All living things behave as though they want to continue living. Plants use a variety of strategies to acquire resources and avoid hazards.
One hazard faced by plants – especially large, long-lived plants like trees – is falling over. Trees have three main strategies for avoiding this life-ending event.
The first, and most well-known, is that trees are able to move with the wind. By bending, instead of standing stiffly, trees are able to absorb the force of the wind without being harmed.
Being flexible, however, comes at a cost of structural strength. A large tree cannot have a flexible trunk, because it would not be able to hold itself up. Thus, trees employ a second strategy in parts of their structure that need to be strong. Over its lifetime, a tree learns which way the wind usually comes from, and strengthens itself in strategic locations. It does this by producing extra cellulose – the tough material that distinguishes plant cells from animal cells.
Finally, trees protect themselves from falling by tightly gripping the soil with their roots. Anyone who has ever tried to pull up a dandelion, or even a handful of turf grass, knows how tenaciously roots hold a plant in place.
A plant’s root system can be the same size as, or even larger than, the aboveground part of the plant. This provides a powerful counterbalance to any forces that may be trying to tip the plant over. Thus, when arborists say that a tree is unbalanced, they are just trying to scare homeowners into paying them money.
Even the healthiest, most structurally sound tree could fall over in a severe storm. Conversely, it is very rare for a tree to suddenly topple on a clear day. Overall, about 30 Americans a year are killed by falling trees or branches – a vanishingly small risk in exchange for all the benefits trees provide, even after they’re dead.