Vegetation and Elevation Surveys of the Gulf of Carpentaria Mangrove Dieback 2017 - 2019 (NESP TWQ 4.13, JCU)

Vegetation and Elevation surveys were conducted at four sites in the Gulf of Carpenteria to provided crucial validation of observations made from aerial surveys and provided further significant insights of the impacts and subsequent changes that occurred across the Gulf coastline up to late 2019. Field studies primarily focused on shoreline fringing stands dominated by the Grey Mangrove Avicennia marina var. eucalyptifolia. A total of eight transects, perpendicular to the shoreline were established at four shoreline sites across the Gulf of Carpentaria. These included matched pairs for each of two severity levels of 90%–100% and 60%–80% dieback of mangrove fringes.A series of profile transects were established and measured from the landward edge to the sea edge of mangroves. Transects were run from a highwater point at the head, directly towards the sea edge. This method captured common reference elevation levels for all sites while maximising coverage of the entire elevation range of the tidal wetland (mangroves plus tidal saltpan and saltmarsh vegetation), from approximately highest astronomical tide levels (~HAT) at the head, to approximately mean sea level (~MSL) at the seaward edge of living mangrove trees.On ground surveys consisted of two components: a) elevation measures from HAT (highest astronomical height – defined by the highwater mark) to MSL (mean sea level – defined by the seaward mangrove edge); and b) vegetation species, structure and density for mangrove and saltmarsh species present along with observations of condition and being likely 2015 dieback. The latter condition was determined from vegetative degradation states of mangrove trees, and as seen in satellite imagery mapping.Methods:Locations:The field studies started with the two locations in Queensland during 4–10 August 2018 andthen moved onto those in the Northern Territory during 11–17 October 2018. A total of eight transects, perpendicular to the shoreline were established at the four shoreline sites across the Gulf of Carpentaria.Limmen – Roper region (NT) - 1A with 90% - 100 % dieback - 1B with 60% - 80% diebackMule - Roper region (NT) - 2B with 90% - 100 % dieback - 2A with 60% - 80% diebackKarumba - SE Gulf (QLD) - 4A with 90% - 100 % dieback - 4B with 60% - 80% diebackMitchell north - W Cape (QLD) - 5A with 90% - 100 % dieback - 5D with 60% - 80% diebackTransect Set Up Summary:Each transect was based or anchored at the observed nominal Highest Astronomical Tide (~HAT) level of the highwater benchmark at each transect ‘head’, via the beach wash zones indicative of the highest reach of tidal waters. A second reference position at the sea edge of mangroves was taken as a proxy relative to mean sea level (~MSL). The location of the head position was chosen so that a straight line transect could be taken to the fringing mangrove stand, and to the sea edge at the proxy position of mean sea level (~MSL).Three additional ‘internal’ ecotone position markers between ~HAT and ~MSL of the tidal wetland zone were recorded for each transect, including the landward fringing mangrove to the saltpan–saltmarsh position (M1-lower); the lower elevation limit of saltpan–saltmarsh bordering the upper dieback mangroveedge (M2-upper); and the lower elevation limit of mangrove dieback (M2-dead/live). Further details about the transect set up can be found in the final report volume 2 (Duke et al, 2020).Surveys:Long plots were used to describe and quantify mangrove and saltmarsh vegetation alongeach transect. The long plot method allowed the plot width to be adjusted during the surveydepending on stem density of particular sections along the transect - where there were closely spaced trees, plots were narrower (2 m wide).Elevation levels were recorded at 20–30 m intervals or more frequently where there were otable changes in topography or there were notable changes in vegetation type and condition. Levels were made using a Topcon construction surveyors rotating laser and staff. Where it was necessary to relocate the laser instrument, additional reference points were taken for each transition point providing offset measures to link each series of measurements. Elevation levels were recorded all the way from the head marker to the sea edge amongst or just beyond the last trees.Vegetation was scored for species, stem diameter, height, condition as well as distance along the transect and distance left or right of the measuring tape. Trees were scored in 30 m sections within a fixed distance from the measuring tape depending on stand density. The width was mostly set at two metres, but on occasion, this was reduced to one metre or up to four metres wide as necessary.Along each transect, at each 30-metre interval or at ecotone points, photographs were taken at four square directions to the transect line – towards the sea, 90 degrees to the right, back towards the ‘head’ and 90 degrees to the left. At these same points, canopy photos were taken using a camera with a fisheye lens.The survey data contains wood sampling and tree coring investigations which could not be completed during the project's reporting time frame. Future project work will include high-level analytical work required, including elemental scans and carbon dating. Evidence of tree cores collected during the field surveys can be found within each vegetation survey sheet and the tree cores tab of the workbooks. Limitations of the data:While terrestrial forestry practice recommended that stem diameter be measured at 1.3 m above the ground – as diameter measured at breast height (DBH) – this was found to be impractical in these and other mangrove forests. The difficulties encountered included the common occurrence of multiple stems, short height mature trees and shrubs (