Seeing the wood for the trees – Is reforesting the Scottish Highlands a good way to sequester atmospheric carbon dioxide?

Biogeochemical Cycles

IAP2-19-052

Overview

This PhD project concerns one of the most pressing issues of our time; the ‘Climate Emergency’, and the proposed use of tree planting to mitigate climate change.

Planting trees for carbon dioxide (CO2) sequestration is a mitigation strategy for climate change that is rapidly gaining momentum in national and international policy contexts (UNEP 2011; New York Declaration on Forests 2014). Recent high profile publications (e.g. Bastin et al. 2019; Lewis et al. 2019) have emphasized the benefits of afforestation, but have attracted serious criticism for oversimplifying the underpinning science, exaggerating the carbon (C) sequestration potential and failing to acknowledge the possible adverse consequences of tree planting in a range of contrasting environmental, ecological and social contexts (see ‘Technical Comments’ on Bastin et al. 2019).

Scotland is currently a relatively sparsely wooded country (~17% of the land area), but could support a much greater woodland coverage, as it has in the past. The Scottish Government has therefore defined annual targets to increase woodland cover – with an assumed parallel increase in C storage – and hence contribute to climate change mitigation.

Tree colonization, however, does not immediately result in increased C storage, as the largest pool of terrestrial organic C, by far, is in soils (He et al. 2016), and the consequences of tree planting for below-ground processes affecting soil organic matter (SOM), and thus C stocks, warrant urgent investigation. Where existing soil C stocks are already very high, such as the UK uplands and in pan-Arctic northern boreal and low arctic tundra ecosystems, it is critically important to know whether the pre-existing SOM is vulnerable to accelerated decomposition when colonised by, or planted with, trees. If this is the case, then the potential for C sequestration in tree biomass needs to be balanced against the possible losses of pre-existing soil C.

In this IAPETUS2 PhD project (http://www.iapetus.ac.uk/aboutstudentships/) you will have access to a unique suite of moorland sites (the oldest established in the 1970s) in the Scottish Highlands which have been experimentally forested using two common native tree species; downy birch (Betula pubescens) and Scots pine (Pinus sylvestris). These sites (known collectively as the MOORCO Experimental Platform; Figure 1 and http://www.hutton.ac.uk/research/groups/ecological%20sciences/research%20facilities/moorco) were established to investigate how woodland expansion onto heather moorland affects biodiversity and ecosystem services (Mitchell et al. 2007).

The overarching aim of the project is to determine the factors controlling soil C storage during tree colonization on moorland. Recent work (Friggens et al., submitted) at several of the MOORCO, and related sites, has shown that tree planting in heather moorlands does not result in net ecosystem C sequestration. Indeed the converse is apparent, particularly when birch is planted, and there is a net loss of C. Based on what we currently know, it is likely that this C is mostly lost to the atmosphere as CO2. In this project, you will unravel the processes and mechanisms involved in this counterintuitive system, with a primary focus on dynamics in the ‘mycorrhizosphere’. We hypothesise that mycorrhizal fungi, and their role in SOM dynamics, may be critically important, along with the process of ‘priming’, whereby recent C inputs into the soil, mediated by C assimilation above-ground, stimulate the soil microbial community, enabling decomposition of pre-existing soil C stores and release of CO2 (Fontaine et al. 2007).

Click on an image to expand

Image Captions

MOORCO planted birch plot on heather moorland, Craggan, Glenlivet, July 2011

Methodology

The project will be centred around the MOORCO and related experimental plantings. These replicated plots provide the opportunity to acquire a unique data set that would otherwise be impossible to within the time-scale of a PhD. In addition, the project will deploy outdoor ‘mesocosm’ experiments, using tree saplings, to study rhizosphere processes in more detail, and indoor (controlled environment) ‘microcosms’ to resolve biotic and abiotic controls on decomposition processes with more refinement. There will also be an opportunity to extend the work to the Arctic through studies in northern Scandinavia.

The data collected will include soil characteristics (e.g. C stocks, organic matter characteristics and radiocarbon age), tree establishment and size, to explore differences in the C budgets, and determination of soil metabolic processes such as respiration, and enzyme activities involved in the decomposition of organic matter. You will also apply C stable isotope (13C) approaches to trace the assimilation, partitioning and fate of C captured in photosynthesis, and the potential role of rhizosphere inputs for the decomposition SOM.

Project Timeline

Year 1

Establishment / instrumentation of mesocosm experiments at the University of Stirling.

Soil sampling at the MOORCO and associated sites and field deployment of monitoring equipment.

Initiation of soil respiration measurements. Development of robust, rapid protocols for the measurement of soil enzyme activity.

Year 2

Maintenance and measurements at the mesocosm experiments and MOORCO. Soil profile sampling. Soil enzyme and microbial analyses.

Establishment of litter (leaf and root) priming experiments and N manipulation experiments (including lab-based microcosm experiments). Above-ground biomass evaluation.

Sample and data analysis; drafting research manuscript(s) and PhD chapters.

Year 3

Maintenance and measurements at the microcosm and mesocosm experiments, and MOORCO.

Potential field campaign in Scandinavia for up-scaling of results; reinstatement of field sites and removal of non-essential equipment.

Drafting of data chapters and research manuscripts to publish.

Year 3.5

Months 37-42:

Completion of analyses.

Completion of PhD writing and research manuscripts. Conference presentation (e.g. British Ecological Society 2023 or European Geophysical Union 2024).

Training
& Skills

1. Field work methods, including soil sampling and processing, soil CO2 flux measurements and experimental design;
2. General laboratory skills and analytical techniques, including soil C and N determinations, and soil water analyses;
3. Soil enzyme and microbial analyses;
4. Application of specialised analytical methods such as C and N stable isotope techniques, and 14C at natural abundance;
5. Numeracy, data analysis and advanced statistics with R, ecological modelling & informatics. These skills will be gained through targeted training courses within the IAPETUS2 consortium and available at Stirling.
6. Complementary training in transferable skills and core scientific skills (data management, analysis, presentations, paper writing).

References & further reading

Bastin J-F et al. (2019) The global tree restoration potential. Science 364, 76-79 (2019). And Technical Comments; eaay8060. DOI: 10.1126/science.aay8060; eaay8334. DOI: 10.1126/science.aay8334; eaaz0388. DOI: 10.1126/science.aaz0388; eaay796. DOI: 10.1126/science.aay7976

Hartley IP et al. (2012) A potential loss of carbon associated with greater plant growth in the European Arctic. Nature Climate Change 2, 875-879.

He Y et al. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419-1424.

Lewis SL et al. (2019) Restoring natural forests is the best way to remove atmospheric carbon. Nature 568, 25-28.

Mitchell, R. J. et al. The cascading effects of birch on heather moorland: A test for the top-down control of an ecosystem engineer. Journal of Ecology 95, 540-554 (2007).

New York Declaration on Forests. Climate Summit 2014 (2014).

Parker, T. C., Subke, J. A. & Wookey, P. A. Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline. Global Change Biology 21, 2070-2081 (2015).

United Nations Environment Programme (UNEP) (2011) The Bonn Challenge.

Further Information

Please contact Prof Philip Wookey with any queries regarding this project:
philip.wookey1@stir.ac.uk

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