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When does a tree form new cells? And how do environmental conditions influence this process? WSL researchers are finding
answers to these questions in wood samples from the Lötschental.

Loïc Schneider turns the wheel of the cutting machine energetically. Wafer-thin strips from a paraffin block fixed in the machine pile up on the cutting surface. Embedded in the block is an approximately one-centimetre-long drill core from the stem of a larch tree. Loïc carefully picks up a strip with his tweezers, dips it in a warm water bath and quickly fishes it out again with a glass plate to put aside to dry.


Loïc is a technician in WSL’s tree-ring lab. The wood samples he is cutting come from the Lötschental valley in Canton Valais. Since 2007, WSL em­ployees have, every week, taken small drill-core samples of larch and spruce from the trees’ cambial and phloem zones, i.e. from where a tree continuously forms new cells during the growing season and thus increases in size. More than forty trees at five different elevations are currently sampled weekly. Sensors beside the trees measure the site conditions, such as the temperature and humidity of the air and soil. Other measuring instruments record how much water evaporates from the trees.

“We want to find out how the environmental conditions affect tree-ring formation and the characteristics of the wood cells, such as the thickness of the cell walls,” explains Patrick Fonti, who is in charge of the investigations. Conclusions can then be drawn from the resulting data about how the wood is functioning – for example, whether the tracheids are large enough to transport sufficient water from the soil to the leaves. Patrick is particularly interested in the influence of changing temperatures on the time in spring when cell formation begins. In the long term, Patrick and his colleagues hope to be able to draw conclusions about how climate change affects the growth of trees and whether trees will be able to cope with the changes in environmental conditions.

The Lötschental is particularly well suited for investigating the influence of global warming on trees because the temperature difference between the tree line and the valley floor corresponds to the predicted temperature increase of 3 °C for the next hundred years. The wood cores from different elevations can be compared to see how the differences in temperature affect tree growth. For a number of years now, Patrick and other WSL staff have travelled to Valais once a week to take samples. “It’s an enormous effort, but it’s worth it. We have obtained a data series with high resolution and can virtually watch the wood grow and the annual rings form.” As the dendro-ecologist says: “No one has ever done this before with so many trees and over such a long period of time.” This is why researchers from all over the world are very interested in being able to work with the data.


Making cells visible

Before Patrick and his team can examine the wood cores, they have to be prepared in WSL’s tree-ring laboratory. This involves complex steps, and the process takes several days. Before cutting the samples, water has to be extracted from the wood. The drill cores are then embedded in paraffin. This wax-like material is solid at room temperature and is used to fix the bits of wood so that they can be cut into slivers that are just seven micrometres thick – about ten times thinner than the average human hair.  The wood cells, however, remain intact. The micro-cuts are then stained with dye so their components can be better distinguished.
“In the past, we inspected the wood samples under the microscope and counted the cells by hand. Today the computer does that,” explains Patrick. He opens a scanner that can process a hundred samples simultaneously. The machine delivers high-resolution images in which every single cell is visible. Staining the samples makes it possible to distinguish the cell components – lignin and cellulose. Special software is used for counting and measuring the wood cells and provides information about their structure.

Wood for the future

On the basis of the data evaluated so far, Patrick and his colleagues have been able to show that the start of cell growth in trees is delayed by two to three days per hundred metres elevation difference. In other words, if it is one degree warmer, the growth phase starts about five days earlier. The starting moment is relevant because this is when the tree begins to store CO2 in its cells. This important quantity enables researchers to calculate how much carbon a tree can bind from the atmosphere (see page 10).
Not all the wood cores from the Lötschental have been prepared and evaluated yet. As soon as sufficient data has been processed, the researchers will be able to develop a model to predict whether a tree will be able to survive at all if it becomes massively warmer or drier. The model will take into account the site conditions in predicting how much wood the tree will form, the structure of its wood cells and what properties the wood will have. This will involve combining the researchers’ data with the results from classical tree-ring research.

Patrick is particularly curious about the samples from the hot summer of 2015, which left its mark in the wood. “We want to find out how the extreme dryness has affected the structure of the wood.” The data will be evaluated in 2020.
(Lisa Bose, Diagonal 1/20)


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