Australasian Plant Conservation
Originally published in Australasian Plant Conservation 21(1) June - August 2012, p 4-6
The use of mitigation planting to achieve strategic planning outcomes in local riparian restoration
Lisa Carter1 and Murray Swales2
1) Parsons Brinckerhoff, Brisbane, QLD, Australia. Email: LCarter@pb.com.au; 2) Cardno, Brisbane, QLD, Australia. Email: firstname.lastname@example.org
The ‘long-shot’ sling shot being used to collect seed from within the site to ensure that the local genetic variation was incorporated into planting. Photo: Murray Swales.
Nursery used to propagate the seed collected from the reserve, 10,000 tubes were ‘grown-on’ from the seed collection, including 24 species of trees and shrubs. Photo: LWA.
LWA staff and contractors participating in the revegetation works after the site’s initial pest plant treatment. Photo: LWA.
The Logan Water Alliance (LWA) is a public and private sector alliance involving Allconnex Water, Tenix, Parsons Brinckerhoff and Cardno. Established in August 2009, it is one of the largest water infrastructure programs of its type in Australia, responsible for delivering water and wastewater infrastructure throughout the Logan district (south-east Queensland) until at least 2013.
The LWA is a planning-led alliance that encourages consideration of ways to avoid, or mitigate, environmental impacts at a planning level, for example by choosing pipeline alignments that minimise vegetation clearing. Significant effort is subsequently made during project design and construction to ensure the extent of impact is minimised and clearance of threatened species is avoided.
Nonetheless, the clearance of Least Concern native plant species protected under the Nature Conservation Act 1992 is necessary to establish water and wastewater infrastructure. The clearance of native plants must comply with policy requirements of the Queensland Department of Environment and Heritage Protection (DEHP), including the provision of compensatory mitigation planting to achieve ‘No Net Loss’ of Least Concern plant species.
Strategic planning objectives
The Greenwood Lakes Reserve rehabilitation program was developed by the alliance in conjunction with Logan City Council’s (LCC) Natural Areas Division and the DEHP, with the aim of identifying an existing reserve with the potential to function as a mitigation site whilst contributing to local environmental and social values. Strategically targeting a large-scale bushland restoration site (as opposed to the traditional piecemeal approach) allowed the alliance to progressively use the site across many projects over the course of the water infrastructure program. The 43 hectare reserve, located on the banks of Oxley Creek in Forestdale, Queensland, consists of two open riverine water bodies supporting 34 ha of fringing riparian vegetation suitable for restoration within Crown (Trust) Land zoned as Public Open Space.
The reserve forms part of the 60 km Flinders to Greenbank-Karawatha corridor which provides important habitat and movement opportunities for native fauna within south-east Queensland, linking larger tracts of remnant vegetation within the Greenbank Military Training Area to the south-west and the Glider Conservation Park to the north-east. The reserve was degraded due to past sand mining and clearing activities resulting in a modified vegetation structure and dense Lantana camara infestation that was significantly limiting habitat connectivity with adjacent remnant vegetation along the riparian corridor.
The reserve also contributes to the visual and social amenity of Forestdale as it is used by residents and visitors for passive recreational activities such as bird-watching, walking and lake-based activities. The rehabilitation program will provide a long term social and environmental asset for the local community.
Pre-restoration condition assessment
Prior to development of the rehabilitation program a comprehensive condition assessment was undertaken to classify vegetation into condition classes, collect empirical data relating to the extent of weed invasion and provide a formal methodology for assessment of vegetation condition for future reserves. The condition assessment used a belt‑transect method to assess the condition of vegetation within 20 x 20 m quadrats that were allocated a score relative to the integrity of each vegetation stratum and abundance of weed species. These were then ranked through comparison to a reference community. The results of the vegetation condition assessment were mapped and used to prioritise bushland restoration areas and define management actions across the reserve.
Riparian restoration methodology
The aim of the rehabilitation program is to improve the quality of habitat and ecosystem functioning of the reserve while stabilising the soils from future erosion. An overarching restoration guideline was developed, namely the Terrestrial Vegetation Management Plan (the Plan), utilising the results of the vegetation condition assessment and relevant LCC and DEHP conditions. The duration of the Plan includes a maintenance phase of three years (inclusive of a six month establishment period) which was defined by the DEHP conditions.
The Plan defines the vegetation management objectives for the rehabilitation program and outlines associated management actions, including:
- a staged program of weed control
- assisted regeneration practices within areas of the reserve identified as supporting sufficient representation of native species
- targeted revegetation for open areas identified as supporting an insufficient representation of native species or exhibiting missing or underrepresented vegetation stratum components
- a detailed monitoring program to check that performance indicators for revegetation, rehabilitation and weed control are achieved
- an adaptive management strategy, including an event contingency plan, to cover issues that can arise from natural disasters, allowing for a ‘natural response’ period prior to performing any further active revegetation following an event.
Erosion controls were identified as a key consideration in degraded areas and where high level weed controls were likely to result in soil disturbance and exposed areas. In order to determine where soil amelioration was required, the underlying soils of the reserve were analysed. The assessment identified a high level of dispersive clays (also evident as gully erosion and sinkholes from past sand mining) and low pH (pH range of 4.2 - 6.2, mean pH of 5.2). Ameliorant regimes were developed and added to the individual infill tubestock, with the goal of improving the general soil structure over a five year period (i.e. increasing pH and improving the clay properties in the upper soil strata), hence improving not only growing conditions but minimising future erosion issues.
In order to facilitate the natural formation of a healthy vegetation community representative of the original landscape, the planting schedule for the mitigation planting was derived from the pre-existing Regional Ecosystem descriptions provided by DEHP mapping and field observations. Seed was collected from the site to ensure that the local genetic variation was incorporated into the planting. Four months prior to the works, the seeds from 24 species were collected within the reserve, comprising nine tree species and 15 small trees / shrubs, for propagation and revegetation activities.
The initial phase of the rehabilitation program commenced in August 2011, targeting vegetation mapped in poor to moderate condition and included the following key activities:
- restoration of 12 ha of riparian forest within the reserve, inclusive of a three hectare area of high level weed infestation (3-4 m high Lantana camara);
- active revegetation totalling 81,516 plantings, including 10,000 tubes propagated from ‘in situ’ seed collection; and
- installation of 34 nest boxes to provide compensatory nesting and denning habitat for a range of endemic species.
The preliminary results of quadrat monitoring (at five permanent and ten random plots), undertaken at the completion of the initial phase of restoration activities in February 2012, indicate that the Plan requirements with respect to weed control targets (<20% non-natives after establishment period) and vegetation benchmarks for active revegetation (>90% survival of infill plantings) have been met. Natural regeneration is still in the early stages and would not be expected to meet relevant performance indicators at this point.
Overall, the Greenwood Lakes Reserve rehabilitation program has been successful in achieving strategic planning outcomes for the Logan district by benefiting broader environmental and social values. Similar strategic mitigation planting models could be used effectively on large scale infrastructure projects to deliver sustainable long-term restoration outcomes that improve the quality of habitat and ecosystem functioning by reconnecting fragmented corridors.
We would like to thank and acknowledge the LWA Environment Team, including former members, for their contributions during LWA’s rehabilitation program and the LCC Natural Areas Division and DEHP for their preliminary direction and defined objectives.