Too little water in the soil – drought and heat as dangers for the Swiss forest – News

The forest is currently sprouting in full green, it looks lush and healthy. But measurements in the forest floor show that the water reservoirs in some places hardly filled up in winter and spring. For the forests there, the drought stress will probably increase in summer, says soil researcher Katrin Meusburger.

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Katrin Meusburger is a researcher in the fields of forest soils and biogeochemistry at the Research Institute WSL in Birmensdorf.

SRF News: You measure the moisture in the forest floor with special measuring devices. How would you describe that?

Katrin Meusburger: There are 16 such measuring devices distributed throughout Switzerland. They determine the water content in the forest floor down to a depth of two meters. We can’t go any deeper because that’s usually where the rock begins.

The measuring stations in Valais are already showing very dry soil conditions.

However, trees can also reach deeper layers with their roots, they force their way further down through the rocky clefts. Overall, however, we still know very little about how much water the trees absorb through their roots because we do not know how deep and where the roots reach.

What do your devices measure in the Swiss forest?

The measuring stations in Valais in particular are already showing very dry soil conditions. The soil water reservoir in the deeper layers could not be completely filled up last winter – which is why the water reserves in the soil are already declining again. One has to reckon with increased drought stress for the forest in the affected areas this year.

Everything is still green at the moment

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The forest is currently in full sap, everything is green, everything seems fine. “The conditions for the forest are good at the moment – it’s not yet too hot and not too dry,” confirms the ecophysiologist Roman Zweifel from WSL. In addition, last year was relatively cool and wet. “This plays an important role in the growth dynamics of the forest this year.” Zweifel uses measuring probes to meticulously examine the growth of the trees in the Swiss forest. He also states that everything is currently “in the green” as far as the tree species examined are concerned. However, that could change during the summer, says Zweifel. “We know from dry and hot years like 2003 or 2019 that they restrict the growth of the trees.”

Why is the water scarce in the ground?

Precipitation in winter is very important for replenishing the water reservoir in the ground. The rains in spring are also decisive. This year, however, there was less rain than the long-term average – and if there is no precipitation in spring, the soil dries out very quickly. This is a very bad starting position for the summer. The Valais and parts of the Jura are particularly affected.

Why are Valais and Jura particularly affected?

Not all the water contained in the soil is available to the plants. The nature of the soil plays a role here: how sandy, stony or clayey is it? This shows, for example, that the forests in the Jura have less water available due to the stony soil conditions and that the soil is already very dry, especially in the northern Jura.

Little rain fell this winter and spring. This is a very bad starting position for the summer.

The ground in the foothills of the Alps and the Alps is generally similarly rocky. Currently, however, there is still enough water in the ground in Graub√ľnden, for example, while it is becoming increasingly dry in Ticino. In the Central Plateau, the trees are still finding enough water in the ground.

What are your expectations for the coming summer?

Where the deep layers of soil are already very dry, the soil will not recover over the summer. The situation will only get worse because there is no buffer, no water reserve for the coming summer. That’s why I rate the situation rather negatively.

Reena Thelly conducted the interview.

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