The secret of deep-rooted plants Dr. Matthias Beyer on his junior research group "Isodrones"

Extreme weather and periods of drought are causing problems for European forests as well. Reason enough to learn more about the vegetation and its water sources. The “Isodrones” junior research group, funded by the Volkswagen Foundation, is therefore investigating the “secret life” of plants and trees below the earth’s surface. The team is investigating processes below the surface with the help of water isotopes and drones. Bianca Loschinsky spoke with Dr Matthias Beyer, Freigeist Fellow and head of the research group, about deep roots, Costa Rica through VR glasses and Stratzlawski.

Mr. Beyer, your research group “Isodrones” deals with the “secret life” of trees. Why do we need to delve deeper here?

As far as the life of trees is concerned, much is still unknown. In our particular case: we cannot simply look under the earth’s surface and see what is happening there. But that is exactly what will play a major role in the context of climate change. And that is the reason why we need to find out more about the “secret life” of plants under the earth. Everyone has observed the forest dieback of recent years and we now need to think about what we can do to make forests, but also urban green spaces, more resilient.

But why do we still know too little about what is going on below the surface?

We know, of course, how plants function. They need CO2, water, light, chlorophyll. And we also know exactly which processes take place. But we are dealing with the question of which depths plants get their water from. Until now, this could only be answered by digging up trees. There is no non-invasive method to determine that. We can look under the earth with radar, with geoelectric methods to a certain extent, but the methods are not yet advanced enough to allow us to see fine roots, for example.

You do not dig up the trees. How do you study the water uptake of plants from the depths?

The aim of our research project is to investigate the water uptake of deep roots without the traditional practice of ‘digging up’. One method that has been established for decades is the water isotope analysis of trees and soils, which we have developed considerably over the past five years. This involves taking a sample from the trunk. The water that is transported upwards by the plant corresponds isotopically to a mixture of all the resources that the plant has underground. This means that if a certain amount of water comes from ten centimetres, from 50 centimetres or three metres, then what we measure in the stem is a mixture of all this water.

There is variation in the isotopes at different soil depths. So the isotope signatures have different concentrations at ten or 50 centimetres. With statistical models we can determine what percentage of the water has to come from a certain soil depth to get this mixture. Scientists have been working like this for decades: Taking samples at different times of the year, bringing them to the lab, extracting them, measuring them. This is incredibly time-consuming and repeated sampling also harms the tree.

Since 2018, with the start of our research group, we have been developing a simpler method. Now, after five years, we have reached the point where we can make this measurement in both soils and plants, using a sensor that we install. We drill a hole in different soil depths as well as in the trunk, install our probes there and can thus continuously measure water isotopes in both compartments. If necessary, I could determine the water isotopes for every day of the year.

How should one imagine these sensors?

We use gas-permeable membranes that look like white tubes and are installed in the tree. The gas phase interacts with the tree and based on that we can determine the isotope signature. In the soil and in the plants, the same methodology works in many climates: Germany, Costa Rica, USA. We have used the method in numerous projects and now we are at the point where we can utilise it and make it accessible.

Who can use this method?

The aim is to bring the method into forestry practice, among other things. Of course, it is not only relevant there. It is the entire hydrological cycle that you learn to understand better with these high-resolution measurements.

You also observe the forests from above, with drones. Why is that important and what insights have you gained from it?

Of course, it sounds contrary at first to look at something that lies very deep in the ground from above. However, we can only use water isotope measurements on a few individuals in our projects: 10 to 15 trees. But how representative is that for a forest or a whole area? This is where I see great potential for the use of drones: We can fly over a representative area and we get detailed images down to the leaf level.

The idea behind this: The tree crown reflects what is happening in the ground. If it is extremely dry for a long time and yet you can still see trees that are still very green, there is a reason for this. It may be that the tree goes into a kind of hibernation and evaporates very little. It adapts by having white leaf surfaces reflect the radiation. Another adaptation is to have as little biomass as possible above the ground and as much as possible below the ground, i.e. deep or widely spreading roots to cover the water demand.

So a green tree crown does not necessarily mean that the plant has deep roots?

No, that would be nice. That’s why we need more information, such as: do the trees evaporate a lot? If they do, it is very likely that they receive water from the depths during an extreme dry phase. That’s why we first did research in very dry areas. We can now determine transpiration from drone data and give a clear indication of water in the subsoil and that there are probably deep-rooted trees there. That alone is a big step forward!

What is a typical deep-rooted tree?

In Europe, for example, the oak is a typical deep-rooted tree. It can root five to seven metres deep.

And shallow-rooted trees are typically rather conifers?

Partly. Pines are also deep-rooted. There are shallow-rooted, heart-rooted and tap-rooted trees. Beech trees, for example, are heart-rooted. Ash and spruce are shallow-rooted. It is very important that they interact with each other. If you plant monocultures, this no longer happens.

In recent years, we have also been able to see the effects of droughts very concretely in German forests, for example in the Harz Mountains. Can you already draw conclusions from your work for the further development of the forests?

Of course we are very concerned about this. This development has also brought about a number of cooperations for us in this area. Of course, the best thing would be to leave the forest to its own devices, which of course requires thinking in decades. And that is certainly not feasible for the forestry sector.

We have just started a new DFG-funded project in which we are collaborating with the municipality of Aperlen in Lower Saxony. Here we are researching a very natural forest that has not been managed for 30, 40 years and where there is this original composition, the “good mixture” of shallow, medium and deep roots. In the long term, the forests adapt. This year we have already seen that the trees have stayed green much longer. Many forest managers say that the beech trees have responded very well. The trees took a break in the summer, when there was little water, and later in the season they grew again and transpired.

One of “Isodrones”‘ research sites is in Costa Rica. Why and what exactly are you investigating there?

We want to study the phenomenon of deep roots in different climates. Costa Rica, with its many climate zones, is one of the best places for this. Our main site is in a dry forest where there is no rain at all for five months. Two hours away, we have a rainforest site where there is a lot of rain throughout the year. That was the main reason why we chose Costa Rica. We also already had a good collaboration there. Costa Rica is very far ahead in the field and has some protected areas.

For example, our main location is in a forest management station where they are trying to re-cultivate endangered tree species and save them from extinction. As researchers, we are also allowed to drill into trees and carry out various examinations there, which is of course not allowed in national parks.

At the station, the first task was to find green trees in the midst of the greatest drought. The staff on site then tipped us off that there is one tree that copes very well with drought – the so-called “guardian of the river”. So we put together the sites with tree species we wanted to study: a rubber tree species and a mahogany species, which is very endangered, but which is still very green in large parts even in the dry season.

At the research site, we built a wooden hut together with the local people. In 2019, we had our first big campaign with doctoral students Malkin Gerchow and Kathrin Kühnhammer. We were on the ground for four months. Then the Corona virus appeared and we couldn’t go into the field for two years because we weren’t allowed to travel to Costa Rica. Fortunately, the sensor system remained active during that time. And we had a Honduran student who was also allowed to control the research station.

Your field research in Costa Rica is also the focus of your science communication project. What is the idea behind it?

The SciComm project was supported by a funding initiative of the Volkswagen Foundation. It was very interesting for us as a project-accompanying measure. It gave us the opportunity to do away with the classic stereotype of a scientist and to show what it’s really like to do science. The SciComm project was also about giving insights into who we are, how we think, what is on our minds and also what is sometimes not so nice. Communicated quite honestly. We wanted to convey an authentic image of scientists.

Also part of the project is an “Interactive Research Site”. What can you explore here?

The “Isodrones 360° Interactive Research Site” is really a great project and almost unique in its kind. Here you can independently explore our research site in the tropical dry forest of Costa Rica. With a 360° VR headset, you get to know the flora and fauna and dive deep into our research. There are nine interactive scenes with explanatory videos, graphics and visualisations of our research data. You can even experience a 360° drone flight!

The “Interactive Research Site” came about by chance. The original plan was to shoot an eight-part documentary series during the second Costa Rica campaign. Unfortunately, because of the Corona virus, this could not take place. But we still wanted to convey an authentic image of our research station.

You have already presented the “360° Interactive Research Site” at events.

Yes, for example at an international conference that we organised. The other scientists thought it was great and wanted to know more about it. And at the anniversary celebration of the Volkswagen Foundation in 2022, people of all ages practically stormed our booth. The product is available as such at any time and will be used many more times – this type of science communication will increase significantly in the future.

Is it worth the effort to manage another large SciComm project in addition to such a research project?

For us, the effort was definitely worth it, even just to document the project. I also use it all the time for presentations. Moreover, communicating our research is becoming increasingly important. As scientists, we should also question what the practical relevance of our work is.

As a hydrologist, what made you decide to do research in this area?

My doctoral thesis was about researching groundwater recharge, i.e. how much water from the rain ends up in the groundwater. During my work in Namibia, I realised how important plants and trees are for the water cycle. The transport of water and the recharge of groundwater in areas with little water is mainly controlled by them. So this is where my interest in roots and trees was awakened.

In addition, I spent my childhood with my grandfather in the forest in the Ore Mountains. That certainly also played a role. I just love being in forests, getting my hands dirty, and valuing trees as equal living beings.

When you examine the trees so closely, can you still enjoy a walk in the forest?

I spend a lot of time in the forest, but you naturally pay more attention to different aspects that also move me in my research. You look at how the forest is doing. Why are so many beech trees broken here? Now I have a more trained eye and observe better. Nevertheless, I still find the forest very healing and pleasant.

You moved from the Federal Institute for Geosciences and Natural Resources (BGR) in Hanover to the Institute of Geoecology in 2018. This was made possible by a Freigeist Fellowship from the Volkswagen Foundation, which runs until the end of 2023. What are your future plans?

I have just acquired two more projects: One is about urban trees, the second is the DFG project already described. Of course, I would like to stay at TU Braunschweig beyond 2023. But I have already had interviews for professorships elsewhere. It is important for me to work on practical aspects with my projects and try to look at complex issues as simply as possible.

The project on urban trees is a joint project with the Julius Kühn Institute. The aim is to develop a kind of vitality index for urban trees. The project title is: Die Maximierung des Kohlenstoffbindungspotenzials von Stadtbäumen (Maximising the carbon sequestration potential of urban trees). It’s about selecting the right tree species that can grow here in the future and setting up a monitoring system that gives us an indication of how the trees are doing. This also fits in well with the research focus “Future City”.

One last question: Who exactly is Stratzlawski?

He is our best friend and helper in the field. He is “the true brain behind Isodrones”. Stratzlawski is an iguana. During our first stay in Costa Rica we were quite fascinated by the many iguanas running around our station. One of them lived in a drainage pipe on a dirt road. He came out every morning to lie in the sun. At some point we started bringing him fruit. We then trained him so that he eventually ate out of our hand. Stratzlawski has become our mascot. You can recognise him quite well because he has a bitten-off tail that has grown back.

Thank you very much!