1 June 2023
The large amounts of silt that have been deposited around Tairāwhiti following Cyclones Hale and Gabrielle have caused numerous problems. As we slowly work to clear silt from around houses, off roads, bridges and productive land, we also need to be aware that silt deposition can seriously compromise the health of our mature native trees.
Following Cyclone Bola in 1988, a large volume of silt was deposited in the former Rakaukaka scenic reserve at Manutuke. This reserve contains some large mature indigenous tree species (titoki and tawa dominant) and some tree deaths and severe canopy dieback was noted in the years following Cyclone Bola.
This reserve was again seriously impacted during Cyclone Gabrielle as were many other areas of significant forest remnants around the district. In town, a small but significant stand of native vegetation adjacent to the Waimata river at Anzac Park was inundated, with around 400mm of silt being deposited throughout the stand of trees.
Council staff visited the reserve shortly after the Cyclone and within 2-3 weeks of the event the smaller, understory trees were already showing signs of severe distress and dieback. Concern was raised as to the long-term impacts on the larger trees, which will take longer to show the effects.
So how does silt impact the health of our big trees?
Silt can cause the cambium (a layer of actively dividing cells that is responsible for the secondary growth of stems and roots of the tree) to break down, exposing the tree to bugs and viruses which leads to dieback. Fine roots can also be suffocated by 200-300mm of silt.
What can we do to save them?
Ideally, silt should be cleared from the root collar of the trees (where the roots meet the trunk) out to the dripline. The challenge is doing this without causing additional damage to the trunk or root network. It is best to do this by hand rather than using a digger wherever possible.
Council’s Parks and reserves team are working on a plan to remove some of the silt from the Anzac Park trees to try and save them from death and serious dieback. The full effects of flooding on some of our more established forest remnants on the flats will not be fully realized for some time yet.
If you have trees on your property that you are concerned about then you can contact an arborist for advice. Removing as much silt as possible from around the trunk and roots as soon as you, it can make all the difference to their long-term health.