BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-14-2293-2017Quantification of dynamic soil–vegetation feedbacks following an
isotopically labelled precipitation pulsePiaydaArndtarndt.piayda@thuenen.deDubbertMarenmaren.dubbert@cep.uni-freiburg.deSiegwolfRolfCuntzMatthiashttps://orcid.org/0000-0002-5966-1829WernerChristianehttps://orcid.org/0000-0002-7676-9057Thünen Institute of Climate-Smart Agriculture, 38116 Braunschweig, GermanyEcosystem Physiology, University Freiburg, 79110 Freiburg, GermanyLab for Atmospheric Chemistry, Ecosystems and Stable Isotope Research,
Paul Scherrer Institut, 5232 Villingen PSI, SwitzerlandUMR Ecologie et Ecophysiologie Forestières, UMR1137,
INRA-Université de Lorraine, Champenoux-54500 Vandoeuvre Les Nancy,
54280, FranceThese authors contributed equally to this work.Arndt Piayda (arndt.piayda@thuenen.de) and Maren Dubbert
(maren.dubbert@cep.uni-freiburg.de)5May20171492293230620October201631October201615March201723March2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/14/2293/2017/bg-14-2293-2017.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/14/2293/2017/bg-14-2293-2017.pdf
The presence of vegetation alters hydrological cycles of ecosystems. Complex
plant–soil interactions govern the fate of precipitation input and water
transitions through ecosystem compartments. Disentangling these interactions
is a major challenge in the field of ecohydrology and a pivotal foundation for
understanding the carbon cycle of semi-arid ecosystems. Stable water isotopes
can be used in this context as tracer to quantify water movement through
soil–vegetation–atmosphere interfaces.
The aim of this study is to disentangle vegetation effects on soil water
infiltration and distribution as well as dynamics of soil evaporation and
grassland water use in a Mediterranean cork oak woodland during dry
conditions. An irrigation experiment using δ18O labelled water was
carried out in order to quantify distinct effects of tree and herbaceous
vegetation on the infiltration and distribution of event water in the soil
profile. Dynamic responses of soil and herbaceous vegetation fluxes to
precipitation regarding event water use, water uptake depth plasticity, and
contribution to ecosystem soil evaporation and transpiration were quantified.
Total water loss to the atmosphere from bare soil was as high as from
vegetated soil, utilizing large amounts of unproductive evaporation for
transpiration, but infiltration rates decreased. No adjustments of main root
water uptake depth to changes in water availability could be observed during
the experiment. This forces understorey plants to compete with adjacent trees
for water in deeper soil layers at the onset of summer. Thus, understorey
plants are subjected to chronic water deficits faster, leading to premature
senescence at the onset of drought. Despite this water competition, the
presence of cork oak trees fosters infiltration and reduces
evapotranspirative water losses from the understorey and the soil, both due to
altered microclimatic conditions under crown shading. This study highlights
complex soil–plant–atmosphere and inter-species interactions controlling
rain pulse transitions through a typical Mediterranean savannah ecosystem,
disentangled by the use of stable water isotopes.
Introduction
Vegetation influences ecosystem water cycling in many ways. Rainfall is
intercepted while at the same time infiltration, redistribution and
translatory flow might be altered depending on rooting depths and soil
structure (Bhark and Small, 2003; Dawson, 1993; Devitt and Smith, 2002;
Dubbert et al., 2014c; Schwinning and Ehleringer, 2001; Tromble, 1988). For example,
a dense vegetation layer can strongly reduce soil evaporation (Dubbert et
al., 2014c; Wang et al., 2012). In turn, plant transpiration is controlled by
soil water availability and distribution, and plant species have different
abilities to use different soil water pools (i.e. surface vs. deep or ground
water). Large parts of ecosystem water losses by transpiration strongly
depend on plant functional types, stomatal regulation and leaf area index
(LAI). Although studies within the last decades have emphasized the
pivotal role of plant roots for soil water redistribution or the role of
plant transpiration in ecosystem water losses (Caldwell, 1987), it remains a
major challenge to quantify dynamic soil–vegetation–atmosphere feedbacks
within the water cycle.
Stable water isotopes are widely used to trace water transfers in soils,
through plants and at the soil–vegetation–atmosphere interface (Werner and
Dubbert, 2016; Yakir and Sternberg, 2000). Fractionation between the heavier
and lighter isotopes occurs during phase changes (from liquid to gaseous,
equilibrium fractionation) and movement (kinetic fractionation). This leads
to different stable isotope compositions (δ2H and δ18O)
in various water pools (i.e. rain, groundwater), along soil profiles, in
different plant species and between water vapour evaporated from soil
compared to water transpired by plants. These differences provide the basis
for tracing water through an ecosystem. The utilization of different water pools
within the soil by different plant individuals may be possible (Dawson, 1993;
Volkmann et al., 2016a). Isotopes can further help to separate transpiration
from soil evaporative fluxes (Dubbert et al., 2013; Yepez et al., 2003) or to
study infiltration or distribution of precipitation in soils (Garvelmann et
al., 2012; Rothfuss et al., 2015). Stable water isotopes have also been used
to study water movement at the soil–vegetation interface (Caldwell et al.,
1998). The isotopic composition of plant water uptake can be determined by
sampling the “output” of the root system, for example the plant xylem, because the water isotopic signatures are usually not altered by plant water
uptake (Dawson, 1993). Compared with values observed in the soil water
profile, the preferential plant extraction depth or the proportional use of
“event water” (i.e. singular precipitation events) can be determined.
Although this method has been successfully used to identify processes such as
hydraulic lift and soil water redistribution (Caldwell et al., 1998), most
datasets were limited in temporal and spatial resolution (Asbjornsen et al.,
2008; Kurz-Besson et al., 2006). Over the last decade, the development of
field-deployable laser spectroscopy has enabled continuous measurements of
water vapour and its isotopic signatures in ecosystem fluxes and atmospheric
concentrations. This opens the door for large-scale assessment of the
soil–vegetation–atmosphere interactions in the water cycle. In particular,
these developments have enhanced the spatial and temporal resolution
tremendously, furthering the understanding in the fields of plant
ecophysiology (Cernusak et al., 2016) and ecosystem physiology (Dubbert et
al., 2014a, c).
In the present study, we focus on disentangling the vegetation effects on
soil water infiltration and distribution as well as dynamics of soil
evaporation and grassland water use in a Mediterranean cork oak woodland. An
irrigation experiment with δ18O labelled water was carried out (1) to
quantify the distinct effects of tree and herbaceous vegetation on infiltration and distribution of event water (freshly introduced
water) in the soil profile and (2) to quantify the dynamic responses of soil
and herbaceous vegetation fluxes to precipitation regarding event water use,
plasticity of water uptake depth and contribution to ecosystem ET.
The following hypotheses were tested:
The presence of understorey vegetation increases evapotranspirative water
loss compared to bare soil but fosters infiltration due to shading.
The preferential root water uptake depth of understorey plants is
unaffected by changes in soil water availability after rain pulses during
drought.
Tree shading fosters the infiltration of event water and reduces
evapotranspiration generating favourable soil moisture conditions for
understorey plants.
Material and methodsStudy site and experimental design
Measurements were conducted between 26 May and 6 June 2012 in an open cork
oak woodland (Quercus suber L.) in central Portugal, approximately
100 km north-east of Lisbon (39∘8′17.84′′ N
8∘20′3.76′′ W; Herdade de Machoqueira do Grou). The trees are
widely spaced (209 individuals ha-1) with a leaf area index of 1.1 and
a gap probability of 0.7 (Piayda et al., 2015).
The herbaceous layer is dominated by native annual forbs and grasses. The
site is characterized by Mediterranean climate, with a 30-year long-term mean
annual temperature of approximately 15.9 ∘C and annual precipitation
of 680 mm (Instituto de Meteorologia, Lisbon). We established two sites: one
directly under the oak crown projected area (tree site, ts) and another one
in an adjacent open area (open site, os). Two types of plots (sized
40 × 80 cm) were installed at each site: bare-soil plots with total
exclusion of above- and below-ground biomass (lateral root ingrowth was
prevented by vertically inserted trenching meshes around the plots; mesh
diameter < 1 µm; Plastok, Birkenhead, UK) and understorey plots
with herbaceous vegetation (four plots per site and treatment). The sites
were kept vegetation free just by regular weeding. We expect no influence of
the mesh on infiltration, since the plots were installed 1 year before the
experiment and processes like preferential flow along the mesh are unlikely
(For further details see Dubbert et al., 2013).
After a baseline observation, all plots were watered with 20 mm of water
within 1 h using watering cans. The water showed an oxygen isotopic
signature of -139.5 ‰ to trace the influence of different
vegetation components on water infiltration. All plots and the surrounding
soil were watered equally to avoid lateral gradients and possible differences
between trenched and control plots. Thereafter, all measurements were
conducted in seven diurnal cycles over the following 10–12 days. The open and
tree sites were watered independently, as the measurement setup did not allow
highly resolved observations of all treatment plots at the same time.
Environmental variables (photosynthetic photon flux density – PPFD; soil water content – θ; vapour pressure deficit – vpd) were not
significantly different between the first and second half of the observation
period.
Environmental variables and plant parameters
PPFD was measured at both sites at
approximately 1.5 m height (PPFD, LI-190SB, LI-COR, Lincoln, USA). Rainfall
(ARG100 rain gauge, Campbell Scientific, Logan, UT, USA), air temperature and relative humidity (rH, CS-215 temperature and relative humidity probe,
Campbell Scientific, Logan, UT, USA) were measured and 30 min averages were
stored by a data logger (CR10x, Campbell Scientific, Logan, UT, USA). Soil
temperature (custom-built pt-100 elements) was measured at -5 cm depth on
vegetation and bare-soil plots at both sites, and 60 min averages were stored
in a data logger (CR1000, Campbell Scientific, Logan, UT, USA; four sensors per
depth and treatment). Temperature at the soil surface was manually measured
on each measurement day in diurnal cycles corresponding with the gas-exchange
measurements using temperature probes (GMH 2000, Greisinger electronic,
Regenstauf, Germany). Volumetric soil water content (θs,
10 h, Decagon, Washington, USA) was measured in 5, 15, 30 and 60
cm depth on vegetation and bare-soil plots at both sites, and 60 min averages
were stored in a data logger (CR1000, Campbell Scientific, Logan, UT, USA;
four sensors per depth and treatment).
Living above-ground biomass of herbaceous plants was determined destructively
on five randomly selected, 40 cm× 40 cm plots at the beginning and
end of the experiment in the open and under the trees. All green fresh
above-ground plant biomass was collected, divided by species, dried
(60 ∘C, 48 h) and weighed. Below-ground biomass was sampled with
soil cores in 5, 15, 30 and 60 cm depth. Oven-dried soil was
sieved and root biomass was determined gravimetrically. In total, 80 % of root
biomass was distributed between 5 and 15 cm depth. Only 5 % was
distributed above 5 cm and 15 % between 20 and 35 cm depth.
Total above-ground biomass was relatively low compared to previous years
(between 42 and 78 g m-2; see Fig. A1 in the Appendix) with a minimal
fraction of dry biomass due to the considerable winter–spring drought in the
hydrological year 2012 (Costa e Silva et al., 2015; Dubbert et al., 2014b;
Piayda et al., 2014). Dry biomass from the previous season was removed from
the plots at the end of summer 2011. While total above-ground biomass was
similar between plots, species composition and relative dominance differed, with the open sites being dominated by Tuberaria guttata and the
tree sites by grass and legume species (Dubbert et al., 2014b).
Cavity ring-down spectrometer based gas-exchange flux and δ18O measurements
Water fluxes and isotopic composition were measured with a wavelength scanned
cavity ring-down spectrometer (WS-CRDS; Picarro, Santa Clara, USA) in
combination with custom-built soil chambers (following the design of Pape et
al., 2009) in an open gas-exchange system (n=3 per treatment and
experimental site). Background and sampling air were measured alternately
after stable values were reached. A 5 min interval average was used
for the calculation of evapotranspiration (ET) and evaporation (E). ET and
E were calculated according to von Caemmerer and Farquhar (1981). Oxygen
isotope compositions of soil evaporation (bare-soil plots) as well as
evapotranspiration of the understorey (vegetation plots) were estimated using
a mass balance approach (Dubbert et al., 2013, 2014c):
δE=uoutwoutδout-uinwinδinuoutwout-uinwin,δE=woutδout-winδinwout-win-winwout(δout-δin)wout-win,
where u is the flow rate (mol(air) s-1), w is the mole fraction
(mol(H2O) mol(air)-1) and δ is isotope value of the
incoming (in) and outgoing (out) air stream of the chamber. Flow
rates are measured with humid air so that conservation of dry air gives
uin(1-win)=uout(1-wout), which
leads to Eq. (2). The second term in Eq. (2) corrects for the increased gas
flow in the chamber due to the addition of water by transpiration. In addition to
isotopic signatures of soil evaporation and understorey evapotranspiration,
the oxygen isotope signatures of ambient water vapour (in 9 m height) were
measured with the cavity ring-down spectrometer (CRDS). All measurements were conducted as diurnal courses
with five to six measurement points between 07:00 and
19:00 CET. For more details about the
chamber design and measurement setup see Dubbert et al. (2013).
Sampling and measurement of δ18O of soil and leaf
water
Soil samples for water extraction and δ18O analysis were taken on
vegetated and bare-soil plots using a soil corer. Samples were collected from
the soil surface (0–0.5 cm depth), at 2, 5, 10, 15, 20 and 40 cm soil depths (n=4 per depth and treatment), usually at midday,
but on the day of irrigation directly preceding the irrigation pulse and
additionally at 18:00. Mixed leaf samples of the herbaceous vegetation for
water extraction were obtained in daily cycles in 2-hourly steps from 08:00
to 18:00 following a destructive sampling scheme affecting the overall amount
of living biomass by less than 5 %. Thus, the effects of destructive sampling on
observed ET fluxes during the experiment are negligible. Soil and leaf water
samples were extracted on a custom-built vacuum line by cryogenic
distillation. Water δ18O analysis was performed by headspace
equilibration on an Isoprime IRMS (isotope ratio mass spectrometer; Elementar, Hanau, Germany) coupled via
an open split connection to a microgas autosampler (Elementar, Hanau,
Germany). Equilibration with 5 % He gas was done for 24 h at
20 ∘C. For every batch of 44 samples, three different laboratory
standards were analysed. Laboratory standards were regularly calibrated
against VSMOW (Vienna Standard Mean Ocean Water), SLAP (Standard Light Antarctic Precipitation) and GISP (Greenland Ice Sheet Precipitation) water standards (IAEA, Vienna). Analytical
precision was 0.1 ‰.
Partitioning of evapotranspiration
Oxygen isotope signatures of soil evaporation were calculated using the Craig
and Gordon equation (Craig and Gordon, 1965; Dubbert et al., 2013; Haverd and
Cuntz, 2010):
RE=1αkα+1-h(Re-α+hRa),
where RE is the isotope ratio (18O /16O) of evaporated
water vapour and Re is the isotope ratio of bulk soil water at the
evaporating sites. The evaporating site is the vapour–liquid interface below
which liquid transport and above which vapour transport is dominant (Braud et
al., 2005). It has been shown for unsaturated soils that this site is related
to a strong enrichment in soil water isotopic composition relative to the
rest of the soil column and an exponential depletion in isotopic signature
within few centimetres of the underlying soil due to evaporative enrichment of the
remaining liquid water (Dubbert et al., 2013; Haverd and Cuntz, 2010). Thus,
for Re and temperature at the evaporating sites (Te),
temperature and oxygen isotope signatures of bulk soil water were measured
along the soil profile and those values along the soil profile were used
where the strongest enrichment in bulk soil δ18O could be detected
(residual soil water volumetric content was only 1 % and therefore
neglected). Ra is the isotope ratio of ambient water vapour,
αk is the kinetic fractionation factor, α+ is the water
vapour equilibrium fractionation factor (αk and
α+> 1; see Majoube (1971); Merlivat (1978); for the
formulation of αk=αdiffnk, see Mathieu and Bariac,
1996). h is the relative humidity normalized to Te. RE can
then be transferred to delta notation as δ=RE-1⋅1000.
Although direct estimates of E and δ18OE were available for
bare-soil plots, vegetation depresses E and also influences δ18OE, for example due to different isotopic signatures of soil water
and also temperature in bare-soil and vegetated soil patches (Dubbert et al.,
2013). Therefore, bare-soil plots only served to validate the Craig and
Gordon equation because on bare-soil plots E contributes entirely to the
evaporative flux and could be tested against modelling results. Finally, the
Craig and Gordon equation was used to calculate δ18OE of
vegetation plots.
The oxygen isotope signature of transpired water vapour δ18OT
was calculated based on the isotopic signature of bulk leaf water
δ18OL using the Craig and Gordon equation (Eq. 3) instead of
measuring xylem or source water isotopic signatures and modelling
δ18OL of leaf water at the evaporating sites. This was done due to the lack
of suberized/lignified plant parts in the herbaceous vegetation. The isotopic
signature at the evaporating site δ18Oe was thus
estimated by
δ18Oe=δ18OL℘1-e-℘with the Péclet number℘=TLeffCD,
where Leff is the effective path length of water movement in the
leaf mesophyll, which we assumed to be 0.05 m, C is the molar water
concentration (55.6 × 103 mol m-3) and D is the tracer
diffusivity in liquid water (2.66 × 10-9 m2 s-1).
T was estimated iteratively with Eq. (5) using ET as an initial value.
Convergence was generally reached after five iterations. Small differences in
isotopic compositions were found compared to a direct use of δ18OL in Eq. (3), which were not significant for the results shown
in this work.
Finally, the contribution of T to ET, ft=T/ET, can be estimated
based on measured understorey δ18OET and modelled soil
δ18OE and herbaceous δ18OT (Moreira et al.,
1997; Yakir and Sternberg, 2000):
ft=δ18OET-δ18OEδ18OT-δ18OE.
This approach is based on the assumption that the isotopic signature of
evapotranspiration is a mixing ratio of not more than the two sources
(evaporation and transpiration) and that no water vapour is lost other than by
the mixing of the two sources with the atmospheric pool (i.e. no
condensation).
Event water partitioning
Event water describes the amount of water in ecosystem pools or fluxes that
originates from a certain rain event. To calculate the amount of event water
in volumetric soil water content θ that originates from the
isotopically labelled watering event, the following linear two-source mixing
model was used:
fθ,eve=δ18Oθ-δ18Oθ,preδ18Oeve-δ18Oθ,pre,
where fθ,eve is the fraction of rain event water in θ at a certain time after the event, δ18Oθ is the
stable isotope ratio in θ at a certain time after the event, δ18Oθ,pre is the stable isotope ratio of soil water
before the rain event and δ18Oeve is the stable isotope
ratio of the precipitation event water. The model assumes no fractionation of
rain event water during infiltration and was solved separately for each
depth. Contributions of infiltrated event water to evaporation fluxes from
soil and transpiration fluxes from plant surfaces were calculated
analogously:
fE,eve=δ18OE-δ18OE,preδ18OE,eve-δ18OE,pre,fT,eve=δ18OT-δ18OT,preδ18OT,eve-δ18OT,pre,
where fE,eve and fT,eve are the fractions of rain
event water in evaporation E and transpiration T.
δ18OE,pre and δ18OE,eve are the
isotopic compositions of evaporation calculated with Eq. (3), assuming that the source water isotopic composition equals either
δ18Oθ,pre at the evaporative site or
δ18Oeve. δ18OT,pre
and δ18OT,eve are the isotopic compositions of
transpiration calculated with Eqs. (3) and (4), assuming that the source water
isotopic composition equals either bulk leaf composition before watering
δ18OL,pre or δ18Oeve.
Root water uptake
The preferential depth of root water uptake by plants along the soil depth
was estimated via a linear three-source model. Therefore, the isotopic
composition of transpiration δ18OT calculated with Eqs. (3)
and (4) from three independent observations of leaf water compositions
δ18OL were compared with three independent solutions for
isotopic transpiration composition δ18OT of Eq. (3),
each assuming the current water source for transpiration originating only
from an observed depth (d1=-5 cm, d2=-15 cm, d3=-30 cm). Soil
depth above and below d1 to d3 showed negligible root density in the
profile and could therefore be excluded from the model. The three possible
source fluxes are related to the resulting transpiration flux mixture via the
following system of equations (compare, e.g., Philips et al., 2005):
δ18OT1=fT,d1⋅δ18OT1,d1+fT,d2⋅δ18OT1,d2+fT,d3⋅δ18OT1,d3+ε1,δ18OT2=fT,d1⋅δ18OT2,d1+fT,d2⋅δ18OT2,d2+fT,d3⋅δ18OT2,d3+ε2,δ18OT3=fT,d1⋅δ18OT3,d1+fT,d2⋅δ18OT3,d2+fT,d3⋅δ18OT3,d3+ε31=fT,d1+fT,d2+fT,d3,
where fT,d denotes the fraction of source water contribution from depths
d1 to d3 to the transpiration flux. The system was solved for fT,d1
to fT,d3 using a shuffled complex evolution algorithm (Duan et al.,
1992) minimizing a multi-objective cost function (Duckstein, 1981) combining
the error terms ε1 to ε3 for each time step.
Error propagation
All results are reported as replicate mean with associated standard error to
achieve comparability between different sample sizes. All model calculations
were applied to single replica and averaged afterwards. Observed effects
were considered statistically different when no overlap of standard errors
was observed.
ResultsEnvironmental and soil conditions
Tree cover significantly influenced diurnal courses of incoming global
radiation Rg during the daytime at the sites. Strong reductions in Rg between 09:00 and 18:00 reduced the daily sum of energy
input ∑Rg by 17.1 MJ m-2 d-1 at the open sites
(os) compared to the tree sites (ts) (Fig. 1). However, air temperature and
relative humidity were very similar in the open area and below trees with mean
values around 66 % and 19 ∘C throughout the experiment. Similar
to Rg, the amplitude of daily mean soil temperatures TS
in the upper soil layer was smaller at tree sites (bare: 7.4 ∘C; veg: 5.5 ∘C) than in the open area (14.9 and 11.3 ∘C
for bare and vegetated soils, respectively, Fig. 1). In contrast, understorey
vegetation cover reduced the soil temperature only by 2–3.6 ∘C at both sites.
Daily cycles, averaged over the experiment period, of
(a) global radiation Rg at 1.5 m height and
(b) soil temperature TS,5cm at 5 cm depth
under bare soil (bare) or vegetation cover (veg). Observations at open sites
between tree crowns (os) and shaded sites beneath tree crowns (ts) are shown.
Uncertainty bands display standard error.
Soil moisture θ prior to the irrigation pulse ranged from 5 to
10 % (Fig. 3), which is low compared to the annual average but typical
for the observation period at the end of May and the beginning of the dry
season. Systematically, lower soil moisture θ at depths below 20 cm
could be observed at the tree sites located close to trees compared to open
sites, whereas the upper soil layers showed comparable values for all sites
prior to the experiment.
Oxygen isotope signatures of ecosystem water pools
Stable oxygen isotope composition of soil water δ18OS for all
plots and all depths ranged between -7.3 and 10.1 ‰ before the
irrigation. Compared to the very depleted irrigation water signature of
-139.5 ‰, only a small enrichment in δ18OS of on
average 0.4 ‰ at the open sites compared to the tree sites was found and 2.9 ‰ enrichment of bare soil compared to vegetation plots
preliminary to the watering (Fig. 2). Irrigation caused a strong depletion of
δ18OS with a peak only 1 h after irrigation in the upper soil
layer. Strongest depletion of δ18OS values were found at tree
sites on bare-soil plots (δ18OS=-106.06 ‰) and
tree sites with vegetation cover (δ18OS=-85.1 ‰), whereas the open sites showed weaker maximum depletions of δ18OS=-79.9 ‰ and δ18OS=-49.4 ‰
on bare-soil and vegetation plots, respectively. The 9 days following the
irrigation event were characterized by a steady increase in
δ18OS, which was slightly depleted compared to pre-event
δ18OS 9 days after irrigation. In addition to the absolute
differences in peak δ18OS between sites, the depletion in
δ18OS was maintained for a longer period at tree sites
(Fig. 2).
Mean daily isotopic composition of soil water δ18OS
during experiment under bare soil (bare) or vegetation cover (veg) at open
sites between tree crowns (os) and shaded sites beneath tree crowns (ts).
Dashed lines mark time of watering event. Interpolation method: linear. The
standard error for soil isotopic composition during the experiment amounts on
average to 1.4 ‰ in natural abundance.
Mean daily soil water content θ along soil depth separated
into pre-event soil water content θpre and infiltrated event
soil water content θeve. Observations are displayed for plots
under bare soil (bare) or vegetation cover (veg) at open sites between tree
crowns (os) and shaded sites beneath tree crowns (ts). Numbers at the top mark
days since the watering event. Uncertainties for soil moisture observations
during the experiment amount on average to 2.3 % vol. propagated from the
observations. Event water partitioning for day 1 on open, vegetated plots
needed to be omitted due to inadequate field data quality.
Oxygen isotope signatures of soil evaporation and leaf water as well as
transpired water vapour (Fig. 4) showed an immediate response to the
irrigation pulse, with peak depletion only 1 h after labelling for soil
evaporation and 3 h for leaf water and transpired vapour. Subsequently, an
exponential rise to pre-event isotope values could be observed in all pools.
Depletion in δ18OE of soil evaporation was much stronger
compared to δ18OT of plant transpiration (and leaf water
δ18OL). δ18OE of soil evaporation and
evapotranspiration δ18OET were both more reduced at the
tree sites compared to the open sites. A similarly strong vegetation effect
could be seen between δ18OE on bare-soil plots in comparison
to understorey vegetation plots.
Mean daily isotopic composition of bulk leaf water δ18OL, soil evaporation δ18OE, plant transpiration
δ18OT and combined evapotranspiration δ18OET from bare-soil (bare) or vegetation plots (veg) at open
sites between tree crowns (os) and shaded sites beneath tree crowns (ts).
Filled dots represent observed values (obs); hollow dots represent modelled
values (mod). Dashed lines mark time of watering event. Uncertainty bars
display standard error.
Infiltration and distribution of event water
Daily mean soil moistures θ throughout the experiment were
characterized by the ongoing drought at all sites (Fig. 3). Watering the
plots with 20 mm increased mean daily soil moisture θ in the upper
layers only by 2 to 6% vol. and had no effect on deeper soil layers.
However, partitioning event water fractions revealed an extensive replacement
of old, pre-event water with new event water up to 4 % vol. and even
down to depths below -30 cm (Fig. 3), in particular on bare-soil plots.
Systematically increased infiltration and deepened distribution of event
water was observed at tree sites compared to open sites. In the course of the
experiment, soil moistures returned to pre-event values and below. The
decrease in event water was stronger than that of pre-event water, leaving
nearly no trace 9 days after the watering.
Event water use by plant transpiration
While pre-event E on bare-soil plots was lower than ET on vegetation plots
at both the open and tree sites, E and ET peaked equally with roughly
3.3 mmol m-2 d-1 at the open sites. However, at the tree sites
the post-event peak of E at bare-soil plots
(2.1 ± 0.1 mmol m-2 d-1) was higher than ET at vegetation
plots (1.5 ± 0.2 mmol m-2 d-1). Moreover, the peak of ET
at both sites was shifted by 24 h compared to E and occurred only 2 days
after irrigation (Fig. 5). Following peaks in E and ET, evapotranspiration
losses declined exponentially to pre-event values 3 days after irrigation on
all sites.
Mean daily flux rates of soil evaporation E, plant transpiration
T, and combined evapotranspiration ET from bare-soil (bare) or vegetation
plots (veg) at open sites between tree crowns (os) and shaded sites beneath
tree crowns (ts). Filled dots represent observed values (obs); hollow dots
represent modelled values (mod). Dashed lines mark time of watering event.
Uncertainty bars display standard error.
Partitioning ET on vegetation plots at both sites into soil E and plant
transpiration T revealed that the time shift of the response of the ET flux
compared to bare-soil plots E was caused solely by a slower reaction of T
to the irrigation pulse. Throughout the experiment the proportion of T to
ET ranged from 9 to 59 % at open sites and 17 to 66 % at shaded
sites.
Event water fraction in soil evaporation fE,eve and plant
transpiration fT,eve differed considerably with T utilizing
only a peak of 12 % of the event water while E is fed up to 62 % by
event water following irrigation (Fig. 6). Nine days after the irrigation
pulse, the event water contribution of T and E converged on average to
10 % of the respective flux and differences between fE,eve
and fT,eve faded. Event water lost by soil evaporation
fE,eve showed no significant differences between open and tree
sites nor between bare-soil plots and vegetated plots except on the day of
watering on the open vegetation plot. Here, fE,eve only reached
about 25 %, corresponding to the limited availability of event water in
the soil (Fig. 2). No significant differences could be observed between
fT,eve on open and vegetation plots.
Preferential root water uptake depth
Prior to the irrigation pulse we refrained from calculations of preferential
root water uptake depth, since the differences in δ18OS along
soil depth were too small (see above) for a sufficient, accurate prediction
power to solve the equation system (Eq. 9), and we derived significant fT,d.
Following the label pulse, soil water uptake by plants was located solely at
soil depths around 30 cm with no change in time or between open and tree
sites despite a small uptake of water for transpiration from soil layers
around -15 cm on day 0 and 1 after watering (Fig. 7).
Mean daily fractions f of event water (eve) and pre-event water
(pre) in soil evaporation E and plant transpiration T from bare-soil
(bare) or vegetation plots (veg) at open sites between tree crowns (os) and
shaded sites beneath tree crowns (ts). Numbers at the top mark days after
watering event. Uncertainty bars display standard error.
DiscussionInfiltration and distribution of event water
Mosaic patterns of vegetation cover by understorey plants and trees are
characteristic for savannah-type ecosystems (Belsky, 1994; Greig-Smith,
1979). Different vegetation cover is known to alter soil hydrological
conditions and microclimate (Scholes and Archer, 1997), which in turn have
effects on vegetation cover and ecosystem sustainability in future climate
change scenarios (Breman and Kessler, 1999; Pueyo et al., 2012). The infiltration
of event water into soil in this ecosystem is strongly altered by understorey
cover and tree shading. The vegetation cover of understorey plants reduced
infiltration on average by 24 % compared to bare soil (Fig. 3), which
clearly contradicts part two of hypothesis I. The reason can be found in
interception, subject to instantaneous plant and litter surface evaporation
before the first flux observations, which took place 1 h after watering.
This water uptake limitation could neither be compensated for by plant roots,
breaking the crust formations which can be observed in the field and are
common for Mediterranean soils and limiting the hydraulic conductivity of top soils
(Eldridge et al., 2010; Goldshleger et al., 2002; Maestre et al., 2002), nor
by beneficial shading effects by the above-ground biomass, which did not
significantly reduce the soil surface temperatures (Fig. 1) and thus the
evaporative demand of boundary layers. The observed infiltration on the day
of watering can further be regarded as insignificantly affected by understorey
root water uptake, which is confirmed by low transpiration fluxes on the day of
watering (Fig. 5). This is in contrast to previous studies, which reported
beneficial effects of plant cover on daily sums of infiltration during the
same period at the onset of drought in 2011 (Dubbert et al., 2014c). However,
Dubbert et al. (2014c) only observed precipitation events of light intensity
during the period of interest. The present study reports on high-intensity
precipitation events. Furthermore, above-ground vegetation cover and biomass
were reduced by 55 and 30 %, respectively, owing to the additional severe
winter–spring drought in 2012. It is thus likely that such a drastic
reduction in understorey canopy cover eliminates much of the beneficial
understorey effects on the ecosystem water balance. This unexpected turn in
effect direction with increasing precipitation intensity, which depends on
vegetation cover and atmospheric evapotranspirative demand, potentially plays
a strong role for the water balance of the ecosystem in the course of ongoing
climate change scenarios since the occurrence of extreme precipitation events
is expected to increase (IPCC, 2013).
Mean daily fractions of root water uptake fT,d of understorey
plants for modelled soil depths. Numbers at the top mark days after watering
event. Uncertainty bars display standard error.
Tree shading had a tremendous impact on the microclimate above understorey
plant and soil surfaces, but effects on infiltration amount could only be
observed on vegetated plots. Reductions in the daily sum of global radiation
∑Rg by 72 % and daily peak soil temperatures
TS,5cm up to 22 % (Fig. 1) generated favourable conditions.
Limited instantaneous evaporation from plant surfaces as described above led
to 71 % higher infiltration amounts (Fig. 3), whereas the high
infiltration amounts on bare-soil plots were unaffected by tree shading. This
confirms part one of hypothesis III on vegetated plots. Previous studies
reported similar, positive feedbacks of tree cover for the hydrological cycle
in savannah-type ecosystems related to shading effects (Eldridge and
Freudenberger, 2005). Effects of altered soil hydraulic properties beneath
tree crowns, like the amount of preferential flow fostering infiltration
(Bargués Tobella et al., 2014), could not be identified in this study.
Supporting findings are given by Bhark and Small (2003) and D'Odorico and
Porporato (2006). Considering the projected shading by crown cover of the
tree layer (minimum of 30 % at noon, increasing during the rest of the
day; Piayda et al., 2015), the infiltration enhancement has potentially large
benefits for the ecosystem level. A previous study of David et al. (2006) under
comparable climatic and stand density conditions estimated only minor
interception losses of 8 % with respect to total canopy throughfall due
to low canopy cover typical for cork oak systems. However, the integral
balance of canopy interception losses, increased infiltration and other
benefits of tree cover (compare Joffre and Rambal, 1993, and Dubbert et al.,
2014c) in this ecosystem could not be analysed in this study and need
further investigations with regard to tree density and age.
The subsurface distribution of soil water θ was systematically lower at
depths below 20 cm at tree sites compared to open sites (Fig. 3). This
clearly indicates the enhanced water extraction by tree roots, similar to the results of Dubbert et al. (2014b). The observed pattern could not be changed
by the event water pulse of 20 mm h-1, equal to a rain event of high
intensity at this site. That explains the intense drought stress understorey
plants suffer during the transition period from moist spring to dry summer,
leading to earlier dieback under tree cover (Dubbert et al., 2014b; Moreno,
2008), and it contradicts part two of hypothesis III. The depth distribution of
event water is very similar on bare-soil plots that show an overall deeper
infiltration of more water than the vegetated plots, caused by the higher
infiltration amounts shown before. This negative effect could partially be
compensated for by higher infiltration amounts below tree shading but the water
was consumed by tree water uptake from deeper depths within 1 day. During these
dry conditions, pre-event water is located in small pores under high matrix
potentials. Infiltrating event water partially displaced pre-event water
downwards (Fig. 3) and additionally filled larger pores in the top soil.
Thus, event water is more subject to evaporation due to lower matrix
potentials in bigger pores than pre-event water. This observation is
supported by a rapid decrease in event water content throughout the
experiment.
Dynamic responses of event water use and plasticity of water
uptake depth
The successful biomass production of herbaceous vegetation depends highly on soil
water availability in upper soil layers hosting the root system. Occasional
precipitation events control the soil water regime (Porporato et al., 2004),
and these are prone to substantial changes in future climate change scenarios
through stronger short-term fluctuations of drought events (IPCC, 2013). Thus, a
rapid adaptation of preferential root water uptake depth is crucial. This is
particularly important for herbaceous vegetation in order to maximize the
utilization of different soil water pools for a successful seed production,
longevity and inter-species competition (Ehleringer and Dawson, 1992;
Rodriguez-Iturbe, 2000). It could be clearly shown that understorey
transpiration T responded slower to an incoming precipitation pulse than
soil evaporation E, with a time lag of about 24 h. ET on vegetated plots
and E on bare-soil plots showed equally high peaks and a comparable decline
until the end of the experiment, providing no evidence for higher water
losses due to the presence of understorey and contradicting part one of
hypothesis I. During the entire experiment, E was the dominant flux on
both tree and open sites, with a comparable contribution of transpiration
T to evapotranspiration ET of 36 and 41 % (Fig. 5), respectively. This
small loss of transpiration water originates on one hand from the longer time
response lag of T and on the other hand from only little event water reaching
deeper soil layers, where understorey plants have their main root water uptake
depth. Event water use of the understorey vegetation was overall low, since no
shift of root water uptake depth could be observed within the 9 days of
the experiments (Fig. 7) leading to comparably small isotopic depletion of
bulk leaf water and transpiration (Fig. 4), which supports hypothesis II.
This is in agreement with previous findings where annual savannah species
were not readjusting their water extraction depth fast enough in order to
exploit precipitation water more efficiently (Asbjornsen et al., 2008;
Kulmatiski and Beard, 2013). More importantly, during that period of the year
the dry conditions in the upper soil layers forces understorey plants in the
direct vicinity of trees to compete for soil water at lower depths where the
trees have their roots (i.e. tree sites). This observation clearly
contradicts the widely discussed two-layer hypothesis, proposing independent ecological
niches for root water uptake of trees and understorey plants in savannahs in
order to avoid competition (Hipondoka et al., 2003; Holdo and Planque, 2013;
Kulmatiski et al., 2010; Walter et al., 1971). Moreover, exponential soil
profiles of plant available nitrogen cause a coupled water and nutrient
competition between herbs and trees in this ecosystem during spring (Dubbert
et al., 2014b). Modelling studies of, e.g., Nippert et al. (2015) already
suggested that understorey plants do not exploit all accessible soil layers
(including the top layers with high drought risk) in order to maximize water
availability. Lower but more resilient production is achieved instead by
limiting root growth and water uptake to deeper depths, which could be
confirmed by this study. Additionally, it has to be considered that the
herbaceous vegetation already reached its growth peak when the experiment was
conducted, and thus maximizing root water uptake might not be a priority for
the understorey community past the growth peak and during seed production.
Dubbert et al. (2014b) showed that the understorey community is strongly
adapted on a small spatial scale to the presence of oak trees regarding its
species composition and overall vegetation period length. This is also
observed in this study, with grasses dominating the understorey community
below the trees and forbs dominating in open areas (Fig. A1). Effectively
this leads to an earlier seed production and senescence of less drought-tolerant grasses in water competition with trees and a longer vegetation
period of drought-tolerant native forbs (i.e. Tuberaria guttata or
Tolpis barbata) in open areas. Consequently, while understorey
species in the open area remained a net sink for carbon during the entire
experiment, the understorey community below the trees was on the verge of
senescence and turned into a net source of carbon by the last experimental
date (Fig. A2), providing an additional explanation for the site-specific differences
in transpiration rate in response to event water (Fig. 5).
Recently, Volkmann et al. (2016a) used a similar flux or isotope approach to
test the widespread dogma that plant water uptake depth is primarily
controlled by root density distribution. While grassland species did not
strongly alter their uptake pattern during the measurement campaign, their
water uptake depth profile was not in accordance with their root density
distribution, with 85 % in the upper 10 cm of the soil profile. This
clearly indicates that adapting the water uptake to soil water availability
plays a role, but probably on longer timescales than what we observed during
the 10-day experiment. The development of membrane-based in situ
methods of soil water (Gaj et al., 2016; Rothfuss et al., 2015; Volkmann et
al., 2016a) and xylem sap sampling (Volkmann et al., 2016b) and transpiration
(Dubbert et al., 2014a, 2017) will advance the studies of
dynamic changes in ecohydrological soil–vegetation feedbacks in the future.
Furthermore, the coupling of isotope laser spectroscopes to gas-exchange
chambers and soil or xylem equilibration probes overcomes the costly and
time-consuming classical destructive sampling methods. Recent studies (Orlowski et
al., 2013) showed significant isotopic deviations between actual soil water
that is available for the plants and water that is cryogenically extracted
from soil samples depending on soil type. While we did not observe this in
sandy soils at our study site, these effects might severely hamper the
usefulness of destructive soil sampling techniques in clay or loam soils. The
newly developed in situ techniques will thus facilitate cost-effective
measurements of soil or xylem isotopic signatures with the highest resolution,
enhancing our capacity to study the dynamics of soil water infiltration, of
the uptake of water by plants and of the partitioning of evapotranspiration.
Conclusion
In this study, the various interactions between understorey vegetation and
trees of a Mediterranean cork oak woodland affecting the ecosystem water
flows could be quantified. The immediate on-site determination (with CRDS)
of the isotope ratios from different soil and ecosystem compartments in
combination with in situ sampling methods enhanced the resolution, precision
and reliability of our results. This facilitated the tracing of the fate of
rain pulse transitions through a typical Mediterranean savannah ecosystem
using stable water isotopes.
Regardless of the presence of vegetation, the total evapotranspirative water
loss of soil and understorey remains unchanged, but infiltration rates
decreased by 24 % (hypothesis I rejected). Still, the amount of
unproductive evaporation is largely reduced in favour of transpiration.
Adjustments of main root water uptake depth to changing soil water
availability after rain pulses could not be observed (hypothesis II
supported). Consequently, the understorey plants could not utilize the applied
precipitation of 20 mm. Hence, these understorey plants were forced into
water competition with trees, rooting at deeper soil layers. The crown shading of
cork oak trees altered microclimatic conditions, thus fostering infiltration
and considerably reducing understorey and soil evapotranspiration
(hypothesis III, part one supported). Despite these benefits, understorey
plants in the immediate vicinity of trees suffer from systematically lower soil
moistures in deeper layers leading to premature senescence at the onset of
drought (hypothesis III, part two rejected).
The underlying research data can be requested from the corresponding authors via email.
Above-ground biomass on vegetated plots during the experiment time
given for each genus. Standard errors are not given for the sake of clarity but amount on average to 30 % of displayed genus biomass.
Mean midday net ecosystem exchange (NEE) of the understorey
vegetation at the open site (white circles) and the tree site (dark grey
circles).
Arndt Piayda and Maren Dubbert contributed equally to experimental work,
data analysis and writing the paper. Matthias Cuntz contributed to data analysis, and he, Rolf Siegwolf and Christiane Werner read and provided feedback on the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
We gratefully acknowledge excellent help in the laboratory by
Ilse Thaufelder. Funding for this study was provided by the DFG grants to Christiane Werner and Matthias Cuntz (WATERFLUX Project: no. WE 2681/6-1, no. CU 173/2-1), as well as
Maren Dubbert (no. DU 1688/1-1). Edited by: X.
Wang Reviewed by: two anonymous referees
References
Asbjornsen, H., Shepherd, G., Helmers, M., and Mora, G.: Seasonal patterns in
depth of water uptake under contrasting annual and perennial systems in the
Corn Belt Region of the Midwestern US, Plant Soil, 308, 69–92, 2008.
Bargués Tobella, A., Reese, H., Almaw, A., and Bayala, J.: The effect of
trees on preferential flow and soil infiltrability in an agroforestry
parkland in semiarid Burkina Faso, Water Resour. Res., 50, 3342–3354,
2014.
Belsky, A. J.: Influences of Trees on Savanna Productivity: Tests of Shade,
Nutrients, and Tree-Grass Competition, Ecology, 75, 922–932, 1994.
Bhark, E. W. and Small, E. E.: Association between plant canopies and the
spatial patterns of infiltration in shrubland and grassland of the Chihuahuan
Desert, New Mexico, Ecosystems, 6, 185–196, 2003.Braud, I., Bariac, T., Gaudet, J. P., and Vauclin, M.: SiSPAT-Isotope, a
coupled heat, water and stable isotope (HDO and (H2O)-O-18) transport
model for bare soil. Part I. Model description and first verifications, J.
Hydrol., 309, 277–300, 2005.
Breman, H. and Kessler, J.-J.: Woody Plants in Agro-Ecosystems of Semi-Arid
Regions Advanced Series in Agricultural Sciences, Springer Berlin,
Heidelberg, 1999.
Caldwell, M. M.: Plant architecture and resource competition, edited by:
Schulze, E.-D. and Zwoelfer, H., Ecological Studies, 164–179, 1987.
Caldwell, M. M., Dawson, T. E., and Richards, J. H.: Hydraulic lift:
Consequences of water efflux from the roots of plants, Oecologia, 113,
151–161, 1998.
Cernusak, L. A., Barbour, M. M., Arndt, S. K., Cheesman, A. W., English, N. B.,
Feild, T. S., Helliker, B. R., Holloway, Phillips, M. M., Holtum, J., Kahmen, A.,
McInerney, F., Munksgaard, N. C., Simonin, K., Song, X., Stuart-Williams, H.,
West, J. B., and Farquhar, G. D.: Stable
isotopes in leaf water of terrestrial plants, Plant Cell Environ., 39, 1087–1102,
2016.
Costa e Silva, F., Correia, A., Piayda, A., Dubbert, M., Rebmann, C., Cuntz, M., Werner, C., David, J., and Pereira, J. S.: Effects
of extremely dry winter on net ecosystem carbon exchange and tree phenology
at a cork oak woodland, Agr. Forest Meteorol., 204, 48–57, 2015.
Craig, H. and Gordon, L. I.: Deuterium and oxygen-18 variations in the ocean
and the marine atmosphere. Paper presented at the Stable Isotopes in
Oceanographic Studies and Paleotemperatures, Spoleto, Italy, 1965.
David, T. S., Gash, J. H., Valente, F., Pereira, J. S., Ferreira, M. I., and David, J.: Rainfall
interception by an isolated evergreen oak tree in a Mediterranean savannah,
GHydrological Processes, 20, 2713–2736, 2006.
Dawson, T. E.: Hydraulic lift and water-use by plants – implications for
water-balance, performance and plant-plant interactions, Oecologia, 95,
565–574, 1993.
Devitt, D. A. and Smith, S. D.: Root channel macropores enhance downward
movement of water in a Mojave Desert ecosystem, J. Arid Environ., 50,
99–108, 2002.
D'Odorico, P. and Porporato, A.: Soil moisture dynamics in water-limited
ecosystems, in: Dryland Ecohydrology, edited by: D'Odorico, P. and Porporato, A., Springer Netherlands, 2006.
Duan, Q., Sorooshian, S., and Gupta, V.: Effective and efficient global
optimization for conceptual rainfall-funoff models, Water Resour. Res., 28,
1015–1031,
1992.
Dubbert, M., Cuntz, M., Piayda, A., Maguas, C., and Werner, C.: Partitioning
evapotranspiration – Testing the Craing and Gordon model with field
measurements of oxygen isotope ratios of evaporative fluxes, J. Hydrol.,
142–153, 2013.
Dubbert, M., Cuntz, M., Piayda, A., and Werner, C.: Oxygen isotope signatures
of transpired water vapor – the role of isotopic non-steady-state
transpiration under natural conditions, New Phytol., 203, 1242–1252, 2014a.
Dubbert, M., Mosena, A., Piayda, A., Cuntz, M., Correia, A. C., Perreira, J. S., and Werner, C.: Influence of
tree cover on herbaceous layer development and carbon and water fluxes in a
Portuguese cork oak woodland, Acta Oecol., 59, 35–45, 2014b.Dubbert, M., Piayda, A., Cuntz, M., Correia, A. C., Costa e Silva, F., Pereira, J. S., and Werner, C.: Stable oxygen
isotope and flux partitioning demonstrates understory of an oak savanna
contributes up to half of ecosystem carbon and water exchange, Frontiers in
Plant Science, 5, 530, 10.3389/fpls.2014.00530, 2014c.Dubbert, M., Kübert, A., and Werner, C.: Impact of Leaf Traits on
Temporal Dynamics of Transpired Oxygen Isotope Signatures and Its Impact on
Atmospheric Vapor, Frontiers in Plant Science, 8, 5, 10.3389/fpls.2017.00005, 2017.
Duckstein, L.: Multiobjective optimization in structural design: the model
choice problem, in: New Directions in Optimum Structural Design, edited by:
Atrek, E., Wiley, Chichester, 1981.
Ehleringer, J. R. and Dawson, T. E.: Water uptake by plants: perspectives
from stable isotope composition, Plant Cell Environ., 15, 1073–1082,
1992.
Eldridge, D. J. and Freudenberger, D.: Ecosystem wicks: Woodland trees
enhance water infiltration in a fragmented agricultural landscape in eastern
Australia, Australian Ecology, 30, 336–347, 2005.
Eldridge, D. J., Bowker, M. A., Maestre, F. T., Alonso, P., Mau, R. L., Papadopoulos, J., and Escudero, A.:
Interactive Effects of Three Ecosystem Engineers on Infiltration in a
Semi-Arid Mediterranean Grassland, Ecosystems, 13, 499–510, 2010.Gaj, M., Beyer, M., Koeniger, P., Wanke, H., Hamutoko, J., and Himmelsbach,
T.: In situ unsaturated zone water stable isotope (2H and 18O)
measurements in semi-arid environments: a soil water balance, Hydrol. Earth
Syst. Sci., 20, 715–731, 10.5194/hess-20-715-2016, 2016.Garvelmann, J., Külls, C., and Weiler, M.: A porewater-based stable
isotope approach for the investigation of subsurface hydrological processes,
Hydrol. Earth Syst. Sci., 16, 631–640, 10.5194/hess-16-631-2012, 2012.
Goldshleger, N., Ben-Dor, E., Benyamini, Y., Blumberg, D., and Agassi, M.:
Spectral properties and hydraulic conductance of soil crusts formed by
raindrop impact, Int. J. Remote Sens., 23, 3909–3920, 2002.
Greig-Smith, P.: Pattern in Vegetation, J. Ecol., 67, 755–779, 1979.
Haverd, V. and Cuntz, M.: Soil-Litter-Iso: A one-dimensional model for
coupled transport of heat, water and stable isotopes in soil with a litter
layer and root extraction, J. Hydrol., 388, 438–455, 2010.
Hipondoka, M., Aranibar, J., Chiara, C., Lihavha, M., and Macko, S.: Vertical
distribution of grass and tree roots in arid ecosystems of Southern Africa:
niche differentiation or competition?, J. Arid Environ., 54, 319–325,
2003.Holdo, R. M. and Planque, R.: Revisiting the Two-Layer Hypothesis:
Coexistence of Alternative Functional Rooting Strategies in Savannas, Plos
One, 8, e69625, 10.1371/journal.pone.0069625, 2013.
IPCC: Climate Change 2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Stocker, T. F., Qin, D.,
Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P. M., Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp., 2013.
Joffre, R. and Rambal, S.: How tree cover influences the water-balance of
mediterranean rangelands, Ecology, 74, 570–582, 1993.
Kulmatiski, A. and Beard, K. H.: Root niche partitioning among grasses,
saplings, and trees measured using a tracer technique, Oecologia, 171, 25–37,
2013.
Kulmatiski, A., Beard, K. H., Verqej, R. J. T., and February, E. C.: A
depth-controlled tracer technique measures vertical, horizontal and temporal
patterns of water use by trees and grasses in a subtropical savanna, New
Phytol., 188, 199–209, 2010.
Kurz-Besson, C., Otieno, D., do Vale, R. L., Siegwolf, R., Schmidt, M., Herd, A.,
Nogueira, C., David, T. S.,David, J. S., Tenhunen, J., Pereira, J. S., and Chaves, M.: Hydraulic
lift in cork oak trees in a savannah-type Mediterranean ecosystem and its
contribution to the local water balance, Plant Soil, 282, 361–378, 2006.
Maestre, F. T., Huesca, M., Zaady, E., Bautista, S., and Cortina, J.:
Infiltration, penetration resistance and microphytic crust composition in
contrasted microsites within a Mediterranean semi-arid steppe, Soil Biol.
Biochem., 34, 895–899, 2002.
Majoube, M.: Fractionation in O-18 Between Ice and Water Vapor, J. Chim.
Phys. PCB, 68, 1424–1436, 1971.
Mathieu, R. and Bariac, T.: A numerical model for the simulation of stable
isotope profiles in drying soils, J. Geophys. Res., 101, 12685–12696, 1996.Merlivat, L.: Molecular Diffusivities of (H2O)-O-16 HD16O, and
(H2O)-O-18 in Gases, J. Chem. Phys., 69, 2864–2871, 1978.
Moreira, M. Z., Sternberg, L. D. L., Martinelli, L. A., Victoria, R. L., Barbosa, E. M., Bonates, L. C. M., and Nepstad, D. C.:
Contribution of transpiration to forest ambient vapour based on isotopic
measurements, Glob. Change Biol., 3, 439–450, 1997.
Moreno, G.: Response of understorey forage to multiple tree effects in
Iberian dehesas, Agr. Ecosyst. Environ., 123, 239–244, 2008.
Nippert, J. B., Holdo, R. M. and Sayer, E.: Challenging the maximum rooting
depth paradigm in grasslands and savannas, Funct. Ecol., 29, 739–745,
2015.
Orlowski, N., Frede, H.-G., Brüggemann, N., and Breuer, L.: Validation
and application of a cryogenic vacuum extraction system for soil and plant
water extraction for isotope analysis, Journal of Sensors and Sensor Systems,
2, 179–193, 2013.Pape, L., Ammann, C., Nyfeler-Brunner, A., Spirig, C., Hens, K., and Meixner,
F. X.: An automated dynamic chamber system for surface exchange measurement
of non-reactive and reactive trace gases of grassland ecosystems,
Biogeosciences, 6, 405–429, 10.5194/bg-6-405-2009, 2009.
Philips, D. L., Newsome, S. D., and Gregg, J. W.: Combining sources in stable
isotope mixing models: alternative methods, Oecologia, 144, 520–527,
2005.Piayda, A., Dubbert, M., Rebmann, C., Kolle, O., Costa e Silva, F., Correia,
A., Pereira, J. S., Werner, C., and Cuntz, M.: Drought impact on carbon and
water cycling in a Mediterranean Quercus suber L. woodland during the extreme
drought event in 2012, Biogeosciences, 11, 7159–7178,
10.5194/bg-11-7159-2014, 2014.
Piayda, A., Dubbert, M., Werner, C., Correia, A. V., Pereira, J. S., and Cuntz, M.: Influence of
woody tissue and leaf clumping on vertically resolved leaf area index and
angular gap probability estimates, Forest Ecol. Manage., 340, 103–113,
2015.
Porporato, A., Daly, E., and Rodriguez-Iturbe, I.: Soil Water Balance and
Ecosystem Response to Climate Change, Am. Nat., 164, 625–632, 2004.
Pueyo, Y., Moret-Fernandez, D., Saiz, H., Bueno, C. G., and Alados, C. L.:
Relationships Between Plant Spatial Patterns, Water Infiltration Capacity,
and Plant Community Composition in Semi-arid Mediterranean Ecosystems Along
Stress Gradients, Ecosystems, 16, 452–466, 2012.
Rodriguez-Iturbe, I.: Ecohydrology: A hydrologic perspective of
climate-soil-vegetation dynamics, Water Resour. Res., 36, 3–9, 2000.Rothfuss, Y., Merz, S., Vanderborght, J., Hermes, N., Weuthen, A., Pohlmeier,
A., Vereecken, H., and Brüggemann, N.: Long-term and high-frequency
non-destructive monitoring of water stable isotope profiles in an evaporating
soil column, Hydrol. Earth Syst. Sci., 19, 4067–4080,
10.5194/hess-19-4067-2015, 2015.
Scholes, R. J. and Archer, S. R.: Tree-grass interactions in savannas, Annual Reviews of Ecology and Systematics, 28, 517–544,
1997.
Schwinning, S. and Ehleringer, J. R.: Water use trade-offs and optimal
adaptations to pulse-driven arid ecosystems, J. Ecol., 89, 464–480, 2001.
Tromble, J. M.: Water interception by 2 arid land shrubs, J. Arid Environ.,
15, 65–70, 1988.
Volkmann, T. H. M., Haberer, K., Gessler, A., and Weiler, M.: High-resolution
isotope measurements resolve rapid ecohydrological dynamics at the soil-plant
interface, New Phytol., 210, 839–849, 2016a.
Volkmann, T. H. M., Kühnhammer, K., Herbstritt, B., Gessler, A., and
Weiler, M.: A method for in situ monitoring of the isotope composition of
tree xylem water using laser spectroscopy, Plant Cell Environ., 39, 2055–2063,
2016b.
von Caemmerer, S. and Farquhar, G. D.: Some relationships between the
biochemistry of photosynthesis and the gas-exchange of leaves, Planta, 153,
376–387, 1981.
Walter, H., Burnett, J. H., and Mueller-Dombois, D.: Ecology of tropical and
subtropical vegetation, Oliver and Boyd, Edinburgh, 1971.Wang, L., D'Odorico, P., Evans, J. P., Eldridge, D. J., McCabe, M. F.,
Caylor, K. K., and King, E. G.: Dryland ecohydrology and climate change:
critical issues and technical advances, Hydrol. Earth Syst. Sci., 16,
2585–2603, 10.5194/hess-16-2585-2012, 2012.
Werner, C. and Dubbert, M.: Resolving rapid dynamics of soil-plant-atmosphere
interactions, New Phytol., 210, 767–769, 2016.
Yakir, D. and Sternberg, L. D. L.: The use of stable isotopes to study
ecosystem gas exchange, Oecologia, 123, 297–311, 2000.
Yepez, E. A., Williams, D. G., Scott, R. L., and Lin, G. H.: Partitioning
overstory and understory evapotranspiration in a semiarid savanna woodland
from the isotopic composition of water vapor, Agr. Forest Meteorol., 119,
53–68, 2003.