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

Wohlleben claims that trees’ “water transport” has been under researched for decades since many scientists offer “simplistic explanations” of this seemingly straightforward phenomenon. The author then examines capillary action, transpiration, and osmosis, the three ways trees can move water from their roots to their branches.

In capillary action, trees’ narrow roots help water “rise against gravity” (56). However, even in very narrow roots this phenomenon could only propel the water three feet high. The author then explains that transpiration, when the tree exhales moisture through its leaves or needles, also helps transport water by creating suction that draws water up from its roots. Finally osmosis helps move water in trees’ leaves and roots, because the water will naturally flow to more sugary neighbor cells.

Here Wohlleben examines a mystery in trees’ movement of water. He observes that trees’ water pressure peaks in the spring, noting that by applying a stethoscope to a tree’s trunk you can even hear the water moving upwards. However, the forceful movement of water at this time of year has not been fully explained. The author rules out transpiration since deciduous trees do not yet have leaves and cannot transpire, and capillary action only moves the water three feet. Osmosis would also not be responsible, since it is only effective in the tree’s leaves and roots. The author admits that scientists know little about this phenomenon, but shares that researchers listened to trees’ trunks and heard a “soft murmur” at night, when trees are very full of water. They attributed these murmurs to the presence of carbon dioxide bubbles in the trunk’s water transport tubes. Wohlleben ends this chapter by concluding that scientists may have to completely rewrite their theories of trees’ water transport once more research is conducted.

In this chapter, Wohlleben focuses on tree bark. He begins by explaining how, just like human skin, bark holds in its fluids and other contents, blocks harmful pathogens from entering its system, and lets out and absorbs moisture and gasses from the air.

The author furthers this explanation by noting that just as humans shed skin cells and grow new ones, trees also shed their bark and continually repair and expand it. Some species flake more than others, and as they age trees’ bark develops wrinkles as well. Wohlleben writes that all trees begin their lives with smooth bark which then wrinkles over time, beginning from the bottom of the trunk. The author notes that birch trees, oak trees, pine trees and Douglas fir trees wrinkle quite young, while silver firs and beech trees remain smooth for longer. He shares that the UV radiation from sunlight can cause trees to wrinkle more substantially, similar to its effect on human skin. Because these wrinkles are a natural and predictable aspect of trees’ aging process, observing bark is another way to determine age. Wohlleben concludes his survey of bark by revealing that it can also be affected by certain pests and diseases, such as bark flies or bacteria.

The author then informs the reader about old trees’ unique contributions to the forest. He cites a study by Dr. Zoe Lindo who examined old growth Sitka spruce trees in British Columbia that were over 500 years old. She found that over these trees’ mossy branches had been colonized by blue-green algae, which absorbed nitrogen from the air. When rainfall caused this nitrogen to fall to the soil, it fertilized the roots of these old trees and the younger trees around them.

According to Wohlleben once a tree has reached a certain age, which could vary from one hundred to three hundred years depending on the species, its new growth is extremely short. Old trees also stop growing in height and only expand in width. As they reach the end of their lives they cannot maintain their outermost twigs, which become brittle and are swept away by the winds, followed by the rest of their branches. Wohlleben explains that after parasitic fungi have successfully invaded these open wounds, it will consume the tree’s stored sugar or even its cellulose and lignin. Eventually, this weakened structure will collapse, though the author hints that even dead trees will remain a productive part of the forest ecosystem for hundreds more years.

While trees are capable of cooperation, some will compete and try to weaken their neighbors. Beech trees in central Europe will “harass other species, such as oak, to such an extent that they weaken” (68). Because Beech trees are dominant in central European forests, their seedlings sometimes encroach upon an oak’s growing space, slowly crowding it out and claiming its spot in the sunlight. Panicked, the oak then sprouts new shoots and leaves at the bottom of its trunk, trying to create more opportunities for photosynthesis. This doomed strategy leads Wohlleben to ask if the oak—considered to be possess great strength—is a “wimp”?

The author transitions to examining Oak trees’ incredible strength and resilience when they grow away from such tough competition. He notes that while beech trees can live only a couple hundred years in isolation, the resilient oak can live for over five hundred years. They can also recuperate from storm damage because of their tannins, which repel the harmful bacteria and fungi that try to invade their exposed wood. Oaks can even regrow branches that were torn away by storms. Wohlleben shares that in the forest he manages, oaks prove their resilience by growing in the most deprived conditions.