The phenomenon known as "windthrow," which uproots trees, is cited by all three experts as a major cause. "Because the tree trunk functions as a lever, the force imparted to the roots and trunk rises with height," Foster explains. "Shorter trees are more vulnerable to windthrow." The weight of the higher branches of taller trees creates greater tension on their trunks, which makes them more likely to break under the pressure of high winds.
Tall trees have thicker trunks that are harder to bend. This means they can withstand windstorms that would kill a thin tree. However, even though they are stronger, they are also heavier, which increases the stress on their trunks and makes them more likely to fracture under the force of strong winds.
Trees grow upward because gravity pulls water and nutrients down toward the ground. If they were to grow straight out from the ground, they would eventually reach the sky and become a skyscraper. Because trees must bend to grow, they use their branches like arms to grab at sunlight and rainwater while avoiding being pulled over by competing limbs or being blown over by strong winds.
Tall trees have wider bases than shorter trees, which gives them better stability when standing alone. But this also makes them more likely to fall over in high winds. Wider bases result from large root systems that spread out far beneath the surface of the soil.
The tree trunk works as a lever between the fulcrum root structure and the treetop canopy in high winds. As trunk height grows, the lever effect becomes more forceful, perhaps leading to tree uprooting. However, heavy trees with large canopies may have their own protection against wind damage. They tend to grow larger trunks that are well-anchored in deep soil.
Trees will often send out new shoots from their roots or nearby branches in order to reach sunlight and obtain water. These sprouts will grow into new trees if given enough time. Sometimes, though, the stress of not being able to reach sunlight quickly enough causes them to die back to the ground where they are destroyed by fire or crushed by animals. This is called "shoot dormancy."
Trees also go through natural death known as "death by old age." An aging tree may become weak and likely to fall during a strong wind or other disaster. When this happens there is no way for another tree to take its place so it is gone forever. Humans also cause death by old age by removing trees from their landscapes. If a tree falls across a road or power line, for example, it can be killed by accident.
Trees also die when they are hit by lightning or burned by fire.
The scientists speculated that because of gravity's downward pull, it may become difficult for a tree to lift water all the way to the top as it becomes older and taller. Gravity's pull diminishes turgor, which might explain why trees cease growing at a certain height. The ability of plants to generate turgor pressure in their cells is important in order to counterbalance the force of gravity.
Trees stop growing when they reach the top of their habitat or when they are deliberately thinned by farmers who want to increase the amount of sunlight that reaches the ground beneath the trees (this tactic is used to encourage new growth). Trees will also stop growing if they are subjected to environmental stresses such as drought, heat, cold, wind, and lack of light. Environmental stresses can cause trees to produce smaller leaves and less food per unit area, which leads to lower rates of growth. However, trees can recover from these stresses; for example, if weather conditions improve and the tree is able to gain back its lost moisture, it will once again grow larger leaves and produce more food.
Trees also stop growing if they are grown under commercial circumstances. When trees are grown for timber production, there is usually a limit on how high they can be grown because they need to remain at an appropriate distance from the ground in order to allow people to cut them without risk of injury.