Tall Trees and High Winds

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Winter wind-storms in the Pacific Northwest can pack a punch. Though they are seldom named until after the fact, they often contain more power than the celebrated hurricanes of the Atlantic, and are much, much larger.

But the wind-storms here have allies that exacerbate the damage they do; soaking rains, and very tall, heavy trees.

The worst damage invariably occurs when strong winds occur after days of rain.

The soil becomes water-logged, and as the wind pushes and pulls the tops of the trees, the roots are unable to hold in the ground. They effectively "drag their anchor" and capsize.

Falling Trees Can Produce Dramatic Results

Since the vast majority of our high winds come from one general direction, the south, the trees located south of a residence (in arborist parlance, we use the term "target", though I don't like that; it sounds so personal; trees hate it when we anthropomorphize them) should be of the most concern. I am guessing here, but I suspect that better than 3/4s of our high winds, winds over 35 miles per hour, come from the south. After that, the west is the most common direction. Our "Double Whammy" storms in December 2007 were an excellent example. The first blew in from the south, and several days later another came howling down the Strait of Juan de Fuca from the west.

While drenching rains are the great enabler of toppling trees, there is another human-caused occurrence that can be just as harmful. Construction.

There are several situations associated with new construction that can cause tree failures within the first few years of the development.

New Construction can be hard on trees.

New construction is a leading cause of tree failures. The building boom, just recently ended, has altered many of our urban forests. The clearing and changing of grades along the ground alters the flow of water, causing pooling and saturation in otherwise drier areas. In the short term, this increases the likelihood of roots slipping and allowing the tree to fall. In the long term, it increases the chances that root rotting pathogens will begin to work on the trees.

Even the clearing of some trees and the construction of buildings alters the flow of the wind, increasing the speed next to buildings and putting stress on parts of trees that are not adapted to that. Wind moves like water, and like water, where the flow is restricted, the speed increases.

In a study at Evergreen State College (naturally), students measured how far individual tree limbs move in the course of a year, movements caused by wind. They discovered a tree effectively travels thousands and thousands of miles.

These winds work on a tree like a mathematical equation, and when one or several variables are changed, either by pruning or the removal of nearby trees, the tree sometimes has to adapt to this by shedding a limb or its top.

The (now) second largest Monterey Cypress in Washington State is located in Port Townsend at the intersection of Lawrence and Monroe Streets. Several years ago a strong wind knocked out the top quarter of the tree. This loss of "sail" changed the way the remaining limbs reacted to winds. A few days later, in a much lighter wind, another very large branch located close to the bottom and on the opposite side of the tree, broke and fell onto Lawrence Street. While there are usually several causes in the failure of any tree, I am convinced this was caused by the enormous changes the tree had recently experienced.

Trees adapt to prevailing winds over time, as is easily seen by observing trees along the coast. They tend to send major anchor roots into the direction of the prevailing wind, and so can bear high wind-speeds from those directions without uprooting. But in our swirling wind-storms, gusts can sometimes come from unlikely directions, and without the anchoring roots, these lesser winds can sometimes topple the tree.

Another practice of land management and development that gives me pause, especially while driving on windy days, is the presence of "Idiot Strips". I do not know the proper name for these thin lines of trees that remain beside roadways after a property has been cleared, but the common name is appropriate. For the first several years after these strips are created by removing all trees but them, they are likely to be the source of tree failure, either by dropping large branches, tops, or even entire trees toppling. After a number of years have passed, typically 4-15 years (which is the published standard amount of time it takes for trees to begin showing stress from development), the remaining trees are mostly stabilized.

Trenching, especially when the digging has been done on the south or west side of a tree, is another cause for concern. By removing the anchoring roots it increases the likelihood of falling. If trenching must be done, it is usually recommended it be dug one foot away for every one inch of tree diameter. We have many 12″ diameter Douglas Firs in this area, so trenching should stay 12 feet, minimum, away from the tree. Of course it's naive to think we will always have that much room to maneuver, so exceptions are the norm. If large roots, 1″ in diameter and larger, can be excavated by hand rather than cut, the tree has a much better chance of long term survival.

There are a couple of long-term issues with trees and construction. These may take a decade or more to reveal themselves as issues, but they are also the most tenacious of problems.

One major culprit associated with construction is backhoe blight, which occurs when large metal machines repeatedly gash trunks and limbs, leaving many opportunities for microscopic pathogens to enter the trees and work their worst. Rots develop in the wood of the trees, which, depending on the health and vigor of the tree, may form weak areas which grow weaker by the year. Strong winds may eventually cause the trees to break at this point.

There is also the ubiquitous ground compaction caused by machines and people repeatedly trampling over the ground. This squeezes the oxygen from the soil, which is critical for the long-term survival of tree roots. With no oxygen, the tree roots begin to rot. The results are easy to see in large developments that leave a number of mature trees but take no or inadequate precautions to mitigate the compaction. There are plenty of newer neighborhoods in this area with tall, declining trees.

This post barely touches on the issues of construction and tree health, and I hope to deal with this issue in more depth and coherence in later posts.

Another component of tree failure is the type of tree. Different genus and species of trees have differing abilities to withstand high winds, and, often, differing methods of "failing". In this case, "failing" is defined as the way in which the tree loses a portion of its canopy or the loss of the entire tree.

For the sake of brevity, most arborists consider the mature Western Hemlock (Tsuga heteropyhlla) the most likely candidate to either break or uproot in high winds.

The Western Red Cedar (Thuja plicata) is generally considered the least likely common native tree to fail in high winds.

This is a generalized statement, and there are so many caveats to list that if I begin now, I might finish by New Years 2011. Suffice to say, I will write more about this.