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Our latest Directors’ blog is by Helen Armstrong, professional ecologist and founder of Broomhill Ecology. In it, she digests important new research on carbon sequestration in afforested ecosystems, and evaluates the implications this might have on woodland expansion policy in Scotland.

Woodland expansion onto open ground is often cited as a good way of capturing carbon relatively quickly. For this and other reasons the Scottish Government currently has a target of creating 12,000 ha of new woodland annually.  Around 70% of Scotland’s land area is classed as upland and much of this consists of open ground. It might be thought, therefore, that the expansion of woodland in the Scottish uplands would be a good way of capturing carbon relatively rapidly.

Two recently published scientific papers have, however, cast doubt on this assumption since they suggest that it may be several decades before woodlands planted on soils containing peat, start to store more carbon than they lose. In the early decades, carbon is lost as the peat dries out and oxidises and this is not always balanced, during the same timescale, by the carbon that is captured by the growing trees. These results lead to the conclusion that woodland expansion onto the peaty soils that cover much of the Scottish uplands may exacerbate climate change, at least in the first few decades, rather than helping to reduce it. In this blog I take a close look at these two studies and discuss the extent to which this conclusion is justified. I then discuss the implications of these results for woodland expansion policy in Scotland, particularly in the uplands.

In the first study, Nina Friggens, and her co-authors,1 assessed carbon stocks at four sites in northern Scotland (Figure 1). Three of the sites had 39-year-old stands of birch and a fourth had 12-year-old stands of both Scots pine and birch. All the stands had been densely planted on open heather moorland and, by the time of the study, the 39-yearr-old stands had a dense tree cover and very little ground flora. The authors estimated the amount of carbon in the soil, roots, ground flora and tree components of the tree stands and, for comparison, also in the adjacent unburned, open heather moorland. The trees had been planted on soils classed as podzols, or peaty podzols. These soils contain some peat but do not have the thick layer of surface peat (>50 cm in the top 100 cm of soil) required for them to be classed as ‘deep peat’ soils. This is important since current guidance is that, for carbon sequestration purposes, only deep peats should be avoided for woodland expansion. The trees were planted by hand so there was minimal soil disturbance, which is known to cause peat to oxidise, releasing carbon.

Figure 1. Map of experimental sites used across Northern Scotland. Taken from Friggens et al. 2020.

The results (see Figure 2) showed that the 39-year-old birch stands at all three sites, as well as the 12-year-old Scots pine stands, stored similar amounts of total carbon compared to the heather moorland plots. The distribution of carbon between the components was different however, with the tree stands storing less carbon in both the soil and the ground flora but storing more carbon in the roots, as well as in the trees themselves. So, the birch stands at thirty-nine years after planting, and the Scots pine stands at twelve years after planting, showed no net sequestration of carbon, but neither had there been a net loss. By contrast, the 12-year-old birch stands stored significantly less carbon in total than the heather moorland control plots because lower carbon storage in the soil and the ground flora was not completely balanced by higher carbon storage in the roots and trees. The difference between the 12-year-old birch and Scots pine may be due to birch producing easily decomposable leaf litter whereas Scots pine produces needle litter that takes much longer to decompose hence carbon is stored for longer in the Scots pine litter.

Figure 2. Mean ecosystem carbon stocks from fours sites across Northern Scotland. Roots and organic soil horizon Carbon stocks are represented beneath the zero-line on the y-axis and tree and ground flora above the line in planted birch and pine plots as well as in un-planted heather moorland ‘Heather’ control plots. Error bars are one Standard Error of each constituent carbon stock mean. Taken from Friggens et al. 2020.

It is tempting to interpret the birch results as a time series and conclude that, when birch is densely planted on heather moorland, carbon loss from the soil will be greater than carbon sequestered by the trees for at least the first twelve years, and possibly for as long as the first thirty-nine years. By contrast, for Scots pine, there may be no net loss of carbon by twelve years after planting. These conclusions must be treated with a high degree of caution, however, since they rest on the results from only one site and the differences may be site-related rather than due to changes over time. It is also important to bear in mind that this study was on densely planted birch and Scots pine stands. We don’t know whether, and how, the carbon dynamics might differ in less densely planted or naturally regenerated woodland composed of one or more tree species that potentially includes more of a ground, and even shrub, layer. Since this is more akin to new native woodland plantings, and to naturally regenerated woodland, it is important that we find this out.

We should also remember that this study compared the carbon stored in stands of trees with that stored in unburned, and largely ungrazed, heather moorland. Much heather moorland in Scotland is burned and most is grazed by deer and /or sheep. It is essential that we also have data on the carbon stored in burned and /or grazed heather moorland since it may be that simply stopping muirburn, and reducing grazing, is the fastest means of rapidly increasing carbon sequestration in the Scottish uplands. This would, of course, result in woodland expansion where there was a seed source. Additionally, we need information on the carbon stored in grasslands dominated by white bent (Nardus stricta) and purple moorgrass (Molinia caerulea) since these habitats are the most likely alternatives to heather moorland for woodland expansion in the Scottish uplands.

In the second study, Keith Matthews and his co-authors2 used computer models to produce maps of Scotland showing the predicted impact of planting different types of woodland on net carbon sequestration over time. The models have been run for a range of commercial and multi-use woodland types as well as for un-exploited native conifer (Scots pine) and native broadleaf (birch, ash or sycamore) woodland. Since not all woodland types can grow successfully in all locations, the maps only show predictions of net carbon sequestration for the locations where each type is most likely to grow well. The impact of establishing each woodland type varies with soil type and climate since these factors affect growth rates and hence the rate at which carbon is sequestered by the trees. The peat content of the soil affects the amount of carbon that is likely to be lost as the trees dry out the soil, allowing the peat to decompose and release carbon dioxide. The balance between these two processes determines whether carbon sequestration is negative or positive.

The models predict that net carbon sequestration increases with time since planting but that, on peat-containing soils, especially in the north and west of Scotland, more carbon may be lost than gained for several decades. For native woodlands, at forty years after planting, there are many areas of upland and marginal land in the east and south of Scotland where net positive carbon sequestration is predicted to be occurring (Figure 3). Much of the suitable upland areas in the north and west, however, where soils are generally higher in peat content (but excluding deep peats), are predicted to still be net carbon emitters after forty years (Figure 3). Even by one hundred years after planting, there are still predicted to be some areas where net carbon sequestration is negative.

The conclusion from these results would appear to be that planting native woodland in much of north and west Scotland, as well as in some areas in the east and south of the country, will not result in net carbon sequestration for at least several decades. These predictions do, however, assume ground preparation (drainage and/or mounding) to enable planting and this is known to release large amounts of soil carbon. By contrast, woodland expansion without ground preparation, or by natural regeneration, is likely to release considerably less carbon. The authors are keen to produce equivalent maps for these scenarios. It should also be remembered that models are only as good as the information that goes into them and, given the importance of these results, it is imperative that more field experimentation is carried out to verify the results and further improve the models.

Figure 3. Predicted net carbon uptake /release 40 years after planting, using ground preparation, with native conifers or native broadleaves. The map shows areas of deep peat, which should not be planted due to the large amounts of carbon that are likely to be emitted as the trees grow and dry out the soil. The maps were kindly provided by Keith Matthews and Douglas Wardell-Johnson, James Hutton Institute, and are reproduced with their permission and cooperation. Providing the maps does not imply any endorsement by the authors of Matthews et al. (2020), nor by the funders of their analysis, of any interpretations or conclusions drawn within this blog.

Both these papers have shown that we cannot assume that new woodlands established on soils containing peat will always capture more carbon than will be lost from the system, at least in the first few decades. Even naturally regenerated, or hand planted, native woodland may not result in net carbon sequestration for at least several decades when established on such soils. Since much of the area of Scotland that is most appropriate for woodland expansion is in the uplands, and much of the soil in the uplands contains peat, more research is needed to test these conclusions and to determine with more certainty the conditions under which new woodland of different types will result in a net carbon loss, the size of any loss and for how long this might occur.

In particular, we need more information on the net carbon balance of burned and /or heavily grazed open ground compared to that of unburned/lightly grazed ground so that we can determine the carbon impact of reducing both these factors. Heavy grazing and burning occur individually, or in tandem, across most of the Scottish uplands and have a major impact on upland habitats. It is surprising, therefore, that so little research is carried out in Scotland on the carbon, and other, impacts of these two factors. One thing we do know, however, is that soil disturbance (e.g. ploughing. draining, clear felling and the use of heavy machinery) increases carbon loss.

Figure 4. Schematic of the benefits of increased cover of mixed species woodland, managed using continuous cover, in the Scottish uplands3,4,,5. Up arrows indicate an increase, down arrows a decrease and a question mark indicates no, or unknown, change.

The expansion of woodland composed of mixed tree species, in mosaic with open habitats, has many potential benefits for the Scottish uplands, aside from possible carbon sequestration (Figure 4). The aim of the Scottish Government should therefore be to increase the area of the uplands covered by the types of woodland that provide most benefit whilst, at the same time, minimising carbon loss through careful site choice and the use of low disturbance establishment and management techniques. The results of these two studies suggest that the former would mean prioritising upland sites, and micro-sites, with relatively low carbon soils. The latter would require woodland expansion to be largely by hand planting or by natural regeneration with the resulting woodland managed using continuous cover methods with the least possible use of large machinery.  

It should also be remembered that climate change itself may result in increased rates of peat oxidation so, even if there is some initial loss of carbon caused by woodland expansion, it may be that, by storing carbon in trees rather than in the soil, woodland expansion now will prevent carbon loss in the future. Perhaps, as suggested by Mike Perks and Elena Vanguelova in a recent article in the Reforesting Scotland journal6, woodland expansion in the uplands should be considered, from a carbon perspective, to be for long-term carbon sequestration and storage, whereas short term carbon sequestration could be achieved in more marginal and lowland areas, without affecting food production, by encouraging the establishment of largely broadleaved wood pasture, agro-forestry, shelterbelts, trees along field boundaries and hedgerows. Although densely planted Sitka spruce grows very fast, and so captures carbon quickly, it also acidifies both soils and run-off. Acidifying Scotland’s productive land would not seem like a good idea.

The millennia of heavy grazing, tree felling and burning to which the Scottish uplands have been subjected is likely to have led to soil wetting and acidification. This, in turn, will have led to an increased soil peat content across much of our uplands. It would be ironic if this human-induced change could not be reversed because of the need to address another human-induced environmental issue. Climate change is not the only environmental crisis that we face and, although tackling it has to be a priority, a balance needs to be struck between this and the many other benefits to be gained from woodland expansion in the uplands. As such, the expansion of mixed woodland, with a large deciduous component, established and managed with minimal soil disturbance and in mosaic with open habitats, should still be a priority for the Scottish uplands.

Acknowledgements

I am grateful to Nina Friggens, Keith Matthews, Mike Perks and Pete Smith for patiently answering my questions on their papers and to Keith Mathews and Douglas Wardell-Johnson for providing Figure 3.

References

  1. Friggens, N. L., Hester, A. J., Mitchell, R. J., Parker, T. C., Subke, J.-A.., Wookey, P. A. 2020. Tree planting in organic soils does not result in net carbon sequestration on decadal timescales. Global Change Biology 26(9): 5178-5188. http://www.sciencedirect.com/science/article/pii/S0264837719304041.
  2. Matthews, K. B., Wardell-Johnson, D., Miller, D., Fitton, N., Jones, E., Bathgate, S., Randle, T., Matthews, R., Smith, P., Perks, M. 2020. Not seeing the carbon for the trees? Why area-based targets for establishing new woodlands can limit or underplay their climate change mitigation benefits. Land Use Policy 97: 104690. http://www.sciencedirect.com/science/article/pii/S0264837719304041.
  3. Armstrong, H. 2015. The benefits of woodland. Unlocking the potential of the Scottish uplands. Part II – Supporting evidence. Report for Forest Policy Group. http://www.forestpolicygroup.org/wp-content/uploads/2015/06/The-Benefits-of-Woodland-Part-II.pdf.
  4. Donkersley, P. 2019. Trees for bees. Agriculture, Ecosystems and Environment 270–271:79–83. https://doi.org/10.1016/j.agee.2018.10.024
  5. Armstrong, H. 2015. The Scottish Uplands: how to revive a degraded landscape. Talk to the Botanical Society of Scotland, 17 September 2020. Available until March 2021 at https://teams.microsoft.com/l/meetup-join/19%3ameeting_MzU5ODgzN2MtZTVjMS00NGJjLWEzYWUtMzNkZWM1NzdiMzlh%40thread.v2/0?context=%7b%22Tid%22%3a%22bb63bb00-175e-46b7-b7b3-bc74158e4fd4%22%2c%22Oid%22%3a%22824033f3-5e3a-484c-8ffc-e6b0beacebfe%22%2c%22IsBroadcastMeeting%22%3atrue%7d.
  6. Perks, M. and Vanguelova, E. 2020. The importance of soil carbon in forest management. Reforesting Scotland journal, 61, Spring /Summer: 18-20
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