Healthy trees are flexible and resilient, so it would be normal to think that it takes a lot for branches to break and even more to uproot trees. The effects of wind speed on trees have been studied and have thrown up some interesting conclusions and arguments. According to the scale presented below (see “Perspective”), winds of less than 54mph will cause a range of damage, whereas wind speeds of 55-63 mph will uproot trees. An important part of understanding the impact of wind on trees is to measure wind speed in miles per second. Those measurements appear on the scale below as well.
What Wind Speed Is Dangerous for Trees?
During a storm wind speeds can reach up to 42 m/s, or 90 mph. This is when most tree trunks will break, regardless of a tree’s elasticity, height, or diameter.
Strong Wind Danger
There are two main ways for a tree to break. In the rain-soaked ground, roots can rot and the wind will uproot the tree. If roots hold, then the trunk will break as a result of torsion and being bent. This is referred to as stem lodging.
Research
Scientists have found that just about all trees break at just about the same wind speed. The reason for this appears to be because of tree mechanics and biology. These include tree ratios of elasticity, tree shape, and fracture characteristics. These factors outweigh tree species, size, and age. That said, some scientists believe that tree height does play a role. Others argue that because trees are living entities, they are constantly adjusting to their environment and acclimatizing. Therefore, trees have a degree of autonomy when it comes to wind resistance and complete branch shredding, thereby undermining the sameness research a little. Scientists argue that trees are not passive engineering structures.
Storm Snapping, Bough Breaking Wind
After the 2009 cyclone that created largescale tree damage in Europe, it was determined that the most damage was caused by sustained winds in excess of 42 m/s. The winds damaged both softwood and hardwood trees of all ages. This led researchers to investigate wood resistance. Although the experiments suggest that short, thin trees are as strong as tall, thick trees, researchers were unable to reach a consensus or definitive conclusion.
Experiments
An experiment was conducted using horizontal beech rods, beech being representative of average wood properties, with only mechanical differences in the different beech species. One end of the rod was fixed and increasing weight was applied to the other end to test curvature until the rods broke. It was observed that most of the curvature occurred at the fixed end and that diameter and rod related to the critical curvature radius. This seemed the case with all the rods, despite different thicknesses and lengths.
The weight was then replaced with model wind gusts. One factor considered was that trees triple in diameter when doubling their height. This experiment showed that critical, damage-causing high wind speeds are only marginally dependent on the physical dimensions of a tree. Height only increased the wind speed requirement for breakage by 9 percent.
Perspective
Here is a more formal definition of wind categories and descriptions, thanks to the Beaufort Wind Scale, in both miles per hour (mph) and miles per second (m/s).
- Calm: Less than 1 mph. 0 m/s. Smoke rises vertically.
- Light air: 1-3 mph. 0.5-1.5 m/s. Smoke drifts.
- Light breeze: 4-7 mph. 2-3 m/s. The wind is felt on your face. Leaves rustle.
- Gentle breeze: 8-12 mph. 2-3 m/s. Leaves and small twigs move. Light flags extend.
- Moderate breeze: 13-18 mph. 5.5-8 m/s. Small branches sway, and dust and loose paper blow about.
- Fresh breeze: 19-24 mph. 8.5-10.5 m/s. Small trees sway. Waves break on inland water.
- Strong breeze: 25-31 mph. 11-13.5 m/s. Large branches sway. Umbrellas are difficult to use.
- Moderate gale: 32-38 mph. 14-16.5 m/s. Whole trees sway. Difficult to walk against the wind.
- Fresh gale: 39-46 mph. 17-20 m/s. Twigs break off. Very difficult to walk against the wind.
- Strong gale: 47-54 mph. 20.5-23.5 m/s. Slight building damage. Shingles blow of the roof.
- Whole gale: 55-63 mph. 24-27 m/s. Trees uproot. Considerable building damage.
- Storm: 63-73 mph. 28-31.5 m/s. Widespread damage.
- Hurricane: over 73 mph. Over 32 m/s. Violent destruction.
Additional Reading: Tree First Aid After a Storm – Arbor Day Foundation
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