Forest-floor litter and deadwood retain around a fifth of precipitation
Extreme weather events are likely becoming more frequent due to climate change. At the forest experimental site “Waldlabor Zürich”, D-BAUG researchers around Dr. Marius Floriancic and Professor Peter Molnar investigate the impact of droughts on trees. In a recently published study, they have shown that litter and deadwood store a significant proportion of annual precipitation and that trees are far more efficient in their water use than previously assumed.
During a precipitation event in the forest, numerous complex transport and storage processes occur along the water cycle which are yet not well studied. One part of the precipitation is retained in the tree canopies and evaporates back to the atmosphere. Another part is drains along the stems. Another fraction of precipitation is temporarily stored in litter, deadwood and in the forest floor itself.
"Only a small fraction of precipitation actually reaches the deeper subsurface and contributes to the recharge of groundwater," explains Dr. Marius Floriancic, first author and project manager of the ecohydrological site. "In our forest lab we are aiming to measure all of these storage and transport processes so that the effect of these processes on the water cycle can be quantified."
A complete record of these different components of the water cycle is ensured with modern and innovative sensors. Using a series of measurements, Floriancic and colleagues have now found that water retention in litter and deadwood plays a far more important role than previously thought. Their study demonstrates that about 18% of annual precipitation is retained in the forest-floor litter and is then evaporated back to the atmosphere. This means that the water does not reach the soil and, thus, is not available for plants or groundwater recharge.
Where and how water is stored in the forest
Monitoring the water content in deadwood (Photo: IfU/D-BAUG, ETH Zurich) The daily weight measurements (Photo: IfU/D-BAUG, ETH Zurich) Deadwood pieces of different decay (Photo: IfU/D-BAUG, ETH Zurich) Soil moisture sensors in different depths (Photo: IfU/D-BAUG, ETH Zurich) Environmental engineer Marius Floriancic takes a water sample from the Holderbach (Photo: IfU/D-BAUG, ETH Zurich)
"The trees are much more efficient in their water use than we thought," says the environmental engineer. "They live on winter precipitation to a large extent. That's part of the reason why trees, unlike grasses or shrubs, can better get through prolonged dry spells in the summer – as long as there is enough replenishment of water during the winter." The evaporation of retained water from the litter layer also has a major effect on the in-forest microclimate.
The litter layer thus plays a much more important role in the water balance of a forest than previously thought. Ignoring this essential retention process in the water cycle leads to a significant overestimation of forest tree transpiration and underestimation of the water use efficiency of trees. This new perspective on the forest water cycle is essential for modeling storage and transport processes and for understanding the impact of droughts on forests.
Original publication
Marius G. Floriancic, Scott T. Allen, Raphael Meier, Lucas Truniger, James W. Kirchner, Peter Molnar
external page Potential for significant precipitation cycling by forest-floor litter and deadwood
Ecohydrology (2022), doi: 10.1002/eco.2493
Test site Forest Lab
In the project “Hydrological Monitoring (P06)», the quantity and quality of all water flows are measured, from precipitation, interception and stemflow, plant and root water uptake, soil water and groundwater to runoff in the "Holderbach". Understanding these processes is essential to predict the impact of climatic change, especially of prolonged dry periods, on forests.
For this purpose, the Laboratory for Environmental Engineering and the Chair of Hydrology and Water Management have set up an eco-hydrological research station on Hönggerberg in recent years. There, the essential processes along the water cycle are examined, innovative sensor technologies are tested and students are involved in scientific data collection as part of field courses. In the future, the forest laboratory should also attract new ecohydrological research at ETH and collaborations with other universities and research institutes.