Gahnia Grove - Site summary and discussion's Journal

Journal archives for May 2021

May 29, 2021

Progress report for Year 3

Less than 3 years since we began the Gahnia Grove Trial, the site is showing excellent results.

In summer it was hard to see the juvenile and seedling natives, hidden for their own protection among dense wildflowers, whose colour and diversity were enjoyed by Reserve users.

Kikuyu was eradicated from the initial Gahnia Grove site within about 5 months, and from a further 3 metre width of kikuyu-free ground added to the cordoned area last year, for aesthetics and practicality.

After cutting down, uprooting or mulching of most of the benign exotic herbs, the number and height of the resulting native trees, shrubs, herbs and groundcovers is now evident, and it is an ideal time to assess the results of the Methodology Trial, and to discuss how the skills and knowledge might be passed on to other volunteers in Reserves, particularly those challenged by kikuyu.

Mass weed invasions were mostly Japanese honeysuckle, Tree and Chinese privet, Cape Honey Flower, Moth plant, Elephant’s ear (Alocasia), Kahili ginger, Arum lily, pampas, wattle, Elaeagnus (severl over 10m in spread, climbing several metres high into canopy trees, and some bushy small trees with trunks to 10cm D and 3mH), blackberry, Syzygium spp, Euonymus japonica, bindweed, Flame Tree, and Tradescantia.

The environmental weeds in Gahnia Grove have mostly been eradicated, or almost eradicated:

  • occasional remaining honeysuckle or blackberry root becomes apparent here or there a few times a year
  • all Cape Honey Flower stumps are now well-decayed
  • all pampas are dead, the largest pampas having had its last few weak live leaves in autumn 2021
  • moth plant seedlings continue to reduce in number each year, and no longer occur in the areas of the first invasions discovered and treated
  • Alocasia has not been found since 2019 except as one or two gradually decomposing but still live pieces of cut stem, which are kept free of the ground as they continue to die
  • the remaining live tubers of ginger and Arum are easily suppressed once or twice a year with deep piles of Tradescantia, which is still being retained in the area surrounding them, with ongoing control to be coordinated with the as-yet unknown timeline for arborist control of the Flame tree.
  • several Elaeagnus, the commonest of the tree weeds, remain as 10cm D tree stumps, with live shoots easily suppressed annually. The majority, however, are now insignificant amongst native vegetation, as broken-down, spindly stems, struggling to produce occasional foliage on their few live branches, after having been allowed until recently to continue producing this small amount of shade during the times of severest drought. They are easily further reduced once or twice a year in during ongoing survey and monitoring. Smaller specimens and those most intensively reduced have already died, and are present only as broken leafless stems identifiable only from our iNaturalist records.
  • Hundredss of Tree privet, Chinese privet, Cotoneaster, Euonymus japonica, Prunus serrulata (wild cherry) and Syzygium have been effectively suppressed. The first discovered and addressed, the tree and Chinese privets, have died and are now decaying, now hidden in the canopy margin among the spreading mapou, Coprosma, hangehange and Gahnia, or as isolated stumps invisible in the shade of the forest.

The hypothesis of the faster demise of woody weeds through leaving trunks and branches partially or (with ringbarking) wholly connected, forcing the tree to feed the upper part while preventing or reducing the downward transfer of nutrients to the root, has been supported by our results, and correspondence with more experienced overseas restorationists through iNaturalist supports the technique, and provides technical explanation.

Flame trees in the canopy and on its margin have had low and prostrate branches pruned, several small trees have died and their decaying bases uprooted easily. Two small stands in the native canopy margin, comprising four tall but slender trunks, have been ringbarked after inspection and authorisation by the Community Ranger and Arborist.

Tradescantia has been mostly removed, ie as much as current soil moisture levels permit, from most of the canopied area, and controlled on the steep sunlit banks above: CHF Bank, now mostly covered by young native trees and shrubs, and Flame Tree bank, where a fallen Flame Tree c. 15 gm H x 40cm D, still live but weakened by ongoing suppression through partial cutting through of branches, has been gradually reduced since Dec 2019 to the lower half of its trunk and one major branch. Most of the removed material has decomposed already, and the remainder is contained, decaying in a single loose pile on high dry ground.

The forest beyond its sun-exposed margin has been thoroughly weeded and is producing many seedlings of species found in the older forest, including many totara and tanekaha, with occasional rewarewa, kauri and kahikatea.

In the third year of the Trial, very little time has been needed for the control of environmental weeds in the initial Gahnia Grove site, while much time has been spent in maintaining and creating soil moisture and shade, and in monitoring and assessing the results of techniques and strategies used to date.

In the same time period throughout the combined area of the Trial - Gahnia Grove (begun June 2018), Tanekaha Ridge (begun June 2019) and Rimu Ridge (begun August 2019) - less time has been required for weed control than for the mitigation of increased heat, sun and drought, through the application of weeds as mulch and the creation of biodegradable shade-screens constructed from plant materials provided by the site.

Benign exotics retained for shade and shelter of regenerating natives have in Autumn 2021 been almost entirely culled, with the exception of Plantains which provide excellent small ground cover between natives and compete splendidly with kikuyu at the mown edge.

Depending on rainfall and sunlight intensity in the coming seasons, the currently established native revegetation, with the recently constructed screens of bamboo poles and harakeke prunings in the ridgetop areas exposed to afternoon sun, may provide sufficient shade and shelter for the continued successful development of seedlings and juveniles. If necessary some Verbena, oxtongue and wild carrot will be allowed to regrow where needed.

The kikuyu technique, in particular, has been surprisingly effective in eradication, more time-efficient than expected, and would be very valuable in the preparation and maintenance of both passive restoration and plantings.

One major cost benefit of the method is that, since the released area is immediately plantable and free from reinvasion, much smaller plants can be successfully planted. This has been shown by several 4-10 cm H tree, shrub and grass seedlings planted in early summer 2020 within 30 cm of the mown kikuyu, in May 2021 they remain weed-free, and are now 30-40cm +H, robust and vigorously growing, some in full sun and some shaded by other native herbs and shrubs.

The mown sward will of course require ongoing control along the edge. Methods we are trialling are aimed at inhibiting the invasion where shade is not yet, or is not planned to be, established.

Particularly helpful so far have been dense plantains, (both broad and narrow-leaved). We have yet to see the establishment at the mown-edge of the creeping ground cover Nahui, and the native grass Microlaena stiploides.

Among the trees, mapou shows the most drought-tolerance, resilience to damage and disease, and dense, light-excluding foliage from ground up. Thus in the remaining treeless areas near the top of the ridge we are aiming to provide the conditions most suited to mapou seedling establishment, with initial ground cover by the prolifically germinating, quick-growing, but short-lived karamu.

Posted on May 29, 2021 10:00 PM by kaipatiki_naturewatch kaipatiki_naturewatch | 0 comments | Leave a comment