A landscape-based approach to optimise carbon sequestration in temperate freshwater wetlands, New Zealand: A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Freshwater wetlands, often overlooked in mainstream carbon policy, function as credible carbon sinks. Referred to as ‘teal carbon zones,’ these ecosystems exceed many terrestrial systems in their capacity to store carbon, owing to high primary productivity and waterlogged, anaerobic soil conditions that slow organic matter decomposition. Yet despite this potential, freshwater wetlands remain marginal in global carbon accounting frameworks, and their design for carbon storage and outcomes is a growing area of interest in landscape architecture. This doctoral research responds to this opportunity by investigating how plant community composition, soil type, and plant–soil interactions influence carbon storage in two temperate inland freshwater wetlands, Travis and Sparks Wetlands, in Ōtautahi Christchurch, New Zealand. The aim is to develop a landscape-informed design framework that places carbon sequestration at the centre of freshwater wetland design and management. A mixed-methods approach is adopted, including case study and survey research methods, field-based vegetation and soil assessments, standard laboratory analyses, and established carbon accounting protocols. Above-ground carbon is estimated using biomass regression equations and volume-derived methods. Soil carbon stocks are estimated through the life belt and the soil type methods to 90 cm depth. Maps assess the spatial variability of soil parameters within wetland areas. A novel ‘carbon-profile’ method is developed to integrate vegetation and soil carbon data at the plant assemblage level, enabling finer-grained insights into the spatial distribution of carbon within freshwater wetland systems. The results reveal that wetland plant communities and soil types have a strong influence on how carbon is stored, both above and below ground. Notably, Travis Wetland stored significantly more soil carbon (51,266 Mg C to 90 cm depth) than Sparks Wetland (3,358 Mg C), due in part to its mix of peat and mineral soils. Plant communities with higher proportions of woody species, particularly those in forest and woodland types, consistently stored more above-ground carbon. Conversely, herbaceous communities, while lower in above-ground biomass, were associated with greater soil carbon concentrations. This inverse relationship highlights the importance of understanding carbon partitioning across biomass and soil pools within wetland plant communities. Plant functional traits, such as root depth, root morphology, litter quality, plant height, size, stem and wood densities, growth rate further shape carbon inputs to both soil and biomass layers. Greater species and structural diversity help distribute carbon more evenly across canopy, understory, and root layers. Canopy structure modulates the microclimate, influencing soil temperature, moisture, and organic matter dynamics. Furthermore, vertical soil carbon density distribution across eight soil types indicate that soil texture, bulk density, and saturation levels play critical roles in stabilising carbon at depth, while pH and salinity influence microbial activity and organic matter preservation. From these findings, a carbon-responsive landscape-framework is proposed. It outlines key considerations for plant species selection, species assemblage design, and soil condition management, with emphasis on aligning vegetation types with underlying soil characteristics. By embedding carbon sequestration into the early stages of freshwater wetland design, this research proposes actionable strategies to elevate freshwater wetlands as climate mitigation infrastructures in urban environments.