The reintroduction of beavers has been a conservation success story repeated across Europe and North America. By damming small rivers and creating pools, ponds and canals, beavers have restored lost ecosystem services such as: enhanced landscape biodiversity (Nummi et al., 2019; Willby et al., 2018), flood and drought alleviation and improved water quality (Brazier et al., 2020). Yet, these ecosystem modifications may also play a role in climate change as a function of the vast quantities of sediment that accumulate in the ponds upstream of beaver dams, in addition to increased biomass of aquatic plants (Law et al., 2016) and large volumes of dead wood comprising the beaver dams. These rewilded landscapes may therefore also act as carbon sinks and, hence, as a potentially valuable tool in creating resilient ecosystems in the face of a changing climate.
However, ponds of all types are simultaneously a disproportionately large source of greenhouse gases (GHGs), a major driver of global warming (Holgerson and Raymond, 2016), due to their shallow depths, frequent mixing, high nutrient loading and fast energy turnover. Our understanding of GHG emissions from ponds, both in terms of magnitude and driving processes, is lacking, even more so in beaver ponds which differ from other freshwater systems in their fundamental physico-chemical characteristics (Cazzolla Gatti et al., 2018). There is currently a dearth of studies that assess the full carbon mass balance of beaver ponds, with none at the UK scale and therefore we are unable to quantify the net effect of beaver reintroduction on carbon cycling. This limits our understanding of the overall impact and influence of beavers in relation to climate change.
Recognising and quantifying the role of ecosystems and individual species in carbon cycling is essential as we work towards net-zero carbon emissions and developing nature-based solutions. Robust evidence of the role of the beavers in carbon cycling is now needed to identify whether carbon emitted is offset by carbon stored in beaver ponds’ sediments, vegetation and the dead wood comprising their dams. The overall aim of this PhD project is to quantify the net effect of beavers on carbon cycling, in order to determine their climate impact. Specifically, the objectives of this project are to:
• Assess beaver ponds as potential sediment and vegetation carbon sinks;
• Quantify GHG emissions from beaver ponds relative to other freshwater habitats in the same landscape;
• Scale up findings to landscape and national scales to provide the first evidence base of the overall role of beavers in carbon cycling and climate.
The PhD fieldwork will be based at an upland farm in Perthshire, Scotland. We have a history of working at this site extending for almost 20 years and excellent relationships with estate management. The site is extensively instrumented and the project can draw on a wealth of supporting data and knowledge assembled in our long-term research and a succession of PhD studentships. It also contains the highest density of beaver dams in Britain, ranging in age from 2-15 years, following the release of animals in 2002 as part of a wetland restoration demonstration project. We have documented many of the ecological changes since this time, e.g. Law et al. (2017). The site includes other freshwater habitats such as streams, ditches, temporary ponds and a small lake and has recently undergone management to open up old ponds and create new ponds and scrapes, thus, giving a wide comparative scope. The site is envisaged as an exemplar of a wetland landscape with restored ecological functions set within a naturalising agricultural landscape. As such it represents a pioneering project within the UK.
The project will compare measurements of greenhouse gas emissions and carbon stored in sediments, plants and dead wood in beaver-impacted freshwaters with suitable natural systems where beavers are not present in order to determine their net effect. Field campaigns will be conducted seasonally to assess variability in carbon cycling at a number of representative sites within the landscape. In each campaign, the student will carry out quantitative surveys to determine dissolved GHG concentrations and fluxes (through the use of the headspace method and floating chambers, respectively), in addition to carbon sequestration in sediments and plants (through the use of sediment traps and core samples, and vegetation biomass measurements, respectively). In addition, water samples will be collected on a seasonal basis to determine water quality, and other relevant site information including pond age, shade, area, depth, and flow will be collated to aid data interpretation.
Samples will be analysed using a suite of laboratory instruments that the student will be trained to use, including a gas chromatograph, TOC analyser, spectrophotometer and ion chromatograph. In-situ continuous data will also be generated from CO2, CH4, temperature and water level sensors deployed at sites of interest within the catchment. The majority of samples will be analysed at laboratories at Stirling, with monthly trips to UKCEH to learn specific lab techniques, use specialised equipment and for supervisory meetings.
Once data are collated, the relative carbon source-sink potential of beaver-engineered freshwaters will be considered in relation to a number of key drivers. The specific focus of experiments and emphasis and timing of sampling will be determined as the student develops their focus and key hypotheses. Possible themes to be explored include: (1) spatial variation within and between beaver ponds; (2) temporal variation in carbon cycling in beaver ponds using ponds of varying age for a chronosequence; (3) biogeochemical drivers; (4) modelling of wider implications.