High variance in community structure and ecosystem carbon stocks of Fijian mangroves driven by differences in geomorphology and climate

Highlights

• We assessed community structure and ecosystem carbon stocks of Fijian mangroves.

• Different geomorphic and biophysical settings resulted in high ecosystem variance.

• Taller mangroves of the Ba region exhibited significant tropical cyclone damage.

• Ecosystem carbon storage of Fijian mangroves is disproportionate to area.

• Carbon storage equates to 73.3% of native forests yet occupies 7.3% of the area.

Abstract

Mangrove ecosystems are particularly important for small island developing states of the Pacific, such as Fiji, which are at the forefront of the impacts of climate change. This is because of the ability of mangroves to mitigate storm surges and floods as well as their high carbon sequestration and storage capacity. However, there are few detailed studies on the spatial variation in mangrove structure and carbon stocks in Fiji, and this information is essential to support decision making by government and communities, enabling the development of effective mitigation and adaptation responses. We assessed mangrove forest structure in contrasting regions around Fiji's largest island, Viti Levu, within sites managed by indigenous (iTaukei) Fijians. Mangroves of the Ba, Nadroga-Navosa, and Rewa and Tailevu regions showed high variance in both structural complexity and ecosystem carbon stocks. Levels of variation were similar to that observed globally due to variable geomorphological and biophysical settings related to orographic rainfall, freshwater influx, tidal amplitude and cyclonic disturbances. High biomass, structurally complex forests occur on the wetter south-east coast (e.g. the Rewa Delta), while structurally uniform scrub mangroves dominate large areas of mangroves along the north-west (e.g. the Ba Delta) and west coast (e.g. the Tuva Delta). Mangroves of the Ba region displayed considerable damage from tropical cyclones, particularly in taller vegetation. All mangrove sites assessed were important reservoirs of carbon, with results when scaled to the spatial extent of mangroves in Fiji revealing that ecosystem carbon storage is disproportionate to area and equates to 73.3% of the carbon held within terrestrial rainforests, despite occupying just 7.3% of the total area. This underscores the importance of mangroves as valuable carbon sinks in Fiji and the need to develop incentives for improved conservation and restoration.

Introduction

Mangrove ecosystems provide a range of important ecosystem services which benefit hundreds of millions of people around the world (Lau, 2013; Mohammed, 2012). Their carbon sequestration functions provide services that mitigate CO₂ emissions, while their coastal protection services support adaption to the impacts of climate change (Duarte et al. 2013, Marois and Mitsch, 2015; Sasmito et al. 2016; Alongi, 2008). Their roles in climate change mitigation and adaptation are particularly important for small island developing states (SIDS) of the Indo-Pacific, such as Fiji, which are vulnerable to sea-level rise and the impacts of tropical cyclones (Nunn and Mimura, 1997; Olsthoorn et al. 2002; Krauss and Osland, 2019). However, there are few detailed analyses of variation in mangrove cover, structure and carbon stocks in small island states, yet these are fundamental for estimating the extent and degree of ecosystem services provided (Kauffman et al. 2011; Donato et al. 2012). For instance, many SIDS such as Tuvalu are very low lying in contrast to the volcanic main islands of Fiji which are much larger, higher and exhibit well developed river systems supporting extensive deltaic mangrove ecosystems. The degree to which mangroves can attenuate storm surge is largely dependent on configuration and width (McIvor et al. 2012; Hochard et al. 2019), with coastal fringing mangroves on lower lying islands providing more limited mitigation ecosystem services than the deltaic systems of higher islands. Developing a comprehensive understanding of mangrove cover, structure and carbon stocks is therefore vital to support inclusion of mangroves within processes that contribute to national climate change adaptation and mitigation. This includes incorporating mangroves within greenhouse gas inventories (Hiraishi et al. 2014; Murdiyarso et al., 2015), carbon offset markets (Needleman et al. 2019), and planning for climate change adaptation (Payton and Weaver, 2011; Pramova et al. 2012).

The carbon capture and storage ability of mangrove ecosystems around the world varies considerably as a function of geomorphic settings and biophysical variables (Rovai et al. 2018). For instance, more productive, high carbon density mangroves generally occur in deltaic systems (Donato et al. 2011; Rovai et al. 2018) which have extensive riverine and hinterland areas typically traversed by a mosaic of rivers and streams (Woodroffe, 1992). Such systems generally have greater biomass and exhibit deeper soils containing more carbon than coastal fringing or oceanic coral atoll sites (Donato et al. 2011; Cameron et al. 2018). Mangrove carbon stocks also vary with tidal inundation gradients which influences species composition and productivity (Hayes et al. 2017), while greater freshwater influx from streams, rivers and direct precipitation can lower salinity levels and stimulate higher productivity (Alongi, 2014). Additionally, natural disturbance regimes within mangrove ecosystems can reduce carbon stocks (Kauffman et al. 2011) and alter community structure. In this regard, Fiji is particularly prone to the impacts of frequent tropical cyclones (TCs) with the archipelago being struck in recent years by TC Gene (January 2008), TC Mick (December 2009), TC Evan (December 2012), TC Winston (February 2016) -a Category 5 event and the most intense in the Southern Hemisphere on record (APDRC n.d) – and most recently TC Harold (April 2020). The conversion of mangroves for tourism development, coastal reclamation, industrial estates, squatter housing, agriculture and construction of sugarcane tram lines, as well as disposal of dredging spoil and harvesting for both fuelwood and construction materials are all recognised anthropogenic drivers of mangrove loss in Fiji (MoE, 2018) which also affects carbon storage and sequestration. Understanding community structure, carbon stocks and the factors influencing variability in both across the range of mangrove habitats comprising a country's mangrove estate, therefore, is vital for informing land-use decision making.

In this regard, Fiji initially developed a two-phased Mangrove Management Plan (Watling, 1985) which was superseded by phase three under the Mangrove Ecosystems for Climate Change Adaptation and Livelihoods Project (MESCAL Project, 2013). Outputs from the MESCAL Project included a preliminary assessment of ecosystem carbon stocks in the Rewa Delta, although the management aspects of the plan are yet to be formally implemented. Lal (1990) assessed the extent of historical loss of mangroves in Fiji which was driven principally through conversion to sugarcane plantations, while Gonzalez et al. (2015) provided a valuation of Fiji's marine ecosystems which included an assessment of the ecosystem service values from carbon sequestration and storage in mangroves. Mangroves are now included within the Environmental Management Plan (2005) and Protected Species Act (2002) and have recently been included as priority habitats for conservation and restoration under Fiji's Low Emission Development Strategy 2018–2050 (MoE, 2018).There are no studies however, that have comprehensively assessed the structural variability and ecosystem carbon stocks of Fiji's mangrove ecosystems. The current study addresses this knowledge gap with the primary objective of assessing community structure and ecosystem carbon stocks (biomass and soil) of selected mangrove ecosystems in Viti Levu across a range of geomorphic settings. We anticipated, similar to findings in other locations, that forest structure and carbon stocks would be highest in deltaic systems compared to those on coral reef flats, and that higher rates of precipitation, freshwater influx and tidal amplitude were key factors driving the formation of more structurally complex forest of higher biomass. We also expected significant changes in both carbon stocks and the allocation of biomass in mangrove forests affected by tropical cyclones.

This study represents the most comprehensive assessment of Fijian mangrove ecosystems yet undertaken and is relevant to resource users at the community level, landowners, land managers, and policy makers. The results help inform governance and decision-making on the management of mangrove ecosystems, particularly in considering the important role they have in climate change mitigation and adaptation. Research presented here also builds on and complements related studies of mangrove coverage, land-use change, and drivers of land-use change in Fijian mangrove ecosystems (Cameron et al. 2020).