Anytime there is a large amount of rainfall for an extended period of time, trees toppling can become a concern, especially if combined with wind.
Structure of Trees
Trees are an engineering wonder suspending tens of thousands of pounds of wood, branches and leaves. Normally, this weight is translated down the trunk of the tree and into the large structural roots, called the root plate. This supports the tree from both vertical (gravity) and horizontal forces (wind).
To better visualize the load-bearing structure of trees, imagine a wine glass sitting on a dinner plate. In this analogy, the top of the wine class is the tree’s canopy, the stem is the trunk and the base is the structural root plate providing stability. The dinner plate, the rest of the tree’s roots, are there to absorb water and minerals to “feed” the tree.
Trees are anchored to the soil by their roots. When the wind blows the canopy, it moves and the trunk acts as a lever translating the forces into the roots. The roots push and pull against the soil and the friction between the roots and soil keep trees upright.
In extreme weather events, when the soil is completely saturated and the wind is blowing, the movement of the tree causes the wet soil to take on a gravy-like consistency. Once this begins to happen, the tree’s roots are no longer held by the soil, but are rather slipping and sliding through it. At this point, if the wind is strong enough, the whole tree might topple.
Factors of Tree Failure
Mismanagement of Landscape Trees
Mismanagement of trees in a landscape can seriously aggravate and increase the risk of whole tree failures. Trees are more likely to fall-over if they’ve had roots cut or damaged. Root removal within 8 to 10 feet of the trunk, has a major impact on tree stability. If trenching of any kind (home foundations, utilities, driveways, sidewalks, irrigation, etc.) has occurred near trunks, roots were severed and the risk is greater. Remember the wine glass analogy, if half or even a third of the base is cut, how stable will the wine glass be? Less roots, equals less holding power for the tree.
Root decay is another factor that aggravates tree stability. If roots are cut or damaged, they become susceptible to rot and decay, further eroding stability. Root decay can progress over a number of years, working its way from the damaged areas towards the base of the tree. Often the only signs of decay are dead branches along the edges of the canopy or mushrooms (the decay pathogen’s fruiting bodies) on the lower trunk or exposed roots.
Trees in Compacted Soils
Large trees growing on heavily compacted soils are also good candidates for toppling in wet, windy weather. Tree roots can only grow in soils that have space for oxygen. The more compacted a soil is, the less pore space for oxygen and the shallower the root system. Shallow roots plates do not provide strong anchoring and are more susceptible to saturated, soupy surface soils.
Trees in Tight Spaces
Similar to compacted soils, trees growing in tight spaces are also primed for toppling. Roots need ample space to spread and provide a radially distributed root system. Trees in tight spaces can develop uneven or one-sided root systems (remember wine glass base). Examples include large trees planted in narrow street medians, parking lot islands or between sidewalks and streets. Soils in these confined areas also tend to be heavily compacted, further aggravating the problem.
Most of these root defects are preventable. Remember, don’t cut large roots and regularly look for signs of rot and decay. If planting trees in compacted soils or in confined spaces, select small to medium size trees more suited to those environments.
Watch for Leaning Trees
Prior to toppling, some trees might exhibit a lean as the adhesion between soil and roots weakens. Leans can be identified by heaving or mounding soil near the base of the tree on the opposite side of the lean. This is caused by the root plate pulling out of the soil and pushing soil upward. In extreme cases, the mounding soil might have cracks and exposed roots. On the inside of the lean, where the root plate pushes down into the soil, a depression around the base of the tree can sometimes be observed.
These trees should be considered dangerous and professional assistance should be sought immediately. It is not uncommon to see trees topple in the days following heavy rains. If concerned about existing trees contact an International Society of Arboriculture (ISA) Certified Arborists. Many of these professional Arborists are Tree Risk Assessment Qualified and can help evaluate tree risk. To find an arborist in your area, visit TreesAreGood.org.