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

Biogeochemical Cycles



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 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 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 ( 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 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 high profile work by our team (Friggens et al. 2020) at several of the MOORCO, and related, sites, has shown that tree planting in heather moorlands does not result in net ecosystem C sequestration over 12-39 year timescales. Indeed the converse is often apparent, particularly when birch is planted, and there is a net loss of soil C at all of the measured plots. Based on what we currently know, it is likely that this C is mostly lost to the atmosphere as CO2. This IAPETUS2 project 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 soil organic matter 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).

The project will address the following questions:
1. How does the rhizosphere associated with birch and pine interact with pre-existing SOM?
2. How does the quality / quantity of the tree inputs (leaves, roots, root exudates and mycorrhizal fungal inputs) influence soil C dynamics, potentially via the process of ‘priming’?
3. Is there evidence of changes in soil C stocks, following tree planting, in mineral soil horizons, rather than solely in organic horizons?
4. What is the relative importance of C loss from soils via respiration and via aqueous fluxes to surface and ground-waters?
5. Can we predict, knowing key information about site characteristics (e.g. drainage, organic horizon depths, soil type), the net effect of tree planting on ecosystem C stocks?


The project will be centred around the MOORCO and related experimental plantings. The core MOORCO experiment is a replicated design with 3 or 4 blocks of each of the two tree species, together with control moorland plots at each of 3 sites. These plots provide the opportunity to acquire a unique data set that would otherwise be impossible to establish 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.

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 may 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 of pre-existing soil C. Related to this, ‘natural abundance’ studies of radiocarbon (14C) in soils and soil-respired CO2 may be deployed to quantify the timescales of C cycling, and the potential for tree colonization to either increase C sequestration in soils, or to accelerate the release of moorland soil C, either to the atmosphere as CO2, or to ground and/or surface waters.

Project Timeline

Year 1

* Training in field and lab methods; establishment and instrumentation of mesocosm experiments.
* Initiation of soil sampling at the MOORCO and associated sites and field deployment of relevant equipment (e.g. mini-rhizon samplers for collection and analysis of soil waters and ‘in-growth’ cores for the quantification of rhizosphere processes).
* Training in soil respiration measurements and development of rapid protocols for the measurement of soil enzyme activity.

Year 2

* On-going measurements at the mesocosm experiments and MOORCO plots. Soil profile sampling (C stocks, natural-abundance 14C). Soil enzyme and microbial community analysis.
* Establishment of litter (leaf and root) priming experiments and N manipulation experiments.
* Sample and data analysis; drafting research manuscript(s) and PhD chapters.

Year 3

* Completion of field data collation and data analysis.
* Drafting of all data chapters and manuscripts to publish findings.

Year 3.5

* Completion of PhD writing and research manuscripts. Conference presentation at the British Ecological Society Annual Meeting 2024 or European Geophysical Union Meeting 2025.

& Skills

IAPETUS2 provides training both as part of the cohort of IAPETUS2 students and at the Host Organizations in which the project is based, and by providing access to external courses. Each IAPETUS2 student will:

* Be registered for a separate qualification – a Postgraduate Certificate in Environmental Methods – which will recognize the transferable skills training and non-project-specific training undertaken.
* Have access to cohort-based activities at three points in the year: Induction in November, our SCENE Research Methods course in January, and our Annual Conference in May.
* Have access to specialized training courses (e.g. lab and field skills training, isotope geochemistry and advanced statistics with R).
* Carry out Entrepreneurship training.
* Take part in virtual discussion groups with the rest of the IAPETUS cohort.
* Have access to funded placements, which can be student-led or designed by end-user partners.

References & further reading

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

Fontaine S et al. (2007 Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 250, 278e280.

Friggens N et al. (2020) Tree planting in organic soils does not result in net carbon sequestration on decadal timescales. Global Change Biology 26, 5178-5188.

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

New York Declaration on Forests. Climate Summit 2014 (2014). Available at:

United Nations Environment Programme (UNEP) (2011) U. N. E. P. The Bonn Challenge. Available at:

Further Information

Philip Wookey (

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