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The University of Stirling and the James Hutton Institute1 recently presented research results into tree planting and carbon fixing in specific soil types. For us at Trees for Life, the report is a welcome step in furthering our understanding of the complex relationships between trees and the carbon stored in our soils. It underlines what we have said since we were established 30 years ago: plant the right tree in the right place.
As the study points out, the narrative around tree planting as a means of fixing or sequestering carbon too often omits the importance of soil carbon and how planting trees affects the flows of carbon between soils, vegetation and the atmosphere. The value of understanding this has become increasingly apparent to us over time as research has shone more light on the issue. Over the years, we’ve responded to the growing knowledge about soils, with the result that we are increasingly mindful about the sites we select for planting. We have also become more sensitive to how trees are planted, seeking to minimise soil carbon loss by disrupting soils as little as possible when planting.
Nature is the most effective tree planter and we know that trees will only grow when all the conditions are right. Where trees exist and produce seed, our aim is to allow natural regeneration to find its own way into a landscape’s soils. For us, this is indisputably the best way to restore a wild forest and it has a lower impact on soil carbon stocks than any planting method employed by people. However, in many cases, including on much of the land on which we work, natural regeneration cannot happen due to the lack of a seed source and browsing pressure from sheep and deer.
At our flagship rewilding estate at Dundreggan and in Glen Affric, by planting native tree species, we are creating a seed source in places where none exists. This planting is an investment in the long term future. It is rooted in the knowledge that the trees we plant can become the forebears of forests and scrublands which will wax and wane across the landscape in response to nature’s slow rhythms and changes. A landscape where such an ecosystem can fully express itself will feature a more natural, and bigger, carbon cycle than is possible under today’s bare, uniform landscape.
Nature works on its own timescales, with soils and vegetation interacting and evolving towards a level of long term stability. Carbon will always flow between soil, vegetation and the atmosphere in the ways described by this research. However, if the number of woody plants growing on the land is increasing, so the system will be storing increasing amounts of carbon as timber in the trees and shrubs. As the trees grow and leaf fall transports more and more nutrients to the ground, so the soils will evolve – a process which is fundamental to rewilding. This ever-changing relationship between soil and vegetation means that the carbon cycle is also in constant flux.
As the authors of this report say, we need more research to fully quantify and understand how the carbon cycle develops over time. For us, it makes sense that, in the long term, natural ecosystems will tend towards net carbon sequestration more often than not. This report’s findings are significant. They tell us that planting trees in some soil types can release carbon for some time in the short term, which is undoubtedly an important consideration in a climate emergency. However, we must also be mindful of the long term and the need to create the stable peatland and woodland ecosystems that will be the carbon stores of the future.
Moreover, for us, carbon capture is one part of a much bigger exercise in rewilding the Scottish Highlands. The extent of the species declines in UK biodiversity revealed in last year’s State of Nature report [2] drives a real need to provide a meaningful scale of positive habitat change to allow plants and animals to grow, shelter and thrive. For instance, we specialise in ‘wee’ trees, the montane specialists that are all but lost from the Scottish landscape. These mountaintop forests, when combined with woodlands on lower slopes create habitat for innumerable species.
For us, the way forward requires us to constantly ask ourselves two questions. Firstly, what is the best blend of actions we can take to balance the need to sequester carbon in a climate emergency, the need to create the carbon stores of tomorrow and the need to recover biodiversity from the all too fragile remnants we have left today? Secondly, what are the best practical ways for us to identify where to restore the right habitats in the right places with the least impact on soil carbon?
We do not expect to find a perfect answer to these questions, but we move with the clear conviction that waiting for such an answer is not an option. Research like this is essential if humans are to navigate a way out of the environmental jeopardies we have so gradually created. We must join with others to act, at scale, and to learn as we go.
1 Friggens et al, 2020, Tree planting in organic soils does not result in net carbon sequestration on decadal timescales, Global Change Biology 2 Hayhow et al, 2019 The State of Nature 2019. The State of Nature partnership