The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries from a Secret World

Wohlleben argues that light is even more important than moisture and soil for a trees’ health. He calls tall trees, such as beech, fir, and spruce trees, the “executive offices” of the forest since they dominate the canopy and therefore receive the most of this precious resource (162). He explains that these trees can outcompete other plants for light by devoting an immense amount of time and energy into growing huge, tall trunks that elevate them above the other forest vegetation. Wohlleben then transitions to examining how other plants have evolved to survive on the little light they receive in the forest understory. Some of these forest floor species include wood anemones and liverwort, which bloom early in the spring before deciduous trees can shade them with their new leaf growth.

Other understory plants include ivy and honeysuckle which can develop into parasitic plants by growing up trees and slowly strangling them and blocking their access to light. Mistletoe can also use trees for their own advantage: when birds deposit their seeds in tree canopies they sprout on branches and use their nutrients to grow, which Wohlleben compares to “bloodletting” (166). Moss also grows on trees but has a more benign presence since they do not take any nutrients from the tree, instead living off rain water droplets and air moisture and the forest’s airborne dust. Wohlleben continues by analyzing lichen, which are a “symbiotic combination of fungi and algae” (168). These plants also grow on wood and can grow very high into the tree branches, though they advance over the tree extremely slowly. Like mosses, lichen are a benign presence in the forest that does not affect the trees’ health in any way.

In this chapter Wohlleben discusses the factors that can prevent urban trees, or “street kids” as he calls them, from thriving as much as their forest counterparts.

He examines a number of factors which negatively affect urban trees’ long-term growth and health. These include being planted outside of their native region, having their roots aggressively pruned before planting depleted and compacted soil, and being overindulged with watering when young. He also points out that being planted in isolation away from trees of the same species means that urban trees cannot develop symbiotic relationships with neighbor trees.

Another risk to urban trees’ health is their poor development of root systems. While their growth above the ground may seem impressive, if their roots are shallow they are more likely to be blown over by a storm. The author explains that these urban trees are also likely to have their growth manipulated by gardeners. Their lower branches are often cut off to maintain a certain aesthetic or view in a city park. Indeed, the author explains that even their crowns are sometimes shorn; this interference is dangerous for the tree. Without the ability to photosynthesize as much as it is accustomed to, parts of the tree will “starve” and become food for parasitic fungi and bacteria which will slowly kill the rest of the tree.

Wohlleben analyzes some of the issues faced by trees planted on city streets. Because the soil underneath city streets is even more compacted, or worse, nonexistent, street trees have a very difficult time developing healthy root systems. In their desperate search for ideal soil, trees like maples and lindens can discover and infiltrate water pipes, causing plumbing issues for the city.

Unfortunately for these city trees, their growth is also stunted by the urban environment’s hot, dry air in the summer, air pollution, dog urine, winter salting, and the absence of beneficial mycorrhizal fungi. Similarly to the trees in urban parks or forests, when these trees are weakened by these unfavorable conditions they are likely to be infested with malicious bacteria, fungi, or insects, and die prematurely.

Wohlleben introduces pioneer tree species, which “prefer to strike it out on their own” (180), since they actively want to leave their parent tree and begin a new forest elsewhere. These pioneer trees have seeds which are designed to be easily airborne and carried on the winds. Some examples of these trees are poplars, quaking aspen, pussy willows, and silver birch, which can create forests in only ten years of growth.

These trees face their own set of unique challenges. One of these is being grazed by deer or horses, since they also prefer meadow environments. Pioneer species protect themselves from grazers by growing quickly and developing bark that is rough and thick. They also infuse this bark with oil whose taste is repellent to deer. Silver birches and quaking aspen have special adaptations developed to counter predators and grow efficiently. For example, quaking aspen can survive being eaten by deer by expanding their root system from which they grow new shoots. This means that what appears to be many trees can in fact be one organism with many trunks. Indeed, Wohlleben notes that there is a quaking aspen grove in Utah which has 40,000 trunks covering 100 acres of land, and yet is a single organism.

As this chapter’s title suggests, once these pioneer species are only a few decades old, however, they can experience burnout. They become exhausted from the hard work of living and worse yet, may have competitor trees growing in their shade. Over time these competitor trees will overtake the 80-foot poplars or birches and they will die soon after. Despite their early deaths these pioneer trees have fulfilled their goal of releasing their own seeds to the wind.