Test: Final Report, Section 2 of 4 "Plant Health" to Weed vines
1:5 Plant health
Tree deaths
We noted in our initial survey numerous dead and declining trees within the Trial site. Most were well below the typical height and girth of adults elsewhere on the site.
Among the dead trees were
- a majority of the observed tanekaha (see earlier reports)
- numerous mahoe at the pathside or within 2 metres
- several kanuka of only 20cm D or less, in locations of at least partial sunlight (and some larger, whose branches and foliage, in canopy breaks, appeared scant but could not be seen close enough to confirm life or death)
- a ti kouka 10-15mH below the roadside planted trees (Zone CaKRS).
Drought stress
In the roadside area, shortly after beginning exploration and intervention in mid-March 2019, we observed leaf wilt in first one or two, then in increasing numbers of mahoe and kawakawa. We also noted pale colour, small leaf-size and secondary disease/insect damage in karo.
These signs, presumably drought stress, were relieved in mahoe and kawakawa by light rains and/or mulching with Tradescantia, but returned intermittently with gradually increasing severity.
Drought stress was not apparent at this time under dense canopy, at the streamside or on lower streambanks, but its later appearance is discussed further below.
Reproduction
Juvenile trees were abundant throughout the streamsides, forest path and lower banks below the roadside.
Seedlings, sporelings or juveniles were rare below the roadside planting in Zones Ca to Eb, and on some eroded or slipped banks, in the immediate area of large tree or branch falls, and on sun-exposed banks (ie where tall trees were absent)
Despite the presence on the lower streambanks of two other apparently healthy mature tanekaha, tanekaha seedlings were not observed in Witheford Reserve outside the defined Trial area except as two depauperate specimens c. 6cm H, in dry soil under canopy margin at (surprisingly) the roadside near a large healthy tanekaha.
We found very few seedlings, exotic or native, in the deep Tradescantia throughout most of the Trial site roadside bank. A few kowhai, kawakawa and Coprosma that were found were at the outer edge of the roadside planted tree canopy, within a few cms of the regularly-mown grassed roadside verge (where grass has now been replaced by a few dried herbs in dust). Just under the dripline bordering mown grass, soil moisture and humus were greater than in the soil further under the canopy, perhaps due to the build up of humus in mown grass.
In late winter and early spring when the ground surface became wet, Tradescantia was uplifted from small patches (eg 0.25 - 1 sq m) under roadside mahoe, kanuka, and karo. This disclosed dozens of newly germinated seedlings of scattered coprosma, abundant kawakawa, and a few kowhai, but these little patches quickly dried out on the next sunny day, even between days of rain, so the bared patches were covered with loose Tradescantia to prevent further desiccation.
After the year of observation and reflection on all possible causes, we suspect chronic desiccation as a major contributor to both the ongoing tree deaths and the absence of seedlings on the roadside bank, but have been unable to obtain specialist advice or assessment to support or negate that. A limited seedbank and soil flora in this constructed roadside may be another factor.
Native vines
Several native vines were found entangled with fallen trees and weed plants alongside and overhanging the path, vulnerable to destruction by the public, both in maintenance of path access and due to the common misperception of all vines as a threat to trees. Some of these native vines alongside the path have attained nearby canopy, and even crossed the path where canopy supports it. Our observations of their presence in canopy and the spread of seedlings indicate these vines, mostly generated since 1999, are providing not only bird food but also much-needed shade and weed-prevention.
A Bush lawyer (Rubus cissoides) trunk about 6cm in diameter lies horizontal near ground-level alongside the path, its upright branches and foliage at times overhanging the path and thus requiring regular pruning for the safety of Reserve users. We discovered this vine during our ad hoc clearance of fallen trees, Elaeagnus and moth plant overhanging the path at that point. We were eventually able to remove the weeds and push several Rubus branches back from the path, leaving several young upright shoots of bush lawyer clear of the path and able to climb the steep pathside bank into the canopy. However, without ongoing manual weed-control, ongoing intervention to direct the Bush lawyer’s growth away from the path, and public recognition of the thorny vine and its horizontal trunk, it is liable to be damaged or killed by a Reserve-user, potentially resulting in permanent loss of its fruit and canopy.
Parsonsia and karaeo are present but not common. Young vines climb both weed and native juvenile pathside trees (Elaeagnus, mahoe and Coprosma), with mature vines observed with difficulty in the canopy. Where these vines are within a metre of the path, identification and directing them away from the path will be necessary to avoid their destruction.
Weed control
2:1 Conservation of soil, flora and water
In the dry conditions it was especially important to reduce weed mass only sufficiently to release existing or imminent native vegetation. This helped maintain shade and moisture which would have been lost in complete weed removal.
Hand-control of Tradescantia at the pathside avoided further loss of native seedlings and soil health. No other weeds were seen on the path, but native Carex, Solanum americanum (native nightshade), native Basket grass (Oplismenus hirtellus imbecilis) and possibly fern and treefern sporelings, may have been perceived, as some previously-healthy examples of these were found decaying in initial survey.
A new occurrence of nightshade, possibly native, was found uprooted in March 2020, in a humus-rich section of pathside area where Tradescantia had been removed and composted during the Trial. This nightshade, whether native or exotic, presented no threat to habitat or path users, and would have provided excellent cover for native seedling development. It was near a sewage manhole that overflows after rain events, where vegetation of any kind is particularly valuable in filtering run-off and restraining the distribution of raw sewage and associated inorganic refuse.
2:2 Conservation of freshwater ecology
The catchment of the Eastern arm of the Kaipatiki Creek above the Trial area is steep and largely paved and piped. In 1997-99 heavy rain events produced powerful currents sweeping debris across the footbridge and into trees up to at least 2m above the normal water level.
Eutrophication of the stream is likely if green leafy material enters the stream, which can happen if plants are cut or uprooted and left to decompose in areas of run off or on banks reachable by floodwaters.
Location of piles and any loose leafy material during the Trial was done, then and now, with that in mind. All loosened nitrogenous material was placed above flood levels, in situations where nitrogen-enriched run-off would have been filtered by carbonaceous ground litter such as logs, sticks and dry dead leaves.
Stream levels were monitored after heavy rains, and colour and opacity observed throughout the Project. The above protocols appeared to have been successfully observed.
2:3 Habitat conservation in tree/shrub weed control
Trial interventions considered Elaeagnus and Tree privet seed production, as both seedlings are moderately shade tolerant and quickly become firmly rooted. We decided to leave one large heavily fruiting branch of the largest Elaeagnus tree, to maintain some of the bird food supply.
Most of the Tree privet we treated were juveniles, possibly approaching fruiting, canopied by Tree privet to 10mH, and surrounded by mature fruiting Coprosma.
During the subsequent severe drought, during which many forest fruits have failed to mature, we regretted having removed any fruit-producing material at all. We also observed that in the extreme dry conditions during this trial and to date, leaving more branches and foliage on both tree privets and Elaeagnus would have adequately suppressed the weed trees’ growth while maintaining the shade they had provided for released natives and the forest floor beyond.
2:4 Trial of tree/shrub weed control techniques
The Trial enabled ongoing monitoring of the results of Trial techniques of Tree/shrub weed control by “partial breakdown”.
We found releasing adjacent native plants and reducing weed mass and reproduction by the trial techniques to be less time-consuming and more effective than anticipated.
Tree privet
While more resilient than their native neighbours, these were similarly sensitive to the extreme drought. Consequently their regrowth during the Trial was slower than observed elsewhere in previous seasons, and the very satisfactory results cannot necessarily be expected in wetter areas and conditions.
Nonetheless, the variety of strategies and techniques used for release of natives and gradual attrition of tree/shrub weeds were found to be practical and effective, allowing adjacent native vegetation to thrive and surpass the weed trees until further intervention. (All these tree weeds will of course recover without further intervention).
Tree privet from 30cmH to 4mH were reduced by partially-breaking and bending down branches or in some cases trunks. Follow-up interventions addressed any branches interfering with native regeneration nearby, while maintaining any foliage supporting adjacent vegetation by providing shade and/or ground cover.
Repeat interventions were ad hoc as each specimen was encountered during general monitoring and restoration activities in subsequent weeks or months, with small reductions requiring little effort or time: eg bending down or snapping a branch, or bending all its branchlets backwards, and making larger breaks and suppressions as the tree weakened. As expected, only a few very small ones became sufficiently weak to be easily uprooted during the year, but many large branches ceased to produce leaves or died. Some partially-broken branches or trunks died and were easily broken off. Where this occurred in a large branch or branches, regrowth from other branches slowed significantly.
Specimens in sunlight showed rapid vigorous regrowth, with substantial foliage developing on partially broken branches.
Those in moderately dense shade were slower to regrow, and had been more spindly with less foliage to begin with. Some of these produced very few leaves after their first follow-up intervention.
After several repeat interventions, and during the ongoing drought, all the tree privet juveniles in the Trial area are, in March 2020, considerably shorter (despite all new growth pointing upward), with scant slow-growing foliage, insufficient to impede adjacent vegetation and not expected to need intervention, or to reach a height prohibiting intervention by volunteers, in the next month or so. The continuing regrowth drains the roots’ resources, as the live but unproductive upper trunk and branches demand ongoing hydration from the roots, while supplying no nutrition in return.
[ iNaturalist member baldeagle has explained the plant processes involved, and established that “girdling” kills privets more quickly than felling. Regarding public health and ringbarking, he observed that dead tree privet many metres high stand intact for years, even supporting other falling trees in a storm.]
Elaeagnus reflexa
A tree several metres high with several trunks to 10cmD was partially felled and left hanging by its upper branches caught in the surrounding native canopy. Regrowth shooting from the stump was in shade, and not obvious by Mar 2020, but will continue, assisted by any canopy loss among the adjacent native trees.
Care and gradual staging were used to avoid the weight of large cut Elaeagnus branches falling on native vines hanging among them. Cutting or ringbarking side-branches of such Elaeagnus and waiting for them to decay gradually reduced their weight so that the eventual removal did not damage the native vines.
Many smaller trees, some supported by native trees or vines, were gradually reduced by the same method as described for Tree privet. Seedlings unable to be uprooted were broken off at base, or partially broken and bent down. 3mH Elaeagnus entangled in native vines were treated by partial breakdown aimed at releasing adjacent habitat and slowing their growth sufficiently to ensure the native vegetation was unhindered.
Several Elaeagnus to 2-3m H, or sprawling, were observed in August 2018 (pre-Trial) under the big pine tree (ZoneBdKRS) were reduced then and found without significant regrowth in March 2019. Many more throughout the site were addressed as found from March-June 2019. Slender specimens to about 2mH showed little or no regrowth by September 2019 and received no further intervention. Larger specimens, eg 3-4cmD, had several follow-up treatments, all simple and brief, leaving small plants reduced to a single stem with any small flexible branches gathered and bent into a loop, and large plants leafing only on one or two stems, which were easily partially broken.
Our experiences with the technique elsewhere (Eskdale Forest paths 2009-12, and Gahnia Grove 2018-29) in less-intensive projects we would plan monitoring and intervention quarterly to annually depending on the conditions and the situation and vigour of the specimen, suggest follow-up monitoring and intervention are initially required quarterly to annually, depending on the micro-climate and the current vigour of each specimen.
Tree/shrub weed seedlings
Few young tree/shrub weed seedlings were seen until summer, and then with a high attrition rate despite the removal of competing Tradescantia. Their attrition was likely increased by the exceptional drought, as shade by this time was not deep due to cutting (by reserve users) of pathside vegetation and leaf loss by drought-stressed trees. The surviving weed seedlings were almost all Tree privet. Judging by their number and survival rate along the lightly forested streambanks, where Tradescantia is absent or sparse we expect thousands more to arise and to develop once soil moisture recovers.
Due to their simultaneous emergence in groups of hundreds and thousands towards the end of the Project, these weed seedlings were not counted, but some particularly striking occurrences were recorded in photo observations.
2:5 Exotic Palms
It was a shock during initial survey (March to August 2019) to find several mature invasions of Chusan and Bangalow palm on the streambank. Numerous Phoenix palm seedlings to 50cmH were easily suppressed by looping and knotting. After some months, the first specimen thus treated died, leaves green and intact, its base and root entirely rotted.
Specimens to 60cmH were observed elsewhere in the wider site during survey, suppressed in the same way during survey, have not been monitored but could be easily located for assessment if required.
Dozens of smaller exotic palm seedlings observed were uprooted, both on forested banks and in the streambed itself, and are expected to be increasingly numerous in this and subsequent autumns, with several bangalow now established along the streambank probably maturing and fruiting this year. We expect the current dry conditions to favour exotic palms over nikau and other native trees.
2:6 Vine weeds
Occasional Moth plant seedlings were found and uprooted on the streambanks and in or beside the path. There are about a dozen pod-bearing infestations at the roadside, and a large streamside infestation throughout the canopy of a mahoe 10mH, just downstream of the Trial area.
A wide invasion of honeysuckle and a few Moth plant vines hung from several kohuhu, karo and mahoe along the roadside at the top of the mostly-treeless streambank below.
Mature moth plant vines look very similar to mature Parsonsia heterophylla vines, and the two are sometimes tangled together in the same tree, so great care had to be taken to ensure foliage was identified before cutting a Moth plant stem.
Beside the forest path, Moth plant hung entangled with Parsonsia or Rubus cissoides. (Here, the Rubus cissoides vine was also entangled with Elaeagnus, whose rough brown stems superficially appeared similar).
Intervention in these cases was by gradual reduction over several months, as parts of the invasion could be located, identified and removed without damage to native vines.
Post-trial, regrowth is expected in any remaining undiscovered parts of such vines.
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