no predetermined place for the dead or dying branch to separate from the main stem, it just hangs on after the branch dies. These branches will persist until they are knocked off by some other branch during a windstorm, or decay to the extent that they can no longer hold up their own weight. The Douglas-fir, growing near Old Main Trail pictured below has a large number of dead branches.
Douglas-fir with lots of dead branches
On the other hand, some trees have trunks which have very few dead branches, such as this tree pictured below also along Old Main Trail.
Douglas-fir with very few dead branches
What Causes This Difference?
Within a given species, the presence of very few dead oftentimes will indicate the tree grew up with lots of competition. With lots of competition, the lower branches die while they are still fairly small. If a tree has very little competition, it will keep its lower branches a long time, and they will be very large when they finally die. Small branches decay and fall off the trunk more rapidly than large branches.
How Might Branch Retention Affect Me?
Dead branches hanging onto the trees represent a possible entry point for tree decay. Additionally, if you are harvesting trees for lumber, the branches are what create “knots.” If the branch was alive at the time a particular section of the tree trunk was growing, the branch wood is tightly integrated into the board in what is called a “tight knot.” An example of a tight knot is seen below.
Tight knot which is integrated into the board
Loose knots form when a branch which is already dead is still attached to the tree. The dead branch is surrounded by the newly developing wood in the tree trunk. Loose knots can seriously impact the strength of a board, depending upon their size. In the split piece of wood pictured below, note how the grain of the wood curves around a now-missing branch. The resulting loose knot has already fallen out.
A “loose knot” has fallen out of this piece of wood
When lumber is cut from trees with loose knots, these knots tend to fall out, creating inferior boards like the one seen below.
Looking through where a loose knot has fallen out of the board
The whole story
Like everything else in the forest at TCSNA, branches have a life cycle: First as the engine that drives the growth of the tree, then as a dead branch hanging on the tree, and finally as dead material on the ground for recycling. Next time you are walking through the park, take a look at the trees and their branches. Notice which parts of the branches’ life cycle you can see.