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Taking a look at the metabolism of trees: What happens under drought stress?

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“Let’s go!” Via radio Arthur Gessler gives the go-ahead for an experiment that, to date, is still unique. He slowly opens the control valve of a gas cylinder. Gas flows through tubes into the plastic enclosure around a Scots pine. For three hours the tree, which is packed airtight from top to bottom, is forced to absorb the gas and incorporate it into its metabolism. The gas is CO2 marked with a stable 13C isotope.

After the three hours, Arthur stops the inflow and the tree is unpacked again. Now the first measurements start. “We can keep track of when the marked CO2 reaches the tree’s leaves, stem and roots during respiration,” says Arthur. He is an ecophysiologist at WSL and is in charge of the large-scale experiment, which has researchers participating from Switzerland, Germany, Finland and China.

The experiment is taking place at the end of August 2017 in the Pfynwald near Leuk (Canton Valais), where it is hot and dry, and where many pines have died. The researchers suspect the mortality is due to a combination of heat, drought, pests and tree diseases. In order to understand the processes in detail, they are investigating the metabolism of the trees.


Tracking the sugar

The experiment will show whether the trees suffering under drought stress produce enough sugar in their needles and whether the transport of the sugar to their stem and roots, as well as further into the soil, still functions. During a dry period, the trees close the microscopically small stomata in their leaves and needles to minimise the loss of water to the atmosphere. This leads, however, to the tree taking up less CO2, which means it can form less of the sugar that is produced from CO2 through photosynthesis. To have a basis for comparison, the researchers are performing the same experiment with some of the 500 pine trees that WSL has been irrigating in the Pfynwald since 2003 to investigate the effects of drought and irrigation on pine forests.

Using portable laser spectro­meters, the researchers can determine directly on site the moment in time when the marked carbon  reaches different parts of the tree. The analyses of the plant material in the lab will show what happens to the sugar formed. Normally, some of it is used to create defence compounds like resin. If not enough sugar is available, the tree will lack these substances and will be more susceptible to pests such as bark beetles. This could explain why the trees die during drought. Since 2017, it has been possible to carry out analyses in a well-equipped isotope laboratory in Birmensdorf with the four mass spectrometers that WSL was able to take over from the Paul Scherrer Institute (PSI).

Everything is linked

In this experiment, the researchers also want to investigate whether the tree passes on the sugar it produces to, for example, seedlings, which are particularly vulnerable to drought. The roots of the forest trees are linked via a network of fungal hyphae that can transport various compounds. Soil specialists analyse how far the sugar produced from marked CO2 is distributed in the soil via such hyphae and whether it also reaches the soil microbes.

Arthur Gessler is satisfied with the day in the field as he and his team were able to collect the samples they required. The evaluation of the data will now take some time, but afterwards the researchers will understand better why the pines in Pfynwald are dying. This will provide an important basis for estimating how the forests will develop in a warmer and drier future. (Lisa Bose, Diagonal 1/18)