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 https://www.hutton.ac.uk/research/departments/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 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?
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Figure 1 – MOORCO planted birch plot on heather moorland, Craggan, Glenlivet.