 Conference
1999
New Zealand Plants and their Story
Revegetation
and Forest Management: Otari - Wilton's Bush
Tim
O'Leary
Otari - Wilton's Bush
Wellington City Council, Box 2199, Wellington
A brief overview of
Otari
Otari was gazetted as
a Forest Reserve in 1906. It included cut over bush, reverting pasture
and 7 ha of mature forest protected in 1860 by Job Wilton, a local
pioneer farmer.
In 1926 with the efforts
of Leonard Cockayne an eminent botanist and native plant advocate,
Otari was officially established as a botanical garden.
The Reserve covers an
area of approx 90 ha, 2 ha of which are devoted to plant collections
and the remainder made up of regenerating and mature Podocarp /
Northern Rata forest association and coastal kohekohe forest association.
Leonard Cockayne's aim
back in the 1920's was, "the forest of the museum be brought
back as far as possible to its original form." It is our job
to carry this aim on today.
In the early years emphasis
was on replenishment, by planting species known to have diminished,
e.g., podocarps that had been logged. Today the strategy is more
holistic with the emphasis on the removal of modifying factors such
as animal and weed pests and the planting of revegetation plants.
The idea is that the subsequent increase in seed viability and bird
distribution will bring about forest restoration in a more natural
way.
Dracophyllum Garden
The Dracophyllum
Garden is located in a portion of the 7 ha of mature forest set
aside by Job Wilton and was opened up by the Wahine storm in 1968.
These cultivated gardens, the Chatham Island Forget-me-not and the
Wild garden were developed in the damaged section.
Today the management
plan we have in place at Otari restricts the expansion into the
forest of such cultivated areas and even in the event of major storm
damage, the policy now requires regeneration to take place.
Plant and tree removal
from the forest is also restricted under the management plan. It
can only be done if certain criteria are met for instance, essential
track maintenance, or if an unstable tree poses a danger to the
public.
Any material felled is
left in the forest to decay; the exception is where it may be wanted
for Maori cultural purposes, e.g., carving.
An interesting aspect
about the forest here is the change in forest association over time.
Stan Reid carried out the first botanical survey in 1934 and he
followed this up with another in 1982. He showed that the kohekohe
forest association was increasing while the tawa-dominated canopy
was not.
He believed this was
due to the removal of neighbouring forest for housing subdivisions
and sports fields, which left Otari exposed to a more coastal influence.
Yvonne Margot resurveyed
Reid's grids in 1992 and showed this trend was continuing. She also
noted the absence of juvenile podocarps, even in the mature areas
and attributed this to low seed viability, low bird numbers and
possums.
Possums
The Rata tree in the
Wild Garden is a site for one of our possum bait stations. We have
approximately 80 stations spread throughout the reserve on a grid
system at 150 m intervals and use Brodifacoum which is an anti-coagulant
poison.
From the early years
up until 1993, possuming at Otari was done on an ad hoc basis. Staff
trapped in the vicinity of the cultivated areas and more extensive
trapping in the bush only took place in periods when skins commanded
good prices, e.g., in the 1978 / 79 season in excess of 800 possums
were taken. When prices dropped however, possum numbers rose, with
all the consequences for the bush.
In 1993 WCC commissioned
Landcare Research to report on possum density and their impacts
on Otari. They found that possum damage on the vegetation was high
overall and that control was necessary.
In November 1993 a two-month
long control operation was initiated in conjunction with WRC, using
brodifacoum, cyanide paste and soft-catch leg-hold traps. This was
followed up between July 1994 and June 1996 with 50 bait stations
being set up along the ridges and gullies and poisoning with brodifacoum.
It was calculated that
with these two operations 1000 possums had been removed. Trees had
been tagged before and after the operations to check impact on the
crowns of indicator species and the results were as good as those
attained on offshore Islands, where similar operations had been
carried out.
The last control operation
at Otari was started in September 1997 and continued through to
November 1998. Bait stations were set up on cut lines in a grid
at 150-meter intervals. Again brodifacoum was used. A survey after
this operation (January 1999) found just 5 possums in the 100 ha.
This was a post-operation trap-catch rate of 1.7%, well below the
5% threshold aimed for.
Part of this success
is attributable to the fact that in conjunction with this last control
operation the Wellington City Council and Regional Council undertook
a poisoning programme on adjacent reserves and this has reduced
possum invasion from neighbouring land.
Control is ongoing; the
bait stations stay in place and are filled every 6 months (pulsed)
to keep possum numbers down.
It is evident by increased
bird life, more abundant fruiting, greater seedling growth on the
forest floor and the improving health of species such as fuchsia,
rata and kohekohe that the possum programme has been extremely successful.
Revegetation
In 1934, 15 plant pest
species were identified at Otari. Today there are in excess of 30.
The more prevalent are convolvulus, willow, blackberry, broom, gorse,
bamboo, and honeysuckle while those that pose a serious threat to
the natural value of the Reserve are Berberis darwinii, Tradescantia
flumenensis and Clematis vitalba.
Stan Reid first noted
Berberis darwinii in the 1920's as a small patch on Johnstons
Hill, which neighbours Otari to the south. Warnings were given then
about its potential to become a problem. Nothing was done over the
years and now it's a major problem, especially on the Southwest
boundary where it has established in large tracts as the major component
of plant reversion.
It grows in full sun
and tolerates shade, and unlike gorse does not act as a nurse crop.
We are attacking it in a piecemeal way using Escort which is effective,
but a long term strategy to deal with it on a large scale needs
to be worked out.
Tradescantia is also
a big problem at Otari. It's found mostly in deep and semi-shade
along path edges through parts of the bush and along the Kaiwharawhara
stream. We have been using Grazon on it effectively. However it
is persistent and requires follow-up spraying.
Clematis vitalba
is present in parts of the bush at Otari, and the Council employs
contractors to eradicate it on all Council owned land. A programme
of eradication has been underway for the past two years with follow-ups
planned over a 7-year period after which time we can safely assume
it has been eliminated.
Native plants can also
pose problems, e.g., on our revegetation sites Muehlenbeckia
australis without control can quickly smother out young reveg
plants. Unsourced revegetation material can also be a problem, e.g.,
Tasmanian Ngaio that was planted years ago on a revegetation site
has to be removed and Karo has also been introduced and self-seeds.
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