Learning from the ridge-top "flood plain" of El Sitio

Palmgrass reduced under mature porokaiwhiri (foreground left), putaputaweta, nikau, mahoe and mapou

An earlier post on the ephemeral stream "El Riachuelo" is here

UPDATES

20 October 2020 Having now attended a course on freshwater ecological testing offered free to the public by PestFree Kaipatiki, we hope to be able to test the water using their testing kit before the ephemeral stream stops. If this is not possible we will test it in Autumn 2021.

October 2020
About a dozen seedlings of Haloragis erecta (toatota), Esler's groundsel (Senecio eslerii) and kawakawa, none taller than 10cmH, were planted on the stream banks where ground could be easily disturbed with a small trowel without encountering tree roots. The area remains well-mulched with decaying plant material, mostly palmgrass.

The stream stopped flowing during a dry early Spring, but has been inundated once or twice since by rain events. The stream continues to flow for at least several days after rain.

The few wild native seedlings and sporelings (including a mamaku c.30cmH) that were revealed by palmgrass reduction have survived subsequent drying and flooding, and leaf litter is arranged around them to retain moisture after each flooding.

The upper portion of the stream remains hidden by the material placed earlier across its banks, ie dried fallen nikau fronds and small logs supporting a loose tangle of dried woody honeysuckle vine.

The bank over the stream's emergence remains exposed at the top, as small Pteris tremula (shaking brake) and a few native tree seedlings have successfully established there. The juvenile karamu and mahoe growing on the top of this bank,and potentially shading the streamhead, have been mulched with woody weed material, and partially sheltered and shaded with a screen of bamboo poles and harakeke leaves, and the young trees are currently doing well. If their growth can be maintained they will supplement the streamhead's partial shade by a wide-spreading rawirinui about 10m H, released from dense honeysuckle in Oct 2019. This forest margin is exposed to the North and East, remains dry from the current drought, and will remain sensitive to sun, heat and wind during the coming summer.

July 29
Small logs (.5 to 1 m long) of recently-cut (home tree-pruning) kohuhu and kanuka have been placed across the stream channel throughout its length, interspersed with a few larger fallen logs, between the ti kouka and putaputaweta trunks, newly arisen fern sporelings and a few Coprosma seedlings, leaving these with access to light while keeping the soil shaded. These logs have been covered with lightweight carbonaceous shading materials including cut honeysuckle vine, fallen nikau leaves, kanuka/manuka brush and cut harakeke leaves. As summer proceeds the shade materials will decay, be replaced and added to with more harakeke leaves, which are abundant as the hundreds of individual plants on Rimu Ridge continue to multiply and need pruning to release adjacent juvenile trees.

Some shorter fresh logs have been placed in the streambed to slow flow, which diminished to 1-10cm depth after the rain, and is no longer evident as overland flow down the adjacent banks, though its Westward path is still wet, about a metre wide and meandering through the pine litter down Pohutukawa Bank and the forest below.

Further exploration found no channel, erosion, flooded plants, or plant material or litter associated with recent flows. However, a rough erosion channel has been seen in that general area when looking up from the forest path below the big rimu, and it is possible this overland flow contributes to that channel.

The flow, both recent and historic, seems to have been widely disseminated across El Sitio by fallen logs and deep pine litter, both of which will be maintained to ensure ongoing absorption here near the top of the ridge in the wet-loving plant community that has developed there.

The streamhead has been loosely covered with multi-branched small dead karamu, dead woody vine, and loose harakeke. Some coarsely woven harakeke mats are planned at one or two points, to allow filtered light and easy uplifting for water inspection.

Fauna:

Rotting logs have been moved in places, to lie across the bank rather than up/downhill, but they are not placed in the stream as they are invertebrate habitat, not to be disturbed. These logs are themselves being covered with loose vegetation to maintain habitat.

Birds and invertebrates will be able to access the stream in gaps between the shade materials, to be left more open where there is tree canopy, and removed as streamside vegetation develops.

Rubbish:
There is little rubbish in the stream -

a disposable medical mask
and a Coke bottle (glass) and a Coke lid (plastic) being among the few items found on the bank, and none in the channel. Newly arriving litter in the stream will be removed before it becomes habitat, unless we are advised to the contrary.

Under remaining palmgrass in the floodpath several large plastic planter bags were found, containing nursery plant medium and some obviously self-sown native carex. The carex will be planted, and the bags removed.

Background:
The presence of a wet-loving streamside community here surrounding an ephemeral stream near the top of a ridge is of great interest. While on either side the soil is dense clay, the 10x10m of ground found here almost totally occupied by palmgrass is topped with a fine black tilth at least 8cm deep, the depth and area of this soil to be further explored.

The area begins 10m downhill of the emergence of an underground stream on El Sitio Top, and is the site of recent, and presumably historic, widespread surface flooding after heavy rain.

The flooded area ends downhill at the top of a steep bank of scattered large rawirinui just outside the dripline of a large radiata pine to the North on Pohutukawa Bank, with partial canopy by mature kohuhu, mapou, porokaiwhiri (pigeonwood) and nikau, a 6mH Syzygium (monkey-apple) and an understorey of karamu (Coprosma lucida, macrocarpa minor and robusta), hangehange and mamangi (C. Rhamnoides) with many separate dense carpets of thread fern, some in shade and some sun-exposed, all moderately infested with honeysuckle, ginger and juvenile Syzygium. (This bank is outside the Trial's boundaries, and not publicly visible or easily accessible).

At the bottom of that steep bank is a large bare area of pine litter below the pine, above an old rimu with diverse streamside-community understorey and ground cover forming the lush and attractive border of the public path through the forest as it descends towards the wider Kaipatiki Stream formed by the confluence of several tributaries along this ridge.

Was this patch of deep, rich soil a naturally silt-trapping niche on the dry ridge prior to the felling and possibly burning of the forest for European forestry and farming? Or was it a steep forest stream, ephemeral or possibly even perennial due to a spring, which, once invaded by palmgrass, slowed flow and retained silt sufficiently to create the deep, rich seasonally inundated soil supporting thee mature putaputaweta, nikau and ti kouka?

Dead decaying trees here indicate canopy over this area, possibly of wattle, lost in the last 20 years.

We appear to have in this 100 sq m a model of waterquality, soil and biodiversity protection in an urban area already subject to air, water and soil pollution, with potentially significant erosion and deforestation.

Through regular monitoring of the results of our suppression of the palmgrass plants and restorative interventions on this site hope to learn how to assist El Sitio's ongoing

• retention and absorption of water
• development of such rich, moist soil
• restoration of dense ground cover as quickly as possible, by nurturing existing, and planting or sowing additional, native seedlings and ground covers (eg the species listed below)
• development of complete canopy by supporting the development of native trees surrounding it and arising in the mini-floodplain

The immediate goal is to shade the stream and the exposed bank between the stumps of palmgrass plants, using available material, eg logs placed across the stream and supporting carbonaceous vegetative material such as fallen nikau, ti kouka and ponga leaves, dried honeysuckle vine, and tied bundles of harakeke leaves.

Suitable native species of ground cover available wild nearby include:

• Carex lambertiana (already present on the floodplain of El Riachuelo)
• Basket grass (already present on the floodplain of El Riachuelo)
• Pteris tremula (already present on the floodplain of El Riachuelo)
• Carex geminata
• Carex lambertiana, dissita and uncinata (we noted C. uncinata dying off during the drought in canopy margins of Gahnia Grove and Tanekaha Ridge, so it is likely suitable for this wetter area)
• Haloragis erecta (shrubby toatoa)
• Senecio eslerii ("Esler's weed" ie Hairy Legs groundsel)
• Solanum opacum (Dark nightshade)
• Alternanthera nahui (Nahui)
• Hydrocotyle moschata (Hairy pennywort)
• Microlaena stipoides (Weeping grass)
• Lobelia anceps

All except Nahui are among species already present in other parts of the Gahnia Grove Trial site. Nahui is present as wild material sourced in the neighbourhood and planted in the Trial site (Arena kikuyu margin)

Gahnia xanthocarpa is already present in the canopied margins of the wet area - water tolerance unknown

Gonocarpus incana, Lepidosperme australis and Schoenus tendo are abundant in the kauri community of Tanekaha Ridge to the South - water tolerance unknown.

[UPDATE A first search was made for a plantable site, from the head of the stream to the lower end of its exposed flood plain, looking for a site to plant a single Carex geminata plant (descendent of Kaipatiki Creek-ecosourced in 1997, about to be dug out from a residential lawn). The soil was porous but contained dense fibrous tree roots throughout, that would have been broken by digging even with a trowel, jeopardising the health of the trees. The C. Geminata was placed in the edge of the stream itself, stabilized with some loose sandy material from the stream bed. It seems sowing seed of fast-growing herbs, and encouraging the regrowth of foliage on the palmgrass, will be necessary to restore ground cover until tree seeds germinate and grow big enough, or the thin low canopy of the existing trees increases and becomes dense enough, to shade the ground.]

The rich fine soil will remain moist while rains continue. Seedlings will undoubtedly arise of the surrounding tree species, ie mapou, mahoe, nikau, putaputaweta, porokaiwhiri, karamu, hangehange and kohuhu, and sporelings of the quick-growing shaking brake (Pteris tremula) and ponga (Cyathea dealbata). Mamaku (Cyathea medullaris) might find sufficient moisture here.

Ongoing selective weeding will protect this regeneration, but additional ground cover is likely to be needed in the short-term to prevent dessication in dry seasons causing further damage to already-drought damaged mature putaputaweta in and beside the stream.

Some soil-stability and ground cover is provided by the bases of the large palmgrass which were suppressed individually by crushing and compacting their leaves onto their bases. These retained bases stand about 30-40cm high and will be used to retain logs, brush and other vegetative material in an effort to prevent erosion by future flooding, and trapping any flood debris to retain moisture for the revegetation.

If needed, low shade tents will be formed of bamboo (growing nearby, under canopy on Pohutukawa Bank), brush, harakeke and ti kouka leaves, decaying honeysuckle etc, allowing varying levels of light for wild and planted seedlings.

Following the Trial Methodology's standard practice for weed control, further reduction of the massed palmgrass, and ongoing suppression of the large plants with retained bases, will be ongoing as ground cover and shade are established, the focus being on the release of existing native vegetation large enough to replace the palmgrass in these functions before the next dry season. This, as always, is a complex assessment based on the expected survival and growth rate of mature and juvenile plants, and of observed or anticipated seedlings and sporelings, in the months following any instance of weed reduction. This assessment has to be combined with the expected rate of growth or reproduction of the weed concerned.

Fortunately we have found the foliage of Palmgrass easy to reduce in a quick operation, and the stumps to weaken and become uprootable if heavily mulched with their own or other plant material.

The seeds are on long slim culms that slip easily out of the plant, and can be piled and heavily covered with plant material to prevent germination. This can be done throughout the infestation at present, and in the future by cutting the flower stalks as they emerge to prevent the development of seed.

Scattered palmgrass plants outside the mass infestation are being uprooted or suppressed (depending on size) as encountered.

Posted on July 21, 2020 11:39 PM by kaipatiki_naturewatch