Restoring grassy understorey under Forest Red Gum – Wolston Creek Bushland Reserve, Riverhills, Queensland

Key words: grassy understorey, assisted natural regeneration, Bushcare, Green Panic.

Carole Bristow and Julie Vejle

Wolston Creek Bushland Reserve, is a 47 ha Brisbane City Council (BCC) Bushcare Site at Riverhills, in the south-west outskirts of Brisbane, Queensland. Although only a relatively small area of bushland remains, connectivity is provided by Wolston Creek, its tributaries Sandy and Bullockhead Creeks and the banks of the Brisbane River.

This report is about just one of the plant communities that the Bushcare group (a handful of dedicated locals) is working on: a 1.55 hectare patch of Forest Red Gum (Eucalyptus tereticornis) forest with a grassy understorey (Fig 1).

Fig 1 (a) Understorey of Eucalyptus tereticornis forest at Wolston Creek Bushland Reserve showing a typical swathe of Green Panic before treatment

In mid-2008 when the project commenced, the understorey of the forest was virtually entirely dominated by the exotic grass Green Panic (Megathyrsus maximus var. pubiglumis) due to past clearing and the sowing of pasture grasses for grazing.   However, as there were some native grasses still evident, we wondered whether the site might respond to an assisted natural regeneration approach that has worked on other sites.   Out of curiosity, we cleared Green Panic by hand around a patch of native grasses to give them space to expand. In the process we found that, under the cover of the Green Panic, were struggling native grasses, sedges and wisps of herbs (Fig 2). This then became a test patch, which in turn became a stand of Pitted Bluegrass (Bothriochloa decipiens). The outstanding results energised us to continue.

Fig 2 (a) Much of the site started with very small native germinants which would have been inadvertently killed had the site been sprayed with herbicide. Photo (a) shows how few natives were sometimes initially revealed – in this case Whisker Sedge (Cyperus gracilis), Common Bindweed (Polymeria calycina) and native Glycine sp.

Our treatments. We further cleared the  Green Panic using mattocks. With the weed cover much reduced, rains brought a massive annual weed response.  We considered spot spraying but while the seedbank response was still being discovered, every native plant was important, so the enthusiasm was there to increase hand weeding on an extended basis. To cope with the volume of hand weeding required, removal was aimed mainly at weed individuals that were flowering and/or seeding.  The result was a gradual reduction of weeds and maintenance of a microclimate for germinating natives.

As this site matured and produced such a vigorous response from natives, it became possible to consider other techniques. In January 2009, an adjacent area was slashed by BCC  to prevent Green Panic from seeding into the work area. By May, with good rains, the slashed area had produced a resurgence of natives, so BCC was asked to stop slashing.  We mattocked out the Green Panic crowns, increasing the original work area by 50%.  We realised that slashing several times proved to be a good preparation for primary weeding.  With the Green Panic tops cut and largely decomposed, the pattern of natives and weeds was better revealed and allowed light to trigger germination and growth.

Subsequently, it was found that initial ‘overspraying’ (taking care to spray the standing Green Panic only) produced a similar response.

Results.  Compared with the former near-monoculture of Green Panic, the ground stratum now has abundant native cover comprising: 18 species of native grasses; two sedges; 19 forbs; one shrub; five twiners, and both Forest Red Gum and Maiden’s Wattle (Acacia maidenii) are regenerating.

The area treated within the first year (250 m2) has now been expanded to 1.55 ha.  Of this, 30% can be considered on ‘maintenance’ i.e. stable native cover, requiring minimal visits for the occasional weeds.  A further 65% is undergoing secondary treatment (still requiring regular work to bring it to ‘maintenance’ stage) and about 5% has undergone primary treatment in 2012.

Lessons learned:
1. It is important to try several patches when testing the resilience of a site.  In our case, making a judgement based on one patch of slow response could have caused misinterpretation of the whole site. (Indeed, areas of slow response eventually filled in and helped to create the diverse mosaic of the ground stratum.)
2. Where appropriate, consider slashing/brushcutting/or spraying weedy grasses as preparation for (or initial phase of) primary weeding in areas that have been found to have strong resilience.
3. Try to view the post-primary weed flush (which often appears to be a ‘sea’ of annual weeds)  as part of the recovery process rather than become overwhelmed by it.  The gradual removal of weed still provides protection for germinating natives.
4. Remove weeds before they seed; keep maintenance stage as the goal;  expand the work area in small steps.  (We were lulled into a false sense of success in dry periods, and possibly expanded too quickly.  So we’ve learned to hasten slowly.)
5.  Plant identification is all important. At the start of the project, the main plant we needed to know was Green Panic. In the continual task of sorting weeds from natives, we remind ourselves that ‘If in doubt, don’t pull it out’.  When a plant is found that we don’t know, it is time to take the interesting journey to the Queensland Herbarium.

The main contributing factors to the success so far have been:
The people: curious; persistent; patient; willing to learn from our mistakes; learning to work with nature’s cycles.
The site: has a strong seedbank that was triggered when rains came and its widely spaced trees made it suitable for preparatory slashing.
The motivation:  the reward and excitement of seeing a native plant community build in integrity and diversity.

We anticipate that there is .5 ha yet to treat. Long term success therefore depends upon the continuation of the above factors and continued support from Brisbane City Council.

Acknowledgements.  Brisbane City Council Habitat Brisbane Section, Wolston Creek Bushcare Group, Queensland Herbarium

Contacts:  Wolston Creek Bushcare Group: For more information, email info@wacc.org.au

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