The botanist Barbara Beikircher is a research associate in the research group for ecophysiology at the Institute of Botany at the University of Innsbruck and researches the drought adaptation and recovery of beech and spruce in the Kranzberg Forest, north of Munich Airport. There she equipped the forest with ultrasonic sensors, among other things. What she hears with it, she reveals in the interview.
Increasingly longer dry periods caused by climate change are affecting the Central European forests. What are you researching exactly?
The aim of our research is to find out how forests will react if summer drought increases in the future. In our case it is beech and spruce forests. It’s also getting warmer. The combination of drought and periods of heat are increasingly stressing the trees. We want to find out how beech and spruce react to this combination. Because these two species are particularly important in our latitudes.
Investigate drought stress in plants
Why are you doing your research in the Kranzberger Forest near Munich?
We can do research in a place where we have a lot of technical possibilities. It is now 13 years ago that scientists there began to identify different sub-areas on which many different measurements take place. There is also a crane there that we can use to get into the crown area. These are opportunities only available in a handful of places in the world.
Die Botanikerin Barbara Beikircher.
(Image: Beikircher)
What exactly did you do?
Our project is a sub-project of Kranzberg Roof Experiments (KROOF). We looked at how the hydraulic system of plants reacts to drought stress.
Water is channeled up the tree in the wooden part. In the wooden part we have tens of thousands of small, very fine tubes that are designed one behind the other, the conductive vessels. When water is lost at the top, it is drawn through the entire trunk. If air penetrates into such a conducting vessel, it fails to transport water. This intrusion of air into a single vessel creates a sound that can be heard in the ultrasonic range. That’s where our first method comes in.
We docked ultrasonic sensors along drought stressed trees and control trees. On a beautiful summer day, when transpiration is very high, we looked to see if there were any differences between drought-stressed trees and control trees. We were able to show that there are significantly more such embolic events in drought stressed trees.
Insight into the water reservoirs of the trees
And the second method?
That was resistance tomography. You can practically look into the trunk without having to take a sample. You apply tension around the trunk and then you get a colored picture showing how the water is distributed in the cross-section of the trunk and how dense the wood is. This allowed us to see that a drought-stressed tree stored less water in the trunk than a control tree.
Taken together: what does this mean for how trees deal with dry seasons?
Through resistivity tomography we have seen that the internal water reservoirs have been tapped in the drought stressed trees. After a year we watered them again and examined them again. We saw that the spruce trees were largely recovering. Nevertheless, their water reservoirs have not been able to fill up completely. So if a drought occurs again after a year, these trees start with a lower water reservoir. After each dry event, the spruce is less well prepared for a new dry season. That was not the case with the beech. If individual conductive vessels are filled with air, this does not mean death for the tree. But above a certain threshold, the tree just goes to death.
Has this threshold not already been exceeded in many forests?
A drought stressed tree does not necessarily die because it is drought stressed. He may be able to recover when he gets water again. But at the same time it is more susceptible to many other factors, including the bark beetle. He goes on the trees that are under stress.
“The spruce can cope with drought for a long time”
Do trees die of thirst from a lack of water, or do they starve because they can no longer get enough nutrients due to the interrupted water transport?
On the one hand it depends on the type, on the other hand the time plays a role. The spruce can cope with drought for a relatively long time because it simply stops losing water completely. But if this drought lasts for a long time, it cannot carry out photosynthesis during this period. Then she might starve to death. The beech behaves differently. It has the stomata of its leaves open for quite a long time and loses water for a long time because it tries to carry out photosynthesis for as long as possible. That way she might not starve to death as easily. However, if the drought stress lasts for a very long time, then at some point it will have reached a threshold at which it will die. But in the end, a tree may not die from drought stress, but from the bark beetle.
Who is more sensitive to drought: beech or spruce?
This is clearly the spruce, because it does not have a deep root system, for example. The beech has the advantage that it has much deeper roots and can therefore also reach water in deeper soil layers.
Another project partner therefore tried to test the hypothesis that spruce could benefit from beech. The beech fetches the water from deeper layers and could release it at a shallower depth. However, the results were not entirely clear. Still, there is the idea that different trees can benefit from each other. It is well known that mixed cultures are much more resistant than single cultures.
What does this mean for forestry?
Today, foresters and forest owners have to decide which trees to plant. And that affects the next generations. Only if we know how trees react, how they cope with drought, whether they adjust their resilience, can we make good decisions that will have an impact decades from now.
(jl)
