Trees and their Associates

Phil Bennett Phil Bennett
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How often do we think about tree associates when we are working on an individual tree?  The sixteen chapters of the Arborists’ Certification Study guide cover many areas of Arboriculture, but there isn’t a chapter dedicated to tree associates.  The problem here is that we may be preconditioned by our training to think of trees as individual entities, rather than the interconnected organisms they are.

Some tree associates cause us problems (or perhaps, opportunities) as Arborists.  English Ivy, fungal root rots such as laminated root rot and invertebrates such as aphids spring to mind.  What about the beneficial, or symbiotic relationships trees have with other organisms?

We know that trees benefit greatly from fungal associations, and that mycorrhizae in the soil greatly increase the uptake of water and nutrients into trees.  However, it doesn’t stop there. The research of Dr. Suzanne Simard has shown that the mycelial networks carry information, including “threat signals” between trees.  Simard has also shown that symbiotic relationships exist between some tree species such as Douglas fir and paper birch, and that they exchange resources below ground, using the mycelial network as an intermediary.  For more information, watch this TED Talk.

Lichens are also important tree associates, and some species provide distinct benefits to trees.  Lobaria species are nitrogen-fixers, and when they shed from the upper parts of the tree, and fall to the ground, this nitrogen is made available for uptake into tree roots.

The last association I’ll mention is between salmon and trees.  Trees provide shade over streams and rivers, which benefit salmon as they need cooler temperatures to survive and thrive.  Young salmon leave their home streams, and swim to the ocean where they feed and grow much larger. They bring the gift of nitrogen back to those same streams, and that nitrogen is distributed into the forest after they spawn and die by other animals such as bears, ravens, and coyotes.  Historically, there was a correlation between the size of annual ring growth in trees and the size of the salmon run that year. The reduction in salmon populations has greatly affected this association, and its benefits. More details can be found in David Suzuki and Wayne Grady’s excellent book “Tree: A Life Story”, which details the life of a Douglas fir from seed to death.

Friends, I know I’m stretching here, and that my last example doesn’t have as many implications for arboriculture as for traditional forestry.  My argument is that we need to consider that trees don’t exist in a vacuum, even when we seemingly put them in one, such as a small tree-planting lawn between the street and sidewalk.  Their associations with birds, invertebrates, lichens, other epiphytes, and fungus still exist and have implications for the tree itself, and the wider environment.

Some questions we could ask when prescribing a treatment, bidding a job, or working on trees are:

  • How will this affect tree associates, and what are the implications of that?
  • Can I mitigate the impacts to associated organisms?
  • How can I communicate these associations and their implications to the customer?
  • How can my work include tree associates as well as trees?

Asking these questions might help us learn more, increase our scope-of-practice, benefit the customer and the environment, and provide a market opportunity.

signed, Phil.