Introduction

A fundamental question for hydrologists and ecologists is how water moves through ecosystems and how and in which quantities water entering the system as precipitation becomes available for plants, replenishes soil and groundwater reservoirs or transforms into runoff. Only recently, the biophysical process of plant transpiration received increased attention by the scientific community as it has been proven to be by far the largest flux of the global water balance (Barbeta & Peñuelas, 2017; Evaristo, Jasechko, & McDonnell, 2015; Jasechko et al., 2013; McDonnell, 2014; Schlaepfer et al., 2014).

recent literature
Recent literature on quantitative estimates of global transpiration. These (amongst many other) scientific statements in the last years initiated to an ongoing discussion and increased research interest on ecohydrological questions

However, fundamental aspects are currently neither completely understood nor quantifiable related to the question how and from which reservoirs plants receive their water from (McDonnell, 2014). One of those is the functioning of deep, potentially groundwater tapping roots (the definition we use here is based on separating a plants’ root zone into a ‘lateral’ and ‘deep’ component). It is widely accepted that such roots are highly efficient in the acquisition of water once tapping the groundwater; however, they are rarely considered in the management of water resources, mainly because of the lack of quantitative methods (Maeght, Rewald, & Pierret, 2013; Pierret et al., 2016).

Deep Roots in Namibia
Exposed tap roots at an excavation site in northern central Namibia. Smaller tap roots extending over the lateral root zone are clearly visible.

Functions of deep roots

Deep roots exist in different forms, of which tap, sinker and descending lateral roots are the most common (Canadell et al., 1996). Their main ecological functions are to provide stability for the plant and supply the it with water and nutrients. For some species (e.g. phreatophytes), survival can completely depend on the capability of roots to access the water table (Canadell et al., 1996; Kalisz & Stone, 1991) Especially when the supply from shallow soil water and precipitation are insufficient, the water uptake function of deep roots is crucial (e.g. in dry and rocky environments). In such cases - even if the fraction of roots reaching the water table is small - the efficiency of tap roots can be hundreds of times larger than roots in drier soil and large amounts of water can be transported (Canadell et al., 1996; Reicosky, Millington, Kute, & Peters, 1964).
Deep roots are not only able to take up water but are also capable to redistribute water between different soil layers. Hydraulic lift (HL) or hydraulic redistribution (HR) is the nocturnal transport of water from moist to dry regions of the soil profile and is well studied since many decades (Burgess, 2011; Burgess, Pate, Adams, & Dawson, 2000; Burgess, Adams, Turner, & Ong, 1998; Dawson, 1993; Ludwig, Dawson, de Kroon, Berendse, & Prins, 2003; Richards & Caldwell, 1987). The morphology of root systems affects the plants ability to take up nutrients. If there is availability of essential nutrients at depth, but not at the surface (e.g. trace elements such as strontium, potassium or iodine) plants might prefer the deeper source even if water availability is less. This mechanism can also work the other way around. Similar to transporting of water, the before mentioned mechanism of hydraulic redistribution can also be used by the plant to redistribute nutrients (da Silva et al., 2011; Lambers, Shane, M.D., Pearse, & Veneklaas, 2006) , but refer to (Maeght, Rewald, & Pierret, 2013) for more examples). Finally, deep roots also play a role in physical and chemical weathering, can influence the biogeochemistry of carbon and have an impact on the soil fauna and microbial communities (Maeght, Rewald, & Pierret, 2013).

Exposed roots Costa Rica
Exposed root system in a riparian area in Costa Rica.

Why more research is needed

The habit of developing tap roots has been shown to be common across all climates; hence, they are of global significance (Lubczynski, 2011). The maximum depth of tap roots of an individual tree has been found to be 68 m (Canadell et al., 1996; Jennings, 1974). Nevertheless, scientific research deep roots is scattered and no significant methodological improvements have been achieved since the global analysis of deep roots by Canadell et al. in 1996 (Canadell et al., 1996; Maeght, Rewald, & Pierret, 2013; Pierret et al., 2016). In fact, there is no generally applicable methodology available to quantify the contribution of deep rooting vegetation to overall transpiration and to evaluate their impact across different climatic and ecoregions. In a recent review on the current understanding of deep roots and their functions in ecosystems published by (Pierret et al., 2016), the authors state that “research on deep roots is hampered by the lack of appropriate methodology to study them accurately”. Further, the authors ask for “…unconventional investigations that involve a larger degree of risk…” (Pierret et al., 2016). This statement seems very conform to the ‘Freigeist’ program and constituted an excellent starting point for the ideas of this project.

“To a large extent, our knowledge of deep fine roots remains extremely limited due to the fact that research on deep roots is hampered by the lack of appropriate methodology to study them accurately. We need “more daring, unconventional investigations that involve a larger degree of risk…” (Pierret et al., 2016)

Project Goals

The overarching aim of this project is to develop a framework for quantifying the impact of deep roots on the water balance across different environments. We divided this main goal into three working packages with distinct goals (expressed as working packages - WP):

(WP1) Develop a methodology to identify groundwater-utilizing plants and quantification of water uptake through deep roots; separate evapotranspiration into transpiration through the lateral root zone, transpiration through tap roots and evaporation using novel stable isotope techniques (WP1). (WP2) Develop drone-based techniques for improving field-scale ecohydrological research (WP2). (WP3) Incorporate deep roots into (eco-) hydrological models and quantify the impact of deep roots on the regional water balance (WP3).

The figures down below depict the WP’s and how they are connected to each other and a conceptual illustration of the underlying ideas.

WP
The working packages of ‘Isodrones’ and how they are connected to each other.
Concept Isodrones
Conceptual illustration of the project ideas.

Hypothesis: “Deep water uptake (underneath the lateral root zone) is an important contributor to the overall transpirative flux of plants across all environments.”

Ultimately, this project will provide a novel perspective on the water budget of the individual plant and for water management under climatic variability. In the context of recent scientific findings related to plant functioning, water acquisition zones and adaptability of the vegetation under various environmental conditions, a better understanding of the coupling between water input (precipitation, groundwater), soil water reservoirs, plants and the atmosphere as water output will be achieved.

Working Package I (WP I): Identification of deep roots and quantification deep root water uptake

The goal of WP I is to develop a methodology to identify groundwater-utilizing plants and to quantify water uptake through deep roots. Essential for this is to separate evapotranspiration () into transpiration through the lateral root zone (); transpiration through deep roots () and evaporation ():

A major element to achieve this are techniques based on water stable isotopes, but also extensive plant physiological and soil hydraulic monitoring. Measurements of isoflux in trees and sap flow, for instance, will be conducted at several positions of the respective tree simultaneously, i.e. at the tap root directly, at the trunk, in branches. Recent advances in techniques utilizing water stable isotopes (Dubbert et al., 2014; Dubbert, Cuntz, Piayda, Maguás, & Werner, 2013; Gaj et al., 2016; Oerter & Bowen, 2017; Werner & Dubbert, 2016; Volkmann, Kühnhammer, Herbstritt, Gessler, & Weiler, 2016; Volkmann, Haberer, Gessler, & Weiler, 2016) are applied and further developed. Whereas insitu methods for measuring soil water stable isotopes have become popular, no major achievements were reported in regard to measuring plant water isotopes directly (i.e. the isotopic composition of xylem). We utilize a novel ‘stem borehole’ technique to measure the isotopic composition of xylem water insitu.

Insitu Sweden
Insitu measurement of soil and tree water isotopes. The image is taken from the first field trip within the Isodrones project in a collaboration project with the University of Freiburg and the SLU in Umea, Sweden.

This will be accompanied by extensive plant physiological (sap flow, water potential, stomatal conductance, etc.) and soil monitoring (soil moisture, matric potential, temperature). In addition, tracer experiments (Beyer et al., 2016; Kulmatiski, Beard, Verweij, & February, 2010; Priyadarshini et al., 2016) for quantitative studies along the soil-vegetation atmosphere interface will be utilized. Finally, an important component of this working package is the investigation of the transport of trace elements (Lintern, Anand, Ryan, & Paterson, 2013) through plants, which might yield an additional indicator and constraint for mixing models. The following questions will be addressed in WP I:

How can we improve the identification deep water uptake?
How can we quantify deep root water uptake?
How can we reduce the uncertainty of current techniques? (e.g. mixing models)

Working Package II (WP II): UAV (Drone-)-based methods in ecohydrology

Research on deep roots has and continues to be struggle for simple reasons: It is extremely labour-intensive, methods are highly invasive and results are difficult to replicate. But maybe there exist above-ground indicators that can tell us if a particular plant has deep roots - or not? The center of WP II is to investigate, if such indicators exist.

Is it possible to identify above-ground indicators for deep rooting strategies of plants??

To provide a simple, comprehensive example, imagine a forest suffering water stress because of hydrological drought. Plants will close there stomata to avoid further water losses and hence, leaf temperatures will increase - unless a particular tree has access to water. This, on the other hand, would be reflected by (comparably) lower leaf temperatures. But not only temperature, also other indicators that can be derived from multi- or hyperspectral information, might be suitable as indicators for deep root water uptake - but which?

Drone_view_Namibia
Drone capture of a forest in northern Namibia in the framework of the SASSCAL project.

Second, sample collection from trees is often limited to the lower part of the tree which can lead to biased interpretations. It also hinders a holistic description of within-plant water transport processes. Therefore, another focal area of this working package is dedicated to the design and assembling of a remote-controlled drone that is capable of collecting samples from both leaves and branches at any desired position within the canopy.

Can drones be utilized for remote sample collection (leaves, branches) from trees? How can such sampling strategies improve the holistic description of within-plant water transport processes?

cutting devices
Some very preliminary ideas of how the cutting devices of the drones might look like

A third aspect that is addressed in WP II is the fact that for the estimation of ET and the application of ecohydrological models, a number of input parameters is required. Most field methods are highly work intensive and often provide data only on a point-scale (e.g. for a single tree). With the abovementioned approaches and a high-resolution camera, a number of parameters for ecohydrological modeling can be derived from systematic flight-overs of the study area.

How can drone-derived parameters assist the parameterization of (eco-)hydrological models?
How can water acquisition through deep roots be regionalized using drone-based measures?

Working Package III (WP III): Incorporating deep roots into (eco-)hydrological models

There is no generally applicable and easily transferable conceptual model accounting for water uptake through both shallow (i.e. the lateral root zone) and deep roots (i.e. through tap roots). Soil-vegetation-atmosphere transfer (SVAT) models commonly used for the simulation of the unsaturated zone (e.g. HYDRUS) often use a simplified representation of plants and the root zone is implemented using a root distribution for a single tree or a ‘mean’ rooting distribution. Consequently, the impact of deep root water uptake on the water balance is often neglected, difficult to represent spatially and error-prone. In WP III the information gathered in the other working packages will confluence and be used to develop a model which is capable of incorporating deep rooting in a spatially distinct manner. A key element will therefore be to account for water uptake through the lateral root zone () and deep water uptake (), respectively.

Upscaling (Regionalization) will be achieved by developing and applying separate models accounting for different functional plant groups (i.e. deep-water-using and non-deep-water-using, respectively). Based on the vegetation surveys (WP II) and the identification and quantification of deep-water-using species (WP I), the number of individuals per functional group can be used to determine the impact of and on the catchment scale and, subsequently, on the regional water balance.

How can water uptake through deep roots conceptually be described in order to allow a flexible treatment in models ?
How can deep roots and the co-existence of deep and shallow rooting plant species be incorporated into SVAT modelling?
How does this affect the water balance at a regional scale (upscaling)?

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