According to the broadly accepted report of the IPCC (2022), the climate change is further progressing, leading to a rise in temperatures and an uneven distribution of rainfall throughout the year. These changes will presumably result in more frequent and intense hydrological events, including extreme droughts and flash floods (Pendergrass and Knutti, 2018; Trenberth, 2011). In central Europe, this trend contributes to the increased incidence of compound droughts (Markonis et al. 2021). Since the precipitation deficit propagates toward the soil water deficit and subsequent plant water stress (Anderegg et al. 2013), retaining the precipitation water in the soil is a key topic. Among other ecosystems, forests are especially effective in precipitation retaining since their enormous ability to enhance soil infiltration and groundwater recharge (Ilstedt et al. 2016). Subsequently, the soil transforms discontinuous precipitation into a continuous water supply to the plant roots, streams, and groundwater (Kutílek and Nielsen 1994). Increasing atmospheric temperatures promote higher evapotranspiration (ET) demand (Vicente-Serrano et al. 2014) and extend the vegetation season (Piao et al. 2015). This can lead to a tight hydrological balance (ET close to precipitation) and potentially result in the depletion of soil water reserves. In the Czech Republic, a central European country, this tight balance manifests notably in forested regions situated below approximately 650 (m.a.s.l.). Here, the interception reduces a significant portion of precipitation input, as reported by Oulehle et al. (2021).
The majority of forests in the Czech Republic are managed with an approximate coniferous to broadleaf ratio of 7:3 and the future targeted ratio is close to 1:1. The dominantly planted species are spruce (with the actual and targeted representation of 48.1% and 28.3%, respectively), pine (16.0% and 13,2%), beech (9.3% and 22.5%), oak (7.6%, and 12.7%) and larch (3.9% and 4.2%) (Ministry of Agriculture of the Czech Republic 2022). The increasing evidence of drought-induced dieback of spruce trees (Zang et al. 2014; Hlásny et al. 2021; Obladen et al. 2021) raises concerns about the resilience of forest ecosystems in future climatic conditions. To account for that, the spruce in lower altitudes is being gradually replaced by native broadleaf species, mainly beech and oak, that might tolerate the upcoming weather conditions better. Consequently, due to substantial changes in tree species composition (mainly for spruce and beech), there is a need to investigate the hydrology of mentioned forest stands and identify the main differences in hydrological behavior.
The overall stand hydrological balance is affected by precipitation input as well as the interception, evapotranspiration, groundwater, and streamflow output, which may differ according to species-specific traits. Since precipitation partitioning plays a fundamental role in regulating the input of water into the soil (Crockford and Richardson 2000), the after-rain distribution of soil moisture can vary spatially, depending on the distinctive structural characteristics of tree species. For example, the upward branch inclination and smooth bark of beech and oak trees promote enhanced stem flow, leading to the formation of infiltration hotspots near the tree base (Hemr et al. 2023); in contrast, the downwards inclination of spruce branches results in dripping water at tree canopy line (Levia and Frost 2006; Levia and Germer 2015). A significant part of the precipitation is intercepted on the surface of vegetation, reducing the throughfall of precipitation and soil water recharge. The interception capacity of deciduous trees distinctly fluctuates through the year depending on the occurrence of foliage (Staelens et al. 2008), which is in contrast with the evergreen trees (Andreasen et al. 2023). The decrease in interception capacity during the leafless period can be the major factor causing lower annual interception totals for deciduous species than for evergreen conifers (Kantor et al. 2003); see also results of Andreasen et al. (2023) or Oulehle et al. (2021).
The water infiltration into the soil can be significantly enhanced by preferential flow through channels (e.g. macropores), particularly around tree roots (Jarvis et al. 2012; Luo et al. 2019; Jačka et al. 2021). Therefore, the configuration of tree root systems determines the preferential flow of water in the soil. While spruce trees typically possess a lateral root system resembling a shallow plate (Dupuy et al. 2007), the heartroot systems of beech and larch tend to penetrate more deeply. This characteristic allows them to facilitate the downward movement of water into the soil's lower layers, resulting in a double funneling effect, particularly notable in beech stands (Schwärzel et al. 2012).
The infiltration and retention ability of forest soil is further influenced by the character of the organic soil layer, as the plant litter of tree canopies alters the soil environment (Frouz 2018; Jačka et al. 2021). Specifically, the carbon–nitrogen (C/N) ratio of tree foliage is determining the decomposition of organic matter and the formation of soil substrate. Deciduous trees usually provide a high amount of plant litter with a low C/N ratio, forming a nutrient-rich environment with a high abundance of soil microfauna that further produce macropores (Albers 2004; Achilles et al. 2021). In contrast, the poor soil habitat of high C/N ratio litter of spruce forest favors the slower fungal decomposition (Frouz 2018), with no distinct preferential pathway formation. Furthermore, the spruce organic soil contains high amount of fine roots and poorly decomposed laterally oriented needles that are tightly bound by mycelia of fungi. This organic layer limits the vertical flow into below-situated mineral soil (Valtera et al. 2023), resulting in lower infiltration rates when compared with the organic soil under the broadleaf trees (Jačka et al. 2021). Forest soil is further altered by understory vegetation. For example, the light-demanding larch stands (Da Ronch et al. 2016) provide a favourable condition for the development of understory vegetation (Barbier et al. 2008), resulting in a spatially variable soil environment (Hiller et al. 2002).
Similarly to the input of precipitation water, the water consumption between tree species varies. The daily transpiration rates in vegetation season accounting for nutrient supply, cooling, and leaf turgor maintenance, are usually higher for broadleaved forests due to the higher stomatal conductance (Weaver and Mogensen 1919). However, the annual amount is affected by the limited period of foliage of deciduous trees, accounting for slightly higher annual transpiration summation of spruce over beech (Rötzer et al. 2017). In terms of water-saving strategies, coniferous spruce adopts an isohydric strategy by closing its stomata early in response to water stress (Pretzsch et al. 2014). In contrast, the anisohydric beech maintains a relatively high transpiration rate, even to the point of xylem cavitation or crown desiccation (Betsch et al. 2011). The spruce's isohydric strategy makes it susceptible to nutrient deficiency and subsequent pathogen outbreaks, such as fungal infections or bark beetle infestations (Matthews et al. 2018; Hlásny et al. 2021). The Larix sp. exhibits characteristics that can be attributed to both water-saving strategies and the strategy is dependent on the specific species (Sasani et al. 2021). The European larch (Larix decidua) is considered an anisohydric (Leo et al. 2014). As the only deciduous conifer in Europe, its annual foliage renewal serves as protection against winter desiccation caused by atmospheric transpiration demand while the soil remains frozen (Da Ronch et al. 2016).
Due to mentioned species-specific variations in organic topsoil properties and in the individual components of the forest hydrological cycle, it is anticipated that different tree species will differ in the soil-water interaction behavior. The aim of the current research is to investigate the impact of spruce (Picea abies), beech (Fagus sylvatica), and larch (Larix decidua) on the forest soil moisture regime under changing climatic conditions. The tree species were selected due to their differences in foliage character and in crown and root-system architecture, which is expected to significantly alter the overall stand hydrology. Our study uses 54 autonomous stations TMS4 manufactured by Tomst for temperature and soil moisture measurement in the topsoil and subsoil layers. The research is carried out as part of the ČZU pilot project “Smart landscape in the Amálie area” situated at a lower altitude approx. 430 m a.s.l, where evapotranspiration requirements are close to the amount of precipitation and the expected rise of compound droughts can lead to increased occurrence of vegetation drought stress. Due to the mentioned tight hydrological balance of the area, the soil moisture level is expected to be highly sensitive to tree species-related traits, which are affecting the overall stand hydrology. Concretely, the hypothesis is that the reduced interception and transpiration of deciduous trees (beech and larch) during the leafless period, will cause more effective soil recharge and subsequently higher soil moisture levels in the winter and spring period than for evergreen spruce.