The research problem
Globally, peatlands are one of the most important terrestrial carbon (C) stores, containing over 500 Gt C. Most research on peatlands has focused on temperate and boreal bogs (rain-fed peatlands) and, recently, tropical bogs (also rain-fen). However, extensive areas of the mid- and high-latitude peatland resource is fenland. Fens are peatlands that receive some of their water from sources other than precipitation, and therefore have a different nutrient status from bogs, which can affect their ecological functioning. One of the least understood types of fen is floodplain woodlands, and it is these that this project will focus on. Stratigraphic surveys in some floodplains have revealed that woodland fens can accumulate peat at a rapid rate (2-4 m in 1000-2000 years) in response to rises in river level caused by sea-level rise, but we lack information on the two key components of the mass balance of these systems: plant productivity and peat decay.
Ecosystem models such as HPM (the Holocene Peat Model) and DigiBog have been, and continue to be, developed to simulate the fate of peatlands under future environmental conditions, and to help improve understanding of peatland-climate feedbacks. These models were developed for temperate bogs and are now being applied to boreal and permafrost bogs, and, more recently, to tropical bogs. However, conditions in wet woodlands are likely to be very different from conditions in these already-modelled systems. Mid- and high-latitude bogs are usually dominated by Sphagnum mosses, while tropical bogs are characterised by tropical hardwood and palm species. The trees found in wet woodlands are deciduous and mostly from the Salix and Alnus genera, and these likely have very different traits from the plants typical of bogs; we know very little about their rates of litter production above (leaves, branches and stems) and below (roots) ground. We also lack information on the decay of the litter and peat formed by these genera. Litter quality and the nutrient loading of the pore water in the peat will be important, as will the position of the water table which defines the boundary between mostly oxic and mostly anoxic conditions.
This project will start to tackle these important gaps by quantifying the aboveground structure of wet woodlands and rates of above- and below-ground litter production. With partners at the University of Cambridge (Dr Emily Lines, @emilyrlines) and Royal Holloway University of London (RHUL, Dr Alice Milner, @alicemmilner), work will take place at a recently-established ‘ecohydrological observatory’ at Wheatfen (https://www.wheatfen.org/) on the floodplain of the River Yare in the Norfolk Broads in East Anglia. There is the possibility that southern boreal sites in Canada will also be used. Detailed measurement of the structure of the wet woodland will be made using a terrestrial laser scanner (TLS) and drone LiDAR imagery and drone photogrammetry. Above-ground litter fall will be measured using adapted GEM plot (Global Ecosystems Monitoring Network) protocols (http://gem.tropicalforests.
ox.ac.uk/files/rainfor-gemmanual.v3.0.pdf), while belowground litter production will be quantified using novel mini-rhizotrons developed for wetland applications by a previous PhD researcher at the University of Leeds (Arnaud et al., 2019; https://link.springer.com/article/10.1186/s13007-019-0489-6). Weather and soil hydrological, chemical, and thermal conditions at the River Yare site are already continuously monitored, so it will be possible to relate measurements of litter production to a suite of environmental variables.
Work from the project will lead to at least three papers on: (1) the above- and below-ground C stock of wet woodlands, (2) factors controlling above-ground litter production, and (3) controls on, and rates of, belowground litter production.
The successful candidate will be part of a wider network of scientists undertaking research on these systems. As well as Drs Emily Lines and Alice Milner, the network includes Maddie Timmins (@msi_timmins), a NERC DTP PhD researcher at the University of Exeter working with Dr Tom Roland (@ThomasPRoland). Maddie will be using a palaeoecological approach to understand wet woodland response to past river- and sea-level changes. We are also hoping to recruit PhD researchers at Cambridge and RHUL, and have links with researchers at Queen Mary University of London who are working on non-wooded floodplain fens. We anticipate six-monthly meetings of this wider network where we will share research findings and plans.
The research approach
This project will use an observational approach to study the above- and below-ground C stocks and dynamics of wet woodland. The main study site is Wheatfen, a floodplain fen on the River Yare in Norfolk and home to the Broadland Ecohydrological Observatory (BEO), established in 2017. The successful candidate will undertake field investigations at the BEO to quantify:
- above-ground forest structure using TLS, drone LiDAR and drone photogrammetry (Structure from Motion);
- above-ground litter fall
- below-ground litter production using mini-rhizotrons.
Ecohydrological monitoring data sets from the BEO will be available to the successful applicant to link C production to ecohydrological variables.
The importance of the work
The findings will lead to improvements in understanding of a hitherto under-studied part of the terrestrial C cycle. The information will help drive improvements of the NERC-funded DigiBog ecosystem model, and, at larger scales, the UK’s land-surface scheme/model – JULES – that is currently being adapted to include a peatland component. Through this project we will start to understand the vulnerability and resilience of wet woodlands and their C stores in response to environmental change.