Can we help forests adapt to a changing climate?

It is an early summer morning outside Bollnäs in the Hälsingland region of Sweden. The morning sun warms the air as Karolina Pehrson steps out of her car. Before her lies a quiet stand of forest, with young larches and pines reaching up towards the sky.

Here, on a hillside where the horizon seems endless and no buildings are in sight, she will collect her data.

– I work with plant physiology and study how plants function internally and how they adapt to a changing climate. My focus is on photosynthesis and how it’s affected by temperature and nitrogen levels. If we can understand how plants naturally respond to new conditions, it’s easier to find the right ways to support them, explains Karolina.

She takes out her saw and approaches one of the selected trees. After measuring its height and noting details, she saws off a thin branch and places it in a water-filled bag. She repeats the process several times before heading back to the temporary field station.

Once there, she places the branches in a bucket of water and attaches the equipment’s nozzle to the needles. With this, she can control the temperature, humidity, and CO₂ concentration – and measure how much carbon dioxide is absorbed by the plant. This is how she calculates the rate of photosynthesis – the process that captures carbon dioxide and produces sugar, the building block of life.

– I often think of photosynthesis as a big factory. When everything’s working well and conditions are ideal, the factory runs at full capacity. But when the plant is stressed, it runs at half speed. That’s when trees become more vulnerable to insect infestations, disease, or drought.

How nitrogen affects photosynthesis

Karolina’s research focuses on how nitrogen, through fertilisation, can affect photosynthesis and trees’ ability to adapt to changing temperatures. Her work is pioneering: connecting fertilisation, photosynthesis and climate change in this way still remains unexplored.

In the peaceful test forest in Hälsingland, Karolina gathers data, which she later analyses at Linnaeus University.

In the nozzle of the portable equipment, temperature, humidity, and CO₂ levels can be adjusted to measure the speed of photosynthesis in the inserted branch.

By studying the effects of fertilisation on photosynthesis in pine, larch, and blueberry, she hopes to contribute to sustainable forestry practices of the future. The first summer’s measurements revealed clear differences between fertilised and unfertilised blueberry plants – those in the fertilised plot had a higher rate of photosynthesis. For larch and pine, however, no differences were observed.

The results from the second summer are still being analysed, but Karolina hopes to see a change. The stands were given another dose of fertiliser in June, which may begin to show in the data.

But the research continues, and each measurement brings her one step closer to understanding how trees adapt to future climate change.

– Being in the field is both magical and challenging. There’s a calm on the hillside, but then you look down at the equipment and realise you need to get the data – and those machines can be quite tricky. It’s a real rollercoaster, says Karolina.

From forest silence to university life

She works in the field for four weeks straight, far from the city and office life. Karolina describes the fieldwork as dreamy but also exhausting.

When she leaves the Hälsingland forest and heads south to Växjö and her workplace at Linnaeus University, the research takes on a new form – in meetings, data analysis, and teaching. Leaning back in her chair, with her office door ajar and colleagues’ voices drifting in from the hallway, she reflects on her everyday life as a doctoral student.

– Being a doctoral student is like having three jobs in one – student, teacher and researcher. It’s freedom with a lot of responsibility. It’s varied and sometimes confusing to juggle so many roles. But most days, I forget that I’m actually working. It’s just so much fun!

Curiosity is the key

The conversation turns to what drives her to pursue research. Curiosity is a key word and something that has been accompanying her since her first encounter with science. Karolina studied natural sciences at upper secondary school, followed by a bachelor degree in biology and a master in zooecology at Lund University.

– When I got to Lund and had the chance to intern in a research lab and shadow a doctoral student, I thought: this is amazing. To pursue your own questions and search for answers no one else has – that immediately drew me in.

Now, two years into her doctoral studies, she’s looking ahead. This summer she is heading to a research station in Minnesota, USA – a unique facility where trees have been growing in climate-controlled chambers for ten years.

It will be an intense few weeks supported by researchers from various countries and universities. There, Karolina will study how long-term exposure to elevated temperature and CO₂ affects photosynthesis, and then compare the results with her Swedish data.

– It feels like the next big step. To see how trees respond over time, and hopefully gain a better understanding of how they adapt to climate change – like getting a glimpse into the future.