When an intense tropical storm passes over a forest it leaves destruction in its wake. Post-damage surveys often show that the tall trees are disproportionately killed in these events. However, it is very difficult to attribute the cause of death of a large tree after the event. A tree may be snapped and lying on the ground, but it could have been killed years before the storm by lightning, or fundamentally weakened by disease. Therefore, we set out to answer a simple question: do tall trees have a higher risk of wind damage?
Tall trees are exposed to higher wind speeds, but they also have wider trunks. The resistance to bending for a cylindrical beam is proportional to the cube of its diameter (this is why hollow scaffolding poles are so strong). We didn’t know a priori which of these two factors would prove the most important, so we took some measurements.
Figure 1 – The experimental setup. Left – Unding bin Jami attaching wind sensors to trees. Top right – looking up a 95 m tall tree (1 m shorter than Big Ben!). Bottom right – strain gauge sensor attached to the trunk of a large tree + an unwelcome visitor.
We measured local wind speeds and the bending stress on the trunks of 17 large trees (you have to start somewhere!). These two measurements were highly correlated and related to each other by a power law. We then compare the maximum bending stress to the breaking stress, which is different
for each species but is well known for the tall timber species in our study. This gave us a measure of wind damage risk for each tree.
Sure enough, we found that the taller trees did have a higher risk of wind damage. This means that the wider trunks of tall trees were not sufficient to compensate for their increased exposure to strong winds. This agrees with previous work in conifer forests and suggests that wind is likely to be an important cause of death for tall trees. The next steps are (1) to increase the sample size and test in different forests and (2) track tall trees over time to see how many are killed during storms.
Link to the paper: https://onlinelibrary.wiley.com/doi/10.1111/btp.12850
Link to the nature highlight: https://www.nature.com/articles/d41586-020-02911-3