IN THE ATMOSPHERE, IN THE TREES, OR IN THE GROUND?
Introduction
In the summer after the first COVID lockdown in 2020, a report was published on the interactions between carbon, trees and moorland in Scotland. It was noted and understood by some, but because of other things going on at the time, it arguably didn’t really get the profile or circulation it deserved. The report is often referred to as Friggens et al, but it was actually the work of a lot of very well respected scientists across a range of organisations.
You can see the report, conclusions and methodology at this link:
https://onlinelibrary.wiley.com/doi/10.1111/gcb.15229
In addition, the main findings are set out in a very useful visual form here, published by the James Hutton Institute (JHI): https://www.hutton.ac.uk/sites/default/files/files/ELSEG-2021.pdf
I will claim an interest. I was working at one of the sites, Ballogie, in 2004, when the then Macauley Land Use Research Institute (MLURI), now the James Hutton Institute (JHI), were looking for potential planting sites.
Having worked there myself, I was sympathetic. The site we had was not quite right in some ways, and it initially went to the bottom of the list, but nothing is ever 100 percent perfect, and they came back to us again after looking at other options, and the site was planted in 2005.
The hypothesis was that planting trees would increase the overall carbon storage of upland areas in Scotland, and this was what was to be examined. Well, it didnae. It didn’t quite turn out that way at all.
The Experimental plots
At Ballogie (plots in the image, above), there was an open, dry heath area at the edge of an expanding pinewood. Plots were fenced off to protect them from grazing, and planted with downy birch or Scots Pine. Each plot had an unplanted heathland plot as a control, and there were three replications to mitigate any subtle differences in ground conditions.
Carbon stocks were to be measured both above ground and in the soil as the trees grew. A whole system approach was taken, and it was to be a long term study. Nobody else was talking much about carbon at the time, so it was well ahead of the curve.
Three areas of older planted native woodlands were located elsewhere in the Highlands for comparison, so that by 2017, there was one site with young 12 year old trees, and three sites with trees 37- 39 years old.
The Results
It is important to say at the outset that these are all moorland, “carbon rich” soils, not mineral soils where the results would have been different, but not peatland either. The peat/ organic layer at Ballogie was only 7.9 cms thick, fairly typical of a lot of our heather moorland. That is about the length of your forefinger.
The headline result was that there was no increase in overall carbon stocks in any of the plots, even after 39 years, largely because any increased carbon sequestration above ground was counteracted by a loss of carbon in the soil.
There were however points of detail to note.
In the young birch plots at Ballogie, there was a significant net loss of carbon overall, with soil stocks reduced by 58% in 12 years, or by 4-5 % a year over the period of the planting until then, but with relatively little incorporated in to the trees.
The Scots Pine planting didn’t lose as much carbon from the soil, and what was lost was more or less made up by increased biomass above ground. In the older plots, what was lost in the soil, was more or less compensated for by increased stock above ground. There was no more carbon overall in the system, it had simply been reallocated from the soil to the trees.
Why?
Birch is widely recognized as a soil improving species. It roots quickly and stimulates activity in the soil, kick starting an increased circulation of nutrients. Most people would welcome this. As a pioneer species, birch can prepare the ground for a range of other species, and indeed, birch is an essential component of almost every other woodland type.
We have both silver birch and downy birch in Scotland, capable of covering both sandy, mineral rich soils in the east, and wetter, more peaty soils in the west. We also have mountain/ dwarf birch at higher levels.
What the experiment has shown is that carbon in the soil has become oxidized and lost to the atmosphere by respiration. Any carbon in the trees will have been fixed there by photosynthesis from the atmosphere, not taken directly from the soil. We might have expected the latter, but probably not the former.
Is this right?
Re- reading the papers three years down the line, several things occur to me.
The first is that many scientific experiments show marginal results, with many caveats included, and there is usually a call for more funding to take forward more research. However, in this case, the results are not marginal at all. They are very clear cut. Although there may be points of detail around the edges, and research will need to continue, the broad thrust of the results is held to be correct. The key action point is not for more funding. Those writing the report have moved directly to management recommendations.
The second point is that in the time since, no-one that I can see has seriously questioned the findings, and no-one has introduced any other data that suggests otherwise. With findings so potentially sensitive, you might have expected a counter argument to have come forward, but this has not happened.
The final point is that in order to minimize ground disturbance at the beginning of the study, the seedlings were planted directly in to the ground with no site preparations; no mounding or ploughing or anything like that. You can see in the photo that tree growth is actually very variable, and this probably reflects subtle differences in ground conditions. If the sites had been mounded, for example, it is very likely that growth would be more uniform, and almost certainly much quicker as well.
There is a suggestion in upland afforestation that ground preparations should be minimized in order to reduce any carbon loss to the atmosphere. However, while this might be the case, it is the trees themselves that have been responsible for a net carbon loss in the early years.
We can imply from this that it doesn’t matter whether your trees are planted with high or low intensity ground preparations or regenerated, the result is likely to be more or less the same, at least with birch.
What does this mean?
The results suggest that in planting birch in particular, you get a rapid loss of carbon from moorland soils in the early years. By 40 years, what is lost from the soil is made up in the trees.
However, birch trees will live until they are 80- 100 years old, so if the total biomass above ground continues to increase, and soil carbon is already depleted, then we might expect a net increase in carbon stocks beyond the 40 years, certainly if the timber was to be harvested and incorporated in to construction or furniture or whatever, and effectively locked up. Such uses and markets are not currently well developed for birch.
It is the trees themselves that create the loss of carbon, as demonstrated by the lack of ground preparations. Ground preparations might speed up the loss. It is probable though that ground preparations will speed up tree establishment and growth, so it COULD be that although loss is exacerbated, the point at which tree carbon equals the loss from the soil comes more quickly too, and maybe there is little negative consequence to ground preparations, just that they speed up something that was going to happen anyway.
This is speculation on my part, but we should be alert to such possibilities.
It seems likely to me therefore that trees probably will increase longer term storage of carbon, but that they will stimulate loss in the short to medium term, which is the period in which we need to be reducing emissions, if we believe that the climate crisis does indeed have to be mitigated. ie: Tree planting on carbon rich, heathland/ moorland soils will make things worse in the short term.
Scots Pine does not appear to have the same affect, so some of our more recognised regeneration projects in Scotland are off the hook, here.
However, if we were serious about introducing a much higher proportion of woodland to our uplands, then inevitably, silver birch and especially downy birch would be key species for that, and account for possibly 70-80% or more of the total.
Should we be planting trees?
All of this applies to carbon rich soils, not mineral soils. More extensive planting on mineral soils may bring us in to conflict with farmers on better ground, but the carbon cost should not be an issue. There may certainly be other issues.
Scots Pine seems to be less of a problem. Lets give it the benefit of the doubt.
Scottish Forestry say that commercial conifers on such soils will be unlikely to have a positive carbon balance until the second rotation, again about 40 years, give or take a few years. They suggest that “carbon rich” can be as little as 5 cms of peat. So, no carbon gain from fast growing conifers either on such soils, although the suggestion here is that if such stands are felled and restocked, then you can start making some progress, but again only if the timber harvested is not destroyed but somehow incorporated in to products and kept intact.
This could be a major boost for commercial forestry if more harvesting was supported by government, and it would add value to woodlands of broadleaved and minor conifer species as well. So, there could be an opportunity in all this.
In his book, An Illustrated Book Of Peat, The Life And Death of Bogs: A New Synthesis, James Fenton suggests that just 12 cms of peat can contain as much carbon as a mature conifer plantation. The various strands of evidence seem to line up around more or less the same theory.
There are many other perfectly good reasons for planting trees of course, even if there is a net carbon loss in the early years/ decades. Trees provide shelter, they diversify habitats, they improve amenity, and they allow for production, which is important if we are to survive ourselves. There is only a problem if we are saying that carbon trumps all of this.
But what if carbon does trump all of this, and our countryside needs to be managed to this end? Widespread planting on carbon rich soils will increase emissions in the period when reduction is important.
Left intact, heathland soils may only be accumulating perhaps a millimetre or less of peat a year, give or take a bit, but at least this is carbon positive, and at a national level, the accumulation will be significant, and may well equate to several million tonnes on an annual basis.
About 20 percent of Scotland is covered in peatland or blanket bog, but possibly twice this area will be wet or dry heath, or upland grassland on a thin layer of peat. The size of the resource is huge. And of course, if managed to gather in CO2, a lot of moorland will evolve towards peatland over several centuries anyway, and lock carbon away for several thousand years to come. We have to be aware of how these habitats relate to one another.
So, if carbon is that important, then we would be better to manage our open ground moorland habitats to that end, and that means keeping them as they are.
Moderate deer browsing would then be useful in keeping the birch at bay, and this should be possible without damaging the moorland vegetation which can withstand moderate grazing, more than trees. The arguments would swing around the other way entirely.
For a woodland advisor, all of this is inconvenient to say the least, but perhaps this is where we are. If moorland habitats are out of reach for planting, or even regeneration, then Scottish Government can say goodbye to their 18,000 hectares a year planting targets. The impact would be profound.
The authors of the report don’t quite say this, but state that we would need to be a lot more discerning in which moorland sites we planted and which we left alone. However, this would require a huge amount of development work for planting proposals, invariably increasing costs and making many unviable.
I have been scoping out a potential planting site in the Highlands at the moment. There is possibly 1000 ha of plantable ground, but excluding “carbon rich” soils would reduce the area by 90%, and the scattered areas left would not be viable. This is the sort of impact that could arise.
Key Message
For me, the key message in all this is that science is important, and while we can often be cynical about science, the world would be a worse place without it.
You often hear people say that scientists are funded to demonstrate a certain outcome. In this case, they expected woodland cover to increase carbon stocks, but this was not the case.
They published the conclusions anyway, because this is what happened. Credit to them for doing so. This is the type of science we need to inform land management going in to the future, and we need to invest in it.
We need to be aware of these discussions, and participate in them as we can. Debate on social media is all very well, but if we can help with experiments like this, then we need to do so.
Ultimately, in terms of this particular issue, where is it that we want our carbon to be? In the atmosphere, in the trees, or in the ground? If keeping carbon in the ground turns out to be most practical solution, then that means looking at our mountain environments in a different way.
That will of course require our politicians to bite their lip a bit and change their tune, but that may be where things are heading now.
It is likely that we might see the pendulum swinging back a bit in the debate between trees and moorland in our uplands. Most gamekeepers will probably be quite happy with that.
Victor Clements is a native woodland advisor working in Highland Perthshire. He advises a number of deer management groups, and has participated in a number of woodland related research projects, albeit in a very small way.
