Judith Doyle, of Wellington, asks :-
The photo I have attached is of a pohutakawa on Oriental Parade, Wellington. I have often wondered why the matted roots (or whatever they are) hang down in great clumps in midair from one particular pohutakawa tree but not all the others?
Nick Stott, a Heritage Arborist with the Auckland Council, responded.
There is no 'evidence' that Pohutukawa trees grow aerial roots for any reason in particular.
Noted that many Pohutukawa trees appear to develop these adventitious roots in an attempt to bind themselves together as the species is known for 'layering' characteristics (layering is where branches either rapidly, or slowly fail to ground level - still attached to the tree).
It is suspected that the aerial roots both/either attempt to bind the tree together, or start the process for re-rooting (when the limb/branch finally gets to the ground). This 'hypothesis' sits adjacent to the species ability to fall down a cliff and continue growing in a new location.
Although we suspect the species ability to fail (or layer) and regrow relates to the development of aerial roots, as mentioned above, there is no concrete evidence. I would also point out that often trees with aerial roots don't layer/fail. Further to this roots (aerial, or underground) will continue to undertake work that roots do - regardless of primary or secondary requirements. That is, trees optimise every situation (and I would expect aerial roots to be providing - as roots do - air, water, etc regardless of the trees need for stability if that were the case).