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Closing the hydrological budget in tropical peatlands

Prof Andy Baird (SOG), Paul Morris (SoG), Dr Omar Lopez (Smithsonian Tropical Research Institute, Panama)

Project partner(s):  Smithsonian Tropical Research Institute, Panama

Contact email: a.j.baird@leeds.ac.uk

Project summary

Tropical peatlands are important global carbon stores and unique ecosystems offering habitat to critically endangered animal species. Their hydrological regime is fundamental to how they function as carbon stores, as ecosystems, and to how they will respond to future disturbance (e.g. climate change). From recently-completed work on peatland sites in Panama we now have a good understanding of the permeability of tropical peat and how water is lost from peat soils via subsurface seepage to peatland margins. However, we still have limited understanding of water losses via evaporation and transpiration. In particular, we do not know how transpiration losses are affected by plant species and by conditions in the soil (the physical properties of the peat in the uppermost half metre and the position of the water table). To address this important research gap, the successful candidate will spend two field seasons on tropical peatland sites in Mesoamerica and SE Asia. At the study sites, measurements will be made of weather variables, water-table dynamics, soil moisture content above the water table and sap flow in the dominant plant species (mostly various hardwood tree species and palms but also some sedges). This study will principally involve data collection and analysis, but the results and improved understanding of the tropical peatland water budget will enable colleagues at Leeds to adapt existing hydrological models to make them suitable for tropical bogs. The information will also be essential to the improvement of the DigiBog model that is used to simulate the decadal to millennial development of peatlands. The study will allow us to manage the tropical peatland resource more effectively and to predict more accurately how these systems will respond to climate change. This project is suitable for applicants with a background in ecology, environmental science, or physical geography who have an interest in tropical forest ecology and/or environmental hydrology.

Background and rationale

Why tropical peatlands?

As much as 90 billion tonnes of carbon is stored in tropical peat which means that the topical peatland carbon stock is as large as that contained in the above ground biomass in the entire Amazon rainforest1,2. The fate of this large carbon store under a changing climate is highly uncertain. As the climate warms and tropical droughts increase in frequency and intensity3, will natural tropical peatlands degrade and become net sources of greenhouse gases, thus exacerbating global warming, or will they show resilience and continue to accumulate carbon, thereby providing a partial buffer to climate change? Many tropical peatlands in Mesoamerica and the Amazon basin remain in a natural or semi-natural state. However, in SE Asia most have been drained and many have been used for tree production and agriculture (oil palm cultivation especially). In these modified peatlands we know that intensive drainage can lead to degradation of the peat and net carbon loss through enhanced peat decay and peat fires especially in drought years4. However, there is interest in managing these peatlands so that they retain more water and function hydrologically in a more natural way. Also, tropical peatlands are unique ecosystems and contain vegetation types not found elsewhere in the tropics5 and provide habitat for rare animal species such as the orang-utan (Pongo pygmaeus and P. abelii), and there is growing interest in how they can be better managed for their wildlife interest.

Tropical peatland hydrology

Critical to the functioning of tropical peatlands as carbon stores and ecosystems is their hydrological regime. We know that these systems are wet (the water table is usually within 20-40 cm of the peatland surface, and surface inundation in some peatlands is common) but considerable uncertainty exists over their hydrological budget. We still lack basic knowledge of the relative importance of water loss pathways in tropical peatlands – subsurface seepage, overland flow, and evaporation and transpiration. Recent work by the supervisory team on peat permeability6,7 has shed light on the importance of subsurface flows. However, few measurements have been made of evaporation and transpiration in tropical peatlands, and virtually no work has been done on how these processes are affected by the properties of the peat, by the depth of the water table below the ground surface and by the species of plant growing on the peatland. For example, during drought conditions we might expect evaporation and transpiration rates to decline as the hydraulic resistance of the peat increases as the water table falls, but we do not know the nature of this relationship, if it exists. Additionally, as the water table falls, different tree species may show different physiological responses (e.g., stomatal closure) to water scarcity, with this response being affected by root architecture and peat properties. By addressing these and related research gaps (see 'Key research questions' below) we will be in a strong position to close the hydrological budget in tropical peatlands and to produce better models of their hydrological regime and response to climate change. Although the focus of the current project will be on innovative field measurements and data interpretation, the findings will also allow us to improve computer models of peatland hydrology and peatland development8 (bog growth and decay). These improved models can then be used to produce more accurate predictions of the response of tropical peatlands to climate change; they can also be used as tools to help peatland managers.

The proposed project

Fieldwork for the project will take place on at least two sites: one in Panama and one in SE Asia. The SE Asian site has yet to be finalised (several candidate sites are being investigated). The Panamanian site is part of a large peatland complex in NW Panama known as the Changuinola peat swamp. This site has been chosen because it contains a mixture of natural and human-modified peatlands and because it is representative of a wide class of domed ombrotrophic (rain-fed) bogs found elsewhere in the tropics9. The site also benefits from being close to the excellent research facilities of the Smithsonian Tropical Research Institute and has been the subject of previous work by ecologists, palaeo-ecologists, soil scientists and hydrologists. Detailed investigations of the permeability of the peat have already been made and a pilot study of water-table dynamics in the peatland was conducted in November 2014. Figure 1 shows pictures of the peatland and some of the equipment used in these previous studies, while Figure 2 shows some of the water table and weather data collected in the pilot work.

Figure 1: Scenes from the Panamanian research site in the Changuinola peat swamp showing peatland vegetation, automatic weather station, permeability testing equipment and automatically-logged water-table wells.

Figure 2: Example data from pilot study work at the Panamanian research site showing rainfall and water-table position. The new study will look at the importance of evaporation and transpiration especially during periods of low rainfall.

Key research questions

  • What proportion of water loss from tropical peatlands is due to evaporation and transpiration?
  • Different peat types form under different types of tropical peatland vegetation; how do evaporation and transpiration losses vary according to peat type?
  • How does transpiration vary according to tree species and growth stage?
  • How do evaporation and transpiration vary according to water-table depth?

Methods

Weather conditions at the study sites will be measured using portable automatic weather stations available from the University of Leeds. Water tables will be measured using automatically-logged dipwells, and soil moisture dynamics will be monitored using high-resolution soil-water probes (equipment again available at Leeds). Instruments for sap-flow (transpiration) measurements will be provided by the third supervisor Dr Lopez at the Smithsonian Tropical Research Institute – who will also provide training in their use.

Requirements

BSc (Hons) and MSc (Merit or Distinction, attained or predicted) in ecology, environmental science or physical geography. Applicants with an MSc involving tropical ecology, forest ecology or environmental hydrology are particularly encouraged to apply.

Training

The successful candidate will be trained in the measurement of weather variables and soil water dynamics on UK wetland sites before starting their tropical fieldwork. Data analysis skills will be acquired through use of the training datasets in sessions with the project supervisors and through formal statistics training courses. Tropical fieldwork can be challenging and detailed risk management training will be provided. The candidate will be part of a large group of peatland researchers at Leeds who can provide informal training in a range of field and laboratory methods – knowledge exchange and mutual support are central to the group's ethos.

Research context

The successful candidate will join a vibrant research environment at the School of Geography which includes the River Basins Processes and Management research cluster and the informal Peat Club which meets weekly.

Potential for high impact outputs

There is increasing interest in tropical peatlands and tropical forests, in particular in their importance as carbon stores and wildlife habitats. The work in this project will provide a fundamental advance in our understanding of how these systems function and should lead to papers in high-profile ecology/hydrology journals such as Journal of Ecology and Water Resources Research and in high-impact inter-disciplinary journals such as Geophysical Research Letters. The supervisory team has experience of publishing in the named journals, and a wide range of similar journals.

Further reading / bibliography

[1] Page, S.E., Rieley, J.O., and Banks, C.J. 2010. Global and regional importance of the tropical peatland carbon pool. Global Change Biology 17, 798–818, doi: 10.1111/j.1365-2486.2010.02279.x.

[2] Fauset, S. and 97 others. 2015. Hyperdominance in Amazonian forest carbon cycling. Nature Communications 6, Art. No. 6857, doi: 10.1038/ncomms7857.

[3] Polade, S.D., Pierce, D.W., Cayan, D.R., Gershunov, A., and Dettinger, M.D. 2014. The key role of dry days in changing regional climate and precipitation regimes. Scientific Reports 4, Art. No. 4364, doi: 10.1038/srep04364.

[4] Page, S.E., Siegert, F., Rieley, J.O., Boehm, H-D.V., Jaya, A., and Limin, S. 2002. The amount of carbon released from peat and forest fires in Indonesia in 1997. Nature, 420, 61–65.

[5] Morrogh-Bernarda, H., Hussona, S., Page, S.E., and Rieley, J.O. 2003. Population status of the Bornean orang-utan (Pongo pygmaeus) in the Sebangau peat swamp forest, Central Kalimantan, Indonesia. Biological Conservation 110, 141–152.

[6] Kelly, T.J., Baird, A.J., Roucoux, K.H., Baker, T.R., Honorio Coronado, E.N., Ríos, M., and Lawson, I.T. 2014. The high hydraulic conductivity of three wooded tropical peat swamps in northeast Peru: measurements and implications for hydrological function. Hydrological Processes 28, 3373–3387, doi: 10.1002/hyp.9884.

[7] Baird, A.J., Low, R., Young, D., Swindles, G.T., Lopez, O., and Page, S. In review. High permeability explains the vulnerability of the carbon store in drained tropical peatlands. Geophysical Research Letters.

[8] Morris, P.J., Baird, A.J., and Belyea, L.R. 2012. The DigiBog model of peatland development 2: ecohydrological simulations in 2-D. Ecohydrology 5, 256–268, doi: 10.1002/eco.229.

[9] Phillips, S., Rouse, G.E., and Bustin, R.M. 1997. Vegetation zones and diagnostic pollen profiles of a coastal peat swamp, Bocas del Toro, Panamá. Palaeogeography, Palaeoclimatology, and Palaeoecology 128, 301–338.

Related undergraduate subjects:

  • Ecology
  • Environmental science
  • Physical geography