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Why Trees Fall — and How You Can Help!

As magnificent, beneficial, and beloved as Atlanta’s dense canopy is, the beautiful trees that shade our streets can pose potential dangers in the midst of heavy rainfall and strong winds, as we saw earlier this week when Tropical Storm Irma swept by. While the sheer force of a strong wind can uproot a tree (called windthrow), there are characteristics specific to urban trees that can affect their susceptibility to windthrow or other weather-related damage. Read on to learn a bit about what factors affect a tree’s stability, and what you can do to strengthen and nurture your trees to best withstand a storm!

That Tree Sure Is Tall…

A tree’s height, age, and canopy might seem the most immediately concerning and visible factors affecting its stability in a storm, but they needn’t be!  A large canopy is better able to absorb and distribute force from strong winds. An older tree might be better adjusted to weather conditions than younger, establishing trees. In addition to a tree’s mass and height, other major factors affecting a tree’s stability include its root structure and the condition of the soil within which it grows.

It’s All About the Roots!

You may be surprised to learn that the majority of our trees’ roots grow in the upper 18 inches of soil. These roots grow laterally, providing strength to their trees while absorbing nutrients not easily accessed in deeper, clay-heavy soil. In spite of efforts taken to protect a tree’s “critical root zone” during construction, its root system may still be damaged by soil compaction, changes to water availability, and severance of far-reaching lateral roots. Should a tree be damaged close to the trunk, not only will any descending roots be adversely affected, but the wide, supportive platform of these lateral roots will suffer as well.

What You Can Do

Mulch, mulch, mulch! Mulching not only helps to provide nutrients to and retain water in the soil, it can reduce soil compaction as well, all things good for our trees’ roots! Be sure not to apply too much mulch (two to four inches is ideal) and leave some room at the base of the tree. If you know a the area above or nearby a tree’s critical root zone will be disturbed (by construction, heavy foot traffic, etc.), lay down cardboard, plywood boards, or — again! — mulch under the tree to distribute the weight.

Soil Quality Matters, Too!

Soil condition affects not only the general health and strength of a tree’s roots, but also the soil’s strength in keeping the roots firmly grounded. As mentioned above, compacted, clay-heavy soils hamper a tree’s capacity for putting out strong, deeper descending roots. Poor drainage can cause roots to rot or develop fungal disease, weakening the overall strength of the system. Roots are more likely to slip through waterlogged soil, but it need not rain heavily for soil to retain damaging amounts of water. Pooling water due to poor drainage can weaken the soil at the base of a tree, effectively imitating conditions of heavy rainfall.

What You Can Do

To tackle drainage issues around your trees, start by amending the planting area with organic matter to improve the quality of our clay-heavy soil. Clay soils are notoriously dense, compact, and water retentive. Adding organic matter helps to break up the tightly-packed clay to make room for air and water to move in the soil. Not only that, organic matter is a source of important nitrogen and helps soil retain nutrients until the tree or plant is ready to use them.

Though the threat of falling trees can be nerve-wracking, taking care to properly maintain and manage urban trees reduces the likelihood of sudden, unexpected tree failure in a storm. We believe the benefits of trees in urban areas far outweigh the risks!

Check out our updated resource guide for dealing with fallen trees and debris after Irma.
Learn how to care for your tree, including best watering, pruning, and mulching practices.

Sources
https://blogs.scientificamerican.com/guest-blog/why-do-trees-topple-in-a-storm/
https://en.wikipedia.org/wiki/Windthrow
http://www.atlantatreecommission.com/a/index.php?option=com_content&view=article&id=58&Itemid=85

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