by Valentina Lagomarsino
figures by Hannah Zucker
When scientists first studied the structure of nerve cells that comprise the human brain, they noted their strong resemblance to trees. In fact, dendrites, the term to describe projections from a nerve cell, comes from the Greek word Dendron, for “tree.” While the connection in the appearance of nerve cells was made to trees, the comparison may have been more apt than originally realized: scientists are starting to uncover that trees have their own sort of nervous system that is capable of facilitating tree communication, memory and learning.
Forests are complex systems
Forests cover 30% of Earth’s land surface and hold over a billion trees. Forests are known as “carbon sinks” because trees absorb carbon dioxide from the air, store the carbon in their trunks, and exhale oxygen. Scientist have leveraged this property to measure the ratio between two naturally occurring forms of carbon (12carbon and 14carbon) to assign an age to trees, a technique termed carbon dating. Using this technique, scientists found that trees living in forests, like the tree colony called Pando, tend to live longer than trees living in urban environments, often in isolation. Dendrologists, scientists who study wooded plants, thought that perhaps trees that lived together were helping each other by sending resources through their roots. To test this out in North American forests, dendrologists utilized a technique called isotope tracing. In this experiment, scientists injected carbon dioxide gas replaced with radiolabeled 14carbon into the trunk of Birch trees (Figure 1). When nearby Fir trees were covered by shaded cloth, to block their ability to acquire nutrients through photosynthesis, scientists found a higher level of radiolabeled 14carbon in their trunk, meaning they must have received sugars from the Birch. These experiments confirmed that trees are indeed communicating with each other and sharing nutrients through their roots, forming a complex system sometimes referred to as the “wood wide web.”
With a little help from my friends
This complex network connecting trees is dependent on a symbiotic relationship with microbes in the soil like fungi and bacteria. Symbiosis is when two separate organisms form a mutually advantageous relationship with each other. Fungi can cover a large surface area by developing white fungal threads known as mycelium. Mycelium spreads out on top of tree roots by up-taking sugars from the tree and by providing vital minerals back to the tree, such as nitrogen and phosphorus (Figure 2). This symbiotic relationship between tree roots and fungi is known as the mycorrhizal network (from Greek, Myco, “fungi” and Rhiza, “root”).
To identify the species that constitute the mycorrhizal network, scientists have utilized recent technological advances in DNA sequencing and big-data analysis. Microbiologists have identified different species of fungi and bacteria that form symbiotic relationships with different species of trees. Scientists believe all trees have a mycorrhizal network, but trees only communicate with each other if the fungal and bacterial species that constitute their mycorrhizal networks are the same. The most common combination of fungi constitute the arbuscular mycorrhizal (AM) network, which has been found to be important for nutrient uptake in 65% of all trees and plant species. The remaining 35% of tree and plant species may have combinations of other fungi varieties that comprise their networks.
By investigating the different interactions between species of trees, scientists found that trees leverage similarities and differences in their microbial “makeup” to recognize other trees of their own species, and they preferentially share nutrients with them through their mycorrhizal network. This behavior, known as “kin recognition,” was recently explored when multiple families of Douglas Fir trees were planted in a plot and carbon tracing experiments indicated that trees of the same family shared more carbon than between trees of different families. Scientists are still investigating why this is happening, but it is hypothesized that all plants evolved to have kin recognition for reproductive purposes. Similarly, there is cross-talk between different species of trees that share the same mycorrhizal network, such as between Birch and Fir trees (Figure 3). Interspecies tree communication has been shown to increase the fitness and resiliency of trees.
Mycorrhizal networks are extremely important for tree health during times of danger. Certain species of fungi can facilitate tree resilience to certain environmental stressors such as predators, toxins, and pathogenic microbes that invade an ecosystem. By using a technique called allelopathy, in which a chemical signal is sent through the mycorrhizal network, trees can warn their neighbors about an invasive predator or to inhibit growth of invasive plant species. Surrounding trees can then defend themselves by releasing volatile hormones or chemicals to deter predators or pathogenic bugs. It was even found that trees can send a stress signal to nearby trees after a major forest disturbance, such as deforestation.
Climate change affects the microbiome of the forest
Trees rely on a healthy forest ecosystem to thrive and protect themselves from danger. Humans rely on a healthy forest ecosystem to be able to inhale clean oxygen. Last year, millions of people around the world experienced the devastating effects of climate change. Not only is climate change impacting human health and wellbeing, but it is also affecting the ecosystem of our oceans and forests. Human-initiated deforestation contributes to climate change by reducing the number of trees that are available to soak up carbon dioxide. Deforestation not only removes the trees that are being cut down, but also impacts trees that are still alive by disrupting the mycorrhizal network that is important for intra-tree communication.
Changes in climate, as seen through increased droughts and extreme temperatures, may further disrupt the biodiversity of the microbes in the forest. This decline in biodiversity is known as human assisted evolution, or “unnatural selection”. The altered microbiota of the forest may then change the nutrients that trees are able to receive and we may start seeing changes in tree morphology, particularly in the shape of leaves. This would change the photosynthetic capacity of the tree; for example, smaller leaves have less surface area for light absorption, which will negatively impact their ability to absorb the sun’s rays and produce sugars through photosynthesis. This could potentially inhibit tree growth and the amount of carbon that trees can share with fungi. Furthermore, without a biodiverse mycorrhizal network, trees are becoming more susceptible to destruction from invasive, harmful insect species. It is clear that the impact we are making on the environment is self-perpetuating and heading in a dire direction for the health of our forests, but there is still hope. Some scientists are trying to combat climate change by using gene-editing techniques to restore ecosystems that have become extinct and by engineering synthetic microbes that are important for a thriving ecosystem.
Trees are considered to be the oldest living organisms on the planet. Over centuries, they have been resilient to changes in their environment due to their symbiotic relationship to fungi and other microbes. There are so many more discoveries to be made to understand the ancient wisdom of our forests and the invisible microbes that keep our ecosystems in harmony.
Valentina Lagomarsino is a first-year PhD student in the Biological Biomedical Sciences program at Harvard University.
Hannah Zucker is a second-year PhD candidate in the Program in Neuroscience at Harvard University.
For more information:
- To learn more about tree communication, check out this book review
- For more on dendrology, or the study of trees, check out this podcast
- For more information on the impact of invasive insect species on our forests, see this piece from The Washington Post
- To learn about the impact of climate change on urban trees, check out this article from the University of Melbourne
- To learn about what trees can teach us about our history, see this New York Times piece