DOI: 10.5061/dryad.7h44j1012
title: Field vegetation dataset: Mangrove expansion on the low wooded islands of the Great Barrier Reef

description: Mangrove forests are the dominant vegetation growing on low wooded
 islands, which occur in the Caribbean, Indian and Pacific Oceans. In the
 northern Great Barrier Reef, we map remarkable, undocumented mangrove
 forest extension on ten low wooded islands in the Howick Group that
 collectively equates to an area of 667, 000 m^2 (66.7 ha). We combine
 extensive field survey with canopy height models derived from RPA imagery
 and allometric scaling to quantify above ground biomass in both old
 (pre-1973) and new (post-1973) forest areas. Forest expansion added ~
 10,233 tonnes of new biomass since the early 1970s. Such substantial
 expansion of mangrove forest has occurred within a short time span in
 response to changing environmental controls. These include sea-level rise,
 sediment transport and deposition, cyclone impact and the development of
 associated reef flat sedimentary landforms, including unconsolidated and
 lithified shingle ridges, which influence reef flat hydrodynamics. Our
 observations highlight the globally dynamic response of mangrove
 distribution and forest structure to environmental change and provide
 timely new estimates from understudied reef island settings.
description: Rapid <em>in-situ</em> mangrove community surveys
 were conducted at seven of the ten reef sites visited in the Howick
 Islands from the 1<sup>st</sup> – 11<sup>th</sup>
 June 2021 (Table S1). Surveys collected information on species
 composition, structure and health status of individual trees; and
 plot-scale vegetation metrics for development of setting-specific biomass
 scaling models. Fifteen shore-normal belt transect survey sites were
 planned using the 1973 mangrove forest maps combined with recent satellite
 imagery to ensure a diverse range of mangrove age classes (i.e. pre- and
 post- 1973 establishment) and species assemblages were assessed.
 Transect start points were located using a real-time Kinematic
 GPS with satellite RTX correction (Trimble R10; Trimble Inc. USA) with
 expected horizontal and vertical uncertainties of ~2cm and ~5cm
 respectively. Transect orientation was measured with a hand-held compass.
 See supplementary Figure S1 for the location of <em>in-situ
 </em>mangrove biomass surveys. Transects ranged
 in length from 54 – 343 m, depending on the forest width, site
 accessibility and time constraints (typically 5-6 hours per island). All
 trees encountered within a 2 m width of each transect that were ≥ 2 m
 height (i.e. 1 m either side of a central line marked by measuring tape)
 were measured for vertical height, maximum branch length (i.e. inclusive
 of lateral biomass growth), and tree diameter at breast height (cm; DBH).
 Tree height and branch length measurements were taken to the nearest 0.1 m
 using a 12 m telescopic measurement pole (Senshin Industry, Japan). Scrub
 mangroves (1-2 m tall) were measured for height (to nearest 0.01 m), and
 basal area diameter measured at 0.3 m height above ground. Mangrove
 seedlings or juveniles &lt;1 m tall were were excluded from the survey
 since their biomass was low. Species identification and status (healthy;
 stressed; dead) were recorded for each individual.
description: # Field vegetation dataset - Mangrove expansion on the low wooded islands
 of the Great Barrier Reef
 [https://doi.org/10.5061/dryad.7h44j1012](https://doi.org/10.5061/dryad.7h44j1012) In the northern Great Barrier Reef, we map remarkable, undocumented mangrove forest extension on 10 low wooded islands in the Howick Group that collectively equates to an area of 667 000 m2 (66.7 ha). We combine extensive field survey with canopy height models derived from RPA imagery and allometric scaling to quantify above ground biomass in both old (pre-1973) and new (post-1973) forest areas. Forest expansion added approximately 10,233 tonnes of new biomass since the early 1970s. We suggest that such substantial expansion of mangrove forest has occurred within a short time span in response to changing environmental controls. These may include sea-level rise, sediment transport and deposition, cyclone impact, and the development of associated reef flat sedimentary landforms including unconsolidated and lithified shingle ridges, which influence reef flat hydrodynamics. Our observations highlight the globally dynamic response of mangrove distribution and forest structure to environmental change and provide timely new estimates from understudied reef island settings. Rapid *in-situ* mangrove community surveys were conducted at seven of the 10 reef sites visited in the Howick Islands from the 1–11 June 2021 (electronic supplementary material, table S1). Surveys collected information on species composition, structure and health status of individual trees; and plot-scale vegetation metrics for development of setting-specific biomass scaling models. Fifteen shore-normal belt transect survey sites were planned using the 1973 mangrove forest maps combined with recent satellite imagery to ensure a diverse range of mangrove age classes (i.e. pre- and post- 1973 establishment) and species assemblages were assessed. Transect start points were located using a real-time Kinematic GPS with satellite RTX correction (Trimble R10; Trimble Inc. USA) with expected horizontal and vertical uncertainties of approximately 2 cm and approximately 5 cm respectively. Transect orientation was measured with a hand-held compass. Transects ranged in length from 54–343 m depending on the forest width, site accessibility and time constraints (typically 5–6 h per island). All trees encountered within a 2 m width of each transect (i.e. 1 m either side of a central line marked by measuring tape) that were ≥2 m height were measured for vertical height, maximum branch length (i.e. inclusive of lateral biomass growth), and tree diameter at breast height (cm; DBH). Tree height and branch length measurements were taken to the nearest 0.1 m using a 12 m telescopic measurement pole (Senshin Industry, Japan). Scrub mangroves (1–2 m tall) were measured for height (to nearest 0.01 m), and basal area diameter measured at 0.3 m height above ground. Mangrove seedlings or juveniles less than 1 m tall were excluded from the survey since their biomass was low. Species identification and status (healthy; stressed; dead) were recorded for each individual. ## **Hamylton-et-al-field-biomass-segment.csv** ## Description of the data and file structure | Data collected as per methods described in Hamylton et al. manuscript, and above | | | -------------------------------------------------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------- | | **FIELD** | **DESCRIPTION** | | record\_ID | unique datset ID | | site\_ID | Island ID. See table below | | transect\_ID | unique transect ID | | segment\_ID | unique segment ID | | age\_class | age class derived at the segment level from historic magrove mapping. 'new' = post- 1973 establishment; 'old' = pre- 1973 establishment | | transect\_position\_m | lateral distance (m) along the transect, where 0m coincides with the start of mangrove canopy | | species | plant species. See table below | | health\_class | health status derived via rapid visual assessment: D = dead; S = stressed; H = healthy | | branch\_no | individual branch number within each measured tree | | tree\_H\_m | maximum vertical height above ground (m). For canopy height purposes | | branch\_L\_m | maximum branch length (m) along it's natural orientation. For biomass estimation purposes only | | stem\_diameter\_cm | diameter (cm) of individual stem | | canopy\_depth\_cm | maximum canopy depth (cm). Measured for shrubs (&lt;1.3m tall) only | | canopy\_W1\_cm | maximum canopy width (cm). Measured for shrubs (&lt;1.3m tall) only | | canopy\_W2\_cm | canopy width perpendicular to canopy\_W1 (cm). Measured for shrubs (&lt;1.3m tall) only | | | | | **site\_ID** | **Island name** | | 1 | Coquet | | 2 | Houghton | | 3 | Newton | | 4 | Pipon | | 5 | Ingram-Bealey | | 6 | Bewick | | 7 | Leggatt | | | | | **species** | **species name** | | RHST | *Rhizophora stylosa* | | OSOC | *Osbornia octodonta* | | CETA | *Ceriops tagal* | | BRGY | *Brugiera gymnorhiza* | | SOAL | *Sonneratia alba* | | AEAN | *Aegialitis annulata* | | AVEU | *Avicennia marina subsp. Eucalytifolia* | | BRCY | *Brugiera cylindrica* | | EXAG | *Excoecaria agallocha* | | TATI | *Talipariti tiliaceum* | | XYMO | *Xylocarpus moluccensis* |