Planted harakeke bordering the forest margin
Harakeke in Gahnia Grove
A lot of detailed observations and questions regarding the harakeke in Gahnia Grove, were not addressed in the Annual Report for Year One or Year Two pending a fuller assessment of their interactions with the trees around them, and the results of pruning. We still hope to learn about their impact on water distribution on a dry ridge after deforestation and roading, but in the meantime we can report on their ongoing management, in these two areas:
Gahnia Grove “proper” (site adopted in June 2018)
The three large stands at the top end of Gahnia Grove are very old, contained many dead plants and in places had forced trees horizontal for up to 3 metres, with live harakeke plants lying horizontal on the outer edge of the ring, on tall bent roots overlying up to 50cm depth of dead plants.
Initially we were able to maintain the natural, unpruned harakeke form of these stands, with long leaves flowing to the ground, but as the plants multiplied the new leaves protruded over the cordon. Despite pruning of outer leaves and flower stems to 4m long overhanging the cordon and obstructing mower and pedestrians, they regrew rapidly, with new plants continuing to emerge at lower and lower angles.
As each successive outer ring on the growing clump emerges at an increasingly horizontal angle, the dead mass or vacant space in the centre of the clump grows larger with the death of each generation of plants in the clump. We have been watching with interest what seems to be the terminal development of these old stands (probably the remains of the 1999 NSCC planting), while trying various methods of preventing them from impeding the growth of the surrounding trees, which are desperately needed for shade of the forest margin behind.
Until shaded out by tree development they will require perpetual weed control and pruning. Weed control is not an issue within Gahnia Grove where kikuyu and vine weeds have been eradicated, but pruning needs increased as each clump grew additional plants each year.
The options have been either to widen the Trial area (exposing more bare ground until native vegetation grows) or to trim the leaves.
We intially trimmed them to fit the cordoned area, but the cut leaf edges became a hazard - and unsightly - as the cut leaves grew. We now cut them very short where possible, but this still results in unsightly cut edges that eventually protrude over the cordon.
Extending the existing band of dense trees, from the top end (open grass area) down towards the flame tree, is the goal for the entire kikuyu margin, to shade out weeds till maintenance requirements are minimal (as in the 10 m of kikuyu margin with dense tree development, ie the Annexe Kikuyu Margin, at the Southern, uphill, end of Gahnia Grove), and for support of the forest reveg below, as the 25 metre wide treeless area is the hottest location on the ridge.
However, whether this can be achieved is uncertain due to the natural aridity and podzolization of this dry kauri ridge, the dessication and growth-inhibiting air pollution common to roadsides, the higher temperatures currently being experienced due to climate change, and hydrological and/or meteorological drought since January 2018.
The few trees that existed here in June 2018 were so heavily covered by honeysuckle that, though they started to leaf out on release, several did not survive the subsequent drought. In the 25m of Arena Kikuyu Margin and CHF Kikuyu Margin, the two harakeke stands have been the only wind-break and shade, and the shade of harakeke will of course remain low, ie 2-3m H. The areas directly below these harakeke stands are also the two driest areas, struggling to produce even wild carrot or ox tongue. During ongoing observation this Winter I have been wondering whether this is due to unseen underground water runoff patterns or the success of harakeke in absorbing the water as it comes down the hill. If the latter, this high point on the ridge may do better after the natural demise of the existing harakeke stands, provided alternate shade has been established by trees. This will be a challenge to achieve while the drought continues.
Due to the uncertainty of adequate rain, the priority of the 2020 workable season, ie winter, was to trial ways of creating shade, including placing bamboo poles and kanuka brush in soil before it became too hard to do so. The resulting shade-fences and shade-tents potentially allow shade to about 2mH from native vines (Ipomoea was removed from the plan after further research led to discussion with Ewen Cameron, but some wild rauparaha, sourced from the nearby forest-margin raingarden, was planted here, the invasive bindweed hybrid having been virtually eradicated over the last 2 years) and/or harakeke shade tents. Loose, natural-looking shade tents (with varying degrees of visual appeal as we discovered what materials and arrangements were stable enough to stay in place in high winds, dense enough to provide shade without creating wind resistance. and sufficiently robust not to crumble before the summer was over).
Several such arrangements of vegetation were successful last year in alleviating critical drought stress for specific trees on the canopy margin exposed to sun by the cutting of honeysuckle vines in the trees.
Harakeke in Rimu Ridge (from the Flame Trees to the Petrol station, adopted for care and included in the Methodology Trial from August 2019)
In the margin of Rimu Ridge the harakeke-dominated banktop was adopted only because it was part of the invasion of honeysuckle and moth plant binding and smothering the trees both at roadside and within the forest margin from ground to canopy, including almost all the juveniles and adults visible from the kikuyu margin since their release, all those down the bank below, and many within the taller rawirinui/podocarp canopy behind that.
Some trees found in the honeysuckle were already dead and others partially dead. Several of the dead trees have since fallen down, and the partially dead have been pruned, where possible and as time permits, to live wood).
This forest restoration trial is intended to demonstrate the long-term economy of a chemical-free weed control methodology, enabling a natural forest succcession through selective weeding over a period of years, culminating in weed eradication and reduced potential for reinvasion with the development of shade (a fundamental principle of forest restoration). Once sufficiently dense shade and intact native plant communities are achieved, minimal ongoing maintenance is required. This principle of establishing shade underpins the Gahnia Grove trial, including the eventual suppression of kikuyu at the mown edge, so that spraying, linetrimming or manual control are no longer required except where light breaks occur, eg due to a tree death.
While releasing the mature trees of the rimu ridge forest margin from honeysuckle we undertook the management of the very, very many harakeke that were uncovered there, including
-small to medium single plants
-small and large groups of small to large plants
-an outer row of small separate plants, many of them with damage or dead due to a single herbicide overspray event in l2019), a few of them projecting into the mown area from an otherwise straight line, The inclusion of these necessitated extending the cordoned area outwards, beyond what would be ideal for the aims of the Trial, and they were included only because they would otherwise impede mowing, and prevent kikuyu control.
-older plants both in large tight groups up to 10 metres across (with honeysuckle rooted throughout, eventually reached as older harakeke plants collapsed), and scattered throughout, with empty space or isolated trees both live and dead in between.
