As you walk under the stately pohutukawa trees on Oriental Parade, do you wonder why one or two of the trees sprout great clumps of matted roots from their branches? Clumps that hang down like masses of coarse matting or, more fancifully, giants' beards?
These are aerial roots, also known as adventitious roots. (Adventitious has nothing to do with being daring. It suggests development in an unusual position, like a root growing down from a branch). They grow out of trunks and branches.
They often develop on pohutukawa growing on banks and rocky cliffs where they can search for crevices, pockets of soil and moisture to help keep the tree anchored and fed, though this isn't applicable to our trees in the Bay.
Wellington is not one of the natural growing areas of mainland pohutukawas — metrosideros excelsa is their formal name. Their natural growing range is north of a line stretching from New Plymouth to Gisborne.
Why do some pohutukawas produce these aerial roots and others don't? I went to the Ask-a-Scientist column in our daily newspaper.
Gerald Collett, an aborist with Geotree Limited, suggests that it might be a genetic variation (a survival adaptation) within the species. Or it might be due to hybridisation.
He points out that northern rata (metrosideros robusta, closely associated with pohutukawa) commonly germinates on another plant and sends roots down the host tree. Sometimes pohutukawa also establish in this way. The two species are known to hybridise.
"I have seen pohutukawa aerial roots descending from wounds on branches and trunks as if stimulated by the wound, " Gerald Collett says, "but this looks to me to be something different to the beards. I have seen pohutukawa stems entirely sheathed in webs of aerial roots. Again I think that this too is something different to the beards.
Nick Stott, a Heritage Arborist with the Auckland Council, considers that there is no 'evidence' that pohutukawa trees grow aerial roots for any reason in particular. But sometimes they appear to develop these roots in an attempt to bind themselves together, as the species is known for 'layering' characteristics where branches fall to ground level, still attached to the tree.
The roots may attempt to bind the tree together, or start the process for rerooting when the branch finally gets to the ground. In any case, the roots will continue to undertake work that roots do — optimising every situation and providing air and water to the tree.
JCD, Bay View newsletter 65, May 2015