Year

2018

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Saline coastal wetlands, particularly mangrove and saltmarsh, are globally recognised as valuable sinks of organic carbon. Increasing awareness of the need to mitigate anthropogenic climate change has led to considerable interest in the capacity of mangrove and saltmarsh to sequester atmospheric carbon. Previous research has focused on quantifying and characterising carbon in coastal wetlands, however carbon storage services provided by coastal wetlands are not homogeneous, and our understanding of the spatial variability remains limited. Spatial variation at smaller scales (i.e. within a wetland) is yet to be adequately described, likely due to current approaches that do not account for variation within broad vegetation units (i.e. mangrove and saltmarsh) across the intertidal zone. This may be particularly pronounced in structurally complex temperate coastal wetlands where mangrove and saltmarsh co-exist, such as southeast Australia. The aim of this thesis was to demonstrate that spatial variation in vegetation structure can describe variation in above-ground biomass and surface soil carbon storage within a wetland.

Spatial complexity in vegetation structure was initially established in this thesis by delineating wetland vegetation using innovative remote sensing analysis. A semiautomated method was developed, using high resolution imagery and Lidar coupled with an object-orientated approach, resulting in greater than 90% classification accuracy for all study sites. Above-ground carbon storage was estimated for mangrove and saltmarsh to identify the spatial scale that optimises efficiency and accuracy. Carbon storage was dependent on three factors: accurate assessment of biomass; quantification of carbon content in biomass; and delineation of vegetation extent that recognised vegetation structure. Terrestrial laser scanning (TLS) was used to develop and apply a nondestructive approach to estimate biomass of structurally complex coastal wetland vegetation. A new modelling method was developed, providing TLS biomass estimates consistent with traditional mangrove allometrics and saltmarsh harvesting. Surface soil carbon storage was explored across an intertidal gradient, demonstrating significant variation can be accounted for by vegetation structure and sedimentary influences on soil carbon content. Carbon in deeper sediment may not correspond to processes operating at the wetland surface and require consideration of previous environmental conditions.

Vegetation distribution and structure, as an expression of broad physiological tolerance to edaphic conditions (i.e. anoxia and soil salinity), exhibit considerable influence on carbon storage across the intertidal zone. Significant spatial variation of biomass and carbon in this thesis was described by vegetation structure, demonstrating that variation at smaller scales (i.e. within a wetland) can be equivalent to variation reported at larger scales (e.g. between sites, regions). Current methodologies that outline sampling and reporting protocols for Intergovernmental Panel on Climate Change (IPCC) tier 3 assessments, required for carbon accounting, do not adequately describe variation of carbon storage within a wetland. This can significantly influence estimates of biomass and carbon and reduce the confidence necessary for reporting carbon storage for national carbon accounts, and carbon off-setting initiatives. As an outcome of this thesis, a stratified sampling approach is provided for future carbon storage assessments in coastal wetlands.

FoR codes (2008)

040601 Geomorphology and Regolith and Landscape Evolution, 050101 Ecological Impacts of Climate Change, 050104 Landscape Ecology, 050301 Carbon Sequestration Science

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.