First impressions review: The Hidden life of Trees, by Peter Wohlleben
This is an engagingly written book that provides a decent introduction to the current state of forest science. Now, being a forest biologist myself, my first inclination was to get nit-picky. The author states that his training as a forester was very narrow, focusing just on stuff that is supposed to produce more timber and not on ecological relationships or tree physiology, and that he rediscovered his sense of wonder about forests later in life. This does show in places, but most of the information is accurate and it is an easy and fun read.
Some of the 100% accurate forest facts include:
- Trees, and plants in general, can sense and respond to their environment, and can send out signals to communicate with other organisms.
- The root systems of trees form underground connections, sometimes directly, more often via networks of mycorrhizal fungi1. Messages and resources (eg. sugars) can be sent from one plant to another through the mycorrhizal network. This has been amusingly dubbed “the wood-wide web”.
- Many trees have genetic systems that allow them to distinguish their own pollen, or that of relatives, from the pollen of other trees, and therefore avoid producing inbred seeds. A few have separate male and female individuals, which accomplishes the same thing.
- Not only can some individual tree trunks live for hundreds or thousands of years, many trees that can sprout new stems from stumps or suckers can persist as a single genetic individual for even longer. The author mentions a spruce that is 9,550 years old at the root!
- Over thousands and millions of years, forests move. As glaciers grow and retreat, or conditions grow drier or wetter, seeds disperse and grow in places that are becoming more suitable, and old trees cease to reproduce and eventually die in places that are becoming less suitable2.
- Understory plants like wildflowers and tree seedlings often leaf out earlier in spring than the adult trees, and in fact get most of their photosynthesizing done before the canopy closes over and plunges them into the shade.
Cool, right? I love blowing my students' minds with this kind of thing in Plant Bio class!
A few statements I would classify as “partly true”, usually due to oversimplification. These include:
- "Conceivably, [the root] is where the tree's equivalent of a brain is located...we know that trees can learn. This means they must store experiences somewhere."
Yes, trees can "learn" to some degree. For instance, like many plants they can count the length of the night or how many days below freezing there have been and use this information to decide whether to leaf out or flower or how to use starch reserves. However, this "memory" probably isn't localized the way it is in animals. One bit of evidence for this is that you can regrow a plant from just a branch or just a root and they don't get total "amnesia". And the "learning" that is mostly discussed in this chapter - which involves how the growth of the trunk responds to environmental factors - would not be preserved if the trunk were cut and resprouted. That being said, roots are used for storage so to the extent that some molecule is used to form that memory they might play an important role.
- A forest would do fine without larger inhabitants like deer or wolves, but not without the microbes in the soil. If you compare a single large species (or even all of them) to ALL the microbes, then yes, this is true. However, not all microbial species are equally important, so if we did a species-to-species comparison the outcome is less clear. An oak tree needs squirrels or jays to disperse its acorns as much as it needs any single species of mycorrhizal fungus, and probably more than it needs a soil pathogen that attacks grass roots, for example.
- "In intact forests, the soil under the trees becomes deeper and richer over time so that growing conditions for trees constantly improve." The German forests that are the focus of this book are in a formerly glaciated area, and when you are going from ground up rock to forest soil the process as described is roughly correct. But if the weathering goes on a long time the soil can become less rich, with many minerals being leached away. That is how you get the nutrient-poor red clays such as are found under many tropical forests. In those forests, most of the nutrients are in the trees, not in the soil - as soon as a leaf hits the ground it decays and the plants snatch up as much nitrogen and phosphorus from it as they can.
We have a few claims that are confusingly worded in a way that doesn’t come out quite right. This could be partly a result of writing a popular science book in one language and then translating it into a second. For instance:
- "The rate of photosynthesis is the same for all the trees [in a particular forest that was studied]...all members of the same species are using light to produce the same amount of sugar per leaf...Whoever has an abundance of sugar hands some over; whoever is running short gets help." Okay. This statement sticks a bunch of physiological statements together in a way that does not make sense, but I'll try to break it down.
Photosynthesis is the process by which plants take CO2, sunlight, and water and use it to make sugars, with oxygen as a by-product. It is not at all unexpected if most individuals in a forest have the same rate of photosynthesis per leaf if those leaves are getting the same amount of sunlight. It would be surprising if an individual that was entirely shaded was photosynthesizing at the same rate as one in the sun, however, because the former has lower energy inputs. Now, even if the per-leaf rate of photosynthesis is the same, a bigger tree will be producing more total sugar because it has more leaves. And the author's statement about redistribution makes it clear that one or both of these things (unequal photosynthesis or unequal total production) is still happening. So I think what he meant to say is that the sugars available to individual trees is more equal than you might think because the sugars get moved from trees that are producing a lot to those that are producing a little. But that is definitely not the same thing as initial photosynthetic rates being equal!
- "Capillary action is what makes the surface of you coffee stand a few fractions of an inch higher than the edge." Transpiration is when water evaporates out of leaf pores and tugs water column up. But, it is said, how can trees be full-to-bursting with water in the spring when there are no leaves?
First, that example is of surface tension, not capillary action. Capillary action combines the tendency of water molecules to stick together with their tendency to stick to the walls of a tube - so water in a straw standing slightly higher than the rest of the drink would be a better example. Second, the author left out a third factor - root pressure. This is when water moves into roots through osmosis because there is less water relative to solutes (sugars, mineral ions, etc.) in the root than in the soil. While this by itself can't move water very high either, I would suspect it has a lot to do with that spring "filling". The tree is already full of water (the tubes don't empty and get refilled) and moist spring soil coupled with no leaves would mean it is taking in a lot more water than it is losing. That is not to say there are no remaining mysteries of how water moves in a tree - there are - but this leaves out some stuff we do know to make it sound even more mysterious than it is.
- The vibrations in a tree trunk that can be measured during drought are a type of communication, or a "scream". Two issues here. First, just because something makes a sound and correlates with an injury doesn't mean it is a warning. A bone breaking makes a sound, but that in itself is not communication...though if someone heard it they might proceed with caution. Second - what would a tree do with that warning? If things are dry enough that water columns are popping in neighboring trees, any other tree in the stand will have already sensed low water levels with their own roots and will have made the only short-term responses they can: closing the pores in their leaves or shedding leaves.
-We know from times of high forest mortality that it is usually the particularly battered individuals that burst into bloom. If they die their genetic legacy might disappear. We certainly do NOT know that - as the lack of citations on this statement indicate - though it is a reasonable hypothesis. My student and I actually wrote a review article proposing that, although this behavior is less likely in trees than in shorter-lived plants because investing in survival would have more long-term benefit in most situations, trees might show a reproductive pulse when the chance of death is high enough. But no study has confirmed that.
- “Despite [the arrival of invasive pests], I am not anxious when I think about the future of our forests. For on large continents…species have to come to grips with new arrivals all the time.” Yes, but at a much slower rate than occurs with humans moving stuff around, and many of the more damaging introductions have been BETWEEN continents!
A few statements are considerably iffier and would require a great deal more research to substantiate. Many of these fall into the category of anthropomorphization. This is REALLY easy to do – I myself have been known to sob over the death of a favorite tree – but at the same time trees are very different organisms than animals. It could almost be considered disrespectful to the trees to conflate our experiences to too great a degree.
Anyway, some of the claims I’d dispute include:
- Trees can feel pain. As mentioned above, trees definitely do respond to damage, but that is not the same thing. When our bodies notice an invader and activate our immune system, that doesn't usually register as pain - at least not unless the battle gets particularly fierce. Even if we are sick or injured and feeling terrible, if we fall asleep (lose consciousness) we no longer feel the pain. Plants don't have nerves, let alone a brain, so whatever they might "experience" (good or bad) would probably be very hard to translate.
- "To enter into a partnership with one of the many thousand of kinds of fungi...fungal threads grow into its soft root hairs. There is no research into whether this is painful or not, but as it is something the tree wants, I imagine it gives rise to positive feelings." Insert tree BDSM joke here.
- “Threatened forests are inherently unstable…And because our Stone Age ancestors were always on the lookout for ideal places to set up camp, it would inherently make sense if we could intuitively pick up on the state of our surroundings.” Having seen many people react positively to overgrown, unhealthy Western forests and negatively to the processes – like understory fire - that restore them, I have my doubts.
Overall recommendation: This is a good book to get people excited about trees. I especially appreciated the information about developing European forestry practices – such as selectively harvesting using mules to reduce the damage of dragging out logs, or transitioning conifer plantations back to native forest – that American readers may be unaware of. But, while at least 90% of the book is mostly accurate, if botany or forest science isn’t your area of expertise, maybe take the statements that don’t have footnotes with a pinch of salt3. Still, I want to give Herr Wollben kudos; Popular science writing isn’t easy – I’ve only made one attempt at it myself so far – and this book gets a lot right both in style and substance.
1. Mycorrhizae are fungi that form connections with tree roots. In exchange for sugars, they pass the tree water and nutrients. The fungi that produce truffles are part of this category of organisms.
2. Here’s something that always amazes me: Giant Forest in Sequoia National Park has the best grove of 2-3,000 year old giant sequoias anywhere. But 6,000 years ago the area was climatically unsuitable for this species, and you can tell from pollen records they weren’t there. So that means that the sequoias were growing further up or down the mountain, and the ancient trees we see today are just 2nd or 3rd generation immigrants!
3. Of course, there are also some true statements the general reader might doubt that DON’T have footnotes. For instance, I know I've heard the story about how acacia trees repel giraffes before and I'm pretty sure it is true, but there is no citation I could use to convince someone else.
