H4Res - Hydraulic Redistribution and forest resilience (2024-2027)
In 2024, the Klaus Tschira Foundation granted this project on investigating hydraulic redistribution through water isotope and geophysical methods as well as modelling its impact on larger scales. We kindly thank the foundation for supporting us! Here’s a project description:
Hydraulic redistribution (HR; Burgess et al., 1998, Caldwell et al., 1998, Fig.1) is a process that can potentially have an impact on the capability of ecosystems to adapt to droughts (Grünzweig et al., 2022, Prieto et al., 2012, Oliveira et al., 2005). HR is defined as the transport of water between different soil parts via plant root systems, driven by water potential gradients in the soil–plant interface (Prieto et al., 2012, Dawson, 1993; Caldwell et al., 1998) that exists in different forms (refer to Prieto et al., 2012 for all types of HR): Hydraulic lift (HL; Richards & Caldwell, 1987) defines the nocturnal upward movement of water from deep wet to shallow dry soil layers and is the most commonly observed type of HR (Prieto et al., 2012). Lateral redistribution (LR) is the horizontal movement of water between soil layers at the same depth but with different water potentials (e.g., Muñoz-Villers, 2020, Nadezhdina et al., 2009, 2006). Downward or inverse HR (DHR) refers to water movement to deep soil layers under certain conditions when shallow layers are wetter than deep layers (e.g., Oliveira et al., 2005, Schulze et al., 1998). Figure 1 illustrates the most common form of HR, hydraulic lift (HL).
![Fig. 1: Schematic illustration of upward hydraulic redistribution: When the shallow soil is dried out, like under drought conditions, its soil water potential decreases, creating a strong suction of the soil matrix. At night, when plants are not transpiring because the stomata are closed and not atmospheric water vapor deficit is induced to the trees, water from deeper, wetter soil layers can be drawn upwards through the root and be released in the shallow soil surface due to the extreme suction ten-sion of the soil. Mycorrhizal fungi living in symbiosis with the plant can further support the exchange of water and nutrients between deep and shallow roots. [Image credit: Christine Faltermayr]](/projects/h4res-hydraulic-redistribution-and-forest-resilience/hydred_hu29e8ba0291f9f2feb0aab4feb9717f37_514903_eb2326282605674fb44968c4cb0e0bb4.webp)
HR is a complex and widespread phenomenon that has been observed in many ecosystems under different climates (refer to the review papers of Prieto et al., 2012 and Neumann et al., 2012). To date, it has been widely accepted that HR is a process with global importance for the hydrological and biogeochemical cycle. The occurrence and magnitude of HR is highly site specific and depends heavily on environmental (e.g., local climate, soil type, soil moisture gradients, temperature) and biotic factors (root architecture, plant species, and mycorrhizal associations) that influence the occurrence and magnitude of HR, causing the process to be “more complex, heterogeneous and patchy than previously thought” (Prieto et al., 2012, Neumann et al., 2012).
Based on the existing evidence of more than 30 years of research on the process, it is indisputable that HR is a process of global importance which has an effect on the hydrological cycle and ecosystem functioning. Despite this proven global importance, evidence-backed modeling approaches are extremely scarce (but see Barron-Gafford et al., 2017, Domec et al., 2012, Lee et al., 2018). As a consequence, we are currently not able to evaluate accurately whether HR will be an important component for the resilience of forests and other ecosystems under predicted climatic changes. Most existing studies report that HR has a buffering effect during droughts reducing plant water stress and increasing plant transpiration (e.g., Dawson 1993, Prieto, 2012), but modeling approaches have found that the relative contribution of HR on transpiration will decline with predicted climatic changes (Bogie et al., 2018) due to a lower water availability in the deeper soil (Prieto et al., 2012). Furthermore, HR is a “heterogeneous and patchy process” (Prieto et al., 2012) that is affected by site specific features like local climatic regimes, soils, vegetation type, plant-plant and plant-soil interactions and mycorrhizal associations. As a consequence, accurate data on HR for each individual site needs to be collected and new ways for data integration need to be found in order to evaluate the effect of HR for a particular ecosystem.
HR has started to be recognized in land surface models, but the availability of quantitative data to parameterize these models on scales relevant for management decisions (i.e., the field to catchment scale) remains a major research gap. Such informative data can only be provided when the research focus and monitoring approaches are moving from looking at individual plants and studies under highly controlled settings to monitoring HR on larger spatial scales (i.e., the field to catchment scale) under field conditions (i.e., in heterogeneous systems).
As stated above, such data are starting to emerge (e.g., Bogie et al., 2018, Priyadarshini et al., 2016, Domec et al., 2012), but most monitoring efforts still rely on measurement techniques that have severe limitations in their spatial (namely, sap flow measurements) and temporal resolution (namely, destructive isotope sampling) making it incredibly expensive and laborious to obtain large representative datasets on HR for larger areas. The only available methods that are cheaper and less laborious (namely, soil moisture/water potential monitoring) have the disadvantage to not be able to separate HR from other processes (e.g., soil water vapor flux), leading to overestimates of HR (see explanation above).
Hence, we postulate:
We are able to accurately measure and monitor HR on the individual tree basis, but projecting these measurements to larger scales is not feasible because the commonly used methods (namely, soil moisture & sap flow measurements and destructive isotope sampling), have severe limitations in their spatial and temporal resolution.
This hinders us to investigate and understand important but potentially relevant processes. For instance, it is a common belief that HR benefits both the redistributing tree and also the surrounding vegetation, including tree seedlings of the lifting tree (e.g., Hafner et al., 2017, 2019, 2021, Prieto et al., 2011). Such studies were carried out almost exclusively under controlled conditions; hence, we do not know how competition and other field implications affect who are the beneficiaries of HR. In addition, the theory that seedlings are supported by their mother tree (mother tree hypothesis) has been questioned recently (Henrikson et al., 2023). Although this critique focused mainly on carbon and to some extent nitrogen sharing, many of the questions raised apply to water sharing as well. In particular, the authors pointed out the need to use strong labelling to overwhelm background fractionation and the need for rigorous exclusion of alternative pathways for label transport.
Developing and applying continuous monitoring schemes to overcome these limitations will help us to provide data to parameterize land surface and plant-hydraulic models, quantify HR on relevant scales and advance the understanding of HR on the (eco-)hydrological cycle. An increased knowledge on the process can inform forest managers on suitable species-combinations and the adaptation capability of current ecosystems favoring HR in order to build resilient ecosystems for the future.
In this research project, we will deploy an in situ monitoring system that is specifically designed to provide quantitative data on HR on the field scale (up to 1 ha, sub-daily to weekly) in order to parameterize a land surface model that is suitable for assessing future implications of HR and management decisions in a mixed broad-leaf temperate forest.
**The main research questions of this project are:
Is it possible to design a combined in-situ water isotope and geoelectrical monitoring experiment to assess the importance of HR on the field scale? Is it possible to use such a design in order to advance towards a standard procedure for assessing the importance of HR for the resilience of specific ecosystems as baseline for management decisions?
Which species and which plant groups are the beneficiaries of HR in a mixed broad-leaf forest?
Are occurrence and amount of hydraulically lifted water tree-specific or site-specific? How heterogeneous is HR in mixed, broad-leaf forests?
Does hydraulic redistribution have a substantial effect in terms of buffering droughts and dry spells now and in the future in mixed broad-leaf temperate forests?**
**Objectives **
The main objective (O1) of this project is to design and establish a field monitoring setup allowing to identify the occurrence and quantify the amount of hydraulically distributed water on the field scale. With the established setup, we will investigate if and where HR occurs at a humid temperate site in northern Germany: a mixed broad-leaf forest located in the Weserbergland.
Our second objective (O2) is to combine the established setup with multi-tracer deep labeling and geoelectrical monitoring experiments in order to provide a species-differentiated quantification of HR (O2a) and identify the beneficiaries of HR (O2b). In order to address this objective, we will leverage on deep-water labeling experiments and add a dedicated geoelectrical monitoring approach.
The third objective (O3) of this project is to quantify the impact of HR on transpiration rate on seasonal and annual time scales (i.e., < 1 y) and assess the effect of HR under predicted climatic changes. To achieve this, we will use End-member mixing analysis (EMMA) and process-based, isotope-enabled land surface modeling.
