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Comments on revegetation works along the lower Barham River
Tidal section near bridge
This area has been heavily grazed over a long period of time, so revegetation in many areas has proceeded from scratch, introducing locally sourced trees and shrubs which have thrived on the higher ground near the river. However, in the lower, increasingly salt-affected soils closer to occasionally saline waters, the diversity and vigour of the new plantings decreases and another suite of indigenous plants, as well as some weeds, takes their place.
Successful revegetation in fenced area at left, grazed areas at right.
This planting can therefore be used as a reasonably precise guide to the indigenous species best suited to varying conditions in this general area, and to fine-tune further plantings. A surveyor’s level could be used to map where these grow best fairly precisely, using the lower fringe of the area where all new plantings have thrived as the benchmark below which these more terrestrial species will not grow well.
Below this point a different set of plants becomes dominant, and includes both indigenous and introduced plants, all of them fairly low-growing. These include significant mats of the edible sea celery (Apium prostratum at left) which can be used as a guide to soil salinity by taste alone, as the plants on the higher, less salt-affected ground are bland in flavour, while those on boggy ground close to the water are distinctly tangy. Other mat-forming indigenous plants here include creeping brookweed (Samolus repens) and on the lowest, wettest ground waterbuttons (Cotula coronopifolia); all three of these species can readily be propagated from cuttings.
There are also several introduced weeds which may be abundant in places, of which the most conspicuous is bushy starwort (Aster subulatus, at left), an annual daisy which spreads by wind-blown seed. Although a prolific seeder, the tiny seed cannot germinate under mats of indigenous plants, which could be established to out-compete it as it is the most visually jarring of the three weeds.
Also present is a form of fat hen (Chenopodium album), a larger seeded annual which could be controlled by pulling out over several years, with an emphasis on preventing seed formation.
Buckshorn plantain (Plantago coronopus) is a fairly deep-rooted perennial and would take more effort to remove, but even the largest patches of this weed are fairly localised and are only likely to spread slowly.
In the shallower water, the indigenous common reed (Phragmites australis) is making a comeback now that grazing has been stopped, and is moving back into seasonally flooded soils at an estimated 2 metres per year. This tall grass may need to be replanted along the barren shore of the river facing the caravan park, but should only be used in small quantities here, to tie in visually with the far bank where it is the dominant plant.
Instead, river clubrush (Schoenoplectus tabernaemontani) which forms a large stand near the bridge should be used more extensively here, and this species can easily be grown from seed sown on waterlogged soil in spring.
Eroded streambanks further upstream
The deeply channelized section of section being revegetated further upstream is a more difficult and longer-term project for restoration. The river here would have been shallower and broader before European settlement, filled with snags in the form of fallen trees and branches which slowed flow rates, and spilling out over the surrounding floodplain during wet periods so erosion was minimised or at least spread out. Clearing of the banks and removal of snags has created faster, eroding flows in the main channel, so that this has cut deeply into the soils below.
Removal of willows has been completed before replanting, and these relatively brittle trees have probably contributed to collapse of the banks as they tear out easily during floods.
Continued planting of blackwoods and eucalypts on the high ground is recommended as this will ultimately provide larger trees which will become snags when they die, slowing flows and allowing silt to build up to higher levels. However, this is a process that will take many decades. As the canopy matures, it will also shade out many of the annual weeds which presently dominate the sloping banks. These include drain sedge (Cyperus eragrostis, below left) and nasturtium (Tropaeolum majus, above left), both of which are introduced, while the status of pale knotweed (Persicaria lapathifolia, below right) is not certain although it is widely regarded as a native.
Although some indigenous plants may be found growing in the streambed itself (including a water milfoil, probably Myriophyllum simulans), these are only likely to increase in numbers as snags gradually refill this section of stream, slowing water flow and allowing a greater diversity of aquatics to grow.
Summary of recommendations: the recent plantings along the two sections of the Barham River discussed in this report provide useful information for future revegetation in the lower reaches of this river, which has been radically affected by grazing, clearing of streambanks, and extensive erosion in the faster-moving sections.
In areas closer to the sea, terrestrial plantings should be confined to higher ground which is less likely to be affected by salinity and tidal events, and the present plantings here provide a fairly precise benchmark which could be mapped with a surveyor’s level. Below this level the emphasis should be on establishing a dense cover of indigenous species which will ultimately compete with the annual weeds present, and if possible removal of buckshorn plantain to minimise its spread.
The bare banks of this side of the river should be replanted against erosion in parts, including with some common reed to tie in visually with the far bank adjacent to the caravan park, where this is the dominant plant. However, the bulk of such plantings would ideally be of river clubrush, a more valuable habitat and nesting plant, and it is also desirable to leave some of the more gently sloping open areas unplanted, as these provide valuable resting places for some waterbirds such as cormorants (seen here on the bar nearest the bridge).
Upstream, continued planting of the higher banks with trees which will become the source of snags as they mature should remain a priority. Plantings on the steeply sloping banks are a more difficult proposition as these may be eroded out during floods, and few indigenous plants are adapted to these relatively unnatural conditions. With time, both of these problems should be mitigated by the gradual build-up of snags, slowing flows and trapping silt so the stream bed rises, while a more extensive overhanging tree canopy will encourage shade-loving perennial plants over the annuals.
Nick Romanowski, 6.5.2011