Inundation and salinity regimes support blue carbon conditions in Australian temperate supratidal forests

In this study we report on new datasets of vegetation structure, carbon cycling parameters, inundation and salinity patterns across 18 sites spanning more than 4,000 km of Australia’s temperate coastlines. We report site-specific ecosystem carbon stocks ranging from 169 to 635 Mg Corg ha-1, with mean aboveground biomass (134 ± 63 Mg DW ha-1) and belowground carbon stocks to 1 m soil depth (193 ± 98 Mg Corg ha-1), which are within the range of national estimates for mangrove and saltmarsh ecosystems. While there are variations in vegetation structure between sites dominated by the genera Melaleuca and Casuarina, this does not lead to discernible differences in above- or belowground carbon stocks. Organic matter decomposition trends within supratidal forest substrates were similar to adjacent mangrove and saltmarsh, though there were differences among study sites and between labile versus recalcitrant tea litters. Soil-atmospheric flux measurements conducted at one site were also within the range of adjacent blue carbon ecosystems. We hypothesise that the high degree of preservation of belowground carbon and low soil-atmosphere flux of greenhouse gases is driven by a combination of infrequent surface inundation, high water tables and typically saline groundwater in supratidal forests, as measured across multiple settings. Supratidal forests are carbon-rich ecosystems influenced by coastal processes associated with tidal inundation. While further research is required to understand the full distribution, carbon cycling and abiotic drivers of supratidal forests, our findings strongly support their inclusion in blue carbon and other management initiatives that support the response and recovery of these endangered ecological communities in a time of change.

Methods
This study collates datasets collected between 2017 and 2023 from 18 study sites spanning a diversity of settings along these coastlines. Within each setting, sites were prioritised where there was adjoining saltmarsh and/or mangrove communities, to allow comparative analyses with either existing or new datasets of those communities. Therefore, our collation of study sites may be under representative of supratidal forests occurring in isolation from intertidal mangrove and saltmarsh, and should not be considered a systematic or comprehensive coverage of forest settings. The measured parameters, measurement frequency and sampling designs varied among study locations according to their specific data collection objectives.

Water level and salinity

Groundwater standpipes were installed opportunistically across five sites (TP, MIN-FP, TIL-F, CI and ONK) to enable continuous logging of water level and/or salinity concentrations and explore the role of tidal inundation and rainfall on these parameters across diverse geomorphic settings. Standpipes were constructed from PVC tubes (1.5 m long; 50mm diameter) perforated with small (2mm) holes and covered with a nylon stocking to minimise infilling by sediments. Standpipes were installed into an augured hole (~50mm diameter with any gaps around the pipes backfilled using extracted sediments) to a depth of approximately 120 cm, enabling data-logging sensors to be suspended at a depth of 100 cm below the wetland surface. Exceptions to this included the mangrove standpipe at TP (sensors at 50cm depth below surface) and the supratidal forest standpipe at ONK (salinity sensor at 87 cm; water level sensor at 69 cm). In all cases, standpipe perforations were limited to sections below the wetland surface. Water table level and groundwater salinity measurements were taken at 15-minute intervals using either integrated loggers with pressure, temperature and electrical conductivity sensors (SOLINST LTC Edge), or via the simultaneous deployment of individual pressure (HOBO U20-001-04) and temperature / electrical conductivity sensors (HOBO U24-002-C).

Surface water-level loggers were deployed at BC, MIN-RC and MIN-FP sites to capture variations in surface inundation across adjacent mangrove, saltmarsh and supratidal forest communities at the scale of individual spring tide cycles. A surface logger was deployed on the surface of fringing supratidal forest at SWAN-I to capture inundation associated with either tides and/or seasonal rainfall at this location. At each site, an additional pressure logger (HOBO U20-001-04) was deployed in a tree well above the inundation limit to enable correction for atmospheric pressure and determination of water depths.

Trimble R8s and R10 real-time kinematic global positioning systems (horizontal error < 8 mm; vertical error < 15 mm) was used to survey the location and elevation of each water level measurement location. Where canopy coverage hindered the use of these instruments (i.e. supratidal forests at TP and MIN-FP) elevation was estimated via manual survey from an adjacent open area using a Leica Sprinter 50 Digital Level. In both instances, elevation is reported in the Australian Height Datum (AHD), where 0 m AHD corresponds to an estimate of mean sea level across Australian coastal waters.

Biomass survey

Field vegetation surveys were carried out at 11 sites to assess variations in forest structure and aboveground biomass across study settings. Replicate survey plots were measured within each site, with plot sizes ranging from 12.5 to 400 m2 depending on the density and homogeneity of vegetation at the site. Within each quadrat all trees > 1.3 m height were measured for diameter at breast height (DBH; Casuarinas) or diameter at 10 cm (D10; Melaleucas) to enable biomass estimation following genus-specific allometric equations created by Paul et al. (2013). The inclusion of such low-statured individuals is consistent with the definition of Australia’s forests as vegetation with mature or potentially mature stand height exceeding 2 metres (ABARES 2023). Biomass values for BC, BUTCK, LES and TP are updated estimates, including new additional plots, of those reported in Kelleway et al (2021). Biomass values are presented as dry weight estimates (Mg DW ha^-1^) and were converted to aboveground carbon stock estimates using a conversion factor of 0.488 (Kelleway et al. 2021) to enable calculation of total ecosystem carbon stocks.

Belowground carbon stocks

Soil cores were collected from one or more supratidal forest plots at 12 sites to assess variations in belowground carbon stocks among settings. Core barrels of either aluminium (74 mm internal diameter) or PVC (82 cm internal diameter) were manually hammered into the ground with care taken to minimise compaction. Compaction was estimated by measuring the difference in elevation of the soil surface within the core barrel and the outer soil surface. A linear compaction correction factor was later applied along the length of each core based on these measures. In the laboratory, soil cores were sub-sampled at compaction-corrected depth intervals of ≤ 5 cm along the entire length of the core. Bulk soil was oven dried at 60°C until constant mass and weighed to determine dry bulk density and then homogenized and ground into a fine powder.

Samples from BC, TP, KWP, QI, RHYLL, OYST-M, WIL-F and NORN-F were assessed for organic carbon (%Corg) via an elemental analyser, following removal of carbonates with HCl digestion, where required. Due to resource constraints, all other samples were assessed via loss-on-ignition (LOI) at 550oC for 4 h (Heiri et al., 2001). For these samples %Corg estimates were derived from organic matter concentrations using a previously developed empirical relationship for Casuarina samples (Kelleway et al. 2021), or a new empirical relationship developed for Melaleuca sites from samples subjected to both LOI and elemental analyses (Appendix S1: Figure S1). Soil carbon stocks were estimated for 0-30 cm, 0-100 cm and 100-200 cm depth ranges where core depths allowed. Stock estimates for BC, BUTCK, CRB, LES and TP were previously reported in Kelleway et al. (2021).

Belowground decomposition

We used a modified tea bag index protocol as part of the global TeaComposition H2O program (Trevathan-Tackett et al. 2021) at two study locations (BC and TP) to compare long-term belowground organic matter decomposition in supratidal forest relative to adjacent intertidal mangrove and saltmarsh ecosystems. We monitored the biomass loss of standardised litters: green tea (Lipton; EAN 87 22700 05552 5) and rooibos tea (Lipton; EAN 87 22700 18843 8), which are proxies for more labile and more recalcitrant plant organic matter types, respectively (Trevathan-Tackett et al. 2024). The green tea contains a higher proportion of water soluble compounds (simple sugars and phenolics), while the rooibos tea consists of a higher proportion of acid insoluble compounds (e.g. lignin; Keuskamp et al. 2013). At T0 (December 2017), all pre-weighed tea bags were buried at approximately 15 cm depth. Four tea bags per plot were manually retrieved (2x green; 2x rooibos) from each site, at each of 3, 12, 24, 36 month intervals after initial deployment, though two of the 64 samples could not be found. Tea bags were rinsed in distilled water to remove attached soil, and any visible root in-growth was carefully removed prior to drying of samples (60 °C until constant mass), and re-weighing.