Why Trees Fall?

White Oak tree toppled over house in Atlanta

Trees eventually fall down but when and why?

Many “urban” trees do not have perfect form, which can be caused by above- or below-ground imbalance. The International Society of Arboriculture defines “urban trees” as being planted, rather than just growing naturally like the trees planted along roads and streets, which are spaced out rather than growing closely in a more dense forest environment, which helps to shield trees from severe winds. As a result very few trees fall in wooded areas when compared to trees hanging over streets or near buildings. Trees planted too close to buildings, streets or structures will also grow crooked, one-sided roots eventually. Trees that do not receive even or sufficient light will grow in a lopsided way, straining to grab sunlight. This ruins their natural symmetrical shape, making them easier to blow over. Tree trimming by utility companies is another hazard—often hurting the canopy and eventually the upper balance of trees.

Most trees have shallow roots.
Although trees like the ones in our historic districts seem indestructible, there is no guarantee that they will not fall down one day. We are more aware of this when it is in our front yard, whereas falling trees are a normal, frequent phenomenon in forests. The International Society of Arboriculture (ISA), explains that virtually all trees will have the bulk of their root system (95 percent), located very close to the soil surface. The roots extend far from the trunk and are typically found just 6” to 12” below ground. This is where necessary oxygen and most of the soil moisture and nutrients are located.

Roots grab into the soil, which is how it stands up. Very strong winds and especially “micro-bursts” can tip that balance once and for all, causing the tree to finally topple over. Some trees are planted in rocky soil where they just can’t grab. Yet Pine trees and Oak trees are natives that usually do well here, despite the soil. And they are able to dig deeper roots over many years because they are so slow growing. There is an excellent PDF chart here from the UK Forestry Commission listing the root depth of many common trees in the US. Of course, this means that the soil components also play a role in that anchoring—in addition to the species unique root structure.

Some trees have weak roots. Most Atlanta soil is “heavy clay” or very compacted below the 6-8” of topsoil. Trees with spongy, soft or brittle roots may not be meant to live in our climate. Non-natives like the Chinese Pistache might do very well in some soils, but should not be planted based on looks alone. It is especially important to plant trees appropriate for the Southeast—both for our climate and soil type.

Roots of some trees have been damaged. This primarily occurs where the tree roots have been cut or crushed by construction, especially from sidewalk, street, driveway, or home construction projects. David Hubbard at the Alabama Cooperative Extension System explains that trees in the urban environment are constantly recovering from damage caused by lawn mowers, weed whips, construction damage, grade changes, de-icing salts and animal damage (rabbits, squirrels and deer). This is the reason that a City of Atlanta ordinance requires fencing around trees prior to construction projects. Sometimes even that is not enough. When a major, shallow root is damaged or severed, the whole tree becomes non-symmetrical and prone to toppling.

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