Ninety percent of tree roots exist in the top 18 inches of soil, and ninety percent of those roots exist in the top 4 inches. This means that tree roots are very shallow and the compression plate of a big tree (the structural root plate that bears the weight of the tree is less than 10 percent of the tree’s total roots) may be stunted, causing a top-heavy tree to be susceptible to tipping.
A tree tugs on roots on one side and presses down on roots on the other side as wind pushes against it. In very wet soil, and if winds are strong enough, the tree can uproot or fall over. If trees have other hindering issues they can even uproot in light or no rain and lighter winds. The first thing to look for when trying to determine if a tree is in danger of falling is disease in the roots. In particular, look for the fruiting structures of decay fungi, such as Ganoderma root rot (http://en.wikipedia.org/wiki/Ganoderma) 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 fungi diseases, the International Society of Arboriculture publishes a guide to decay fungi, and they all, if it has been eating away at the tree for years, can mean that the tree’s root system has been injured and can’t withstand shear and compression pressures.
The age and health of a tree has a lot to do with how resistant it is to these decay fungi. It may take many years for a fungi to severely damage a young, healthy tree’s roots, however it may only take a season to damage an old tree weakened by several pathogens. It is important to inform us the tree’s history and age when inquiring about health issues.
Construction or other disturbances taking place around the tree may damage roots, such as trenching, excavation, root pruning or the application of chemicals around the tree. It is also important to think back many years into the tree’s history because it may take 5 or 10 years for a disturbance to damage roots enough to cause toppling.
Health and structure are two different issues. A tree may be healthy, but also may have some hidden structural problem(s) that could lead to failure. The tree’s roots may be fine, but a hollow core or decay in limbs could lead to failure in a storm. When examining a tree it is good practice to inspect old pruning cuts and sometimes to even 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. Also when considering the building of new developments or commercial or industrial complexes, trees that are left from a once full forest are no longer protected from the forest walls from the brunt of winds, leaving them prone to failure because their root structures are not adapted to the changed environment.
Trees missing about 30 percent of its lower branches is approaching the frightening 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.
A related problem is that trees are often grown in restricted spaces, such as between a sidewalk and a curb, which leaves them with little space for root growth. Trees want to put their weight evenly in all directions, and confining them can be deadly.
Soil compaction in many cases can actually lead to tree toppling and is particularly true in clay soils found in our region. The compaction 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.
There is also the issue of species. Simply there are just some species that are more vulnerable to toppling than others. There are too many species to list, but in general the species that have shallower root systems are more vulnerable.
Another facture that can be used to determine tree vulnerability is its placement relative to the prevailing winds of the region. This can be a compounding factor if a tree’s roots are restricted along the wing 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.
When assessing a tree as an uprooting hazard you must start with the big picture and work your way closer to the tree. Observe the configuration of the canopy and the tree’s weight distribution. Is it leaning? Does it have an asymmetrical limb structure? Are there co-dominant trucks with one over-weighing another and do any of them have cavities?
“There are certain structural characteristics we look for”, say Matt Wilson(Owner of Good 2 Go Lawn Care), because trees have evolved to support their own weight under normal weather conditions. However, in severe weather conditions and with unbalanced structure, they can fail or uproot easily. This can be difficult to predict, but there are indicators. Some big picture factors are the location of the tree and its exposure to severe winds, such as on top of a ridge, soil depth and soil compaction. Cracks can also appear in the soil around the base of the tree close to the trunk, which indicate soil slippage due to root movement. It usually takes several of these factors to make a tree uproot.
The trees that normally uproot are usually large ones, which can do a lot of damage, so the risk is very significant. The most common measure for a severely compromised tree(s) 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 ticking time bomb.