I was listening to an excellent podcast last night called Ologies. Every week, presenter Alie Ward meets with an -ologist for a deep-dive into their field. I highly recommend it!
I’m quite behind on the episodes, so last night I got to Dendrology, the study of trees. The guest of the week was urban arborist Casey Clapp, who may know more about trees than anyone on the planet. So much of what he said was ridiculously interesting, but one thing in particular really caught my attention. He said that trees can use electrical and chemical signals to communicate with each other.
I just had to find out more about this, and oh my there is a lot of info out there. Because there’s so much, this article will just focus on one thing, because it’s the thing that makes me happiest.
The world is a hostile, harsh and sometimes violent place, but it’s tolerable when you’ve got your family beside you, helping you through it. And it turns out trees need the same loving support as you or I!
In a viewpoint paper published in the Journal of Experimental Botany, Meredith Biedrzycki and Harsh Bais describe the ability of an organism to differentiate related members of the same species from non-related organisms. This phenomenon, known as kin recognition, was originally investigated in the context of animal behaviour. In recent decades, however, scientists have begun to postulate that this concept might also have relevance for some plant species.
Early studies in the 1990s revealed that plants are actually able to recognise themselves; root growth continues if they are exposed to other roots from the same plant, but becomes inhibited when the roots are exposed to roots from a different plant of the same population, from a clone plant, or from plants of a different population (Mahall and Callaway, 1991, 1992, 1996).
Then in the mid 2000s, scientists identified parasitic plants which could not only detect other plants, but actually seek them out as hosts by sensing secretions in the soil (Awad et al., 2006; Runyon et al., 2006;
Lopez-Raez and Bouwmeester, 2008). Sort of like Day of the Triffids, but if the triffids just wanted a piggyback ride.
Since then, dendrologists have gone into overdrive trying to determine exactly when kin recognition occurs, why this might be and how plants can benefit.
A 2014 literature search brilliantly entitled ‘Kin recognition by roots occurs in cycads and probably in conifers’ (Gorelick and Marler, 2014) summarises some of this research nicely. For instance, they describe one study from the 90s in which Cycas edentata seedlings were placed in pots with other seeds exhibiting varying degrees of relatedness. An inverse relationship was found, whereby increased relatedness predicted low root proliferation and vice versa. And no, it wasn’t a fluke! These findings have been replicated in many other studies.
Extensive root grafting has been documented in conifers which further supports the idea of kin recognition in trees. Conifers tend to graft more on strangers than relatives. Grafted trees can share resources and also affect growth, but whether this is for the benefit of the grafter or the graftee is unclear. That conifers graft more on strangers would indicate that perhaps this is a more competitive than altruistic behaviour.
At a TED summit in 2016, forest ecologist Suzanne Simard detailed some additional findings from her research which indicate that trees really do try to support their kin. Her team performed an experiment where they grew a ‘mother’ tree alongside kin and stranger trees. As expected, mother trees colonised their kin with bigger mycorrhizal networks.
What’s that you say? Mycorrhizal whatnow?
Mycorrhizal networks are underground webs of fungi which connect individual plants together and act as a delivery system for all sorts of yummy goodies, like carbon, water, nitrogen and other nutrients. These networks have been wonderfully described by the journal Nature as the ‘Wood-Wide Web’ – whoever came up with that needs a gold star.
Fungi in this network benefit from a symbiotic relationship with the trees. They enjoy a tasty snack of carbon, and trees need all the nutrients they can get. During the Ologies podcast episode mentioned at the top, Casey Clapp described one scenario where a tree needed water but its roots were too chubby – so it said ‘Hey fungi, fancy getting me that drink? I’m parched.’ And water was delivered!
Other research has also supported the notion of kin support via mycorrhizal networks. A thesis by Monika Gorzelak in 2009 found that Douglas fir trees preferentially transfer carbon through the mycorrhizal networks to their kin rather than stranger trees.
Along with nutrients, trees within a mycorrhizal network can also pass information. In Suzanne Simard’s TED talk, she explained that an injured mother tree passed defense signals to her kin, which directly increased the resistance of those seedlings to future stresses.
This messaging system is further supported by Peter Wohlleben, a German forester and author of The Hidden Life of Trees: What They Feel, How They Communicate. He explains that trees send distress signals about drought and disease, or insect attacks, and other trees alter their behavior when they receive these messages.
Aside from simply being awesome, these findings are actually really important and could have a big impact on our forestry practices. Wiping out swathes of forest all in one go might mean that mother trees are destroyed before they can pass their wisdom to their kin. As a result, after deforestation, the trees remaining are weakened which can lead to a system collapse. Forests are extremely resilient, however, so if we’re able to keep some mother trees standing, that knowledge can still be passed on to younger trees to ensure our forests thrive.