Following the water cycle in the forest

The forest on Hönggerberg in the north of Zurich is both, a recreational space and a laboratory. The Forest Laboratory called “Waldlabor” considers itself to be an experience-oriented education and research site that is open to everyone. The public, experts and scientists experience, manage and observe the cultivated forest with the overriding goal of better understanding and preserving this habitat.

A thematic focus of the Waldlabor is climate change. In a multi-year project ETH Zurich researchers are currently investigating how warmer and drier conditions affect the water balance of the forest. For this purpose, Marius Floriancic, Senior Scientist at the Institute of Environmental Engineering, and the ETH Hydrology Professors James Kirchner and Peter Molnar are operating a comprehensive measurement and monitoring system on Hönggerberg near the Holderbach in order to precisely record and trace the relevant water fluxes between the soil, trees and the atmosphere.

In a recently published study in the journal Ecohydrology, Floriancic, Kirchner and Molnar were able to show that dead plant material on the forest floor plays a far more important role in the water balance of a forest than previously thought.

How much water is left for trees?

When it rains in the forest, various transport and storage processes take place that still have to be investigated in more detail. “In order to understand the effect of droughts for forest ecosystems, we need to know on which paths and in what quantities precipitated water moves through the forest,” explains Floriancic, lead author of the study and head of the hydrological project.

Floriancic and colleagues are dealing with fundamental questions: Where does the rainwater end up when it rains in a forest? How much of it ends up in the soil? What proportion is accessible for vegetation? How long does it take for a drop of rain to be transpired through the leaves of the trees?

The researchers' water balance for the Waldlabor provides initial answers: around 20 percent of annual precipitation is retained in tree canopies and evaporates back to the atmosphere – that's what they had expected. What surprised them, however, was that almost the same fraction, around 18 percent of precipitation, is retained in the litter layer of fallen leaves, needles and deadwood on the forest floor. “This is much more than we expected based on previous research results,” says Floriancic. This portion of the water also evaporates back into the atmosphere.

In total, about 38 percent of annual precipitation is retained by canopies and the litter layer. This water does not contribute to soil water recharge and therefore is not available to plants. Only 62 percent of the rainwater actually infiltrates into the forest soils. A large part is then taken up by grasses and shrubs that root in the upper soil layers. "In the end, only a very small part of the precipitation reaches deeper soil layers, where it becomes accessible for forest trees or recharges groundwater," Floriancic notes.

Understanding changing forests

According to the researchers, storage in the litter layer and its effect on the forest water cycle was generally underestimated in the past and therefore often neglected in water balances. “This means that trees have much less water available for transpiration than previously thought,” Floriancic concludes. Trees live from winter precipitation to a large extent. This is one reason why, unlike grasses or shrubs, they are better able to survive longer dry periods in summer, as long as there is enough supply in winter, says the environmental engineer. Meanwhile, the evaporation of the retained water from the litter layer also favours the cool, moist microclimate in the forest.

Why is this important? Because knowledge of the water balance is an essential prerequisite for understanding the effects of drought on forest ecosytems. The findings also feed into water balance modelling to better predict future changes of water availability in forests.

For their study, Floriancic, Kirchner, Molnar and collegues combined hydrological experiments with physiological measurements on plants, weighed the wet and dry forest-floor litter material and analysed the chemical composition of various water samples. In this way, they recorded all water fluxes during and after precipitation, from retention by tree canopies and forest-floor litter to soil water, plant and root water, groundwater, to runoff in the Holderbach stream.

The team is actively supported by students from the Departments of Environmental Systems Science and Civil, Environmental and Geomatic Engineering. Thanks to the proximity to the ETH Hönggerberg campus, the young scientists in the Waldlabor are actively involved in practical research. “A group of student assistants helps us with taking samples and maintaining the experiments,” Floriancic says.

In field courses and theses, students can also independently investigate aspects of the water cycle and apply theoretical knowledge. In this way, the Waldlabor also makes a significant contribution to the practical training of ETH students.