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REGULAR ARTICLE
Water acquisition, sharing and redistribution by roots:
applications to agroforestry systems
J. Bayala &I. Prieto
Received: 2 April 2019 /Accepted: 14 June 2019
#The Author(s) 2019
Abstract
Aims In the face of problems caused by ‘intensive agri-
culture’dominated by large areas of monocultures, mixed
intercropping mimicking natural ecosystems has been re-
ported to constitute a viable solution to increase and stabi-
lize productivity. When designing such systems, root niche
separation was thought to be a prerequisite to optimize
production.
Methods This paper reviews the beneficial and adverse
effects of trees and crops on water acquisition and redistri-
bution in agroforestry ecosystems using the concepts of
competition and facilitation between plants in link with
root functional traits.
Results The results of the review showed that the reality
was more complex leading agroforestry practitioners to
adopt management practices to induce a separation in root
activities thus avoid competition, particularly for water.
Water uptake by plant roots is triggered by the water
potential difference between the soil and the atmosphere
when leaf stomata are open and depends largely on the root
exploration capacity of the plant. Thus, root water uptake
dynamics are strongly related to root-length densities and
root surface areas. In addition, plants with deep roots are
able to lift up or redistribute water to the upper layers
through a process known as hydraulic lift, potentially
acting as “bioirrigators”to adjacent plants. The
redistributed water could be of importance not only in
regulating plant water status, e.g. by enhancing transpira-
tion,butalsoinincreasingthesurvivalandgrowthof
associated crops in mixed systems.
Conclusions Even though some more work is still needed
to assess the volume of water transferred to neighbors,
hydraulic lift could constitute an ecological viable mecha-
nism to buffer against droughts and ensure productivity in
regions with erratic rainfall. Giving the difficulty in mea-
suring the above-mentioned aspects in the field, modeling
of some of the most relevant parameters to quantify them
might inform the design of future empirical studies.
Keywords Ecosystem services .Hydraulic lift .Mixed
cropping .Productivity.Sustainability
Introduction
In the face of the world current challenges to sustain good
living conditions including increasing population numbers,
a changing climate or the degrading agro-ecosystems as-
sociated with a declining agricultural productivity, there is
a need for approaches that could ensure food security.
Indeed, ‘intensive agriculture’is dominated by large areas
of high-yielding monocultures, i.e. cultures using a single
https://doi.org/10.1007/s11104-019-04173-z
Responsible Editor: Remi Cardinael.
J. Bayala (*)
World Agroforestry (ICRAF), ICRAF-WCA/ Sahel Node, BP
E5118 Bamako, Mali
e-mail: j.bayala@cgiar.org
I. Prieto
Departamento de Conservación de Suelos y Agua, Centro de
Edafología y Biología Aplicada del Segura –Consejo Superior de
Investigaciones Científicas (CEBAS-CSIC), Murcia, Spain
/Published online: 26 June 2019
Plant Soil (2020) 453:17–28
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
species, that are resource-consuming and have led to many
environmental problems that intercropping could help to
address (Brooker et al. 2015; Machado 2009). For in-
stance, agroforestry or the combination of trees with agri-
cultural crops or pasture, was listed among the climate
smart technologies (Dinesh et al. 2017). This is due, in
large part, to the fact that intercropping has the potential to
promote an efficient use of two limiting factors for plant
growth, i.e. water and nutrients, by an improved acquisi-
tion of these resources through complementary root distri-
butions, water sharing mechanisms such as hydraulic re-
distribution (HR), common mycorrhizal networks, and/or
reduced runoff leading to increased water infiltration
(Bayala and Wallace 2015; Brooker et al. 2015;Mao
et al. 2012;Prietoetal.2012b). To take advantage and
promote the use of these mechanisms in agricultural sys-
tems there is a need for a careful design of the species
mixtures used (Altieri et al. 2015;Duprazetal.1997;
Malézieux et al. 2009; Ong and Leakey 1999;van
Noordwijk and Ong 1999). In agroforestry systems, annual
crops are typically shallow-rooted while perennial plants
include trees that have root systems extending deeper in
the soil giving foundation to the safety-net hypothesis
(Rowe et al. 1999; van Noordwijk and van de Geijn
1996; van Noordwijk et al. 2015). The safety-net hypoth-
esis, i.e. intercepting mobile nutrients leaching from crop
root zones, is also complementary to the nutrient pump
hypothesis which refers to the taking-up of nutrients, gen-
erally relatively immobile, from deep soil layers, their
incorporation in aboveground plant tissues (i.e. leaves)
and their transfer to upper soil layers through leaf shedding
and decomposition (Ong and Leakey 1999;van
Noordwijk and Ong 1999; Jobbagy and Jackson 2004).
However, these mechanisms can be far more complex and
dynamic (Akinnifesi et al. 2004; Brooksbank et al. 2011a;
Schroth 1998;vanNoordwijketal.2015) and depend on
seasonal shifts of water availability and nutrient extraction
from different soil depths (Bargués Tobella et al. 2017).
This complexity has prompted an increased interest of
the scientific community in agroforestry systems that
started with more descriptive work and then moved to
more complex studies trying to understand the mecha-
nisms behind resource sharing between intercropped plants
and the quantification of these processes for plant growth
(Black et al. 2015;vanNoordwijketal.2004). The fast
development of different tools and approaches in this field
have helped researchers to investigate plant root distribu-
tions and structure in these systems for a better understand-
ing of the interactions and processes involved
belowground (Cardinael et al. 2015;Ongetal.2014;
Tracy et al. 2015). Despite these advances, root research
in mixed tree-crop agroforestry systems remains challeng-
ing because of the difficulty in finding the relevant spatial
and temporal scales for thehigh heterogeneity of soil
conditions in these systems. Besides spatial differentiation
of root systems between plant species, some functional
traits linked to resource acquisition, including selective
root placement (SRP), root length density (RLD) or spe-
cific root length (SRL), are increasingly being used to
evaluate the competitiveness of plant species (e.g. acquis-
itive fast-growing versus conservative slow-growing strat-
egies) for the different resources in mixed systems
(Bardgett et al. 2014;Prietoetal.2015; Ravenek et al.
2016; Reich 2014). However, to date there is no consensus
on which are the most relevant traits that confer a true
advantage in belowground plant competition (Ravenek
et al. 2016).
Putting more efforts in these under-researched areas
will be pivotal for designing the best mixtures of tree and
crop species for agroforestry practices that provide benefi-
cial outputs both for the communities and the environment
(Altieri et al. 2015;Gutoaetal.2012; Malézieux et al.
2009). Based on the key concepts of interspecific compe-
tition and facilitation between plants in link with root
functional traits, this paper reviews the beneficial and
adverse effects of trees and crops on water acquisition
and redistribution in agroforestry ecosystems.
Water acquisition by roots
Sampling patterns to measure soil water contents in agro-
forestry systems vary according to the studied agroforestry
practice (transect, random, etc.) but taking soil water con-
tent measurements at various distances from the plant and
at different soil depths and comparing situations with
agroforestry practice against monoculture practices
(control) has been the most common approach (Bayala
and Wallace 2015; Ong et al. 2014;Fig.1). Measurement
methods have also undergone a tremendous development
from gravimetry measurements based on soil weighing to
automated and remotely controlled measurements using
wireless networks that allow a continuous monitoring of
the soil water status (Bogena et al. 2007; Tracy et al. 2015).
Even though the relationship between tree cover and water
supply is not straightforward, the tree components in ag-
roforestry practices are in general expected to reduce runoff
and thus increase soil water infiltration favoring the uptake
18 Plant Soil (2020) 453:17–28
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of water from deep soil layers by plant roots and a potential
recycling of this water in upper soil layers through water
redistribution (discussed in more detail in the next section).
Kuyah et al. (2016) reviewed the roles of trees on agrofor-
estry farms in semi-arid sub-Saharan Africa and found that,
in most cases, trees had beneficial effects on associated
crops by enhancing soil water availability in 58% of the
studies considered in the their review. These correspond to
situations in which a strategic placement of trees in the
landscape at an optimal density induces desired positive
effects on local hydrology (Brooksbank et al. 2011b; Siriri
et al. 2010,2012). For example, it has been reported that a
moderate tree cover could enhance groundwater recharge
(Ilstedt et al. 2016) because of increased infiltrability and
higher preferential flow due to the channels left by decayed
roots and an increased faunal activity (Bargués Tobella
et al. 2014; Ghestem et al. 2011). In the same site, it was
found that trees obtained 90% of their water from upper
soil layers during the rainy season and switched to taking
up 50% of their water from groundwater during the dry
season (Bargués Tobella et al. 2017; Roupsard et al. 1999).
That could partly explain why some tree species in dry-
lands can flower and fruit during the dry season, but this
remainsanassumptionthatneedstobeverifiedinfuture
studies. Thus, this source of blue water (water from rivers,
lakes and groundwater) being recycled as green water
(rainwater absorbed by the plants) may be playing an
important role in the overall performance of the agro-
ecosystems in these areas. Furthermore, it has been found
that an increase of tree cover beyond the optimum to
maintain a stable groundwater recharge, decreased infiltra-
tion and recharge but remained at higher levels than in
scenarios without any trees (Ilstedt et al. 2016). In addition,
tree roots help aggregate soil particles increasing soil sta-
bility and reducing runoff and increasing water infiltration.
This effect is due to tree root exudates that could explain
approximately 20%–75% of the variation on the anti-
erodibility of soils (AES) according to Wang et al.
(2017). Exudates and decayed roots are also a source of
carbon to the soil which, together with root mass densities,
were reported to influence the formation of soil aggregates
and increase their stability (Kalhoro et al. 2017;Le
Bissonnais et al. 2018).
The roots of plants have several functions such as
anchorage, storage (carbohydrates), resource acquisition
(water and nutrients), and communication (cytokinins,
gibberellins and other growth regulators) but most of the
controlling factors of these functions are still not fully
known (Hopmans and Bristow 2002;Ryanetal.2016).
Information on the root distribution of tree species is
mostly concentrated on the total root mass, that generally
includes both fine (< 2 mm) and coarse roots (> 2 mm)
while it is the finest part of the root systems (i.e. the finest
roots and lower root orders) that are most dynamic and
most actively involved in water and nutrient uptake and
hence of greater interest in agroforestry (van Noordwijk
and van de Geijn 1996; Wajja-Musukwe et al. 2008).
Water uptake by the roots is triggered by a demand of this
resource from the leaves creating a tension that propagates
down to the roots and induces an inflow (or outflow in the
case of redistribution) of water from the rhizosphere fol-
lowing paths of low soil hydraulic resistance (Lobet et al.
2014;McElroneetal.2013; Steudle 2001). Thus, root
uptake dynamics are usually related to root-length densities
and root surface area (Lobet et al. 2014;vanNoordwijk
1993), even if we do not account for the fact that uptake is
also controlled by root age and other mechanisms taking
place within the roots and in their surrounding rhizosphere
(mucilage exuded by roots, the gradient in soil water
potential around the roots, changes in hydraulic
conductivities along the roots, etc., Carminati et al. 2016;
Hopmans and Bristow 2002).
In agroforestry systems crops take up water mostly
from the upper soil whereas trees may display a more
opportunistic strategy obtaining most of its water from
the upper soil during the wet season and switching to water
stored deeper in the soil, e.g. groundwater, during the dry
season when upper soil layers are dry (Bargués Tobella
et al. 2017). Many studies have revealed that the root
distribution of most tree species in agroforestry systems
overlaps in the upper soil layers with the shallower root
systems of associated annual plants (Schroth 1995;Smith
et al. 1999). This root distribution allows accumulating a
high proportion of their fine roots in upper soil layers and
gives the plant access to intermittent soil moisture and soil
nutrients in topsoil layers while the primary root grows
deeper to facilitate extracting soil water from deeper wetter
layers (Dhyani et al. 1990). Such accumulation of fine
roots in the upper soil layers may generate a competition
between the tree and associated annual crops for water
(and nutrients) since root placement is a strategy for com-
petitiveness (Ravenek et al. 2016). Despite this, selecting
tree and crop species and provenances according to root-
related criteria with deeper roots for agroforestry systems
(Schroth 1995) is rarely done (Rowe et al. 1999;Smith
et al. 1999) but a few studies have found separate niches
for tree and crop absorptive root systems in response to
agricultural practices with tree roots developing below the
Plant Soil (2020) 453:17–28 19
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
crop rooting depth (Akinnifesi et al. 1999; Dupraz et al.
2005;Howardetal.1997;Roweetal.1999;Ruhigwa
et al. 1992; Singh 1994). A good option may therefore be
to manipulate tree root systems to maximize benefit while
minimizing competition with crop species (Bayala et al.
2013; Cardinael et al. 2015; Hou et al. 2003; Siriri et al.
2013), which implies reducing niche overlap by limiting
tree root growth in the crop rooting zone (Bayala et al.
2013;Namirembe1999). Tree pruning has been demon-
strated to reduce root growth in shallow layers but requires
tedious work and labor and is not a desirable practice for
carbon sequestration and nutrient cycling (Schroth 1995;
Smith et al. 1999; Wajja-Musukwe et al. 2008). Another
possibility would be to manipulate the tree crown because
roots do not act as simple pumps and their activity is
regulated by the demand in the aboveground parts of the
plant (Jeschke and Hartung 2000; Jones et al. 1998). For
instance, Kizito et al. (2012) showed that a partial coppice
of the canopy of Guiera senegalensis reduced daytime
sap-flow rates, i.e. plant transpiration. Plant water uptake
is determined by a combination of soil water availability
and potential transpiration by the canopy (Hoad et al.
2001). There is emerging evidence pointing to crown
pruning as an effective tool to manipulate tree root dy-
namics of shallow-rooted tree species (Bayala et al. 2002,
2004; Bazié et al. 2012; Jackson et al. 2000;Jonesetal.
1998;Siririetal.2013). All these studies revealed a
decrease in tree root densities in shallow soil because of
the forced reduction of the tree aboveground demand.
Indeed, Jones et al. (1998) observed a marked reduction
in root length densities of Prosopis juliflora trees 60 days
after pruning, a reduction that was more pronounced in
upper soil layers. This reduced competition from tree roots
in shallow soil layers allowed for a better development of
crop roots and associated sorghum plants developed
higher root densities under pruned P. juliflora trees even
though this occurred only in the uppermost shallow layers
(10–20 cm depth). Another effect of tree crown pruning is
that a reduction or removal of the tree crown will lead to
an increase in soil temperature and a reduction of soil
water in the upper soil layers around the tree (Bayala et al.
2004)thatwillaffectfinerootgrowthanddevelopment
since fine roots are highly sensitive to changes in soil
temperature and moisture. Because root functioning is
also under the control of aboveground parts of the plant
(Friend et al. 1990;Hoadetal.2001; Jeschke and Hartung
2000; Steudle 2000), crown reduction will also lead to a
decrease in water and nutrient demand and consequently a
reduction of fine roots in shallow soil layers. Water and to
some extent nutrients are the most limiting factors in semi-
arid areas. If, because of crown pruning, less water and
nutrients are taken up by trees from the topsoil layer, this
means greater availability of these resources for the asso-
ciated crops, at least in the short term. Consequently, this
may lead to more space for crop roots and thus higher
crop root density and ultimately to better crop production
under pruned trees (Bayala et al. 2004). A decrease in the
production of tree fine roots in upper soil layers and their
increase in deeper soil layers may not only reduce tree/
crop competition and improve crop production but also
help improve the benefits of the “safety-net”hypothesis
described by Cadisch et al. (1997), Rowe et al. (1999)and
Vanlauwe et al. (2002). This hypothesis postulates that
deep rooting trees can utilize more efficiently leached
Description of
agroforestry systems
Process/mechanisms of
resources s haring
Sampling patterns for
water measurement
approaches: with and
without
Measurements
methods: gravimetry to
automated remotely
controlled wireless
networks
Root
distribution/structure/f
unctional traits
Soil layers of water
uptake by trees and
associated crops
Use isotopes for
dynamic partitioning of
ecosystem fluxes and
identification of water
sources and inks in soil-
plant-atmosphere
continuum: safety net
hypothesi s
Hydraulic lift
magnitude, ecological
importance and
volume of water
transferred to the
associated crops
Manipulate tree root
systems to maximize
benefits and minimize
competition: tree
placement and density,
crown and root pruning
Modelling of relevant
root paramete rs to
inform future field-
based empirical studies
on water acquisition
and sharing
Fig. 1 Current knowledge on
water acquisition and sharing and
their potential applications in
agroforestry systems
20 Plant Soil (2020) 453:17–28
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
nutrients beyond the range of crop roots when the demand
for these nutrients is high (Cadisch et al. 1997;Gathumbi
et al. 2002; Lehmann et al. 1998a,b;Suprayogoetal.
2002). A high safety-net efficiency requires a minimal tree
RLD to a certain depth, a minimal level of activity of tree
roots present in the soil layers considered, and a minimal
demand by the tree for the nutrient considered (Cadisch
et al. 1997;Vanlauweetal.2002).
Thus, pruning has been shown to reduce root
biomass and this was ascribed to translocation of
carbohydrates from roots to shoots to maintain
their growth and the functional equilibrium be-
tween the two compartments (Govindrajan et al.
1996;Hoadetal.2001; Namirembe 1999). Nev-
ertheless, the impact of pruning is variable and
depends on the species considered (Akinnifesi
et al. 1999;Jonesetal.1998;Odhiamboetal.
2001; Schroth and Lehmann 1995), on manage-
ment practices (Govindrajan et al. 1996;
Namirembe 1999) and on environmental conditions
(Vanlauwe et al. 2002; Vogt et al. 1998). The
distribution of plant roots within soil layers is also
a dynamic process disagreeing with Jones et al.
(1998) and Smith et al. (1999) who found that tree
roots dominated agroforestry plots all year-round.
Indeed, Odhiambo et al. (2001)reportedthattree
roots were only the most abundant about half of
the cropping period. In some circumstances, prun-
ing induces no effect on tree roots and may even
increase root production (van Noordwijk and
Purnomosidhi 1995). In any case, the effect of tree
pruning and root reductions on well-established
trees is transient and needs to be repeated in
subsequent periods since root growth and produc-
tion will recover as the tree crown regains its size
(Bayala et al. 2004;Namirembe1999;Odhiambo
et al. 2001).
Water sharing and redistribution by roots
In addition to improving soil conditions for enhanced
water infiltration and storage, trees reduce solar ra-
diation underneath leading to lower evapotranspira-
tion and greater soil water contents under their can-
opies. Another process leading to increased soil wa-
ter content in the upper rooting zone under trees is
the process of hydraulic lift (HL) or water redistri-
bution by roots, which is defined as the passive
movement of water through the roots of trees, from
deeper and wetter soil layers to shallower and drier
horizons, along a gradient of soil water potential
(Richards and Caldwell 1987). The HL process usu-
ally occurs during the night when plant transpiration
is low or null (Prieto et al. 2010b; Hultine et al.
2003) and the water potential gradient is established
between wetter and drier soil layers, but has been
reported to occur during the day under particular soil
conditions, e.g. along water potential gradients creat-
ed by variations in soil salinity (Hao et al. 2009).
Although the direction of water movement is typi-
cally upward, towards drier, shallow soil layers,
measurements of sap flow in taproots and lateral
roots of trees have demonstrated that roots can also
redistribute water downward or laterally from moist
surface soils to drier regions of soil (Burgess et al.
1998; Hultine et al. 2003;Nadezhdinaetal.2006).
This bidirectional movement of water was termed
“hydraulic redistribution (HR, Burgess et al. 1998).
The process of HL is generally found in species
with dimorphic root systems with access to water
from both shallow and deep soil layers (Scholz et al.
2008). Hydraulic lift has been shown to be ubiqui-
tous as long as conditions favors its occurrence and
has been reported in a large number of species from
various climates including Mediterranean ecosystems,
cool temperate regions and seasonally dry tropical
and subtropical habitats (Horton and Hart 1998;
Jackson et al. 2000; Neumann and Cardon 2012;
Prieto et al. 2012b). One important aspect is that
HL has also been described in many woody species
widely used in agroforestry systems such as
Azadirachta indica,Acacia tortilis,Eucalyptus kochii
subsp. borealis, Guiera senegalensis, Parkia
biglobosa, Piliostigma reticulatum, Vitellaria
paradoxa(Bayala et al. 2008;Brooksbanketal.
2011a; Kizito et al. 2006,2012;Ludwigetal.
2003; Smith et al. 1997).
The ecological or agroecological relevance of HR
both to the plant engaged in HR and for neighboring
plants has been widely reported by many authors (see
reviews by Burgess 2011; Caldwell et al. 1998;Liste
and White 2008; Neumann and Cardon 2012;Prieto
et al. 2012b). For the plant, HR can improve plant
transpiration rates and plant water status (Dawson and
Pate 1996); reported estimates of the total volume of
water lifted range from 5% to 30% of the total plant
transpiration suggesting that hydraulic redistribution
Plant Soil (2020) 453:17–28 21
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
could postpone the development of water stress and
mitigate the impact of soil drying in the surface horizons
(Caldwell et al. 1998; Emerman and Dawson 1996;
Oliveira et al. 2005; Warren et al. 2005,2007). Hydrau-
lic lift has other side benefits such as higher fine root
survival (Bauerle et al. 2008), a limited root embolism
during the day by refilling xylem conduits at night
(Domec et al. 2004,2006; Prieto and Ryel 2014)or
improved soil nutrient recycling that could benefit both
the woody species and associated crops (Aanderud and
Richards 2009; Armas et al. 2012; Cardon and Gage
2006;Cardonetal.2013;Prietoetal.2012a). For
neighboring plants, water that is redistributed to shallow
soil layers can also be taken up by plants living in close
association to plants engaged in HL enhancing their
transpiration and ultimately increasing their survival
and biomass production (Bogie et al. 2018; Izumi et al.
2018; Lee et al. 2018; Prieto et al. 2011). Nevertheless,
the net effects that species engaged in HR have on their
neighbors is still unclear (Prieto et al. 2012b). There are
reports of positive, neutral or even negative effects
depending on ecosystem type, plant life form or whether
donor and receiver species shared common ecto- and
endomycorrhizal networks. We need to note here that
common mycorrhizal networks are strongly involved in
water and nutrient sharing between plant species (Allen
2007; Montesinos-Navarro et al. 2019) and play an
important role in the transfer of lifted water between
species and within individuals of the same species
(Querejeta et al. 2003,2012; Egerton-Warburton et al.
2007; Prieto et al. 2016; Warren et al. 2008). However,
our knowledge on the importance of mycorrhizal path-
ways in agroforestry systems is still very limited (de
Carvalho et al. 2010) and will not be discussed in this
review but opens up new areas of research to differen-
tiate between root-only- and root-mycorrhizal-based
transfer of HL water (Fig. 2). Several studies have
shown that HL enhances water availability to plants
(McMichael and Lascano 2010;Hirotaetal.2004;
Prieto et al. 2011;Shenetal.2011; Smart et al. 2005;
Wan et a l. 2000), thereby promoting microbial processes
and decomposition processes which release nutrients
from organic matter and soil minerals (Aanderud and
Richards 2009; Armas et al. 2012;Austinetal.2004;
Cardon et al. 2013;Matimatietal.2014; Munoz et al.
2008; Prieto et al. 2012a; Smith et al. 1999; Yoder and
Nowak 1999) that could also benefit the growth and
survival of plant neighbors. The process of HL may
therefore contribute significantly towards meeting the
water, and or nutrient supplies needed to support growth
and grain filling in associated crops in agroforestry
systems with limited water availability (Bogie et al.
2018; Izumi et al. 2018). The potential application and
exploitation of HL to improve water availability to crops
or seedlings/saplings in forestry and ecological restora-
tion has been already suggested (Burgess 2011;Liste
and White 2008) but, to date, real-world experimental
tests in agroforestry systems are still limited but show
promising results. For example, Kizito et al. (2012)
estimated that, in two species commonly used in agro-
forestry (Guiera senegalensis and Piliostigma
reticulatum), HL alone could account for up to 35–
47 mm of water annually, which represents between
8% and 10% of the mean annual precipitation
(450 mm) in the Sahel. Under similar environmental
conditions, Vadez et al. (2013) showed that pearl millet
(Penissetum glaucum) yield could increase by up to 3.5–
4.7 g per plant for each additional mm of water provid-
ed, which means that, in regular planting densities of
10,000 plants ha
−1
, the additional water provided
through HL could result in substantial yield increases
of 35–47 kg ha
−1
mm
−1
. Experimental evidence of such
increases in plant crop biomass when grown in associ-
ation with species engaged in HL was reported in a
subsequent study by Bogie et al. (2018). These authors
used an enriched deuterium (
2
H) water tracer to demon-
strate that the associated pearl millet crop took up water
that had been lifted by the shrubs. They concluded that,
along with other potential beneficial factors of growing
crops in association with the shrubs (shade or lower tem-
peratures), HL contributed largely to the 900% increase in
pearl millet biomass from below 200 kg ha
−1
in the control.
In addition to the extra water provided by HL, crops could
also benefit from other side benefits of improved soil
moisture conditions such as a faster nutrient recycling
and uptake under plants engaged in HL (Sun et al. 2014).
Also, competition for water between crops and associated
species has been observed in agroforestry systems when
tree root systems had no access to groundwater (Rao et al.
1998;Duprazetal.2005) and a requirement for species
selection is that facilitative species have deep roots and
access to deep water, or they will probably otherwise
outcompete crops for a limited water supply (Barron-
Gafford et al. 2017; Van Noordwijk and Ong 1999). Thus,
the relative importance of complementarity for water via
HR or competition may therefore depend on particular soil
conditions such as soil moisture content, as well as other
factors known to shape inter-plant interactions (Bayala
22 Plant Soil (2020) 453:17–28
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et al. 2008;Hirotaetal.2004;Ludwigetal.2003,2004).
These reported benefits for plant production and survival,
however, may be thus largely dependent on the context
and the species involved (Prieto et al. 2012b), which may
be particularly true in mixed agroforestry systems where
shading and micro-climatic effects could have an equally
important influence on plant growth and survival (Prieto
et al. 2011).
One interesting aspect of the use of HR in agrofor-
estry systems is promoting positive plant interactions by
an active management of agroforestry systems. One
example of such management was provided by Sekiya
et al. (2011) that grew a crop in association to deep-
rooted forage plants in an agricultural field. To remove
competition for water and promote the uplift of water
from deep soil layers, they removed the shoots of deep-
rooted plants forcing HL 24 h a day, which resulted in a
better crop water status and greater biomass production.
As these shoot-less plants were found to be effective
bio-irrigators, the mechanism at play could be HL until
root death and a capillary or “wick”effect afterwards
(Leffler et al. 2005). This calls for a better understanding
of the fundamentals of HL before we can apply effective
bio-irrigation techniques such as total plant removal.
Although this example has not been replicated since,
common agricultural practices in agroforestry systems
that do not completely remove the canopy, such as tree
pruning or shading, can be tested as effective mecha-
nisms favoring the process of HL since reducing above-
ground canopy transpiration can enhance the amount of
water lifted to topsoil layers (Prieto et al. 2010a,b;
Sekiya and Yano 2004). Furthermore, the remnants from
cutting the shoots of deep-rooted forage plants can be
recycled into forage for animals providing an additional
benefit for such systems. Therefore, a directed selection
of pairs of species in intercropped systems along with
agricultural practices, may help overcome this problem
and promote water sharing between the tree and neigh-
boring crops and the use of HR as a bioirrigation system.
However, our knowledge in the incorporation of species
engaged in HL in agroforestry systems is still very
limited and further efforts in applied research are still
needed to assess which species would be better suited to
be used as “bioirrigators”, in which context or environ-
ments, the magnitude of the volumes effectively trans-
ferred to the associated crops/plants and what crops
would benefit more from this lifted water along with
its impact on plant productivity (Fig. 2).
Fig. 2 Future investigation areas for water acquisition and sharing in agroforestry systems
Plant Soil (2020) 453:17–28 23
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Conclusion
This review provides an overview of our current knowl-
edge on water acquisition and sharing mechanisms in
agroforestry systems. Based on key concepts such as
interspecific competition and facilitation through water
transfer between plants, this paper reviews water acqui-
sition and redistribution patterns and their potential ap-
plication in agroforestry ecosystems. In promoting ag-
roforestryas an ecologically adapted farming system for
addressing environmental issues caused by intensive
monoculture, the central assumption that underpinned
the science and practice is that trees take up water from
deep soil layers and recycle it in upper soil layers
through water redistribution, thus making it available
to nearby shallow rooted associated crops. Therefore,
research actions have covered a range of topics includ-
ing safety net hypothesis, root distribution, root func-
tional traits, HL/redistribution and its ecological impor-
tance, hydraulically water lifted transfer between spe-
cies, water source using isotopes, and modelling of
relevant root parameters to inform future field-based
empirical studies on water acquisition and sharing. In
this sense, a better characterization of the hydraulic
resistances of the soil-plant system, particularly at the
soil-root interface in one hand and in another hand a
better characterization of the soil system and hydraulic
architectures are pivotal to improving our quantitative
understanding of HL (Meunier et al. 2017). Despite
clear advances, root research in mixed tree-crop agro-
forestry systems remains challenging because of the
difficulty in finding the relevant spatial and temporal
scales as well as a lack of consensus about the most
relevant traits that confer a true advantage in below-
ground plant competition. Other under-researched areas
which equally deserve more attention to be able to
design the best mixtures of tree and crop species for
agroforestry practices that provide beneficial outputs
both for the communities and the environment include
(Fig. 2):
–Investigating and determining the controlling fac-
tors of the root functional traits;
–Clarifying and quantifying the role of common
mycorrhizal networks in the mechanism of HL,
including the proportion of water transferred
through root-only- and root-mycorrhizal-based
HL, and their importance and applications in agro-
forestry systems;
–Characterizing hydraulic resistances along the soil-
plant system, particularly at the soil-root interface,
and root hydraulic architectures to gain knowledge
on the potential for water uptake and water transfer
patterns in agroforestry systems;
–Quantifying the volume of water transferred to
neighboring plants through HL and their species
and environment, including soil type and
dependence;
–Disentangling the advantages and potentially ad-
verse effects of hydraulically redistributed water
(e.g. in saline systems).
Acknowledgements JB was supported by the Livelihood Sys-
tems flagship of the CGIAR research program on Forests, Trees
and Agroforestry (FTA) and IP acknowledges funding from the
Juan de la Cierva program (FPDI-2013-16221) and the Spanish
Ministerio de Economía y Competitividad (project CGL2013-
48753-R). The anonymous reviewers are highly appreciated and
thanked for their constructive comments.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestrict-
ed use, distribution, and reproduction in any medium, provided
you give appropriate credit tothe original author(s) andthe source,
provide a link to the Creative Commons license, and indicate if
changes were made.
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