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 and
then 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% dieback
Mule - Roper region (NT)
- 2B with 90% - 100 % dieback
- 2A with 60% - 80% dieback
Karumba - SE Gulf (QLD)
- 4A with 90% - 100 % dieback
- 4B with 60% - 80% dieback
Mitchell north - W Cape (QLD)
- 5A with 90% - 100 % dieback
- 5D with 60% - 80% dieback
Transect 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 mangrove
edge (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 along
each transect. The long plot method allowed the plot width to be adjusted during the survey
depending on stem density of particular sections along the transect - where there were closely spaced trees, plots were narrower (<2 m wide) than where trees were larger and further
apart (>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 (<1.3 m), multiple forms of plant types (shrubs and trees), low branching (<1.3 m), and high placed roots and
buttresses (>1.3 m).
A more appropriate standard was applied in these studies of measuring stem diameters above highest prop roots and buttresses and below lowest branching
Special consideration was taken in measuring stem diameters because slight differences in these measures could create considerable differences and errors in calculations of biomass and carbon content when using allometric equations.
Format of the data:
The data are complied in two excel workbooks detailing the QLD and NT surveys. The workbooks contain three tabs for each survey type (Vegetation transect data, transect elevation profile and undercanopy surveys). Both workbooks contain a Totals/Summary tab with statistics from each site, as well as a tree cores tab detailing the core samples.
Data dictionary: see data package
For the map layer:
LOCATION: Name of the site
HEAD_LAT: Latitude of the transect head point in decimal degrees
HEAD_LONG: Longitude of the transect head point in decimal degrees
SEAWARD_LA: Latitude of the transect head or seaward point in decimal degrees
SEAWARD_LO: Longitude of the transect head or seaward point in decimal degrees
IMPACT_SEV: Whether the site is a high dieback or moderate dieback site
LENGTH : Length of the transect in meters
ELEVATION: Elevation range in meters (m)
CANOPY_DOM: Dominate species of tree for the transect
P__DEAD_CA : Percent of dead trees in the canopy survey. Recorded in the % Damage (tree loss) cell at the top of each vegetation survey tab. The total live and dead trees are calculated, then summed to show the total survey trees. The equation of total dead trees/total trees100 is used to them present the % damage (tree loss) figure.
TOTAL_CANO: Shows the total number of live and dead trees in the transect
MAX_CANOPY: Records the tallest tree surveyed in the transect (uses MAX formula in each Vegetation TREEs datasheet)
UC_DOM__SP: Records the dominate undercanopy speecies, found on each site Undercanopy tab - columns Live & dead: AM Sap / AM Seedl, AA shrub/ AA seedl, Other Sap Species, Other Seedl species
P__DEAD_UC : Percent of dead growth in the undercanopy. From the undercanopy tab of each transect fite, the total counts of live and dead Sap / Seedl are calculated for each species, then totalled in UC Live, UC Dead. The % Dead UC then uses equation of total dead UC/Total UC100 to present the % UC Dead figure.
TOTAL_UC: Records the total UC figure (total counts of live and dead Sap / Seedl are calculated for each species)
MEAN_UC_HG: Mean height of the undercanopy in meters (m)
TREE_CORES: Number of tree cores for the transect
MEAN_STEM: The mean steam diameter (cm) for the transect
References: Duke N.C., Mackenzie J., Hutley L., Staben, G., & Bourke A. 2020. Assessing the Gulf of Carpentaria mangrove dieback 2017–2019. Volume 2: Field studies. James Cook University, Townsville, 150 pp.
eAtlas Processing:
The original data were provided as two excel workbooks. No modifications/ minor modifications to the original data were performed. This metadata was created using the above referenced report. The map layer is derived of summary data extracted from the workbooks. Included in the data download package is our best estimate of the descriptions for the data attributes as a draft data dictionary which will be updated when further information is made available by the project team.
Location of the data:
This dataset is filed in the eAtlas enduring data repository at: data\custodian\2018-2021-NESP-TWQ-4\4.13_Assessing-gulf-mangrove-dieback
