Rob Swanson, senior arborist with Caldwell Tree Care in Roswell, Georgia, says he’s seen a lot of toppled trees in his 20 years of tree work. In his state, it isn’t a rare event. That’s partly because of the heavy clay soils in the Piedmont area around Atlanta.
“Ninety percent of our tree roots exist in the top 18 inches, and 90 percent of those roots exist in the top 4 inches,” he says. That means that roots are very shallow, and the compression plate of a big tree — the structural root plate that bears the weight of the tree, but is less than 10 percent of the tree’s total roots — may be stunted, causing a top-heavy tree to be susceptible to tipping. Such conditions only add to other factors, such as disease or damage.
Swanson says that a tree tugs at roots on one side and presses down on roots on the other side as wind pushes against it. In very wet soil, if winds are high enough, the tree can go over. Trees with certain problems can uproot even in light or no rains, and in light winds. He says the first thing he looks for when trying to determine if a tree is in danger of falling is disease in the roots. In particular, he looks for the fruiting structures of decay fungi, such as Ganoderma root rot or any of the dozens of possible root pathogens, which show up on exposed roots or in the soil around the tree.
Conks, puffballs and mushrooms are indications that roots may be shortened or weakened. There are many of these fungal diseases — the International Society of Arboriculture publishes a guide to decay fungi—and any one of them, if it has been eating at the tree for years, can mean that the tree’s root system has been curtailed and can’t withstand shear and compression pressures.
“If you damage any part of that system, that’s what we want to know about,” Swanson says.
The age and health of the tree can have a lot to do with how resistant it is to these decay fungi. It may take many years for fungi to severely damage a young, healthy tree’s root structure, but it may only take a season to damage an old tree weakened by several pathogens. For this reason, it is important to quiz the tree’s owner about its age and history, especially in an area where high rainfall and humidity are conducive to decay diseases.
Another reason to do so is to find out whether any construction or other disturbances have been going on around the tree. Roots damaged by construction installations, trenching, root pruning or chemicals can make the tree susceptible, and it is important to look for these situations, as well as ask the property owner about past disturbances that may not be visible at the time of inspection. It’s also important to go back many years in its history, because it may take five or 10 years for a disturbance to damage roots enough to cause toppling.
“Health and structure are two different issues,” Swanson emphasizes. A tree may be healthy, but may have some hidden structural problem that could lead to failure. Roots may be fine, but a hollow core or decay in limbs could lead to failure in a storm. He thinks of this as he would a soda can; try to crush it end to end and you can’t do it, but put a cut or bend in it, and it will crush easily. So, when he examines a tree, he also looks at old pruning cuts, and he will drill into the trunk to look for a hollow or decayed core.
Another major factor in uprooting is the percentage of trunk that has no branches. In Georgia, this is particularly troublesome in pine trees that are left over from a forest when an area is developed for housing. The forest protects the individual trees from wind stress, Swanson says, but when most of the “forest wall” is cleared and a tree is left standing alone or at the forest edge, it may not be able to withstand heavy winds.
He says that a tree missing about 30 percent of its lower branches is “approaching the scary mark.” The fewer low branches it has, the less it is able to withstand storms, because it will be top-heavy and configured badly. Wind pressure will not be distributed well over the length of the tree. In new developments, the moving of soil and reconfiguring of slopes may also leave a tree vulnerable through damaged or exposed roots. If developers would consult with arborists prior to excavation, they could save a lot of tree trouble later, he says.
A related problem is that trees are often grown in restricted spaces, such as between a sidewalk and curb, which leaves them with little space for root growth. When he examines a tree he always looks at its site. “Trees want to put their weight evenly in all directions,” he points out, and confining them can be deadly.
One sneaky problem that Swanson has noticed in his 20 years of tree observation is that soil compaction can actually make a tree susceptible to toppling. That’s particularly true in the clay soils of Georgia, where even foot traffic can compact soil around a tree. That reduces air spaces in the soil and causes the tree roots to grow closer to the surface, leaving the tree with less grip on the earth around it.
Of course, there is always the issue of tree species. Some species are simply more vulnerable to toppling. In the Southeast, Swanson says, the worst are water oaks and willow oaks, partly because they are usually old trees that have been planted in constricted areas or where the soil has later been modified. Pines can also be put in this category. The Bradford pear is a popular tree in Georgia; they have multiple trunks and are known for failing because of weak crotches.
Another factor that can be used to determine tree vulnerability is its placement relative to the prevailing winds of the region. Swanson says that this can be a “compounding factor” if a tree’s roots are restricted along the wind direction. Shortened roots on the windward side may not give the tree enough hold, and shortened roots on the leeward side can leave it without enough of a compression plate.
Nelda Matheny says that when assessing a tree as an uprooting hazard, she always starts with the big picture and works her way closer to the tree. She observes the configuration of the canopy and the tree’s weight distribution. Is it leaning? Does it have an asymmetrical limb structure? Are there codominant trunks with one overweighing another, and do any of them have cavities?
“There are certain structural characteristics we look for,” she says, because trees have evolved to support their own weight under normal weather conditions. However, in severe weather, and with unbalanced structure, they can fail or uproot easily. This can be difficult to predict, but there are indicators. Other big-picture factors are the location of the tree and its exposure to severe winds, such as on top of a ridge. Soil depth is also a factor, as is soil composition.
Matheny, owner of HortScience, Inc. in Pleasanton, California, and a certified consulting arborist, says she then works her way down the tree. She looks for decay in the trunk and bark buckling. Decay can mean a structural problem, and bark buckling on the downward side of a leaning tree is a good indicator that the bark is being compressed on that side. There may be an offsetting bark pop off on the opposite side. This indicates that the tree is being compressed unnaturally in the direction of eventual fall.
Evidence can also be seen in the soil. She says that many uprootings occur during storms after heavy rains and wind, and cracks can appear in the soil to indicate soil slippage due to root movement.
“The roots can’t hold in the soil. The cracks will form near the trunk,” Matheny says. Uprooting is more likely when a tree has a narrow base and/or no buttress roots to give its trunk strength and good weight distribution. When several of these “aggravating factors” are apparent, it is an indication that a tree could be headed for a fall, because it usually takes several factors to make a tree uproot.
Matheny says that species like sweet gum, which she has done research on, seem to be stronger in regions where there are severe conditions like high winds or heavy snow. Trees adapt over time to their environmental needs, but that species, when planted in a benign climate, may be weaker and not able to respond to sudden, severe pressure.
In the book that Matheny wrote with her partner James Clark, “A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas,” she lists many hazards. Uprooting is only one, but it is one that involves a lot of risk for a property owner, and for an arborist evaluating it. Trees that fall are usually large ones, which can do a lot of damage, and so the risk is significant.
Some of the species that tend to uproot in her urban area, east of San Francisco Bay, are valley oak and coast oak, as well as Deodar cedar and Italian stone pine, but there are usually human factors involved. The cedars generally fall after their roots are cut, the pines fall because of poor roots right from the nursery, and the oaks often uproot when their roots are diseased. In parks and campgrounds, trees that topple are often pines that are diseased. In the wild, there can be situations like stilt root, which is when a tree grows on top of something like a rotted tree.
Finally, an arborist should be aware of the climatic patterns in his area. Elements such as prevailing winds or snow should be factored into the equation, and trees at higher elevations are subject to more winds. Even mild areas, like coastal California, can have events that dump huge amounts of rain onto a region. That is a time when the trees that are susceptible to uprooting will topple. Each region will have its own conditions where uprooting will become more probable.
There are limited measures that can reduce the chances of trees uprooting, such as pruning an asymmetrical tree, but Swanson says the most common measure for a severely compromised tree is to remove it. That is often the only way to make sure people and property are protected, because a tree with several aggravating factors is a time bomb waiting to happen.
Editor’s note: This story was originally published in May 2008 and has been updated.