Dynamic niche partitioning in root water uptake facilitates efficient water use in more diverse grassland plant communities

Accepted Article

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may DR MARCUS GUDERLE (Orcid ID : 0000-0002-9653-7333)

DR CHRISTIANE ROSCHER (Orcid ID : 0000-0001-9301-7909)

Article type : Research Article Editor : Katie Field

Section: Ecosystems Ecology

Dynamic niche partitioning in root water uptake facilitates efficient water use in more diverse grassland plant communities

Marcus Guderle^a,b,c*, Dörte Bachmann^d, Alexandru Milcu^e,f Annette Gockele^g, Marcel Bechmann^a,c, Christine Fischer^a,h, Christiane Roscher^i,j, Damien Landais^e, Olivier Ravel^e, Sébastien Devidal^e, Jacques Roy^e, Arthur Gessler^k,l, Nina Buchmann^d, Alexandra Weigelt^,m,j and Anke Hildebrandt^a,c,j

a Friedrich-Schiller-University Jena, Institute of Geosciences, Burgweg 11, 07749, Jena, Germany

b Technische Universität München, Department of Ecology and Ecosystem Management, Hans-Carl-von-Carlowitz-Platz 2, 85354 Freising, Germany

c Max Planck Institute for Biogeochemistry, Biogeochemical Processes, POB 100164, 07701 Jena, Germany

d ETH Zurich, Institute of Agricultural Sciences, Universitaetsstrasse 2, 8092 Zurich, Switzerland

e CNRS, Ecotron - UPS 3248, Campus Baillarguet, 34980, Montferrier-sur-Lez, France

f Centre d’Ecologie Fonctionnelle et Evolutive, CEFE-CNRS, UMR 5175, Université de Montpellier –Université Paul Valéry – EPHE, 1919 route de Mende, 34293 Montpellier Cedex 5, France

g University of Freiburg, Faculty of Biology, Geobotany, Schaenzlestr. 1, 79104 Freiburg, Germany

h UFZ, Helmholtz Centre for Environmental Research, Department of Conservation Biology, Permoserstrasse 15, 04318 Leipzig, Germany

i UFZ, Helmholtz Centre for Environmental Research, Department of Physiological Diversity, Permoserstrasse 15, 04318 Leipzig, Germany

j German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, 04103 Leipzig, Germany

k Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland

lBerlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Altensteinstr. 6, 14195 Berlin, Germany

Accepted Article

m Systematic Botany and Functional Biodiversity, Institute of Biology, University of Leipzig, Johannisallee 21, 04103 Leipzig, Germany

* Corresponding author: mguderle@bgc-jena.mpg.de Department of Biogeochemical Processes

Max Planck Institute for Biogeochemistry Hans-Knöll-Str. 10

07745 Jena, Germany

Summary

1. Efficient extraction of soil water is essential for the productivity of plant communities. However, research on the complementary use of resources in mixed plant communities, and especially the impact of plant species richness on root water uptake, is limited. So far, these investigations have been hindered by a lack of methods allowing for the estimation of root water uptake profiles.

2. The overarching aim of our study was to determine whether diverse grassland plant communities in general exploit soil water more deeply and whether this shift occurs all the time or only during times of enhanced water demand.

3. Root water uptake was derived by analyzing the diurnal decrease of soil water content separately at each measurement depth, thus yielding root water uptake profiles for 12 experimental grasslands communities with two different levels of species richness (4 and 16 sown species). Additional measurements of leaf water potential, stomatal conductance, and root traits were used to identify differences in water relations between plant functional groups.

4. Although the vertical root distribution did not differ between diversity levels, root water uptake shifted towards deeper layers (30 cm and 60 cm) in more diverse plots during periods of high vapor pressure deficit. Our results indicate that the more diverse communities were able to adjust their root water uptake, resulting in increased

Accepted Article

water uptake per root area compared to less diverse communities (52% at 20 cm, 118% at 30 cm, and 570% at 60 cm depth) and a more even distribution of water uptake over depth. Tall herbs, which had lower leaf water potential and higher stomatal conductance in more diverse mixtures, contributed disproportionately to dynamic niche partitioning in root water uptake.

5. This study underpins the role of diversity in stabilizing ecosystem function and mitigating drought stress effects during future climate change scenarios. Furthermore, the results provide evidence that root water uptake is not solely controlled by root length density distribution in communities with high plant diversity but also by spatial shifts in water acquisition.

Key-words

Introduction

Root water uptake and evapotranspiration of ecosystems link the water and carbon cycles between soil and atmosphere (Feddes et al., 2001; Chapin et al., 2002; Feddes and Raats, 2004; Teuling et al., 2006; Schneider et al., 2009; Seniveratne et al., 2010; Asbjornsen et al., 2011). Consequently, soil water access is a key factor for ecosystem functioning and ecosystem services (Schwartz et al. 2000; Cardinale et al. 2012; Hooper et al. 2005).

Grasslands, which provide various ecosystem services, are strongly affected by climate change, especially due to increased drought stress (Barkaoui, Roumet & Volaire, 2016).

However, highly diverse grassland communities are able to mitigate drought stress effects and thus stabilize their ecosystem functions (Cardinale et al. 2012; Klaus et al. 2016; Knapp

Accepted Article

et al. 2002). For example, water stress of individual plant species alleviates if they are able to adjust their root systems to site-specific conditions, such as a shallow water table (Feddes et al. 2001). This also holds true for plant communities, where the diversity of plant species shape ecosystem evapotranspiration and root water uptake (Zea-Cabrera et al. 2006).

Until now, only a few studies have investigated patterns of root water uptake in plant species mixtures, and little is known about the effect of plant species richness and particular functional groups as well as their root traits on ecosystem evapotranspiration and root water uptake patterns. Among these, Van Peer et al. (2004) and Milcu et al. (2016) found enhanced ecosystem transpiration with increased plant species richness. In general, enhanced transpiration with species richness could be explained by higher leaf and thus transpiring area (Li et al. 2005; De Boeck et al. 2006), which, in these studies, coincides with higher plant diversity and biomass production (Van Peer et al. 2004; Roscher et al. 2005). In addition, enhanced transpiration could be explained by more efficient water extraction (Kulmatiski and Beard 2013), by which we mean the optimal acquisition of soil water within plant communities instead of the common ecophysiological concept of water use efficiency, which describes the productivity of plants depending on the volume of water that they use (Stanhill 1986; Milcu et al. 2014).

Furthermore, niche partitioning has been suggested as the basis for species complementarity in diverse communities (Hooper 1998; Loreau & Hector 2001; Schenk 2006) and for the positive relationship between biodiversity and ecosystem functioning (Cardinale et al. 2007;

Hooper et al. 2005). For example, plant communities containing species with different root distribution patterns might be able to occupy a larger niche space and thus can acquire more unexploited soil resources compared to communities containing species with similar root distributions (Berendse 1982; Mommer et al. 2010; De Kroon et al. 2012; Mueller et al.

2013). In this context, diversity in root traits such as root length density (RLD), maximum

Accepted Article

and minimum root depth, root foraging behavior and root system architecture possibly plays a key role in ecosystem functioning (Schenk 2006). Furthermore, research shows that the diversity of root traits is even more important than the taxonomic richness, as it allows accounting for different spatial belowground niches in diverse plant communities (Cody 1986; Silvertown 2004; Stubbs & Wilson 2004; Schenk 2006). Such niche differentiation in rooting patterns may result in community-specific root water uptake profiles, soil water fluxes, and storage, which are probably key to achieving more efficient water acquisition in diverse plant communities (Zea-Cabrera et al. 2006; Schwendenmann et al. 2014).

Investigations of belowground niche partitioning typically rely on assessing root length density at different depths, thus implying that root distribution reflects capacity for resource uptake. However, several studies show that root water uptake profiles may shift independently of root length distribution (Hamblin & Tennant 1987; Garrigues et al. 2006).

This is because root water uptake is also related to root hydraulic properties (Nimah & Hanks 1973; Kulmatiski et al. 2010; Kulmatiski & Beard 2013), soil salinity, soil water distribution (Dousson et al. 2006; Garrigues et al. 2006; van der Ploeg et al. 2008), the amount of fine roots, and the architecture of root systems (Nepstedt et al. 1994; Williams et al. 1998;

Schneider et al. 2009; Rosado et al. 2011; Javaux et al. 2013) as well as the transpiration of leaves (Jackson, Sperry & Dawson 2000; Steudle 2000; Quijano et al. 2012). For example, Rossatto, Sternberg and Franco (2012) found that herbs are able to adjust their water uptake in dry and wet seasons in a Neotropical savanna. Similarly, Volkmann et al. (2016) observed that tree species, i.e. oak (Quercus petraea Liebl.) and a mixed culture of oak and beech (Fagus sylvatica L.), had the ability to shift water uptake towards deeper soil layers when needed and proved that RLD alone does not control water uptake distribution. Thus, plant species identity (Hamblin & Tennant 1987), root architecture (Bechmann et al. 2014), and temporal and spatial plasticity in belowground resource use could be major drivers of niche

Accepted Article

partitioning or competition for soil resources (Callaway, Pennings & Richards 2003;

Hildebrandt & Eltahir, 2007, Ashton et al. 2010; Kulmatiski & Beard 2013) and independent approaches are warranted to assess differences in root water uptake between ecosystems.

However, how species-specific strategies interact to affect the community-level soil water uptake in species rich communities remains poorly understood. Although root water uptake profiles provide detailed information on resource uptake, plant growth, nutrient cycling, and species coexistence (Kulmatiski et al., 2017), the majority of studies measured only total water fluxes, and did not differentiate it according to depth. We address this knowledge gap by using a data-driven method to estimate root water uptake profiles, such as by combining measurements of short-term fluctuations (Guderle & Hildebrandt, 2015) with lysimeter- measured evapotranspiration.

The aim of our study was to investigate characteristics of root water uptake profiles in grassland plant communities with different species richness and productivity to gain a deeper understanding of how plant species richness affects efficient use of available soil water. We hypothesized that increasing plant species richness leads to (1) increased evapotranspiration rates due to increased transpiring area, and (2) spatial and temporal complementarity in root water uptake, such that plants acquire more unexploited soil resources to cover the enhanced water demand. Specifically, we investigated how communities with 4 and 16 grassland species extracted from a long-term biodiversity experiment (Jena Experiment; Roscher et al.

2004) differed in their water uptake profiles as estimated in a controlled environment facility for ecosystem research (the CNRS European Ecotron of Montpellier; Milcu et al 2014).

Accepted Article

Materials and methods

PLANT COMMUNITIES

The Jena Experiment is a long-term grassland biodiversity experiment located in the floodplain of the Saale River north of Jena, Germany (50° 57' 4.25" N, 11° 37' 28.52" E, 130 m a.s.l.; Roscher et al. 2004). The mean annual precipitation is 610 mm and the mean annual temperature is 9.9 °C (Hoffmann et al., 2014). The experiment was established in 2002 and the site was previously used as arable land for growing wheat and vegetables since the early 1960s (Roscher et al. 2004). The experiment consists of 82 large plots (20 x 20 m), which are arranged in four blocks according to variation in the soil texture (i.e. from sandy loam near the river to silty clay far from the river). The 82 plots are comprised of grassland communities with different plant species richness (1 to 60 species) and functional group richness (1 to 4 functional groups: grasses, tall and small herbs, and legumes) assembled randomly from a species pool typical for the local Arrhenatherion grasslands (Roscher et al., 2004).

Twelve of the plots were selected for an Ecotron study according to the following criteria: (1) including the four functional groups, (2) realised species numbers were close to sown species richness, and (3) plots were equally distributed across the experimental field site to account for different soil textures. The selected plots met the aforementioned criteria with the exception of one plot where no grasses had been sown. The soil-vegetation monoliths (2 m²surface area, diameter of 1.6 m, 2 m depth with a weight of 7 to 8 tonnes) were cut with a steel cylinder, excavated, turned upside down for sealing at the bottom, and returned to an upright position in December 2011. The cylinders were buried to the top edge in the experimental field, before being transported and installed in the lysimeter system of the Macrocosms platform of the Montpellier Ecotron, France, in March 2012, before the start of

Accepted Article

vegetation growth. This allowed for recovery from the disturbance of excavation such that the soil could settle within the steel cylinder in the same environment as the experimental field site (Milcu et al. 2014). For more information on the Ecotron facility please see Supplementary methods in the Appendix.

ESTIMATION OF EVAPOTRANSPIRATION AND ROOT WATER UPTAKE PROFILES Ecosystem evapotranspiration (ET) was measured from the lysimeter weight changes (later referred to as measured ET) to validate the ET estimated with a water balance method (later referred to as modelled ET). The weight measurements (6 minutes resolution) were smoothed using a moving average over 30 minutes to reduce noise due to the experimental setup (Milcu et al. 2016). The investigated period was constrained to those five days with reliable (noise- free) lysimeter weight records and without irrigation. A detailed description of these constraints is given in the Supplementary methods in the Appendix.

A water balance method was used to estimate daily root water uptake profiles and thus daily ecosystem ET (modelled ET) from diurnal fluctuation of soil water content measurements (Guderle & Hildebrandt, 2015). The method consists of applying a running regression over multiple time steps on soil water content time series at each measurement depth. Here we used measurements with a temporal resolution of 1 minute from 10 cm, 20 cm, 30 cm, and 60 cm depth (Figs. A2 and A3). During the day, evapotranspiration leads to a decrease of volumetric soil water content. This extraction of soil water extends over the entire active rooting depth. Additionally, soil water flow occurs both at night as well as during the day (Khalil et al., 2003; Verhoe et al., 2006; Chanzy et al., 2012), following water potential gradients in the soil profile. Thus, during dry weather conditions, the time series of soil water content shows a clear day-night signal. We split up the time series by fitting a linear function to each day and night branch of the time series in order to disentangle soil water flow and

Accepted Article

actual root water uptake. In a prior investigation we found the main transpiration time lasted from 5:30 am to 6:30 pm, so the onset of the day and night branches were fixed to these times. Nighttime transpiration was low (< 23 % of daytime transpiration) and therefore neglected (Milcu et al. 2016). Subsequently, the root water uptake profile was integrated over the entire soil profile to determine the modelled ET per one m surface and day. The modelled ET was furthermore multiplied by a factor of two in order to upscale the modelled ET to the surface of one lysimeter, which is two m .

The water balance method was validated by estimating the coefficient of determination (R ) of the regression between the measured ET (predictor variable) and the modelled ET (response variable) on 25th, 28th, and 29th June 2012. Since the modelled ET is calculated from the integrated root water uptake profile, good fit between the measured and modelled ET indicates that the predicted root water uptake profile represents the true profile, which was shown in a prior study by Guderle & Hildebrandt (2015). A detailed description and evaluation of the method can be found in Guderle & Hildebrandt (2015) and Guderle (2015).

The predicted root water uptake profiles were used to investigate whether differences in depth and quantity of water uptake between 4-species and 16-species mixtures occurred.

Furthermore, we compared the normalized root water uptake profiles and the normalized root length density profiles. This gives insight into dynamic niche partitioning in the examined ecosystems, which is indicated when root water uptake profiles differ from root length density profiles.

SOIL WATER CONTENT MEASUREMENTS

Soil water content measurements used for estimating root water uptake (RWU) were taken with time domain reflectometry (TDR) sensors TRIME®-PICO 32 (IMKO Micromodultechnik GmbH, Germany), which were placed horizontally at six soil depths into

Accepted Article

the soil monoliths (one sensor at 10 cm, 20 cm, 30 cm, and 60 cm, respectively). The used sensors had a rod length of 110 mm and a rod diameter of 3.5 mm. The total length of the sensor was 328 mm. The measurements were taken every minute from mid-June to the end of July. The sensors were installed with a minimum distance of 22 cm to other sensor installations, and the edge of the lysimeter steel wall. Before estimating root water uptake profiles, outliers (defined as differing ± 1.5 standard deviations from the mean of 30 measurements) of the soil water content measurements (1 minute resolution) were removed.

To reduce the noise of the TDR sensors output, we applied a third-order Savitzky-Golay finite-impulse response (FIR) smoothing filter over 240 minutes for 10 cm and 20 cm depths and a moving average over 720 minutes for 30 cm and 60 cm depths (Figs. A2 and A3). The larger window in deeper soil was necessary because daily soil moisture changes were small and the signal to noise ratio was correspondingly smaller, which required more smoothing to allow meaningful estimates of RWU at those depths (Guderle & Hildebrandt, 2015; Guderle 2015).

LEAF WATER POTENTIAL AND STOMATAL CONDUCTANCE

Leaf water potential was measured with a pressure chamber (Pressure Chamber Instruments Model 600, PMS Instrument Company, Albany, Oregon, USA) at predawn (04:00 am – 06:00 am) and midday (01:30 pm – 04:00 pm) for the most abundant species on each plot. Two to three species were chosen in the 4-species mixtures, and five to eight species in the 16- species mixtures (Table A1). All measurements were performed within three days (17^th to 19^th July 2012). Measurements were carried out on young, but fully expanded leaves from four individuals per species per plot. Stomatal conductance (gs, mmol m^-2 s^-1) was measured with a portable leaf porometer (SC-1 Leaf porometer, Decagon Devices, Pullman, USA) on three leaves from different individuals of all available species per plot (Table A1). Since the

Accepted Article

instrument’s reliable upper measurement limit is 1000 mmol m^-2 s^-1, measurements above this threshold were replaced with 1000 mmol m^-2 s^-1 as suggested in Caplan and Yeakley (2010).

Measurements were done in the auto mode using the first 30 s of stomatal conductance data to predict the final stomatal conductance under true steady state conditions. Values of leaf water potential and stomatal conductance of the individual measurements were averaged per species per plot.

MEASUREMENT OF VEGETATION STRUCTURAL VARIABLES

Community leaf area index (LAI)

Community leaf area index (LAI) was measured with a portable LAI-2000 plant canopy analyzer (LI-COR, Lincoln, USA) on 15^th July 2012. LAI was measured under diffuse light (evening) by taking one reference measurement above the canopy and five measurements near ground level within the center of each lysimeter. More detailed information on the LAI measurements can be found in Milcu et al. (2014).

Leaf area index per species (sLAI)

The leaf area index per species (sLAI, cm² cm^-2; Eq. 1) was estimated from leaf dry weight per soil surface (mdl; g cm^-2) and specific leaf area (SLA; cm² g^-1) of the respective species (Table A1).

(1)

More information on the estimation of SLA can be found in the Supplementary methods in the Appendix.

Accepted Article

Root length density (RLD), belowground biomass (BGBM) and root area index (RAI)

Three soil cores (diameter 3.5 cm) were sampled to 60 cm depth in each plot. Each core was divided into six layers (0-5, 5-10, 10-20, 20-30, 30-40, and 40-60 cm) such that the bottom of the soil cores from 5-10, 10-20, and 20-30 cm aligned with the depth of the TDR sensors. The respective layers were pooled per plot, and washed with tap water over a sieve (mesh size 200 µm) to separate roots from soil. Cleaned roots were weighed and a subsample of the fresh roots (approx. 2 g) was stored in 70 % ethanol before it was dyed (with neutral red solution) and scanned (Scanner Optical STD4800 Regent Instruments Inc. with top light unit and an image resolution of 600 dpi). Root length density (RLD) and root surface area were estimated with WinRhizo (Reg 2009c, Regents Instruments Inc.). The smooth off area of WinRhizo was set to 0.0001 and the length-width ratio to 3.0. In order to obtain RLD values for soil layers represented by the water balance method, the RLD values of 0-5 and 5-10 cm depths were summed, as were RLD values from 30-40 and 40-60 cm. Belowground biomass was estimated as described above but the samples were then dried at 65°C for three days.

The ratio between root area index (RAI) and LAI provides information on the ratio between transpiring and water acquiring areas (Sperry et al. 1998). Therefore, we calculated the RAI from measured root surface area Ar (cm² cm^-3) and thickness of the respective soil layers zi

(cm; Eq. 2):

, (2)

where i is the index of the soil layer.

Accepted Article

STATISTICAL ANALYSIS

We used the statistical software R 3.0.2 (R Development Core Team, http.//www.R- project.org) to perform linear mixed-effects model analysis using the R package lme4 (Pinheiro et al. 2014). First, we investigated the relationship between evapotranspiration as the response variable and the fixed effects of species richness, measurement day (Time), and their interactions (model 1; Table 1). Macrocosm (= plot) and replicates per day were defined as random effects. Second, we analysed the relationship between root water uptake (response variable) and the fixed effects of species richness, depth, measurement day (Time), and their interactions (model 2; Table 1). Again, plot and replicates per day were used as random effects. Third, we analysed the relationship between root water uptake in each single depth (response variable) and the fixed effects of species richness, measurement day, cover of tall herbs, and their interactions (plot and replicates per day as random effects; model 3; Table 2).

The statistical analysis for the effects of species richness on evapotranspiration and root water uptake was done separately for June and July to account for differences in environmental conditions (e.g. temperature, vapour pressure deficit, and irrigation). Furthermore, the separate analysis was conducted because the selected days in June coincided with periods selected by Milcu et al. (2014) and the days in July coincided with the measurements of leaf water potential, stomatal conductance, and root traits, which were done on these days. Fourth, we investigated the relationship between percentage of root water uptake and percentage of root length density (both as response variables) and depth and measurement day (Time; fixed effects; plot, replicates per day and depth as random effects; model 4; Table 3). Fifth, effects of species richness and functional group identity on leaf water potential and stomatal conductance (response variables, respectively) were tested using species richness (SR) and functional group identity (FG) as fixed effects and plot and species identity as random effects (model 5; Table A4). Differences between functional groups were identified with Tukey’s

Accepted Article

HSD tests using the function ghlt in the R package multcomp (Hothorn et al. 2008) based on models fitted with the restricted maximum likelihood method. To account for the dependency of evapotranspiration and root water uptake on measurement day and to exclude pseudoreplication due to measurements taken on the same day, the measurement day was considered a fixed effect and replicates per day as random effects. All fixed effects were added stepwise to a constant null model containing only the random effects (Roscher et al.

2014). Significant improvement of the model after adding the fixed effects was determined using likelihood ratio tests (χ² ratio). Effects of species richness on RLD, BGBM, RAI, and RAI:LAI ratio were tested using a one-way ANOVA, while a simple linear regression was used to analyse the relationship between leaf area index (LAI) and evapotranspiration (ET) and the relationship between root water uptake and root length density over depth.

Inspection of the residual plots did not indicate any obvious deviations from homoscedasticity and normality. Exceptions were root water uptake as the response variable in June (model 2) and stomatal conductance (model 5), which were log transformed in order to obtain homoscedasticity and normality for the linear mixed-effects model analysis in Table 1 and Table A4, respectively.

Results

PLANT SPECIES RICHNESS EFFECTS ON EVAPOTRANSPIRATION

In general, the modelled community evapotranspiration (ET) explained measured evapotranspiration very well (R² = 0.77, p < 0.001; Fig. A4), thus giving confidence in the ability of the model to derive root water uptake profiles.

On all investigated days, the measured ET of the 16-species mixtures was significantly higher than the ET of the 4-species mixtures (Fig. 1). On average, ET was 18% higher in June and 27% higher in July in the 16-species mixtures compared to the 4-species mixtures.

Accepted Article

LAI and ET (mean over all measurement days) were positively related (R = 0.73, p < 0.001;

Fig. 2), and the 16-species mixtures had significantly higher LAI than the 4-species mixtures (Fig. 3b).

PLANT SPECIES RICHNESS EFFECTS ON ROOT WATER UPTAKE PATTERNS

In both June and July root water uptake (RWU) decreased with depth (Table 1). The interaction between species richness and depth was significant in June, but not in July, indicating that uptake profiles were different between the 4-species and the 16-species mixtures in June (see also Fig 4). Separate analyses per depth showed that 16-species mixtures had a higher root water uptake at 20 cm depth (+86%) and 30 cm depth (+124%) compared to the 4-species mixtures in June (Table 2). In July when the vapor pressure deficit increased (Fig. A5), we found a higher root water uptake at 20 cm depth (+52%), 30 cm (+

118%) and 60 cm depth (+570%) in the 16-species compared to the 4-species mixtures (Table 2).

ROOT WATER UPTAKE DISTRIBUTION

Over 90% of the sampled root fractions in 4- and 16-species mixtures were fine roots (0-0.5 mm; Table A3). Depth distribution of root water uptake was clearly related to root length density (RLD) in the 4-species mixtures on 17^th and 18^th of July. This is also supported by a significant positive relationship between mean RWU and mean RLD over depth (R² = 0.907, p = 0.032; Table 3). Both RWU and RLD followed the same curve shape over the entire soil profile (Fig. 4a, c). On the contrary, the RWU profiles of the 16-species mixtures deviated significantly from RLD profiles (Fig. 4b, d, Table 3). Here, proportional RWU is substantially smaller than the proportion of RLD at 10 cm depth (about 32%), whereas it is considerably higher than the proportion of RLD in deeper layers (about 17% at 20 cm and 10% at 30 cm).

Accepted Article

This difference strengthened on 17^th July 2012, the day with the highest evapotranspiration among the considered days. Furthermore, root water uptake in the 16-species mixtures varied more between individual ecosystems, as evident from the broad range interval shown in Fig.

4b and 4d, which is also supported by the results of the fitted linear model (R = 0.626, p = 0.133). Interestingly, we found no significant difference in root length density distribution and belowground biomass distribution between the two diversity levels (Fig. A6, Table A2).

Root area index (RAI), a measure of root surface per area soil surface, was not significantly different between diversity levels (Fig. 3a, Table A2). In contrast, the RAI:LAI ratio was marginally significantly higher in 4-species mixtures than in 16-species mixtures (p = 0.084, Table A2), suggesting that the transpiring surface areas were higher in 16-species mixtures while the relative surface for water uptake, the RAI, was similar at both diversity levels.

Hence, uptake velocity (flux per unit root area) probably increased in 16-species mixtures.

Furthermore, we found a higher cover of tall herbs in 16-species mixtures than in 4-species mixtures as well as a significant effect of cover of tall herbs on root water uptake at 30 cm in June and in July (Table 2) for 16-species mixtures.

LEAF WATER POTENTIAL AND STOMATAL CONDUCTANCE OF FUNCTIONAL GROUPS

Plant species richness (SR) and functional group identity (FG) had no effect on measured predawn leaf water potential (ᴪp), but species richness effects varied among functional groups (significant interaction SR x FG, Table A4). Midday leaf water potential (ᴪm) differed among functional groups (Table A4). Grasses had lower ᴪm than the other three functional groups in both diversity levels (Fig. 5b). Tall herb ᴪm was greater in 4-species mixtures than in 16- species mixtures (p = 0.003), but ᴪm was not affected by diversity in the other functional groups (Fig. 5b).

Accepted Article

Stomatal conductance (gs) differed between functional groups (Table A4). Tall herbs had higher and grasses much lower stomatal conductance than the other two functional groups.

Tall herbs in more diverse mixtures had higher gs values than in less diverse ones (p = 0.002).

Stomatal conductance of small herbs, grasses and legumes did not differ between diversity levels.

Discussion

Briefly, our results indicate dynamic niche partitioning in diverse plots, likely involving primarily tall herbs, which decrease their leaf water potentials, thus maintaining high flow rates throughout the root system (Doussan et al., 2006), extract water from deeper soil layers, and contribute to enhanced transpiration.

EFFECTS OF SPECIES RICHNESS ON EVAPOTRANSPIRATION RATES

For all studied days we found an enhanced actual evapotranspiration for the 16-species mixtures compared to the 4-species mixtures (Table 4), similar to studies of Kreutziger (2006) and Van Peer et al. (2004), and in agreement with Milcu et al. (2016). An increase in transpiration with greater leaf area has been frequently reported (e.g. Eavis & Taylor 1979;

Irritz & Lindroth 1996; Rosset et al. 1997; Li et al. 2005; Li et al. 2006; Zhongmin et al.

2009; Milcu et al. 2016) and our study confirms this finding at two levels of diversity (Fig.

2). The strong correlation between LAI and evapotranspiration indicates that transpiration is a substantial portion of evapotranspiration, which corroborates the findings of Milcu et al.

(2016), who used the Shuttleworth and Wallace energy-partitioning model to distinguish transpiration and soil evaporation in the same experiment. The increased leaf area in 16- species mixtures and thus the higher evapotranspiration rates could be due to an increased productivity of diverse plant communities, which has been shown repeatedly in recent studies

Accepted Article

on biodiversity in ecosystems (Hooper 1998; Marquard et al. 2009; Roscher et al. 2012;

Cardinale et al. 2012; Naeem et al. 2012). In general, two main drivers are discussed: (1) complementarity, which is described as higher resource use efficiency in diverse mixtures, and (2) that diverse communities have an increased probability to contain particular species, which have an above-average effect on ecosystem processes (referred to as “sampling effect”;

Marquard et al. 2009; Roscher et al. 2011). Our results support previous research that both processes can act together (Marquard et al. 2009).

HIGHER EXPLOITATION OF DEEPER SOIL LAYERS IN MORE DIVERSE MIXTURES DUE TO DYNAMIC NICHE PARTITIONING

At higher levels of plant species diversity, we found significantly higher root water uptake at 20 cm and 30 cm depths in June and at 20 cm, 30 cm and 60 cm depths in July. This shift of root water uptake to deeper layers can be explained by the increased VPD in July, which leads to a higher evapotranspiration demand during this time, especially for the 16-species mixtures with their higher LAI values compared to the 4-species mixtures. Thus, the 16- species mixtures need to exploit soil water resources more efficiently to reduce water stress, and may increase spatial niche differentiation for water use in diverse mixtures. These results are in line with those of Verheyen et al. (2008), who suggested a more exhaustive exploitation of soil water in more diverse grassland communities. However, Bachmann et al.

(2015) found no indication of niche partitioning for water in the Jena Experiment. As will be discussed below, the different findings could be related to the time of measurement.

Remarkably, in our study root length density (RLD) was similar between the two diversity levels throughout most of the soil depths, and differed (slightly) only in the topmost soil (0-5 cm). The same applies for belowground biomass. Thus, there is no evidence for a spatial belowground niche differentiation with regard to root abundance within the two considered

Accepted Article

species-richness levels. This is in line with findings of Ravenek et al. (2014), who investigated the root biomass of the Jena Experiment (from which our lysimeter data originate) over nine years. Standing root biomass was sampled with soil cores of 4.8 cm and 8.7 cm diameter down to 40 cm in all 1-, 2-, 4-, 8-, and 16-species mixture plots (in total 76 plots per year) in 2003, 2004, 2006, 2008, and 2011. They found no difference in root distribution over depth dependent on particular functional groups, species, or functional group richness. It is commonly assumed that vertical root water uptake distribution is proportional to the distribution of root length density over soil depth (e.g. Perrochet 1987;

Prasad 1988; Jarvis 1989, Feddes et al. 2001). Niche differentiation could result from vertical differentiation of root systems between different species and/or functional groups (Berendse 1982; Brassard et al. 2011). However, root distribution may also be driven by other resources, in particular nutrient availability. Thus, one explanation for the similar RLD distribution between the two considered diversity levels could be the nutrient rich soil of the Jena Experiment’s site (Bessler et al. 2009, Ravenek et al. 2014).

However, although we found no spatial root niche differentiation, water uptake shifted towards deeper, less densely rooted soil layers. This finding suggests a dynamic (functional) niche partitioning (shift in uptake patterns) in more diverse plant communities. Similar dynamics were repeatedly observed on single plants (e.g. Lai & Katul 2000; Dousson et al.

2006; Garrigures et al. 2006; Rosado et al. 2011) and is in accordance with models of root water uptake (Schneider et al. 2009; Bechmann et al. 2014, Dousson et al. 2006). For instance, Dousson et al. (2006) showed that the contribution of different parts of the root system to total water uptake changes dynamically with time and depends on hydraulic properties and arrangement of the root system. Moreover, changes in soil moisture over time affect the root water uptake depth of different species differently (Volkmann et al. 2016).

Another mechanism could be the improvement of water uptake by arbuscular mycorrhizal

Accepted Article

fungi (AMF) due to direct water uptake and transport via the fungal hyphae from soil pores, which are not accessible to the plant roots themselves (Khalvati et al. 2005; Birhane et al.

2012). Mellado-Vázquez et al. (2016) found significantly higher concentrations of the neutral lipid fatty acid (NLFA) 16:1ω5N, which acts as a biomarker for the presence of AMF, in 16- species mixtures than in 4-species in this lysimeter experiment. Unfortunately, soil samples were only taken down to 10 cm depth, whereas we found an enhanced root water uptake at 20 cm, 30 cm and 60 cm depth. However, the decrease of this NLFA with depth was much lower than that of root biomass, which suggests that AMF were likely present in deeper soil layers as well.

On the other hand, our findings are in contrast with observations using stable isotopes, which found no evidence of enhanced niche partitioning in more diverse plant communities with regard to root water uptake in the Jena Experiment (Bachmann et al. 2015). This discrepancy may have several causes. First, our measurements were conducted during a period of high transpiration demand, whereas the tracer experiments of Bachmann et al. (2015) were performed at the beginning of the growing season and shortly after mowing in summer and autumn when leaf area was decreased and transpiration subdued. As will be discussed below, niche partitioning may be limited to periods of high transpiration only. Second, while providing a direct way of observing root uptake depth, isotopic signals could only be observed in the sap water of single plants and do not account for the abundances of species in a community, while our measurements were conducted at the whole-community level.

WHICH PROCESSES DRIVE THE DYNAMIC NICHE PARTITIONING IN ROOT WATER UPTAKE?

Overall, the ratios between root and leaf area are high (Table 4) compared to values from literature (Sperry at al. 1998; Tyree, Velez & Dalling 1998), indicating that the root system

Accepted Article

poses little limitation to water uptake, rather reflecting requirements of nutrient uptake (Boot

& Mensink 1991; Sperry et al. 1998). Although root area was similar in 16-species compared to 4-species mixtures, LAI was greater in the more diverse mixtures. With increased water uptake per root area in 16-species mixtures, xylem potentials at the root base and in leaves need to be more negative to maintain flow (Kalapos 1994; Hacke et al. 2000). Thus, the limiting factor for water uptake might be the higher water flux density in diverse mixtures rather than the absolute availability of soil water (Sperry et al. 1998).

Since root xylem and leaf water potentials are related, decreasing RAI:LAI ratio should affect leaf water potential. In general, the leaf water potential prescribes the potential gradients between leaves and soil, driving water uptake (Sala et al. 1981; Kalapos 1994; Sperry et al.

1998). It usually shows a daily pattern with a minimum in the afternoon where transpiration reaches its daily maximum (Sala et al. 1981). In our experiment, species of tall herbs showed particularly negative leaf water potentials and greater stomatal conductance in diverse plots (Table 4, Fig. 5). Furthermore, the LAI of tall herbs in 16-species mixtures was higher than in 4-species mixtures (Fig. A7), and the total cover of tall herbs in 16-species mixtures was higher than in 4-species mixtures. Additionally, we found a significant effect of cover of tall herbs on root water uptake at 30 cm. These results strongly suggest that tall herbs used their differentiated tap root systems (Fig. A8) to obtain a greater proportion of soil water from deeper layers even if the root distribution between functional groups as well as between different species mixtures was similar (Ravenek et al. 2014) and thus likely caused the recorded shift of root water uptake in 16-species mixtures. This might explain the wide range in the root water uptake patterns in the 16-species mixtures, because these plants adjust their uptake depth according to their demand within the community. These findings are strongly supported by Leimer et al. (2014), who also found low water content in deep soil related to the presence of tall herbs in the Jena Experiment. Furthermore, Marquard et al. (2009)

Accepted Article

reported a strong positive effect of tall herbs on community productivity in the Jena Experiment, which was attributed to both complementarity as well as “sampling effects”. The latter refers to the higher biomass expected of tall herbs, and the higher likelihood of presence of those species diverse communities, which by itself increases the expected biomass.

Complementarity refers to additional processes supported by tall herb presence, which increase the ecosystem productivity beyond effects expected from selection alone. As already discussed, increased biomass productivity leads to higher LAI and likely increases evapotranspiration and root water uptake. Thus, our results corroborate that both a sampling effect and complementarity act in concert.

In summary, the more diverse communities in this experiment consisted of more plants with the capacity to extend their root water uptake deeper into the soil, such that the water use of the entire community was more uniformly distributed over the depth profile (“sampling effect”). Presumably, plants with root systems structured in transport and uptake roots, like tall herbs, have an advantage because they can explore the soil water more dynamically.

Furthermore, the comparison between June and July showed that some species in more diverse plant communities could escape possible competition for water by exploiting deeper water resources, especially if the evapotranspiration demand is increased (complementarity).

The effect of AMF on water uptake in 16-species mixtures is likely secondary since we found a significant relationship only between the abundance of tall herbs and elevated root water uptake in deeper soil layers, but not for the other functional groups in the plant community.

WHICH ECOLOGICAL CONCLUSIONS CAN BE DRAWN FROM THESE FINDINGS?

Our results reveal that grassland communities with higher plant diversity have improved water uptake over the soil profile due to dynamic below ground niche partitioning. This

Accepted Article

ability of higher diversity grassland communities to mitigate drought stress and level off the related C cycling by photosynthesis and aboveground biomass production could stabilize their ecosystem functions during future climate change scenarios (Cardinale et al. 2012;

Klaus et al. 2016; Knapp et al. 2002). Thus, conservation of plant diversity and increasing the number of species in species-poor agricultural grasslands is an essential management strategy that needs to be put into practice (Schwartz et al. 2000; Klaus et al. 2016).

Conclusions

Our study suggests that the increased transpiration of diverse grassland plant communities, associated with higher productivity and LAI, was achieved by complementary root water uptake in the absence of corresponding complementarity of root distribution. This study underpins the idea that root water uptake is not solely characterized by root length distribution but is also affected by dynamic shifts in water uptake due to temporal changes in environmental conditions. A pre-requisite for such plasticity is the ability of specific plant species to explore deeper soil layers. This dynamic water uptake requires a low leaf water potential in combination with a particular plant root structure to transport the water from the soil to the atmosphere. In our experiment, such a shift in water uptake was enhanced in more diverse plant communities, likely evoked by specific plant species such as tall herbs, since their abundances were higher in the more diverse grassland plant communities. These facts corroborate the growing discussion advocating conserving plant diversity in natural and agricultural grasslands in order to maintain good ecosystem functioning and services, especially against the background of future climate change and related drought stress of grassland ecosystems. Haines-Young and Potschin (2009) highlighted that sustainable management strategies and policies are essential for maintaining ecosystem functions.

However, clear understanding of the influence of biodiversity on ecological processes is

Accepted Article

crucial.

Acknowledgements

Financial support through the ProExzellenz Initiative from the German federal state of Thuringia to the Friedrich Schiller University Jena within the research project AquaDiva@Jena for conducting the research is gratefully acknowledged. This work was also supported by the German Research Foundation (FOR1451). This study benefited from the CNRS human and technical resources allocated to the Ecotrons Research Infrastructure as well as from the state allocation ‘Investissement d’Avenir’ ANR-11-INBS-0001. The

‘Transnational Access’ of M.G. to the Ecotron was funded by the ExpeER I3 project (7th Framework Programme of the EC). We are thankful to the Ecotron team for running the experiment and providing data. M.G. was also supported by the International Max Planck Research School for Global Biogeochemical Cycles (IMPRS-gBGC). We thank the Handling Editor Katie Field, the Senior Editor Alan Knapp and the Assistant Editor Jennifer Meyer, for handling the manuscript as well as the two anonymous referees for their thoroughly discussion of the manuscript and their very helpful comments. We also thank Andrew Durso for text editing.

Authors' Contributions

MG analyzed the data and wrote the manuscript. AM coordinated the experiment. CR, DB, AG, NB and AW provided plant and root trait data. MG, DL and AH provided soil, evapotranspiration and root water uptake data. JR, DL, SD and OR provided Ecotron-related data and methods. All authors contributed substantially to revisions. The grant was written by the Jena Forschergruppe including JR, AH, AG and NB.

Accepted Article

Data Accessibility

Environmental data, soil water content, root water uptake, evapotranspiration, plant trait and root trait data of the Jena-Ecotron experiment are published at the Open Access Repository PANGAEA:

Guderle, Marcus; Milcu, Alexandru; Landais, Damien; Ravel, Olivier; Escape, Christophe;

Bachmann, Dörte; Gockele, Annette; Weigelt, Alexandra; Gessler, Arthur; Scherer-Lorenzen, Michael; Roscher, Christiane; Buchmann, Nina; Hildebrandt, Anke; Roy, Jacques: Collection of environmental data, soil water content, root water uptake, evapotranspiration, plant trait and root trait data of the Jena-Ecotron experiment. PANGAEA, Dataset:

https://doi.org/10.1594/PANGAEA.877687

References

Asbjornsen, H., Goldsmith, G.R., Alvarado-Barrientos, M.S., Rebel, K., Van Osch, F.P., Rietkerk, M., Chen, J., Gotsch, S., Tobon, C., Geissert, D.R., Gomez-Tagle, A., Vache, K. & Dawson, T.E.(2011) Ecohydrological advances and applications in plant-water relations research: a review. Journal of Plant Ecology, 4(1-2), 3-22, doi:10.1093/jpe/rtr005.

Ashton, I.W., Miller, A.E., Bowman, W.D. & Suding, K.N. (2010) Niche complementarity due to plasticity in resource use: plant partitioning of chemical N forms. Ecology, 91(11), 3252-3260.

Bachmann, D., Gockele,A., Raenek, J.M., Roscher, C., Strecker, T., Weigelt, A. &

Buchmann, N. (2015) No evidence of complementary water use along a plant species

Accepted Article

richness gradient in temperate experimental grasslands. PlosONE, 10(1), e0116367.

doi:10.1371/journal.pone.0116367.

Barkaoui, K., Roumet, C. & Volaire, F. (2016) Mean root trait more than root trait diversity determines drought resilience in native and cultivated Mediterranean grass mixtures.

Agriculture, Ecosystems and Environment, 231(122-132), doi:10.1016/j.agee.2016.06.035.

Bechmann, M., Schneider, C., Carminati, A., Vetterlein, D., Attinger, S. & Hildebrandt, A.

(2014) Parameterizing complex root water uptake models – the arrangement of root hydraulic properties within the root architecture affects dynamics and efficiency of root water uptake. Hydrol. Earth Syst. Sci. Discuss., 11, 757-805, doi:10.5194/hessd-11-757- 2014.

Berendse, F. (1982) Competition between plant-populations with different rooting depths. 3.

Field experiments. Oecologia, 53, 50-55.

Bessler, H., Temperton, V.M., Roscher, C., Buchmann, N., Schmid, B., Schulze, E.-D., Weisser, W.W. & Engels, C. (2009) Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs. Ecology, 90(6), 1520-1530.

Birhane, E., Sterck, F.J., Fetene, M., Bongers, F. & Kuyper, T.W. (2012) Arbuscular mycorrhizal fungi enhance photosynthesis, water use efficiency, and growth of frankincense seedlings under pulsed water availability conditions. Oecologia, 169, 895- 904, doi:10.1007/s00442-012-2258-3.

Accepted Article

Brassard, B.W., Chen, H.Y.H., Bergeron, Y. & Paré, D. (2011) Differences in fine root productivity between mixed- and single-species stands. Funct. Ecol., 25, 238–246, doi:

10.1111/j.1365-2435.2010.01769.x.

Boot, R.G.A. & Mensink, M. (1991) The influence of nitrogen availability on growth parameters of fast- and slow-growing perennial grasses. In: Atkinson, D. (ed.) Plant root growth: an ecological perspective, pp. 161-168. Blackwell Scientific Publications, Oxford.

Callaway, R.M., Pennings, S.C. & Richards, C.L. (2003) Phenotypic plasticity and interactions among plants. Ecology, 84, 1115-1128.

Caplan, J.S. & Yeakley, J.A. (2010) Water relations advantages for invasive Rubus armeniacus over two native rzderal congeners. Plant Ecol., 210, 169-179.

Cardinale, B.J., Wright, J.P., Cadotte, M.W., Carroll, I.T., Hector, A., Srivastava, D.S., Loreau, M. & Weis, J.J. (2007) Impact of plant diversity on biomass production increase through time because of species complementarity. Proceedings of the National Academy of Sciences (USA), 104, 18123-18128.

Cardinale, B.J., Duffy, J.E., Gonzalez, A., Hooper, D.U., Perrings, C., Venail, P., Narwani, A., Mace, G.M., Tilman, D., Wardle, D.A., Kinzig, A.P., Daily, G.C., Loreau, M., Grace, J.B., Larigauderie, A., Srivastava, D.S. & Naeem, S. (2012) Biodiversity loss and its impact on humanity. Nature, 486, 59–67 doi:10.1038/nature11148.

Chanzy, A., Gaudu, J.C. & Marloie, O. (2012) Correcting the temperature influence on soil capacitance sensors using diurnal temperature and water content cycles. Sensors (Basel, Switzerland), 12(7), 9773–90, doi:10.3390/s120709773.

Accepted Article

Chapin, F.S., Matson, P.A., Chapin, M.C., & Mooney, H.A. (2002) Principles of Terrestrial Ecosystem Ecology. New York: Springer, 472.

Cody, M.L. (1986) Structural niches in plant communities. Community Ecology (eds.J.

Diamond & T.J. Case), 381-405, Harper & Row Publishers, New York.

De Boeck, H. J., Lemmens, C., Bossuyt, H., Malchair, S., Carnol, M., Merckx, R., Nijs, I.

and Ceulemans, R. (2006) How do climate warming and plant species richness affect water use in experimental grasslands?. Plant and Soil, 288, 249-261.

Doussan, C., Pierret, A., Garrigues, E. & Pagès, L. (2006) Water uptake by plant roots: II - Modelling of water transfer in the soil root-system with explicit account of flow within the root system - Comparison with experiments. Plant and Soil, 283, 99-117, doi:10.1007/s11104-004-7904-z.

Eavis, B.W. & Taylor, H.M.: Transpiration of soybeans as related to leaf area, root length, and soil water content. Agron. J., 71(3), 441-445.

Feddes, R.A., Hoff, H., Bruen, M., Dawson, T., De Rosnay, P., Dirmeyer, P., Jackson, R.B., Kabat, P., Kleidon, A., Lilly, A. & Pitman, A.J. (2001) Modeling root water uptake in hydrological and climate models. Bulletin of the American Meteorological Society, 82, 2797–2809.

Feddes. R.A., and Raats, P.A.C. (2004) Parameterizing the soil-water-plant root system. In Feddes, R.A., de Rooij, G.H., and van Dam, J.C. Unsaturated-zone Modeling: Progress, Challenges and Applications. 95-141, Dordrecht. Kluwer Academic Publishers.

Accepted Article

Garrigues, E., Doussan, C. & Pierret, A.: Water Uptake by Plant Roots: I – Formation and propagation of a water extraction front in mature root systems as evidenced by 2D light transmission imaging. Plant and Soil, 283(1-2), 83–98, doi:10.1007/s11104-004-7903-0.

Guderle, M. (2015) Towards understanding belowground resources acquisition: Applying data driven methods for deriving root water uptake profiles in grasslands of different diversity. PhD Thesis, Friedrich Schiller University Jena, Jena.

Guderle, M. & Hildebrandt, A. (2015) Using measured soil water contents to estimate evapotranspiration and root water uptake profiles – a comparative study. Hydrol. Earth Syst. Sci. Discuss., 11, 10859-10902.

Guderle, Marcus; Milcu, Alexandru; Landais, Damien; Ravel, Olivier; Escape, Christophe;

Bachmann, Dörte; Gockele, Annette; Weigelt, Alexandra; Gessler, Arthur; Scherer- Lorenzen, Michael; Roscher, Christiane; Buchmann, Nina; Hildebrandt, Anke; Roy, Jacques: Collection of environmental data, soil water content, root water uptake, evapotranspiration, plant trait and root trait data of the Jena-Ecotron experiment.

PANGAEA, Dataset: https://doi.org/10.1594/PANGAEA.877687

Hacke, U.G., Sperry, J.S., Ewers, B.E., Ellsworth, D.S., Schäfer, K.V.R. & Oren, R. (2000) Influence of soil porosity on water use in Pinus taeda. Oecologia, 124, 495-505.

Haines-Young, R. & Potschin M. (2009) The links between biodiversity, ecosystem services and human well-being, in: Raffaelli, D. and C. Frid (eds.): Ecosystem Ecology: a new synthesis. BES Ecological Reviews Series, CUP, Cambridge.

Accepted Article

Hamblin, A. & Tennant, D. (1987) Root length density and water uptake in cereals and grain legumes: How well are they correlated?. Aust. J. Agric. Res., 38, 513-527.

Hildebrandt, A. & Eltahir, E.A.B. (2007) Ecohydrology of a seasonal cloud forest in Dhofar:

2. Role of clouds, soil type, and rooting depth in tree-grass competition. Water Resour.

Res., 43(11), 1–13, doi:10.1029/2006WR005262.

Hoffmann K, Bivour W, Früh B, Koßmann M, Voß P-H (2000) Klimauntersuchungen in Jena für die Anpassung an den Klimawandel und seine erwarteten Folgen. Deutscher Wetterdienst, Offenbach/Main

Hothorn, T., Bretz, F. & Westfall, P. (2008) Simultaneous inference in general parametric models. Biometrical Journal, 50, 346-363.

Hooper, D.U. (1998) The role of complementarity and competition in ecosystem responses to variation in plant diversity. Ecology, 79(2), 704-719.

Hooper, D.U., Chapin, III, F.S., Ewel, J.J., Hector, A., Inchausti, P., Lavorel, S., Lawton, J.H., Lodge, D.M., Loreau, M., Naeem, S., Schmid, B., Setälä, H., Symstad, A.J., Vandermeer, J. & Wardle, D.A. (2005) Effects of biodiversity on ecosystem functioning: a consensus of current knowledge. Ecological Monographs, 75(1), 3-35.

Jackson, R.B., Sperry, J.S. & Dawson, T.E. (2000) Root water uptake and transport: using physiological processes in global predictions. Trends in Plant Science, 5, 482-488.

Accepted Article

Jarvis, N.J. (1989) A simple empirical model of root water-uptake. Journal of Hydrology, 107, 57-72, doi:10.1006/0022-1694(89)90050-4.

Javaux, M., Couvreur, V., Vanderborgth, J. & Vereecken, H. (2013) Root water uptake: From three-dimensional biophysical processes to macroscopic modeling approaches. Vadose Zone J., doi:10.2136/vzj2013.02.0042.

Kalapos, T. (1994) Leaf water potential –leaf water deficit relationship for ten species of a semiarid grassland community. Plant and Soil, 160, 105-112.

Khalil, M., Sakai, M., Mizoguchi, M. & Miyazaki, T. (2003) Current and prospective applications of zero flux plane (ZFP) method. J. Jpn. Soc: Soil Physics, 95, 75–90.

Khalvati, M.A., Hu, Y., Mozafar, A. & Schmidhalter, U. (2005) Quantification of water uptake by arbuscular mycorrhizal hyphae and its significance for leaf growth, water relations, and gas exchange of barley subjected to drought stress. Plant Biol., 7, 706- 712, doi:10.1055/s-2005-872893.

Klaus, V.H., Hölzel, N., Prati, D., Schmitt, B., Schöning, I., Schrumpf, M., Solly, E.F., Hänsel, F., Fischer, M. & Kleinbecker, T. (2016) Plant diversity moderates drought stress in grasslands: Implications from a large real-world study on 13C natural abundances. Science of the Total Environment, 566-567, 215-222, doi:10.1016/j.scitotenv.2016.05.008.

Knapp, A.K., Fay, P.A., Blair, J.M., Collins, S.L., Smith, M.D., Carlisle, J.D., Harper, C.W., Danner,B.T., Lett, M.S. & McCarron, J.K. (2002) Rainfall variability, carbon cycling, and plant species diversity in a mesic grassland. Science, 298.

Accepted Article

Kreutziger, Y. (2006) Rückkopplungseffekte verschieden diverser Grünlandökosysteme auf die Komponenten des Bodenwasserhaushaltes an einem Auestandort der Saale – Ergebnisse des Jenaer Biodiversitätsexperiments, PhD Thesis, Friedrich-Schiller- Universität Jena, Jena.

Kulmatiski, A., Beard, K.H., Verweij, R.J.T. & February E.C. (2010) A depthcontrolled tracer technique measures vertical, horizontal and temporal patterns of water use by trees and grasses in a subtropical savanna. New Phytol, 188, 199–209.

doi:10.1111/j.1469-8137.2010.03338.x.

Kulmatiski, A. & Beard, K.H. (2013) Root niche partitioning among grasses, saplings, and trees

measured using a tracer technique. Oecologia, 171, 25-37, doi:10.1007/s00442-012- 2390-0.

Kulmatiski, A., Adler, P.B., Stark, J.M. and Tredennick, T. (2017) Water and nitrogen uptake are better associated with resource availability than root biomass. Ecosphere, 8(3):e01738.10.1002/ecs2.1738.

Kumar, R., Jat, M.K. & Shankar, V. (2013) Evaluation of modeling of water ecohydrologic dynamics in soil-root system. Ecological Modelling, 269, 51-60.

Lai, C.T. & Katul, G. (2000) The dynamic role of root-water uptake in coupling potential to actual transpiration. Advances in Water Resources, 23(4), 427–439, doi:10.1016/S0309- 1708(99)00023-8.

Accepted Article

Leimer, S., Kreutziger, Y., Rosenkranz, S., Beßler, H., Engels, C., Hildebrandt, A., Oelmann, Y., Weisser, W.W., Wirth, C. & Wilcke, W. (2014) Plant diversity effects on the water balance of an experimental grassland. Ecohydrology, 7, 1378-1391, doi:10.1002/eco.1464.

Li, S.G., Lai, C.T., Shimoda, S., Yokoyama, T., Higuchi, A. & Oikawa, T. (2005) Evapotranspiration from a wet temperate grassland and its sensitivity to microenvironmental variables. Hydrol. Process., 19, 517-532, doi:10.1002/hyp.5673.

Li, S.G., Eugster, W., Asanuma, J., Kotani, A., Davaa, G., Oyunbaatar, D. & Sugita, M.

(2006) Energy partitioning and its biophysical controls above a grazing steppe in central Mongolia. Agricultural and Forest Meteorology, 137, 89-106, doi:10.1016/j.agrformet.2006.03.010.

Loreau, M. & Hector, A. (2001) Partitioning selection and complementarity in biodiversity experiments. Nature, 412, 72-76.

Marquard, E., Weigelt, A., Temperton, V.M., Roscher, C., Schumacher, J., Buchmann, N., Fischer, M., Weisser, W.W. & Schmid, B. (2009) Plant species richness and functional composition drive overyielding in a six-year grassland experiment. Ecology, 90(12), 3290-3302.

Mellado-Vázquez, P.S., Lange, M., Bachmann, D., Gockele, A., Karlowsky, S., Milcu, A., Piel, C., Roscher, C., Roy, J. & Gleixner, G. (2016) Plant diversity generates enhanced

Accepted Article

soil microbial access to recently photosynthesized carbon in the rhizosphere. Soil Biology & Biochemistry, 94, 122-132, doi:10.1016/j.soilbio.2015.11.012.

Milcu, A., Roscher, C., Gessler, A., Bachmann, D., Gockele, A., Guderle, M., Landais, D., Piel, C.,

Escape, C., Devidal, S., Ravel, O., Buchmann, N., Gleixner, G., Hildebrandt, A. & Roy, J. (2014) Functional diversity of leaf nitrogen concentrations drives grassland carbon fluxes. Ecology Letters, 17, 435–444, doi:10.1111/ele.12243.

Milcu, A., Eugster, W., Bachmann, D., Guderle, M., Roscher, C., Gockele, A., Landais, D., Ravel, O., Gessler, A., Lange, M., Ebeling, A., Weisser, W.W., Roy, J., Hildebrandt, A.

& Buchmann, N. (2016) Plant functional diversity increases grassland productivity- related water vapor fluxes: an Ecotron and modeling approach. Ecology, doi:

10.1890/15-1110.1.

Millennium Ecosystem Assessment (2005) Ecosystems and Human Well-being: Biodiversity Synthesis,World Resources Institute, Washington, DC.

Mommer, L., van Ruijven, J., de Caluwe, H., Smit-Tiekstra, A.E., Wagemaker, C.A.M., Ouborg, N.J., Bögemann, G.M., van der Weerden, G.M., Berendse, F. & De Kroon, H.

(2010) Unveiling below-ground species abundance in a biodiversity experiment: a test of vertical niche differentiation among grassland species. Journal of Ecology, 98, 1117- 1127, doi: 10.1111/j.1365-2745.2010.01702.x.

Accepted Article

Mueller, K.E., Tilman, D., Fornara, D.A. & Hobbie, S.E. (2013) Root depth distribution and the diversity–productivity relationship in a long-term grassland experiment. Ecology, 94(4), 787-793.

Naeem, S., Duffy, J.E. & Zavaleta, E. (2012) The function of biological diversity in an age of extinction. Science, 336 (6087), 1401-1406, doi:10.1126/science.1215855.

Nepstedt, D.C., Carvalho, C., Davidson, E., Jipp, P.H., Lefebvre, P., Negreiros, G.H., Da Silva, E.D., Stone, T.A., Trumbore, S.E. & Vierira, S.A. (1994) The role of deep roots in the hydrological and carbon cycle of Amazonian forests and pastures. Nature, 327, 666- 669, doi:10.1038/372666a0.

Nimah, M.N. & Hanks, R.J. (1973) Model for estimating soil water, plant and atmospheric interrelations. I. Description and sensitivity. Soil Sci. Soc. Am. Proc., 37, 522-527.

Ogle, K., Wolpert, R.L. & Reynolds, J.F. (2004) Reconstructing plant root area and water uptake profiles. Ecology, 85(7), 1967-1978.

Perrochet, P. (1987) Water uptake by plant roots — A simulation model, I. Conceptual model. Journal of Hydrology, 95 (1-2), 55-61.

Pinheiro, J., Bates, D., DebRoy, S., Sarkar, D. & R Core Team (2014) nlme: Linear and Nonlinear Mixed Effects Models. R package version 3.1-118, http://CRAN.R- project.org/package=nlme.

Accepted Article

Prasad, R. (1988) A linear root water uptake model. Journal of Hydrology, 98, 297-306.

Quijano, J.C., Kumar, P., Drewry, D.T., Goldstein, A. & Misson, L. (2012) Competitive and mutualistic dependencies in multispecies vegetation dynamics enabled by hydraulic redistribution. Water Resour. Res., 48, W05518, doi:201210.1029/2011WR011416.

Ravenek, J.M., Bessler, H., Engels, C., Scherer-Lorenzen, M., Gessler, A., Gockele, A., De Luca, E., Temperton, V.M., Ebeling, A., Roscher, C., Schmid, B., Weisser, W.W., Wirth, C., De Kroon, H., Weigelt, A. & Mommer, L. (2014) Long-term study of root biomass in a biodiversity experiment reveals shifts in diversity effects over time. Oikos, 123, 1528–1536, doi: 10.1111/oik.01502.

Rosado, B.H.P., Martins, A.C., Colomeu, T.C., Oliveira, R.S., Joly, C.A. & Aidar, M.P.M.

(2011) Fine root biomass and root length density in a lowland and a montane rain forest, SP, Brazil. Biota Neotrop, 11(3), 203-209, http://dx.doi.org/10.1590/S1676- 06032011000300018.

Roscher, C., Schumacher, J., Baade, J., Wilcke, W., Gleixner, G. & Weisser, W.W. (2004) The role of biodiversity for element cycling and trophic interactions an experimental approach in a grassland community. Basic and Applied Ecology, 5, 107-121, doi:

10.1078/1439-1791-00216.

Roscher, C., Scherer-Lorenzen, M., Schumacher, J., Temperton, V.M., Buchmann, N. &

Schulze, E.D. (2011) Plant resource-use characteristics as predictors for species contribution to community biomass in experimental grasslands. Perspectives in Plant Ecology, Evolution and Systematics, 13, 1-13, doi:10.1016/j.ppees.2010.11.001.