When is a tree a trees?

We all know what a tree is, don’t we? But when does a tree stop being ‘a’ tree and start being something bigger, more complicated and perhaps even plural? Francis Hallé has approached the question from a morphological perspective by showing that through reiteration, trees can actually be forests, with branches functioning as individual trees or trees-in-waiting, and I was fascinated to read this paper which touches on this same question from a different perspective, showing that up to 40% of the carbon in a tree’s fine roots might actually have been produced by another tree.

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Schefflera branch attachment, showing the ‘roots’ of a branch within the tree

 

Lots of research has been published about signalling between trees being mediated by mycorrhizal associations, and similar research has shown carbon-sharing in seedlings but this is the first research to show how this works in mature trees. And even more mind-blowing, the trees don’t even have to be the same species for the process to work.

It poses all sorts of questions that would be fascinating to study but one that’s immediately relevant to my research is to ask what the impacts are on street trees. If trees have evolved to use symbiotic relationships to trade resources and respond to external stimuli, what is the effect on the tree if we plant it in solitary confinement in a street tree pit? Newly planted street trees are often said to have a life expectancy of 12 years- the are lots of reasons why, but perhaps the answer isn’t just to do with soil media and rooting volumes.