Organic matter production and accumulation along tidal gradients of Australian coastal wetlands

<p dir="ltr">Coastal wetlands can store substantial carbon through organic matter accumulation, thereby influencing substrate volume. These processes collectively contribute to climate change mitigation through carbon sequestration and climate change adaptation by increasing the vertical position of substrates while sea level rises. Although, uncertainty about carbon fluxes via organic matter production and the contribution of sequestered carbon to surface elevation change remain. Variation in tidal behaviour along tidal gradients influences spatial vegetation structure, sedimentation and organic matter production. However, relationships between these factors and tidal position are not well defined. Holocene sea-level history partly underpins carbon storage and the contemporary position of coastal wetlands in the tidal frame, but limited information of organic matter accumulation processes in some settings reduces capacity to account for variability. Information about variation in organic matter production along tidal gradients is required, particularly for coastlines where sea level has been relatively stable for millennia.</p><p dir="ltr">The aim of this thesis was to quantify spatiotemporal variation in organic matter production and stocks along tidal gradients in coastal wetlands, providing crucial information regarding associations between organic matter accumulation and relative sea-level rise (RSLR). The objectives of this thesis are, to: (1) characterise the influence of vegetation zonation and tidal position on root mass and volume dynamics within substrates; (2) characterise relationships between above-ground biomass, production, forest age and tidal position on a coral reef island; and (3) examine mangrove substrates and organic matter accumulation in the context of decadal- to millennial-scale sea-level history on a coral reef island. Short-term contributions of below-ground root mass and volume to production, standing stock and turnover were quantified using the root ingrowth technique and sediment coring along a tidal gradient at Kooweerup, Victoria, Australia. Root zone dynamics were primarily explained by variation in vegetation structure and composition, whereas tidal position imposed a secondary influence. Field-based mangrove structural measurements, and LiDAR and multispectral data were used on Low Isles, Great Barrier Reef, to investigate mangrove above-ground biomass production and canopy production. Above-ground biomass was positively correlated with forest age and tidal position, whereas above-ground biomass production and canopy production declined along this gradient. Substrate organic matter accumulation patterns were also examined using radiometric dating techniques (analysis of ²¹⁰Pb, ^239+240Pu, ²³⁶U and ¹⁴C dating), across mangrove age zones and tidal positions on Low Isles. Mangroves established on Low Isles within the past -200 years, and shallower below-ground carbon stores contrast sites where RSLR for millennia encouraged the development of deep mangrove peats.</p><p dir="ltr">Mangrove age and/or structure influenced organic matter production at Kooweerup and Low Isles; sites where mangrove shoreline progradation established conditions for forest age and/or structure to co-relate to tidal gradients. Links to tidal gradients may be less defined elsewhere, particularly where tidal position and forest age are not coupled. At Kooweerup and Low Isles, below-ground organic matter production peaked in older, taller mangroves at higher tidal positions, indicating capacity for root volume storage and vertical adjustment with RSLR is enhanced at optimal tidal positions. Continued above-ground biomass accumulation in the oldest stand on Low Isles (-135 years old) contrasts to previous studies indicating mangrove biomass asymptotes at younger forest ages. Despite being positioned in a setting optimal for production (i.e., wet tropics), mineral sediment supply may constrain carbon accumulation patterns on Low Isles. Older, taller forests at sites where mangroves are expanding seaward continue to accumulate above- and below-ground organic matter and store carbon. Ongoing carbon accumulation at both sites demonstrates resilience to recent rates of RSLR. However, resilience under accelerating RSLR may be tested sooner on Low Isles where processes increasing mangrove substrate elevation are dominated by plant organic matter inputs. Knowledge of organic matter dynamics along tidal gradients is important, allowing for space-for-time substitution and modelling of coastal wetland substrate development with RSLR. Moreover, information about above- and below-ground carbon fluxes will improve accuracy of net ecosystem production accounts and forecasting potential carbon additions associated with blue carbon restoration initiatives.</p>