The New Zealand mangrove: review of the current state of knowledge. May 2007 ARCTP325 Auckland Regional Council Technical Publication No. TP325 May 2007 ISSN 1175-205X ISBN -13 : 978-1-877416-62-0 ISBN -10 : 1877416-62-2 Printed on recycled paper The New Zealand mangrove: review of the current state of knowledge Donald Morrisey Catherine Beard Mark Morrison Rupert Craggs Meredith Lowe Prepared for Auckland Regional Council by National Institute of Water & Atmospheric Research Ltd (NIWA) NIWA Client Report: HAM2007-052 May 2007 . NIWA Project: ARC07211 Recommended Citation: Morrisey, D. et al (2007). The New Zealand Mangrove: Review of the Current State Of Knowledge. Auckland Regional Council Technical Publication Number 325 Contents 1 Executive Summary 1 2 Introduction 11 3 Mangrove ecology and coastal processes 13 3.1 The status of the mangrove (Avicennia marina) as an indigenous species 13 3.1.1 Classification and Taxonomy 13 3.1.2 Indigenous status of Avicennia and history of its presence in New Zealand 14 3.2 The geographic distribution and habitat requirements within New Zealand of mangroves, and key drivers of this distribution 15 3.2.1 Global connections 15 3.2.2 Latitudinal distribution of Avicennia marina in New Zealand 15 3.2.3 Key drivers of mangrove distribution: latitudinal effects 18 3.2.4 Key drivers of mangrove distribution: habitat requirements 19 3.2.5 The productivity of mangroves and their carbon/nutrient contribution to the marine environment 21 3.2.6 Mangrove productivity compared to other New Zealand coastal habitats 28 3.2.7 Mangrove productivity compared to terrestrial primary producers 29 3.2.8 The importance of mangrove productivity within estuaries and harbours 29 3.3 The intrinsic and unique values of mangroves to the marine environment 38 3.4 The role of mangroves as a food source or habitat for other species 40 3.4.1 Benthic macrofauna 40 3.4.2 Fish 48 3.4.3 Terrestrial invertebrates 80 3.4.4 Reptiles 82 3.4.5 Birds 82 3.5 The sediment trapping role of mangroves 83 3.5.1 The role of mangroves in trapping sediment and contaminants 83 3.5.2 Erosion protection by mangroves 85 3.5.3 Sediment organic matter generated by mangroves 85 3.5.4 The role of mangroves in the natural ageing of estuaries 85 4 The effects of mangrove spread and management 86 4 The effects of mangrove spread and management 87 4.1 Historic loss of mangrove habitat in New Zealand through human activities 87 4.2 Recent changes in mangrove distribution 89 4.3 How does the current expansion compare to historical losses? 94 4.4 Causes of mangrove spread 94 4.4.1 Increased inputs of sediment to estuaries and harbours 94 4.4.2 Nutrients 96 4.4.3 Climate change 96 4.4.4 Artificially altered estuarine hydrodynamics 98 4.5 Factors affecting the likely maximum extent of mangrove spread 99 4.5.1 Reproductive success of existing plants 99 4.5.2 Propagule supply (dispersal) 101 4.5.3 Seedling establishment, survival and population recruitment 101 4.6 Evidence for mangroves colonising other valued habitats 102 4.6.1 Bird feeding, roosting and breeding habitat 102 4.6.2 Intertidal flats and seagrass areas 103 4.6.3 Saltmarsh and fringing wetlands 105 4.6.4 Do mangroves provide habitat for rats? 107 4.6.5 Are areas of mangrove expansion reducing biodiversity? 108 4.7 Does the value of mangroves to the ecosystem vary with their location? 108 4.7.1 Gradients in mangrove age 108 4.7.2 Gradients in elevation across the shore 109 4.7.3 Gradients along an estuary 112 4.7.4 Gradients of human impact and environmental degradation 113 4.7.5 How do mangroves at either end of the estuarine gradient compare to the other habitat values present? 115 4.8 Minimal “critical mass” of mangroves required to retain a sustainable mangrove ecosystem 116 4.9 Mangrove management initiatives in New Zealand 117 4.9.1 Background 117 4.9.2 Management initiatives 118 4.9.3 Effects of mangrove removal 120 4.9.4 Mangrove restoration and enhancement 121 4.9.5 Effectiveness of mangrove management initiatives 122 4.9.6 Regulatory approaches to mangrove management 123 5 Future research 125 6 Acknowledgements 129 7 References 130 _________________________________________________________________________ Reviewed by: Approved for release by: Donna Sutherland David Roper Formatting checked 1 Executive Summary The present review provides a baseline of information on mangroves in New Zealand to inform the debate on their management. What are mangroves and are they native to New Zealand? Mangroves are a taxonomically diverse group of halophytic (salt-tolerant) plants that, worldwide, comprise approximately 70 species within some 19 families. They are typically woody trees or shrubs taller than 0.5 m, and inhabit the intertidal margins of low-energy coastal and estuarine environments over a wide range of latitude. The New Zealand mangrove, or Manawa, is one of several taxa within the genus Avicennia. Avicennia are true mangroves in that their habitat is defined solely by the intertidal zone, and they also possess specialized physiological and reproductive adaptations which allow them to grow there. Manawa is presently ranked as a sub-species (Avicennia marina (Forsk.) Vierh subsp. australasica (Walp.) J. Everett) within Avicennia marina (grey mangrove), a species occurring in both northern and southern hemispheres of the globe. Avicennia marina subsp. australasica qualifies as an indigenous member of the New Zealand flora, given that its existence here can be dated some thousands of years before humans inhabited, or even visited, these islands. Mangroves have inhabited New Zealand coastlines for approximately 19 million years, as indicated by the presence of Avicennia-type silicified woods associated with lower Miocene rocks from the Kaipara Harbour. The origin of New Zealand’s mangroves remains in question. It has been suggested that some populations may have arisen from sporadic introductions of propagules (the young plantlets) rafting across the Tasman Sea. Habitat requirements of mangroves Mangroves in New Zealand are confined to the northern coastlines of the North Island, forming often dense stands along the sheltered littoral margins of most major estuaries, shallow harbours, lagoons, tidal creeks and rivers north of about latitude 38oS. Their southern natural limit occurs at almost identical latitudes on both east (Kutarere, Ohiwa Harbour, latitude 38o03΄S) and west (Kawhia Harbour, latitude 38o05΄S) coasts of the North Island. Historically, the range of Avicennia extended much further south. Pollen in sediment cores from Sponge Bay and Awapuni (Poverty Bay) puts the range of mangrove approximately 140 km south of its present southernmost natural limit. Frosts play an important, additional and episodic role in defining the range of mangrove in New Zealand, but the main driver enforcing their latitudinal limit appears to be physiological stress. Avicennia distribution may be constrained by its physiological limitations under chilling (non- freezing) conditions, or latitudinal limits may be set by constraints on internal water transport as a result of vessel size, and embolism in the xylem vessels following freezing. It is also possible that establishment of plants in suitable microclimates further south of the present limits may be prevented by dispersal barriers (unsuitable coastline) and unfavourable ocean currents. Shallow, low-energy and gently-shelving shorelines are required not only for mangrove seedlings to anchor, but to allow these plants to persist in an area. Mangroves have significant value in the coastal zone as a buffer against erosion and storm surges. Mangroves themselves require some protection from high-energy waves and currents. In open estuaries in southeastern Australia, the upper and lower limits of A. marina distribution lie between mean ARCTP325: The New Zealand mangrove: review of the current state of knowledge. 1 high water and mean sea level. The seaward limits of mangrove growth in New Zealand have not been examined in great detail, but in locations where they are known mangroves are limited to slightly above mean sea level, possibly because of inhibitory effects of waves on establishment of propagules. The preferred habitat for mangroves in general is soft, muddy, waterlogged sediment, but some, such as Avicennia, will also grow successfully on a wide variety of other substrates including volcanic rock, sand and soil. Seedlings are only shade-tolerant up to the end of their cotyledon stage and high light intensity is crucial for their ongoing development and survival. Further development from this stage may be arrested if conditions are not suitable, with plants remaining in a 'seedling bank' for several years before progressing to the next size class. Seedling development is affected by salinity of the substrate in which they grow, with optimum growth occurring between 10 and 50% seawater. Photosynthetic performance of adult plants is also affected by salinity. Despite these limitations, A. marina exhibits a wide range of salt- tolerance and an ability to grow in waterlogged soils of salinities ranging from 0 to 90 psu. Elevated nutrient loadings (particularly nitrogen) have been thought to play a part in the acceleration of mangrove spread in some New Zealand estuaries, by contributing to enhanced growth rates and biomass, and increasing their potential to produce more propagules. As yet, however, there is no conclusive evidence that nutrients are the main causal factor of the observed expansion. Occasionally ‘natural’ disturbances have severe impacts on mangrove stands, resulting either in severe dieback or death of mangrove. Identified causes of dieback of A. marina in New Zealand and overseas include the pathogenic fungus Phytophthora and severe winter frosts. Mangrove productivity Comparisons of the productivity of mangroves from different latitudes worldwide suggest that productivity and plant biomass decreases with increasing latitude.