UNIVERSITY OF PORTSMOUTH
Mangrove forests play a vital role in the health of our planet. The trees and shrubs absorb a substantial amount of greenhouse gas emissions, help protect communities from rising sea levels, and act as nurseries for baby fish.
These coastal forests are the second most carbon rich ecosystem in the world, being able to store more than 1,000 tons of carbon in just one hectare; that’s about the size of a football pitch. They do this by capturing the chemical element from the air and storing it in leaves, branches, trunks and roots.
But despite environmental efforts to prevent the loss of these important ecosystems, they are still at risk. A new study, by the University of Portsmouth and facilitated by research organisation Operation Wallacea, has revealed how the stored carbon from atmospheric CO2 in large woody debris is processed by organisms. The findings suggest climate change can significantly impact this ‘blue carbon’ system.
Scientists from the University of Portsmouth analysed large woody debris (LWD) in four mangrove forests in Indonesia’s Wakatobi National Park with differing intertidal zones. Each survey area had up to 8 sections (transects) – each revealing their own way of processing carbon.
In the upper reaches of the ecosystem, closer to land, the team discovered organisms typically found in tropical rainforests are breaking down fallen wood. These include fungi, beetle larvae, and termites. Further towards the ocean, the LWD is being degraded more quickly by worm-like clams with calcium carbonate shells, known as shipworms.
Two consequences of climate change can affect the delicate process of fixed-carbon degradation in the mangrove forest. The first being rising sea levels, as the carbon cycle is driven by tidal elevation. The second is an increase in ocean acidity caused by rising CO2 in the atmosphere, which can dissolve the shells of the marine organisms degrading the wood in the lower reaches.
Lead author of the study, Dr Ian Hendy from the University of Portsmouth’s School of Biological Sciences, said: “This data highlights the delicate balance between wood-biodegrading organisms and fallen mangrove wood. Mangrove forests are crucial to mitigating climate change, and alterations to the breakdown of fallen wood in the forests will change the above-ground carbon cycles which may have an effect on mangrove carbon stores”.
Dr Hendy and his team now have their sights set on taking part in large-scale mangrove forest restoration in Mexico. The joint biodiversity initiative rePLANET is working exclusively with a group of scientists at Portsmouth, Brighton, Singapore, and CINESTAV to fund a series of PhD projects examining the innovative approaches being taken to preserve and protect forests.
“The team’s goal now is to use the findings from this study to guide large-scale restoration of mangrove forests across the globe”, added the study’s co-author, Dr Simon Cragg from the University of Portsmouth.
The ‘Biodegraders of Large Woody Debris Across a Tidal Gradient in an Indonesian Mangrove Ecosystem’ study, published in Frontiers in Forests and Global Change, was supported by experts from the University of Plymouth, Brighton University, the Eden Project, UK Centre for Ecology & Hydrology, and Estonian University of Life Sciences.
Frontiers in Forests and Global Change
METHOD OF RESEARCH
SUBJECT OF RESEARCH
Biodegraders of Large Woody Debris Across a Tidal Gradient in an Indonesian Mangrove Ecosystem
ARTICLE PUBLICATION DATE
The paper is open access and has a creative commons license. Here’s some of it.
Biodegraders of Large Woody Debris Across a Tidal Gradient in an Indonesian Mangrove Ecosystem
1Institute of Marine Sciences, University of Portsmouth, Portsmouth, United Kingdom
2Centre for Blue Governance, University of Portsmouth, Portsmouth, United Kingdom
3Eden Project Learning, Green Build Hub, Eden Project, Bodelva, United Kingdom
4School of Biological and Marine Sciences, University of Plymouth, Plymouth, United Kingdom
5UK Centre for Ecology & Hydrology, Gifford, United Kingdom
6Centre for Aquatic Environments, University of Brighton, Brighton, United Kingdom
7Department of Landscape Management, Estonian University of Life Sciences, Tartu, Estonia
There has been limited research on the breakdown, recycling, and flux of carbon from large woody detritus (LWD) in mangrove forests. The breakdown of LWD is caused by guilds of terrestrial and marine biodegrading organisms that degrade wood at a range of rates and efficiencies. Spatial variations in environmental factors within mangroves affect the distribution and community of biodegrading organisms, which, in turn, impacts carbon flow and sequestration. We reveal the role of biodegrading organisms in LWD breakdown and the environmental factors that influence the distribution of biodegrading guilds within a mangrove forest in South East Sulawesi that supports a diversity of mangrove species typical of Indonesian mangrove forests, which constitute 20% of Global mangrove cover. Within the high intertidal regions, terrestrial biodegradation processes dominated upon LWD. After 12 months exposure on the forest floor, experimental wooden panels in these areas remained unchanged in mass and condition. In the low intertidal region, marine wood-boring animals belonging to the family Teredinidae were the dominant biodegraders of LWD, and their activity reduces LWD volume and speeds up the loss of LWD volume. More than 50% of the experimental wooden panels’ weight in these areas was lost after 12 months exposure on the forest floor. Although different biodegrading guilds occupy the same LWD niche, their distribution throughout the mangrove forest is influenced by inundation time. The change of biodegrading guilds within LWD between the terrestrial and the marine organisms was distinct, creating a biodegradation boundary in a distance as narrow as 1 m on the mangrove forest floor. These results are important, as rising sea levels have crucial implications for biodegrading guilds. A full understanding of factors affecting the biodegradation processes of LWD in mangrove forests is critical to accurately assess mangrove carbon stores and the fate of mangrove derived carbon.
Mangrove forests form the characteristic vegetation of low wave energy tropical and subtropical shorelines. Due to their location they play an important role in land-to-sea transfer of organic carbon derived from the breakdown of vascular plant detritus (Cragg et al., 2020). Some of this detritus is brought by rivers, but much is derived from the high primary productivity of these forests (Duarte, 2017). Mangrove productivity is channeled into biomass compartments of leaves, branches, main stems and roots, with the proportion of woody tissue increasing as the tree grows, with over 70% of total biomass being composed of trunk wood in larger trees (Cintron and Schaeffer-Novelli, 1984; Ong et al., 2004). These biomass components eventually fall to the forest floor, where they are processed in situ or tidally exported. In situ processing drives the remarkably high capacity of mangrove forests to store carbon in their sediments (Donato et al., 2011; Breithaupt et al., 2012; Sanderman et al., 2018) where anoxic conditions in fine grained sediment tend to limit further breakdown (Jennerjahn, 2020; Kauffman et al., 2020). Crabs can play a major role in leaf breakdown (Thongtham et al., 2008). However, the fate of mangrove carbon derived from large woody detritus (LWD) is less understood.
Woody tissues can represent a high proportion of the above ground biomass of mangroves that increases as the tree grows (Rovai et al., 2021), but woody components are shed in increasing amounts throughout the life of the tree and eventually the tree falls to the forest floor where it will be partially submerged during the tidal cycle (Cragg et al., 2020). Measures of the woody component of litter fall at multiple sites in tropical Australia indicate that woody tissues represent 8–15% of total small litter fall (Duke et al., 1981). Litter traps are not designed to capture the rare events of falling trees or large branches, so the main input of wood onto the forest floor can only be measured by surveys of LWD, which show that quantities vary widely between sites (Allen et al., 2000; Donato et al., 2011). Kauffman et al. (2020) undertook a remarkable systematic study of organic carbon in mangrove forests at numerous sites around the world. They report that LWD (downed wood in their terminology) generally constitutes less than 10% of the above ground C store in live and dead tissue, with quantities ranging from 0 to 150 Mg C ha–1, with the upper values coming from forests where LWD quantities exceed standing biomass.
Large woody detritus enhances soil pedogenesis, provides habitat for both germinating seeds (Allen et al., 2000; Krauss et al., 2005) and animals (Hendy et al., 2013, 2014), and increases the mangrove forest nitrogen budget (Robertson and Daniel, 1989), thus contributing to ecosystem function and productivity. LWD also plays a crucial role in carbon flow and storage within mangrove ecosystems. Carbon fluxes originating from LWD within mangrove ecosystems are driven by fungi and woodborers, depending on the mangrove tidal zone (Kohlmeyer et al., 1995). These biodegrading organisms promote fragmention of LWD (Filho et al., 2008), facilitating incorporation into anoxic sediments, where the majority of mangrove carbon typically resides (Kauffman et al., 2020) and is generally slow to degrade due to mostly anoxic conditions (Jennerjahn, 2020). However, there are few studies on biodegradation of LWD in mangroves, and the processes of LWD biodegradation are complex and require the ability to unlock the enzyme-recalcitrant lignocellulose complex characteristic of woody plants (Cragg et al., 2020).
Rates of wood biodegradation vary between tidal zones within mangrove forests (Kohlmeyer et al., 1995). In the high intertidal region of a mangrove forest, decay of LWD is slow (Benner and Hodson, 1985) and caused mainly by white and brown rot fungi (basidiomycetes) (Kohlmeyer et al., 1995) and termites (Vane et al., 2013). In the mid to low intertidal regions, degradation of LWD is rapid (Middleton and McKee, 2001), driven mainly by teredinid bivalves commonly known as shipworms (Robertson and Daniel, 1989; Filho et al., 2008). Decay due to bacteria and ascomycete fungi also occurs in waterlogged wood, but at a much slower rate than that achieved by white and brown rot fungi (Singh et al., 2022).
Gradients of inundation and salinity may alter the distribution of biodegrading guilds as mangrove forests are habitats where terrestrial and marine influences interact. Teredinid wood-boring activity is particularly important in areas of mangrove forests with high levels of LWD (Robertson, 1990; Kohlmeyer et al., 1995). Teredinids convert fallen logs into fine fragments of fecal material (frass) that contribute to carbon cycling and reduce build-up of LWD in mangrove ecosystems (Filho et al., 2008). However, teredinids cannot tolerate the prolonged emersion that occurs in the high intertidal (Robertson, 1990). In the mid to low intertidal zones of a Rhizophora-dominated Australian mangrove forest where there is less organic content in the sediment, Robertson (1990) found that half of the original LWD was consumed by teredinids within 2 years, whereas in the high intertidal, where teredinids were absent, only 5% of the original mass of fallen logs was lost. Teredinids and isopod wood-borers are sensitive to salinity levels. In an extensive study of teredinids in LWD and wooden panels around Papua New Guinea, Rayner (1983) categorized the numerous teredinid species in these waters into stenohaline marine, euryhaline marine, euryhaline brackish water or stenohaline brackish water with the latter three categories occurring in mangrove ecosystems. Limnoriid crustaceans bore into wood in the intertidal zone, but are not tolerant of brackish conditions. They occur in the seaward regions of the forest investigated in this study (Cookson et al., 2012). Global-mean sea level (GMSL) has increased by approximately 1.5 mm yr–1 over the 20th century (Hay et al., 2015; Dangendorf et al., 2019) and is predicted to increase by 65 ± 12 cm by 2100 (Nerem et al., 2018). How climate change and sea level rise affect the distribution and activity of LWD biodegrading organisms, and by proxy, rates of carbon flow and carbon storage in mangrove ecosystems, is an open and urgent question. In the West Pacific region, geostrophic forcing causes seawater to accumulate along the western margin of the Pacific Basin, resulting in sea levels and rates of sea level rise in Southeast Asia that are higher than the global average (Weller et al., 2016). For example, the rate of sea-level rise in coastal Indonesia from 1992 to 2015 was approximately 7 mm per year (Surya et al., 2019), and climate change models predict a sea level rise of 74 cm by 2100 (Karondia et al., 2019) which would inundate large areas of coastal mangrove. Shifting tidal ranges will impact biodegradation of LWD through changes in the distribution of wood-boring guilds.
This study uses a mangrove ecosystem with a range of tree species and wood degrading organisms that are typical of the mesotidal mangrove forests of Indonesia (Cragg and Hendy, 2010), a country that contains 20% of the global mangrove forest area (Bunting et al., 2018). It aims to identify the organisms that initiate break down of woody detritus within four mangrove forests of Sulawesi, Indonesia, and to determine how tidal elevation, inundation and salinity affect their distribution and rates of biodegradation. Understanding the baseline structure of LWD biodegrading guilds and how they vary with tidal elevation, and how ecological conditions influence rates of biodegradation, is essential to understand carbon cycling and carbon sequestration in mangrove ecosystems.