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The skies have been sunny for days, but the tumult of last week’s storms have caused some Southern California trees to uproot in soil-eroded neighborhoods. Have you ever wondered why some trees fall over during a storm while others remain standing? This week, we’ll get to the root of the question!

After several days of heavy rainfall, HWF Founders Alex Havasi and Marilyn Fordney encountered the fallen tree photographed above just after it crushed a car parked in its periphery.  The fallen oak is presumed to be over 100 years old and its impressive weight caused the unoccupied vehicle to buckle and crack. Shattered glass and bent metal are all that is left of the Chevy truck which proved no match for the giant oak.

Falling trees are commonplace during stormy seasons and can wreak havoc on houses, fences, and vehicles parked beneath their limbs, which begs the question:

What causes a tree to uproot and fall?

According to experts, when a tree falls the roots are usually to blame.  In the plant kingdom, roots have several important jobs including the transportation of water and nutrients, food storage (think edible roots like carrots and potatoes), and stabilization. When it comes to stabilization, roots are notoriously thought of as anchors for plants and trees, but they also provide stability for the surrounding soils and help prevent erosion or soil movement.

During severe wind storms, the leaves and branches that make up the tree canopy can act like a sail in the wind. As excessive winds pass through an area, they get caught in the canopy and trees begin to sway. When the surrounding soil is wet, a tree whose roots have difficulty anchoring themselves securely in the soil may be uprooted by the high winds. Though rotted, shallow, damaged, and young roots are at an increased risk for uprooting, any tree is susceptible to a fall during violent weather.

Oaks growing near a man-made path. When concrete is poured near the root system of a tree, roots can suffocate over time and eventually, the tree may fall.

Wild oaks need ample space for root growth since their roots can extend up to seven times the length of their crown.

The Anatomy of a Root System

Plants and trees generally conform to one of two root systems— the taproot system in which one large, vertical root branches out into smaller horizontal roots, or the fibrous root system in which many thin roots branch out to form a mat undergrown. Oak trees like the one above start their lives as acorns. When the acorn first sprouts, most of its energy is spent on root development and very little growth is seen above ground. A taproot plunges deep into the ground in search of reliable moisture and once access to moisture is secured, leaf and branch growth can begin.  A taproot cannot deliver all of the nutrients that the budding oak tree needs, so a secondary web of roots known as the lateral roots begins their horizontal growth. Lateral roots provide the tree with moisture and nutrients for its lifetime and help anchor trees and soil in place. Though fibrous root systems are most effective at preventing soil erosion, the lateral roots in a taproot system can prevent soil movement when there are many of them.

A plant’s health is very closely tied to its roots and a healthy tree’s roots will continue to grow throughout its life. Since the lifespan for trees varies from decades to millennia, the size of different root systems also varies. Typically, a tree’s roots will occupy an area two to four times the size of its crown (the crown is comprised of the branches that extend from the trunk) but many trees have root systems that extend up to seven times the diameter of their crown! According to encyclopedia.com, the longest known tree roots belong to a species of South African fig whose roots are buried at least 400 feet deep.

As humans expand and build their homes in wild spaces, the root systems of the trees that remain can become damaged. In some cases, contractors cut roots to avoid interference with pipes or construction plans.  Similarly, when asphalt or concrete is spread, nearby soil can become compacted and impede the exchange of gases, thereby damaging the roots. Critical root zones that have been cut or smothered by concrete or compacted soil are more likely to give way during a storm.  Take a look at the photo below and you will notice that this giant oak was growing very close to a paved road and concrete path. It is likely that some root damage may have occurred during urban expansion.

Reduce The Risk of Uprooting

Water Slowly Over a Long Period of Time- Frequent watering in short spurts encourages roots to grow closer to the surface and makes deep anchoring less likely. Instead, use a drip irrigation hose or turn your water down low and allow it to penetrate the soil for 2-3 hours.

Don’t Over Water- Overwatering can cause root rot which in turn can make your tree susceptible to falling. Ideally, you want the soil around your tree to be moist but not wet enough to form a ball of mud.

Don’t Over Mulch- Mulching is great for your trees because it locks in moisture and encourages the growth of microbial life (a win-win). However, over mulching can cause suffocate roots or cause shallow roots to grow, so keep mulch away from your tree’s base and only spread 2-4 inches deep.

If you have a relative or friend who loved animals and/or had pets, make a memorial contribution in their name HWF and help our program extend to more elementary school students who want to go on field trips into the wilderness and learn more about nature, wild animals, and our environment. 

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