How water use by trees responds to the winter with less snow? Sensors installed inside pine trees in Oulanka reveal forest water fluxes under changing seasonality

Boreal forests cover 33% of the global forest area, and 66% of the water used by forest covered boreal land is through forest water uptake. In Finland, 74% of the land is covered by forest. Understanding boreal forest water fluxes is crucial under changing snow-rain seasonality in northern environments.

“When trees start to grow in spring, they use not only spring rainwater, but also the melted water from winter snow. Under climate change, the amount and frequency of winter snowfall and summer rainfall are changing. This may influence the forest and regional water resources”, explains Associate Professor Pertti Ala-aho.

At the Oulanka Research Station, where the water cycle and boreal forests are not directly affected by human activity or settlement, the Boreal Forest Water Fluxes and Boreal Tree Water Use research projects aim to renew our understanding of how ongoing changes in snow and rain affect boreal tree water use strategies and how water moves through boreal forests.

For comprehensive understanding of the complex forest hydrology processes, up-to-date instruments were installed last summer at Oulanka on several Scots pines (Pinus sylvestris) with different snowpack thicknesses, from 25 cm to more than 100 cm of snow for comparisons. Currently, the air, soil, root, and tree trunk temperatures are measured, as they are the direct triggers for trees to start using water from soils. Most of the data will be captured when tree water uptake starts.

The sap flow sensor has three needles that go into the root and the trunk of the tree. The middle needle sends a heat pulse at regular intervals and the needles on both sides measure temperature. This way the sensors can tell how much and in which direction the water inside the tree flows. Another measurement system tracks the source of water to find out which soil layers and which seasons trees prefer water from.

Water fingerprints, the stable isotopes of water, can be used to understand where the water comes from and where it goes to in the water cycle

One possible adverse effect of the winter with less snow could be damage to the tree roots due to the thin snowpack that cannot protect the roots from freezing temperatures during winter and early spring. If the water intake capability of the roots is compromised, the growth of the tree could be negatively affected in spring. The failed spring water intake could have further effects on the summer water intake.

Although there is little snow now to melt and be absorbed by the roots this spring and summer, the previous autumn's rainfall may also affect tree water uptake in spring. The study will use water isotopes to determine the origin of the water. Different isotopes have atomic weight differences that act as a kind of water fingerprint, telling us how much of the water trees use in spring is snow from the past winter, rain from the previous autumn that has been preserved in different soil layers, or new rain from the spring.

“The new measurements will tell us exactly when trees start to use water in spring. That's when real-time data on the trees' water use will also start to flow into our three-dimensional model of the trees and the forest", describes Postdoctoral Researcher Zuosinan Chen. "The data will immediately show how differently tree roots under different snowpack depth start to use water."

Snow is an important part of the water cycle, which is also being studied in the national Digital Waters flagship coordinated by the University of Oulu.

See also a fascinating video about snow research

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