Wohlleben focuses on the types of pest attacks and illnesses trees may endure throughout their lives. He begins by explaining that it can be difficult to predict the lifespan of a tree. While healthy trees could live for up to 400 or 500 years, they rely on the “stability of the forest ecosystem” to make this possible (155). If factors such as temperature, light and moisture change suddenly, it can be difficult for trees to adjust and thrive in their new conditions. Wohlleben adds that parasitic species of fungi and bacteria are a constant threat to trees’ health as well. He continues by explaining that trees remain healthy by rationing their energy use into these categories: breathing, processing food, feeding beneficial fungal partners, and growth. It also stores energy for emergency defense from pests. These “secret reserves” of energy can be used whenever necessary and sometimes have “defensive compounds” that are special to each species of tree (155).

Wohlleben explores how even seemingly beneficial changes to the forest environment, such as suddenly receiving more light to fuel fast growth, can have harmful consequences for trees. He explains that this is due to how environmental changes can induce trees to stop carefully balancing their energy.