Why do trees form girdling roots?
In the urban landscape, where trees are subjected to the will of humans, trees are often too deep in the soil. When we add soil around the base of an established tree for a raised garden bed or plant a tree too deep, there is a high probability that the tree will form girdling roots.
These girdling roots on this maple have been left alone for too long. They are no longer fibrous, and are becoming woody. Royal Oak, MI.
Girdling roots can be tricky in that symptoms often appear long after the issue manifests. Without getting into detail here, girdling roots pose potentially severe and often chronic fatal problems for trees. Simply put, the tree eventually can “choke” itself.
Why would this mechanism have evolved if it is of such great consequence to trees?
Read on to learn why trees form girdling roots, and how this mechanism, in the right context, is actually advantageous to trees.
Dormant and Adventitious Buds
There are dormant and adventitious buds scattered all over trees beneath their bark. These buds exist to deal with stress and damage. Ordinarily, they are suppressed under normal growing conditions. But in the presence of certain environmental or biological triggers, those buds can grow into different things, depending on the stimulus. Kind of like human stem cells.
Adventitious roots triggered around the base are not an ideal place for adventitious roots to form.
When living wood is exposed to moisture and warmth for the right length of time, dormant or adventitious buds will “turn on”, and begin the growth of adventitious roots. When a branch falls from a tree, a dormant bud on the tree near the breakage can be activated to grow a new stem, for example.
Ordinarily, a tree outside of the urban landscape rarely has to deal with being too deep in the soil, so the adventitious buds at the base are inconsequential. However, they are incredibly consequential in the urban setting when trees are too deep, which is why fixing and avoiding girdling roots is so common in proper tree care.
Only in very few natural environments do trees have to deal with fluctuating soil levels. The trees that have evolved in these areas are well suited to dealing with this environmental stressor. You can find these tree species on alluvial riversides or dune environments.
Going back to that branch that fell from the tree: There are still dormant buds on the branch. This is where adventitious roots prove to be an amazing adaptation.
Layering
The development of adventitious roots is ideal in this scenario.
If that branch was living, and if it lands just the right way, adventitious roots can be activated! This can happen both if the branch remains partially attached to the tree and if the branch entirely comes off. Even if an entire tree fails, it can generate a whole new root system from adventitious and dormant buds!
When the tree’s trunk is not in the way, there is no risk of girdling itself. This is where adventitious roots give a tree a second chance. This phenomenon is called layering or air layering.
Even if no branch fails, contact with the soil triggers adventitious roots to develop. This secondary root system on this oak contributes nutrients and water to the tree, just like its primary root system. Rochester Hills, MI.
Willow trees are a great example of how aging is not a one-way process in trees, and this is made possible by their eagerness to form adventitious roots. With their heavy branches, they tend to break apart as they age. As that happens, the tree can either live on or be cloned in new iterations supported by adventitious roots. Even small twigs from willows that fall off in the wind can root and eventually become full trees themselves!
Takeaways
Trees are extraordinary, but we already knew that. I wrote this article to explain that girdling roots are not a malfunction of trees, but a malfunction of the way we care for trees.
The stress put on trees in the urban landscape is not what they have evolved to cope with. We’ve got ideas of what trees or landscapes are supposed to look like, but it is important to remember, above all, tree biology stretches across whatever context we put them in.
Sources
Bidlack, James E., and Shelley Jansky. Introductory Plant Biology. McGraw-Hill, 2018.
Costello, Laurence R. Abiotic Disorders of Landscape Plants: a Diagnostic Guide. University of California, Agriculture and Natural Resources, 2003.
Dujesiefken, Dirk, et al. Trees - a Lifespan Approach, Contributions to Arboriculture from European Practitioners. Fundacja EkoRozwoju, 2016.
Hirons, Andrew D., and Peter Thomas. Applied Tree Biology. John Wiley & Sons, Inc., 2018.