Deciduous forests in the hills are more effective at mitigating floods than coniferous forests
In recent years, society has faced clear signs of long-term climate change, including a rise in average annual temperatures while total precipitation levels have remained relatively constant. However, extreme climatic events, such as heavy rainfall and prolonged droughts, are occurring with increasing frequency. Forecast models suggest that total precipitation will rise slightly—by 10 to 13% by the year 2100—but the number of days with precipitation is likely to decrease, particularly during the summer months. This trend indicates a heightened risk of severe rainstorms and flooding. As a result, researchers are exploring various strategies to reduce the impact of flood waves.
Photo: Křtiny, spillway with measuring device, author Petr Kupec
Scientists from Mendel University in Brno, along with partners from various institutions, addressed this topic in the NAZV project No. QK21010198, titled “Adaptation of forestry for sustainable use of natural resources” (Adaptace lesního hospodářství pro udržitelné využívání přírodních zdrojů). They published their findings in the article “Different behaviour of paired spruce upland micro-catchments in two hydrologically different years” (Rozdílné chování párových smrkových pahorkatinných mikropovodí ve dvou hydrologicky odlišných letech), which appeared in the journal Reports of Forestry Research, Volume 4, 2025.
The objective of their research was to compare the annual patterns of basic hydroclimatic variables during the year 2023 and the flood year 2024. This study focused on two forest micro-catchments located in the uplands of the Bohemian-Moravian and Drahanská Highlands, where spruce is, or was, the dominant tree species.
All flow measurements were conducted on stabilized specific spillways within two small forest catchments. The Račín micro-catchment (MP) is located in the Kinský Žďár Local Historical Collection (LHC) near the city of Žďár nad Sázavou, while the Křtiny micro-catchment is situated within the Křtiny LHC ŠLP ML, which is adjacent to the northern border of the city of Brno.
Forests, which currently cover 34% of the Czech Republic’s surface area and are continually expanding, can be a key element in flood protection. In addition to providing timber and various other benefits for society, forests play a crucial role in the water cycle within the landscape.
Photo: Kanice, forest stand, author Petr Kupec
Scientific research has confirmed the essential role of forests in water management and their ability to mitigate flood waves during periods of intense rainfall. Factors such as species composition, spatial arrangement, age structure of the stand, and particularly the condition of forest soil significantly influence the water management functions of forests.
Scientists have found that forest complexes can absorb significant amounts of water and can reduce peak wave levels by up to half compared to agricultural land. This is largely due to the high infiltration capacity of forest soils and their ability to retain precipitation. The retained water drains slowly as subsurface runoff, helping to maintain base flow in watercourses. As a result, the minimum flows in small forest streams increase during droughts, making this water available in downstream landscapes.
One key hydroecological parameter that can be consciously managed in forestry practices at the river basin level is the tree composition of forest stands. To effectively improve landscapes, it is essential to understand how individual tree species influence water dynamics. A critical distinction exists between coniferous and deciduous tree species, primarily due to their different physiological characteristics and, especially, their varying assimilation systems. Generally, it can be stated that more water flows annually from deciduous catchments compared to coniferous ones.
In hilly landscapes, researchers have documented that during dry periods (lasting more than a week without rain), catchments with deciduous and mixed forest cover can maintain stable flow rates. In contrast, catchments dominated by spruce see flow rates drop to below 10% of their annual average.
Photo: Žďár nad Sázavou, forest stand, author Petr Kupec
The situation in the same areas responding to extreme precipitation is notably different. Research indicates that mixed micro-catchments (1.5 times more effective) and beech micro- catchments (twice as effective) outperform spruce micro-catchments in managing runoff. Local studies suggest that the effects of deciduous and coniferous trees on runoff water vary significantly throughout the year and can often reverse.
During drier periods, when transpiration is limited due to reduced soil moisture, runoff from deciduous areas tends to be higher. Conversely, in wetter periods, runoff from these areas is lower. Existing research supports the notion that in upland regions, deciduous trees contribute to a greater volume of water in forest streams, making them more favourable for hydroecological purposes compared to spruce forest stands.
This is likely because drier conditions are more prevalent in the Czech uplands throughout the year, and spruce trees are often at the limit of their ecological optimum in this region, preventing them from meeting their water management potential as effectively as they do in mountainous areas. The study utilized data from two micro-catchments, where spruce predominated as the main tree species. Before 2020, spruce represented 56% of the tree population, while beech accounted for 39%. However, by 2024, remote sensing data indicated that spruce representation had decreased to 29%, with beech rising to 59%. This shift is attributed to the decline in bark beetle infestations between 2020 and 2023.
Photo: Mixed forest with a stream in the uplands, author Jan Řezáč
In this study, spruce is being replaced by beech, which are expected to improve the water management efficiency of the catchment area in retaining extreme precipitation. However, the effects of this significant change, which affects 27% of the vegetation composition, were not recorded in the study results. It can be assumed that the originally spruce-dominated catchment will retain low water management efficiency for several years following the change to a different controlling tree species.
The findings indicate that spruce catchments in the lower hills of the 3rd LVS achieve significantly lower water management efficiency than those closer to their ecological optimum, represented by the 5th LVS. This important observation highlights that we cannot generalize the issues related to spruce; it is crucial to consider local conditions. This consideration is especially relevant to changes in tree species composition, which can have a significant positive impact on hydrological conditions over the long term, but this effect may be delayed for several years.
The study presents data from two forest micro-catchments observed across two hydrologically distinct years. Given the localized nature of the study and the short time frame, it should be understood within the broader context of global climate change. It serves as an experiment aimed at enhancing our understanding of the evolving role of forest stands in flood protection and the implications of radical changes in tree species composition.
The paper “Different behaviour of paired spruce upland micro-catchments in two hydrologically different years” (Rozdílné chování párových smrkových pahorkatinných mikropovodí ve dvou hydrologicky odlišných letech) can be downloaded here.
Authors: Jan Deutscher, Jan Podlipný, Petr Čech, Petr Kupec, Mendelova univerzita v Brně, Lesnická a dřevařská fakulta, Ústav inženýrských staveb, tvorby a ochrany krajiny, e-mail: petr.kupec@mendelu.cz
Prepared based on the original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz