Mangrove Response to Environmental Change in Northern Australia

Mangrove forests are critical for carbon storage, primary productivity, coastline protection and biodiversity, however have been degraded across their range. One exception is northern Australia, where mangroves are protected and undisturbed. Hence, the majority of changes can be attributed to natural events.Focusing on three areas in northern Australia (the Gulf of Carpentaria, Kakadu National Park and Hinchinbrook Island), the primary aim was to quantify changes in the extent, cover and height of mangroves over temporal and spatial scales using time-series data from airborne and/or spaceborne optical, radar and LiDAR sensors. Secondary, was to establish the reasons for changes in relation to climate, coastal inundation and sea levels. For the Gulf of Carpentaria, the mangroves exhibited progressive landward and seaward extension from 1987 to 2014. These extensions were attributed respectively to a) regular supply of sediment through freshwater input and b) combined effects of sea level rise and prolonged tidal and freshwater inundation. In Kakadu NP, maps of mangrove extent were generated using aerial photography (1991), hyperspectral (2002), LiDAR (2011) and RapidEye (2014 and 2016). Changes in the annual mangrove area between 1987 and 2012 indicated a close correspondence with sea level and river discharge. In 2015, extensive dieback of mangroves was reported in the Gulf of Carpentaria. A drone survey in Kakadu NP in September 2016 confirmed dieback had also occurred, primarily affecting landward Avicennia marina. The immediate and long-term implications of Tropical Cyclone Yasi were investigated at Hinchinbrook Island. Based on a combination of Landsat, Queensland Globe aerial imagery and RapidEye imagery, 16 % of the 13,795 ha of mangroves were found to be damaged. Damage was primarily inflicted on Rhizophora stylosa, little/no recovery was attributed to an inability of R. stylosa, to resprout and changes in hydrological and sediment regimes.This investigation observed significant changes in mangrove extent and structure. Of concern is that, mangroves showed limited recovery. Global climate projections suggest enhanced: temperatures, Sea Surface Temperatures (SSTs), storm intensities and changes in rainfall. For this reason, there is urgent need to establish baselines of extent, structure and species composition and to monitor changes