Do the “natural pumps” of trees help during droughts? Project on the hydraulic redistribution of water by plants

Press release of the Klaus Tschira Foundation

Many people fear climate change. Others deny that it is happening. Some try to find out how humans, animals and plants can learn to deal with it better. One of them is Matthias Beyer, a PhD hydrologist and head of the research group ‘Isodrones’ at the Institute of Geoecology at TU Braunschweig. He is working on the project together with Professor Matthias Bücker from Christian Albrechts Universität in Kiel.

PhD student Alberto Iraheta during the novel in-situ measurement of stable water isotopes in the field. The measurements can be used, among other things, to investigate the water uptake depth of trees. Photo credits: Matthias Beyer/TU Braunschweig

In a three-year project funded by the Klaus Tschira Foundation, the research team is trying to better understand the hydraulic redistribution of water by plants and then make the results usable for agriculture and forestry. Hydraulic redistribution refers to the passive, nocturnal transport of soil water by plant roots from moist to drier soil layers.

This is about nothing less than adaptation to periods of drought and temperature extremes, which are becoming more frequent even in temperate central Europe. “The redistribution of water in the soil made possible by plants,” says Beyer, “is a matter of life and death in agricultural, urban and forest systems during periods of drought.”

If understood and harnessed effectively, this water redistribution system has the potential to make entire ecosystems more resilient to extreme droughts. However, this requires a much better understanding of how hydraulic redistribution works and how it affects ecosystems. And not just on a small laboratory scale, but under real-life conditions, for example in an entire forest over an extended period of time.

Installation of a 200 metre long geoelectric monitoring profile in the forest. Cables and spear electrodes are buried in the forest floor to protect the long-term experiment from damage, for example by wild animals. Photo credit: Matthias Bücker/TU Braunschweig

Water dispenser for the ecosystem

The research team is interested in the redistribution of water by plant roots from a place in the soil where there is a lot of water to a place where there is little. The classic example: in summer, the topsoil is completely dry, while water is still present in deeper layers due to the abundant rainfall in spring. There are certain groups of plants whose roots are so long that they can reach the water supply at depth.

Over hill and dale – the installation of the monitoring profile was hard work, but it has paid off: the geoelectric experiment has been providing monthly data since May 2023. The picture shows the plastic protective tube in which the actual measuring cables run. Photo credit: Matthias Bücker/TU Braunschweig

Using a hydraulic effect, they can transport water into the upper layer of soil. This effect is based on the phenomenon of suction tension. In dry soil, this is much higher than in the lower, water-rich soil layers. This leads to a pumping effect where water is drawn up from the bottom. During the day, this suction comes from the atmosphere and draws water from below up into the canopy, where it evaporates through the leaves. At night, the suction from the atmosphere is reduced because the stomata of the plants are closed.

When the difference in suction tension between the topsoil and the subsoil reaches a certain level, the dry soil draws water up through the roots. This benefits the deep-rooted plants themselves, as the water pumped up from below is available for photosynthesis first thing in the morning. At the same time, the water is made available to other shallow-rooted plants that would otherwise not be able to reach the deeper water.

The project will use two methods: one will use measurements of stable water isotopes to track the path of water molecules as they fall as precipitation through the soil and back into the atmosphere. The second method – geophysical monitoring – involves installing electrical sensors in the forest floor. These sensors are designed to pick up tiny, time-varying electrical signals generated by the movement of water in roots and soil as it is hydraulically redistributed. In addition, small ‘nails’ driven into the ground and connected to measuring devices are used in so-called geoelectric measurements to record spatial and temporal variations in water content.

An area of research with great potential for the future

Geoelectric monitoring measurements in the research forest. Repeated geophysical measurements record variations in the electrical conductivity of the forest floor, which can be traced back to spatial and temporal variations in soil moisture. Photo credits: Matthias Bücker/TU Braunschweig

How did this research idea come about? As a hydrologist, Beyer himself had not had much to do with plants for a long time. It was during his PhD in Namibia, where he studied local groundwater recharge, that he learned that the root depth of plants can play an important role in the water balance of a soil.

What are the findings at the end of the project? Matthias Beyer: “We will know which tree species have deep roots and what this might mean for other plants.” Some tree species already benefit from this process. This is also known from agroforestry, where trees provide shade and water for fruit and vegetables. The research group also wants to find out what role fungal networks play in the soil.

“We are dealing with a broad and complex field of research, which is becoming increasingly important in the light of global developments,” says Alex Seuthe, head of research funding at the Klaus Tschira Foundation. “The project is an excellent piece of basic research with a clearly defined goal. It promises not only to significantly advance agriculture and forestry, but also to significantly expand our understanding of the adaptability of ecosystems to climate change.”

Text: Kirsten Baumbusch, Klaus Tschira Foundation