Test: Final Report Section 4 of 4 "A, Moderate shade..." to End
A. Moderate shade under tall canopy
Under large old pine, kanuka and tanekaha, where leaf litter was longstanding, deep and dense, Tradescantia cover was initially light, superficial, and maybe relatively recent. Tradescantia was removed completely from these areas, with no drought-stress or other impact observed, and released juveniles and seedlings continuing to grow. Few new seedlings or sporelings were observed until March 2020, and are not numerous now, in the undisturbed deep litter.
B. Quadrats of Tradescantia removal
Several Quadrats of Tradescantia removal, each about 80cm sq, were marked with ground staples and plastic tape in the midst of deep dense Tradescantia under the roadside trees, from 1 to 5 metres from the road.
A few dead leaves and pieces of kanuka brush were placed in each to provide micro-shade. These quadrats remained dry and bare throughout the year, with only one or two producing one or two seedlings of Coprosma or kawakawa, and a few Crepis capillaris (benign exotic) seedlings, all of which died soon after.
Several smaller (c.40cmD) similar areas were briefly released in winter, releasing vigorous new seedlings of scattered coprosma, dozens of kawakawa, several toatoa (Haloragis erecta) and a few kowhai and possible mahoe. These newly-bared areas quickly dried out when the sky cleared, so the seedlings were unlikely to survive and the areas were re-covered with loose Tradescantia to prevent further stress to nearby trees.
C. Streamsides
Tradescantia was carefully removed from several areas of streamside with dense lush native vegetation. In these areas few seedlings were found other than those already evident emerging from light Tradescantia coverage. The released juveniles and large seedlings (eg 10cmH) flourished as expected, at least until late summer, when some individuals started to be affected by drought.
Throughout a level sedimentary sandstone bank less than a metre above water level in zone CaKSS, moderate regrowth of Tradescantia had occurred by March 2020s. Strands, gently uplifted, revealed dozens of ti kouka, several coprosma, a few puriri and mahoe, and some carex seedlings, with numerous unidentified fern sporelings.
D. Steep bank above stream
As significant rain resumed in winter, Tradescantia was also gradually removed from the steep bank above the stream, and eventually all carried uphill to reinforce the mulch under the roadside mahoe, kawakawa and karo.
Numerous patches of the creeping native fern Lastreopsis hispida were found under deep undisturbed Tradescantia in March, and left undisturbed to avoid moisture loss. They were gradually released from the Tradescantia with the return of steady rain in winter, and grew rapidly. The Lastreopsis were joined in spring by new sporelings.
Some Lastreopsis grew to c. 1mH. Some of the other sporelings (ponga, Pteris tremula, Deparia) developed to c. 30cmH.
Many were still vigorous in March 2020, but some may not have survived the Spring/Summer drought without Tradescantia cover.
E. Forest pathsides
Where the path had been widened through plant damage and trampling, Tradescantia was absent or reduced, leaving compacted and bared clay holding a reduced number of juvenile trees and the exposed above-ground roots of numerous mature mahoe and pate.
After on-site discussion with Council and contractors, the path was re-defined using ground staples and plastic marking tape at ground level. Piles of Tradescantia were applied outside the taped edge, to restore soil health and moisture and to define the path for Reserve users. It was hoped that the remaining juvenile trees would then be permitted to grow freely with vertical pruning only, with ground covers (carex, Basket grass, fungi, mosses etc) recovering or returning, supporting soil health and hydration for the health and ongoing development of the native trees along the path.
We considered creating signs to educate Reserve users about the need to support shade and canopy development, and the role of shade in preventing weed invasion. However, we found we had not the resources to do this thoroughly and effectively.
The pathside Tradescantia piles were monitored at least weekly through winter, and regrowth scooped back onto the piles, with some turning of small piles to restrain growth and accelerate decomposition, to avoid requests for herbicide spraying.
Piles were gradually combined and reduced through Spring and early summer, preventing regrowth and to permit sporeling/seedling germination in released areas of pathside.
Due to the shortness of winter, Spring/Summer drought and increasing soil moisture deficit, the Tradescantia along the pathsides had mostly decomposed by early summer. Exposed and compacted clay along the pathsides remained bare. In areas of leaf litter, humus or light Tradescantia regrowth, generally around the bases of trees or rotting logs, native seedlings began to appear during the summer (Coprosma, pate, mahoe).
In March 2020, many pathside Carex partly or completely yellow or brown in May 2019 are growing healthy new leaves. Those with Tradescantia piles beside them are vigorous and lush. Basket grass is increasing from remnants in the path edges.
Some pathside juvenile trees have grown slightly, some are little changed. Drought stress is common among the broad-leaved trees, especially mahoe, hangehange and kanono.
Of three rawirinui juveniles observed growing vigorously beside the path in June 2918, only one survived the pruning and felling of pathsides in early 2019. This one, set furthest back from the path, is growing well.
The horizontally-pruned and sun-invaded stretches of pathside have lost some of their smaller trees and ferns to drought; particularly hangehange, pate, kanono, mahoe, and their epiphytes, including petako and huruhuruwhenua.
Many of the juvenile trees and ferns epiphytic on pathside trees have grown increasingly pale and flaccid during the summer, with some unlikely to recover.
Review
3:1 Plant health and reproduction
After the year of observation and reflection on various possible causes, we suspect chronic desiccation as a major contributor to both the tree deaths noted in Initial survey, and the absence of seedlings on the roadside bank.
A limited seedbank and soil flora in this constructed roadside may also be a factor in the absence of seedlings, but note that Coprosma and kawakawa seedlings did germinate both under Tradescantia and in patches released earlier from Tradescantia, but none survived.
The several juvenile kotukutuku (Tree fuchsia) along the forest path became vigorous and leafy during winter, before losing most of their leaves in Spring.
“Adult kotukutuku #\1” (ZoneFbWSS), in an area of run-off that remains largely Tradescantia-covered and relatively well-hydrated, produced sparse foliage and flowers, but the foliage blackened and crumbled early in summer, possibly due to the tree’s advanced age, and perhaps poor root instability and ill-health in an area of sewage overflow and stormwater run-off after heavy rain.
3:2 Climate change, soil moisture deficit and low rainfall
Over the course of this trial, we observed and struggled to comprehend plant responses and conditions previously unknown to us.
In late 2019, as the Australian bushfire disaster worsened, news media began to provide background on the extent of climate change and drought in that country. We then learned that the effects of the earlier Antarctic warming and Extreme Postive Indian dipole events contributed not only to the Australian drought and subsequent fires, but to the current ongoing drought in the North and East of New Zealand, and such climate events are expected to occur with increasing frequency with ongoing global warming.
We then understood our observations and concerns to reflect the unprecedented depth of soil moisture deficit in the currently ongoing 2019-20 hydrological and meteorological drought.
“The lowest rainfall for summer was recorded in Dargaville (63mm), Leigh (59mm), Whangaparaoa (60mm), North Shore (53mm), Western Springs (62mm) and Auckland Airport (73mm)”.
https://www.stuff.co.nz/environment/climate-news/120017585/north-drought-weather-records-tumble-in-northland-auckland-over-summer)
We also noted throughout the year that Metservice temperature and rainfall records reflect Whangaparaoa, not Glenfield, where rainfall was often less or completely absent, and maximum daily temperatures on North-facing ridges were informally recorded up to 10 degrees higher than temperatures reported by weather stations.
Locally at least, the climate and soil conditions are very different from even recent years. The loss of even small amounts of groundcover, shade or shelter can cause critical soil desiccation, tree death and habitat loss in these conditions. If ground cover and/or shade are lost and cannot be restored, we envisage escalating desiccation leading to devegetation, erosion and land slips, particularly on ground contoured by road construction using loose porous materials, and without run-off due to road guttering and piping directly into the stream.
We believe the impact of soil moisture deficit and continuing drought on the Trial site was exacerbated in some places by both interventions for amenity, and by our own manual weed removal and the removal of large areas of plastic and other inorganic refuse.
In these conditions the Methodologies moisture-retention techniques, including the use of all material as mulch, failed to completely mitigate drought-stress.
From March to at least June, intense heat and sun recurred between rainy periods, immediately drying all surface moisture in the roadside area, and on lower banks arresting the development of seedlings both native and exotic.
Ongoing drought reduced canopy density through leaf wilt, leaf drop, smaller leaf size, and in some cases tree death.
Piles of Tradescantia brought uphill from the lower streamside banks quickly lost most of their mass, leaves completely disappeared (helped by earthworms and other invertebrates) and the remaining stems did not shoot, instead drying and disintegrating, producing little humus.
Piled Tradescantia not only failed to regrow and spread beyond the pile, but much of the remaining Tradescantia alongside it stopped growing, even in winter.
By January 2020, areas of undisturbed Tradescantia had also become sparse, letting light in and moisture out, and on the drier banks became discoloured (purplish) and wilted.
Our reduction of the relatively small amount of vine and several juvenile tree privet at the roadside of the Tradescantia trial area likely contributed to tree stress by letting light angle in under the dehydrated and shrinking canopy of the much taller planted trees lining the road.
Thus the trial unintentionally demonstrated the negative impact of weed removal, prior to or during meteorological drought, from an area of deep soil moisture deficit, despite moderately dense tree canopy, normal tree leaf turgor, and ground cover and soil surface intermittently wet from light to moderate rain.
On streambanks and in [previously deeply shaded streamside forest, Tradescantia regrowth was much less than expected. In most areas of intervention, follow-up removal was contraindicated by drought.
Having observed these effects, and learned more of the recent and current climate change in Auckland, we now recognise the need to assess deep soil moisture, (noting we lack the expertise or equipment to do this), before planning or undertaking weed control or inorganic refuse removal.
We also suspect there has been a long-term (10-60 years?) increasing desiccation of the roadside bank.
Causes may include:
- artificial contouring with introduced porous materials during construction
- absence of trees on the roadside bank until the 1999 planting of the upper roadside (but not the bank below)
- guttering and piping directly into the stream of all rainwater from the road and the land across the road
- thinning of canopy due to tree deaths (tanekaha, ti kouka, kanuka, and some unidentifiable) from 1999 (or earlier) to 2019
In the current climate of prolonged drought with short winters and long hot summers, weed control, and any other intervention affecting shade or ground cover in the vicinity of vegetation, needs to be even more restrained and gradual, and the ecological services currently provided by weeds and refuse considered even more carefully before intervention. Some of the locally wild plant species may no longer be viable, especially if planted.
Unable to directly correlate species, tree size, site conditions, run-off patterns and interventions with all observed incidences of severe drought stress, we have been trying to determine the factors influencing each tree’s response to desiccation.
4: Trialling interventions to reduce ongoing desertification of local environments
Since January 2019 we have increasingly focused our Gahnia Grove trial on ways of maintaining or replacing shade and ground cover to allow safe removal of weeds presenting an actual or imminent threat to existing habitat. When time permits we hope to discuss these issues further, along with both trialled and proposed interventions. As always, we welcome any suggestions and experiences, in our Gahnia Grove Trial reports.
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