Why do our forests suffer from a lack of water, even though it rains and snows more?

It is becoming increasingly clear that Sweden's forests suffer from a lack of water, which leads to reduced growth and an increased risk of pest attacks and forest fires. Hjalmar Laudon wants to understand the movements of water in forests by mapping the interactions between winter climate, spring snowmelt, water storage in soil and groundwater, uptake by trees and stream runoff.

The most reliable forecasts predict that Swedish forests will grow faster in a warmer climate, yet the trend is the opposite in the Swedish field inventories of the last decade. Reduced forest growth creates problems for the industry that needs a long-term supply of raw materials, and also makes it harder to reach Sweden's goals for biological diversity and for the annual carbon sequestration that we report to the EU.

We need to know what warmer winters lead to

It is evident that the reduced growth is related to water scarcity, but the trend cannot be explained by changes in the amount of precipitation or the weather of the growing season. The answer must likely be found in some other part of the water cycle. We know that the accumulation of snow in winter and the melting of snow in spring are key to the replenishment of soil and groundwater supplies, but we do not know how winter conditions affect the availability of meltwater and the water uptake of trees. This makes it difficult to assess how our forests will grow in the future. 

Understanding the relationship between snowmelt and groundwater levels is important, as winter is the season that is most affected by climate change. Warmer winters will not only cause snow to melt earlier, but this will also make winters rainier, reducing the amount of snow that is converted to groundwater in spring. Whether and how changing winters can be linked to water shortages in forests is, however, an open question. To answer it, we need an overall picture of the water's movements in the forest.

Hjalmar Laudon's goal is a less vulnerable management of forests and water in a future climate. He intends to achieve this by combining a unique research infrastructure in northern Sweden, with sophisticated water isotope analyses, large-scale field experiments and advanced ecohydrological modelling.