Expanding Arctic forests could release more CO2

A study, funded by the Natural Environment Research Council (NERC), has discovered that the expansion of forests in the European Arctic could exacerbate the release of carbon dioxide (CO₂) into the Earth’s atmosphere. Researchers from the Universities of Exeter, Stirling and Sheffield, the NERC Radiocarbon Facility, the James Hutton Institute and Heriot-Watt University, have found evidence to suggest that carbon currently stored in Sweden’s Arctic tundra may be released into the atmosphere by new trees.

Because the warming climate is accelerating Arctic plant growth, many have contended that more carbon will be stored in the increasing biomass. It was therefore believed that Arctic greening would result in higher levels of CO₂ being removed from our atmosphere, thus helping to combat the rate at which global warming is occurring. The new study, however, published in the journal Nature Climate Change, has found that this phenomenon could have the opposite effect.

I spoke to Dr Iain Hartley, lead author of the paper and Lecturer in Physical Geography at the University of Exeter, to find out how more trees could result in the release of more CO₂. I began by asking Dr Hartley whether he and his colleagues were surprised to find that the expansion of Arctic forests might actually lead to more CO₂ being released into the atmosphere.

"Yes, I think that we were," he said. "It was surprising to find that such a large increase in plant biomass could be cancelled out by a reduction in carbon storage below ground."

By measuring the radiocarbon content of CO₂ being released into the atmosphere, the team discovered that birch trees seemed to be stimulating the decomposition of soil, and in doing so, contributing directly to the reduction of carbon storage below the ground. Dr Hartley and his colleagues compared the carbon stocks in vegetation and soils between tundra and a neighbouring birch forest, and found that the two-fold increase in carbon storage created by plant biomass was more than outweighed by reduced carbon stocks in forest soils. I asked Dr Hartley whether he believed that birch trees were unique in their capacity to reduce carbon storage in soils.

"We don’t know whether or not birch trees are unique in this respect," he explained. "We know that birch trees are generally considered to be quite special. When they have been grown in other ecosystems – even in more temperate locations such as Scotland – they have also been shown to reduce carbon storage below the ground. They act as what we call ‘ecosystem engineers’, dramatically changing the ways in which ecosystems function.

"Having said that, in other regions of the boreal Arctic fringe, where different types of tree species have also been expanding, there has been some evidence to suggest that carbon is being lost as a result. I think that it’s fair to say that birch trees are particularly capable of changing the ways in which ecosystems function, but to categorically state whether or not this phenomenon is entirely confined to birch trees would be very difficult at this time."

I asked Dr Hartley to explain more about the possibility of this process occurring in more temperate locations.

"This particular process probably requires trees to be growing in ecosystems where there is a lot of carbon stored below the ground," he said. "Obviously, if you are in ecosystems where there is less subterranean carbon storage, you would expect the change in plant biomass to have the most prominent impact. However, in moorland and tundra ecosystems, we should perhaps expect this type of thing to happen – certainly in northern parts of the United Kingdom. As I said, there has already been some research into the potential impact of birch trees on decomposition rates in Scotland."

The most obvious strategy for counteracting this phenomenon seems to be to reduce rates of climate change. However, this is certainly easier said than done. I asked Dr Hartley whether any other tactics could be employed to counteract the potentially harmful effects of forest expansion.

"I think that we should be careful in saying that these effects are harmful," he warned. "My colleagues and I were only interested in trying to determine what impact these trees could have on carbon storage. There is a whole suite of other factors that influence climate change; the rate of transpiration from birch leaves versus the albido effects, and other things like that. We just wanted to find out more about an instance in which carbon storage appeared to have been reduced.

"In terms of counteracting this phenomenon, as a lot of the areas that we are talking about are so remote, I don’t think that there is anything practical that we can do on the ground. If the climate is going to change, the ways in which ecosystems are going to function are going to change as well. I don’t think that there is much that can be done directly apart from as you have suggested, trying to reduce the rate of climate change itself."

I concluded our interview by asking Dr Hartley whether he had any further research planned, to find out more about how the mechanisms involved in this process operate.

"I would like to look into getting more funding so that further research can be conducted," he replied. "At present, we do not have the formal research funding that will be necessary to investigate more areas, or to try to calculate the rate at which carbon will be released. We would certainly like to look at different areas of this ecosystem, and attempt to discern how local or general an effect this might be."