Carbon measurements in old-growth forests

The environmental movement often hails old-growth forests as saviours of the climate, whereas the forestry industry argues that it is young, growing forests that soak up carbon dioxide. We followed along as the exhalations of an old forest were measured – using steel cages and gas masks.

Strapped to the pine trunk is something resembling a Second World War gas mask. However it’s not there to help the tree breathe better, but to measure its exhalations – how much carbon dioxide is breathed out through the bark, explains researcher Hanna Axén.

A graph is slowly drawn on a small measuring device attached to the “gas mask”. This pine tree exhales 0.24 grams of carbon dioxide per hour from each square metre of bark, reports Hanna Axén. The neighbouring spruce tree produces 0.17 grams.

“You might not normally think of the bark of a tree breathing, but all living things breathe out carbon dioxide, just like us. But trees also suck in carbon dioxide.”

Her research involves mapping the movement of carbon in Swedish old-growth forests. Behind tall blueberry bushes and moss-covered fallen trees, we see heads moving. The site looks like a woodland workshop where everyone is busy with tape measures, callipers, knives and more obscure high-tech instruments.

Barrier tapes on the trees mark the edges of a 20 by 20 metre grid inside which all the carbon dioxide produced by the forest will be measured. We are at Vithult nature reserve, a few tens of kilometres northeast of Växjö, in one of the areas least affected by forestry in southern Sweden. The ground is criss-crossed with cords and studded with small plastic pegs that we have to avoid stepping on.

Hanna Axén treads skilfully between the cords. This work is part of her doctoral thesis in physical geography at Umeå University, and she flits between her five assistants who need help with various tasks. The team is on a six-week field trip, visiting old-growth forests in 13 nature reserves stretching from the north to the south of Sweden. Her thesis is part of a larger project on the carbon cycle and ecology of natural forests at Lund University. The aim is to build up a detailed picture of the carbon budget of Swedish old-growth forests – discovering where the carbon is stored and how it moves between different parts of the forest. This is a field that has been studied in much greater depth in tropical forests than in our northern latitudes, and the two systems function quite differently. Most things happen faster in tropical forests – the trees absorb carbon faster, but they also release it faster. Ultimately, this basic knowledge is needed to answer various questions, such as how much more carbon is stored in a damp forest than in a drier one, and how much carbon is stored in slow-growing old-growth trees compared to cultivated fast-growing trees. This in turn should hopefully reveal how conservation efforts should be prioritised to capture carbon dioxide.

The question of whether old-growth forests act as sinks or sources of carbon dioxide has been hotly debated in the scientific community, but in recent years research has reached a fairly strong consensus that old-growth forests absorb more CO2 than they release. But to really understand how carbon flows between the soil, trees and the atmosphere requires a great deal of work, like that going on around us now.
Field assistant Kajsa Rosén is kneeling in front of four sawn-off plastic tubes inserted in the soil to record its carbon dioxide emissions. Around three of the tubes, Kajsa has used a knife to cut away all the lingonberry scrub and moss. Around two of them, all roots have also been excluded by means of a fine mesh net that extends thirty centimetres deep. And around the fourth, all fungal hyphae are also excluded by a thick tarpaulin. She measures the emissions from each of the tubes for two minutes by attaching a device that looks like a grey thermos on top of the tubes. A cable runs from the thermos to a carbon dioxide meter that looks like a mini radiator.

This work is important, because in coniferous forests like this only one third of the forest’s carbon is stored in the trees. The rest is stored in the soil, and the CO2 emissions from the soil are considerably greater than from the trees, explains Hanna.

All the scrub that is cut away is placed in a paper bag to be weighed later to determine how much carbon is stored in the undergrowth.

“We end up carrying quite a lot of gear,” comments Kajsa.

She pulls out two more paper bags to store the moss from the soil measurement grid, which will also be weighed later. The outermost, pale green shoots that have grown this year are put in one bag, and the rest go in the second bag.

“Compared with where we were yesterday, the moss has grown much better here,” says Kajsa as she removes small moss shoots that are barely a centimetre long.
“Yes, it’s more humid here,” says Hanna.

In each nature reserve, they measure three grids in areas with different humidity. Hanna took the same measurements in the same locations last year and will repeat them next year. Possibly even for a fourth year.

Behind them, Ella Hambeson extends a tape measure rather cautiously. Her task is to lay a straight line and then measure all the dead wood that crosses that line, but soon the tape measure will pass right over a stump circled by irritable social wasps.

Lourdes Martínez-Garcia from Mexico, who is doing her postdoc in Sweden, is sorting branches that crossed a previously laid line. The freshest are placed in plastic bag number 1 while the most decomposed are placed in bag number 5. The reason for separating them into different degrees of decomposition is to calculate their respective carbon contents and carbon dioxide emissions.

It is also necessary to count the large fallen trees that cross the line but are too big to be bagged. Hanna pulls out a metal cylinder that looks like something from Star Wars, and with a deafening bang shoots it straight into a tree trunk. She explains that it is a hardness meter and that this tree measured four on the decomposition scale. A few pieces with different hardness values will then go into small measuring chambers to determine how much carbon dioxide is released from wood at different stages of decomposition.

Hanna then moves on to a white plastic sponge that looks like an unusually shiny parasol mushroom. She sits on a fallen tree and connects the sponge to a laptop. The screen shows graphs of temperature and soil moisture that the sponge has recorded every half hour over the past year. Heat and moisture stimulate the tiny fauna and fungi in the soil, speeding up decomposition, which in turn accelerates carbon dioxide emissions, she explains.

It’s not a simple task to measure and classify all the emissions from life in a forest, or trying to draw straight lines across crooked dry trees and large boulders. Isn’t there an easier way?
An alternative way to measure carbon dioxide emissions from different forests is to set up monitoring towers, but they are expensive installations that require power and are unfeasible to erect in old-growth forests. The results they obtain can also be affected by variations in air currents, adds Hanna.

“If everything works perfectly, you can identify which gases are emitted from large forest areas, but what we are doing here gives us a different picture of what is happening in the forest – of what comes from where.”

She would have liked to have a tower at each test site, but they are expensive and it is difficult to get a permit for such a large construction in a nature reserve.

Hanna takes out a small steel cage that is designed to measure how quickly roots penetrate through the soil. Her supervisor has used it in tropical studies, where roots grow very quickly, but here in Småland the roots that had grown into the cage in one year were so small and thin that they were almost invisible.

They must wait another year for the roots to grow large enough to collect and weigh. The needles, leaves and branches that fall to the ground are also collected in small plastic trays and taken back for weighing. Everything up to a centimetre in diameter has to be weighed. Anything larger than that is captured along the “social wasp line”.

“We have to measure everything. Everything that the trees and undergrowth emit and produce within this grid.”

Hanna takes a short break on the slope just outside the sample grid. If the results of her study were to show that these old forests are carbon sources, what would that mean, would Swedish forest owners be able to harvest more?

“No, we have such a tiny proportion of old-growth forests left, so the few fragments that are left need to be preserved regardless. We are also seeing a biodiversity crisis, and an acute shortage of older reindeer grazing forests in northern Sweden. And because we need to reduce emissions rapidly over the next ten years to meet the 1.5 degree target, it will be difficult to carry out intensive clear-cutting, because areas that are clear-cut release carbon dioxide for the first ten years.”

So why are you doing all this?
“We have lots of research on production forests, but we also need to know what is going on in the more pristine forests to understand their role in the carbon cycle and how they could be affected in a future climate. I hope to contribute a piece of this big puzzle.”

The rustling from the crowns of tall trees is mixed with the shrill beeping of a wayward thermometer. Hanna needs to get back to work. The results of this study will take a few years, but it is already clear that analysing the breathing of an entire forest involves a lot of work.

A Swedish version of this article has previously been published in Sveriges Natur #1-2024: https://www.sverigesnatur.org/aktuellt/forskarna-som-lyssnar-pa-skogens-...