WEED RISK ASSESSMENT FOR THE WAIHOLA-WAIPORI WETLAND COMPLEX, OTAGO JULY 2009 Report No. 2182 Prepared for: LAKES WAIHOLA WAIPORI WETLANDS SOCIETY INC. C/- PAULINE BACON 26 NORE STREET WAIHOLA 9073 WILDLAND CONSULTANTS LTD, DUNEDIN OFFICE, 764 CUMBERLAND ST, DUNEDIN Ph 03-477-2096, Fax 03-477-2095, email [email protected], website www.wildlands.co.nz 11 Contract Report No. 2182 HEAD OFFICE: 99 SALA STREET,© P.O. 2009 BOX 7137, TE NGAE, ROTORUA. Ph 07-343-9017; Fax 07-343-9018 CONTENTS 1. INTRODUCTION 1 2. METHODS 2 3. ECOLOGICAL CONTEXT 3 3.1 Hydrology 3 3.2 Ecological significance 3 4. LAND TENURE 4 5. VEGETATION TRENDS 5 6. THREATS TO INDIGENOUS VEGETATION AND HABITATS 6 6.1 Indigenous vegetation 6 6.2 Nationally uncommon and regionally important plant species 7 6.3 Fauna 10 7. RECREATIONAL AND CULTURALLY IMPORTANT SITES 10 8. STAKEHOLDER INVOLVEMENT 10 9. WEED CONTROL STRATEGY 11 9.1 The need for weed control 11 9.2 Limitations of current weed control 11 9.3 Weed control in outlying areas 11 9.4 Timing of control 12 9.5 Where should control start? 13 9.6 Control methods 13 9.6.1 Aerially-applied herbicide spray 13 9.6.2 Ground- and water-based control 14 9.6.3 Use of volunteers 14 10. WEED CONTROL PRIORITIES 15 10.1 Pest management strategy for Otago 2008 15 10.2 Setting priorities 15 11. PRIORITY 1 WEEDS 17 11.1 Alder ( Alnus glutinosa ) 17 11.2 Crack willow ( Salix fragilis ) 19 11.3 Grey willow ( Salix cinerea ) 20 11.4 Reed sweet grass ( Glyceria maxima ) 21 12. PRIORITY 2 WEEDS 22 12.1 Elder ( Sambucus nigra ) 22 12.2 Red alder ( Alnus rubra ) 22 12.3 Wilding conifers 22 12.4 Scotch broom ( Cytisus scoparius ) and gorse ( Ulex europaeus ) 23 12.5 Montpellier broom ( Teline monspessulana ) 24 12.6 Silver birch ( Betula pendula ) 24 22 © 2009 Contract Report No. 2182 13. PRIORITY 3 WEEDS 24 14. ESTABLISHMENT OF INDIGENOUS VEGETATION 25 14.1 General 25 14.2 Raupo 25 15. RECORD KEEPING, MONITORING, AND RESEARCH 26 15.1 Records 26 15.2 Monitoring 26 15.3 Research 27 16. OTHER ISSUES 27 16.1 Dead trees blocking waterways 27 16.2 Financial resources 27 16.3 Grazing 28 16.4 Dumping of rubbish 28 17. CONCLUSIONS 28 18. RECOMMENDATIONS 28 ACKNOWLEDGMENTS 29 REFERENCES 30 APPENDIX 1. Weed distribution maps 31 PROJECT TEAM Steve Rate - Vegetation survey, report compilation. Kelvin Lloyd - Internal peer review. Reviewed and approved for release by: _______________________ W.B. Shaw Director/Principal Ecologist Wildland Consultants Ltd Wildland Consultants Ltd 2009 This report has been produced by Wildland Consultants Ltd for Lakes Waihola Waipori Wetlands Society Inc. All copyright in this report is the property of Wildland Consultants Ltd and any unauthorised publication, reproduction, or adaptation of this report is a breach of that copyright. 33 © 2009 Contract Report No. 2182 1. INTRODUCTION The Lakes Waihola Waipori Wetlands Society Inc. was formed to promote the sustainable management and enhancement of the Waihola Waipori wetland complex on the Taieri Plain near Waihola, Otago. This wetland complex is nationally and internationally significant for its indigenous habitat and wildlife values, fishery and hunting values, and cultural and recreational values. The wetlands are also very significant to local M āori. One of the key issues identified by the community and stakeholders at a planning meeting in November 2007 was the need to control invasive weeds in the wetlands. In particular, crack willow ( Salix fragilis ), and reed sweetgrass ( Glyceria maxima ) have become major invasive weeds (crack willow for the last 60 years), and there are new threats from species such as alder ( Alnus glutinosa ) and grey willow ( Salix cinerea ). Control of crack willow has been undertaken by the Otago Regional Council, Department of Conservation, and Fish and Game New Zealand over the last eight years, but the approach to management of weeds in the wetlands, and protection of biodiversity values has been piecemeal. This is, in part, due to the large size and complex nature of this tidal lowland wetland, and also due to the number of landowners within and adjacent to the wetland and the many different community interests. This has made it difficult to plan for and implement comprehensive weed control within the wetlands. The Lakes Waihola Waipori Wetlands Society held a recent weed and native vegetation workshop that brought the community together and established a community mandate with the following goal for weed management and indigenous vegetation and wildlife enhancement in the Waihola Waipori wetland complex: “A wetland system where weeds are being actively controlled according to a plan in order to maintain and enhance the diversity of habitats including a balance of open water and land, for the benefit of native plants, fish, birds and other fauna and for recreational purposes”. The Society intends to apply for funding to assess, plan, and undertake weed control in the wetland complex. The first task is to undertake a comprehensive weed risk assessment. This report comprises the weed risk assessment, and addresses the following issues: • Identification of significant weed species in the wetlands that could potentially invade habitats within the project site, either in its present or restored state, and descriptions of the ways in which these species are spread. This report specifically addresses crack willow, grey willow, reed sweetgrass, and alder, but also addresses other weeds that are potentially of concern. • Sources of weed invasion in the vicinity of the project site have been identified. • Zones around the project site that are susceptible to weed invasion are identified. • The distributions of ecological weed species have been described and mapped. • The threats posed by each weed species have been identified, and the potential for eradication or control of these species is assessed. 1 © 2009 1 Contract Report No. 2182 1 • Existing weed sources and areas that are at risk of weed invasion are mapped and/or identified. • Significant ecological features within or near the areas potentially affected by weeds are identified. • Threatened indigenous plants that need to be protected when undertaking weed control, or may be under threat from the spread of weeds, are identified. • The direct and indirect effects of weeds and weed control programmes on indigenous vegetation, habitats, and species are evaluated. • The feasibility of eradicating or controlling the weeds identified is assessed, including follow-up control methods and long-term control. • The potential for weed suppression by establishing indigenous vegetation is assessed. • Control methods are evaluated for potential use in sites of important cultural and cultural harvesting significance (i.e. flax and raupo harvesting by local iwi) and in fishing, hunting, and recreational boating and activity sites. This report also provides recommendations on: • Whether the proposed project should proceed. • Appropriate timing for the proposed weed control project, taking into consideration the need to eradicate weed sources. • Whether weed control or eradication work should be undertaken within a weed dispersal (buffer) zone and whether this needs to be done before or after other vegetation restoration work. 2. METHODS Relevant information on ecological values of the Waipori/Waihola wetland complex and adjacent areas was reviewed. This included previously commissioned botanical and hydrological reports. In most cases these covered only parts of the wetland complex. Stakeholders (Otago Regional Council, Department of Conservation, Fish & Game, local iwi, and other members of the community) were consulted to establish existing knowledge on weeds and potentially affected values. Field surveys were undertaken during April and June 2009, within the study area marked in Figure 1. The study area conformed to ‘natural’ boundaries along much of its circumference: SH1, Titri Road, Berwick Road, and stop banks. Field work was challenging due the large size of the wetland and the difficulty of traversing dense wetland vegetation and deep drainage ditches on foot. Sites were observed from a boat, traversed on foot, or viewed from a distance using binoculars. Particular attention was paid to the distribution of invasive weed species. The location and identity of threatened and uncommon indigenous plant species was recorded when they were encountered. An attempt was made to visit a representative range of vegetation/habitat types and infestations of each major weed species on the ground. Weed distributions and important areas of indigenous vegetation and habitat within the Waihola/Waipori wetland complex were mapped using field observations, aerial 2 © 2009 2 Contract Report No. 2182 2 photographs, and photographs taken from the ground and during a helicopter flight. The distribution of potentially invasive weeds in the surrounding landscape was also noted. An assessment of the risks and practicality of weed control was based on field observations, literature review, and stakeholder consultation. There are several limitations in this current study. Not all sites could be visited and it was difficult to determine some vegetation types from aerial photographs or from a distance. This has resulted in some uncertainty regarding the distributions of weeds and the distributions of uncommon species and vegetation types. These uncertainties are identified in the text and maps contained within this report. 3. ECOLOGICAL CONTEXT 3.1 Hydrology The major hydrological input into Lake Waipori is from the north via the Waipori River, but smaller flows enter from the Meggat Burn, Taieri Main Drain (Contour Channel), and several small streams. The Waipori River also drains Lake Waipori at its southern end, flowing east before joining the Taieri River c.10 km from the coast. Lake Waihola is drained by the Waihola River, which joins the Waipori River within the wetland complex, between Lake Waipori and the Taieri River. Tidal flows travel up the Taieri River and enter the wetland complex, raising lake levels c.0.5 m at high tide.