Tradescantia - Streamside banks
Most streamside banks from water’s edge (in dry season) to Kaipatiki roadside or Native Plant Trail were observed in June 2018 almost fully canopied by native vegetation with occasional 10-20mH tree privet or wattle, with a dense understorey of trees, shrubs and vines, scattered ferns in the forest and among Tradescantia. Some of the more recently eroded banks were bare or held only exposed tree roots, some held numerous ferns and tree seedlings lightly covered with Tradescantia.
This cyclic washing-out and sedimentation of banks essential to a natural meandering stream, maintaining water quality and freshwater habitat, was observed 1997-99, with the subsequent cycles of young and old native vegetation, and annual fresh supplies of Tradescantia from further upstream.
In 2018-19, there are few areas of heavy sedimentation, and many more of deep scouring of a wider stream. This is presumed due to the continued increase of stormwater, whether piped from housing and roads or channelled from paths, or unchannelled via deep crevasses cut into the clay of the almost-sheer banks above.
If our impressions are correct, this bodes poorly for any revegetation to be established now or in the future in this narrow strip of streamside forest surrounded by dense housing, roading and, up stream, commerce.
However, the exploration of what might be growing under the Tradescantia quickly resulted in the removal of most visible Tradescantia from large areas of streambank in several zones, and produced dozens of Tradescantia piles initially up to 2mx2m and 1.5mH.
Parts of the streambank are not suitable for weeding in a short-term (ie up to 3 years) 1-person Project as bank collapses, tree falls and resulting partial dams make the full extent of Tradescantia extremely difficult to access and the banks subject to ongoing entrapment of flood-borne Tradescantia.
One such area holds a large clump of ginger which will not be addressed in this Trial, though other clumps have been suppressed by stem-cutting and/or breaking and tuber-rotting.
Several tall slender woolly nightshades and wattles overhanging the stream among native canopy have been cut down (wattles) or partially-cut or broken (woolly nightshades).
The stream and much of the streamside vegetation, including a little of the areas dominated by ferns, are now visible from the Native Plant Trail. We have defined the Trail at ground-level with yellow plastic tape secured by ground staples, and identified it as a Trial chemical-free area with a few home-made signs, also encouraging Reserve users to report sewage overflows and water pollution events, with the numbers for Watercare and Council.
The large loose piles of Tradescantia are nitrogenous material that should be minimised as it decomposes to avoid nitrification of the streamwater. We are gradually moving the piles further from the stream, using them to border the path, and are also placing some of the abundant excess wheki and ponga fronds between them and the stream both to slow run-off of nitrogenous liquid and to replace the ground cover. As the streamside forest here appears well-hydrated and holds plenty of humus already, ideally the reducing Tradescantia mass would be gradually relocated to the dry under-nourished roadside margin. Not having the resources for this i the current project, we are gradually mio
This sandy soil is porous, holds much decaying plant material and is continually hydrated by the stream beneath, but it also provides habitat for many invertebrates, though surprisingly we have not in 2018-19 yet seen a lizard of any kind. Moving of Tradescantia exposes spiders, insects, and earthworms of the type commonly observed in both deep and superficial soil, so removal is done incompletely at first, and piles of Tradescantia are left at frequent intervals to allow self-relocation.
To reduce the size of the piles, we usually keep other plant material in the piles to a minimum, but with such a vast amount of nitrogenous material being piled near the stream here we are allowing tree fern litter etc in the piles, with the idea of increasing the carbon-nitrogen ratio of breakdown products.
Most of the areas of dense Tradescantia thus cleared held scattered juvenile native trees and ferns and occasional weed shrubs/trees, no vines either weed or native, and no small native seedlings.
Areas where the Tradescantia coverage was light or moderate held numerous juvenile native trees, carex and ferns, with a greater diversity and some small seedlings, including pigeonwood, karaka, tawa, karaeo and maire.
Detailed observations are yet to be made of seedlings in an area of light Tradescantia coverage not yet removed. Monitoring will continue in areas where Tradescantia was removed earlier and few or no seedlings were found.
Tradescantia piles will continue to be monitored and aggregated as they decompose.
The rate of regrowth of Tradescantia will be observed, and from past experience is expected to be rapid on streambanks from which it is not possible to remove its stems buried under many layers of sandy sediment.
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