# Root growth and biomass partitioning of nine juvenile Sahelian agroforestry tree species under drought and irrigation treatments

Fatou Gning, Christophe Jourdan, Diatta Marone, Daouda Ngom, Anders Ræbild

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Fatou Gning, Christophe Jourdan, Diatta Marone, Daouda Ngom, Anders Ræbild. Root growth and biomass partitioning of nine juvenile Sahelian agroforestry tree species under drought and irrigation treatments. Plant and Soil, In press, 10.1007/s11104-024-07155-y. hal-05026372

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# Root growth and biomass partitioning of nine juvenile Sahelian agroforestry tree species under drought and irrigation treatments

Fatou Gning Ⓓ · Christophe Jourdan Ⓓ · Diatta Marone · Daouda Ngom · Anders Ræbild Ⓓ

Received: 6 July 2024 / Accepted: 12 December 2024
© The Author(s) 2025

## Abstract

*Background and aims* Root traits of tree species are of great importance for selection of trees in dry zone agroforestry systems. Climate models predict increased frequency and severity of drought in the Sahel but our understanding on the strategies of root growth of tree species under drought is limited. To increase our knowledge of species differences in rooting patterns we investigated root growth of nine dry-land tree species in West Africa during one year when exposed to drought stress.

*Methods* Seedlings were planted in 2 m tubes inserted into the soil and subjected to irrigation and

drought (no irrigation during the dry season). After 4, 8 and 12 months, we assessed root growth and root biomass, supplemented with assessments of water potential, leaf phenology, and above-ground growth. *Results* Tree species showed significant differences in rooting profile. Drought induced deeper root growth, especially in deciduous species. *Adansonia digitata*, *Sterculia setigera* and *Anacardium occidentale* had shallow roots when irrigated. *Acacia senegal*, *Acacia nilotica* and *Faidherbia albida* had deeper root growth and showed clear differences in leaf water potential between treatments while *A. digitata* and *S. setigera* appeared to be water conserving under drought.

*Conclusions* Our results link root growth with phenology, physiology and above-ground growth and

Responsible Editor: Bruno Rosado.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s11104-024-07155-y.

F. Gning (✉) · A. Ræbild
Department of Geosciences and Natural Resource Management, University of Copenhagen, Rolighedsvej 23, 1958 Frederiksberg C, Denmark
e-mail: fag@ign.ku.dk

A. Ræbild
e-mail: are@ign.ku.dk

F. Gning · D. Marone
Centre National de Recherches Forestières/Institut Sénégalais de Recherches Agricoles (CNRF/ISRA), Route Des Pères Maristes, BP 2312, Dakar, Sénégal
e-mail: diatta.marone@isra.sn

C. Jourdan
UMR Eco&Sols, Univ Montpellier, CIRAD, INRAE, IRD, Montpellier SupAgro, Montpellier, France
e-mail: christophe.jourdan@cirad.fr

C. Jourdan
CIRAD, UMR Eco&Sols, 34398 Montpellier, France

C. Jourdan
LMI IESOL, Centre IRD-ISRA de Bel Air, Dakar, Sénégal

D. Ngom
Department of Plant Biology, Cheikh Anta Diop University, BP: 5005, Dakar Fann, Dakar, Senegal
e-mail: daouda11.ngom@ucad.edu.sn
provide an attempt to group dryzone tree species in
functional types based on their root growth.

**Keywords** Drought stress · Phenology · Root traits · Root biomass allocation · Soil depth

**Introduction**

Water is a crucial requirement for plant growth and a principal determinant of vegetation distributions worldwide as it plays a central role in all physiological processes of the plant (Stephenson 1990). South of the Sahara broad vegetation bands shift from desert in the north to rainforest in the south, and plant species' distributions in this region are thus strongly correlated with availability of water (Magill et al. 2013). Plants require water for growth and except a small amount that may come from aboveground foliage, trunk and branches during raining events (e.g. Jensen et al. 2020), most of this water is taken up through the root system. Root systems thus play a critical role in determining the capacity of trees to use water (Landsberg 2000) and the ecological ranges of species. Coarse roots provide nutrients and water transports, stores carbohydrates and anchors the plant in the soil (Bayala et al. 2008; Bayala and Prieto 2020). Fine roots on the other hand represent the majority of length and surface area of the root system (Rewald et al. 2011) and are thought to be the most important organs for nutrient and water uptake and supply to the rest of the plant (Lu et al. 2022).

Root growth may be influenced by both biotic and
environmental factors (Schenk and Jackson 2005).
Plants vary in their strategy in response to different
environmental conditions, and trees with deep root
systems may thus have the opportunity to maintain
high transpiration rates in zones with irregular rain-
fall particularly during the long dry season (Bleby
et al. 2010). In these zones, characterized by low
rainfall and where water is often not available in the
upper layers, many tree species have developed deep
root systems to reach the water table (Dupuy and
Dreyfus 1992; Canadell et al. 1996; Alvarez-Flores
et al. 2018). In a review of global rooting depths,
Canadell et al. (1996) found that the average maxi-
mal rooting depth may reach more than 13 m depth
in desert vegetation. In addition, species in drylands
are generally thought to have a higher root/shoot ratio

to balance water availability with water use (Barbour
1973). Thus, especially under semi-arid conditions,
water acquisition by roots is affected by the root mor-
phology and root distribution, and it seems likely
that trees may experience a strong selection pressure
to optimize root development and water absorption
capacity (Bayala and Prieto 2020).

Several studies found that the root growth is spe-
cies-dependent and increases with soil tempera-
ture (Steinaker and Wilson 2008; McCormack et al.
2014). However, low soil water content limits the
fine root growth, and the number of fine roots were
found to increase following years with high precipita-
tion (Comas et al. 2005; Withington et al. 2021). In
addition, Radville et al. (2016) reported that the root
phenology was controlled by the soil moisture in dry
sites and that root production was highest during the
rainy season. Extensive research has been performed
to study the plant responses to drought stress and most
of them show that water deficits reduce both shoot
and root biomass (McDonald and Davies 1996; Cana-
les et al. 2019, Álvarez et al. 2012; Faye et al. 2019;
Wijewardan et al. 2019). However, while drought usu-
ally lead to a reduced root growth, the allometric pat-
terns may change, leading the plant to allocate a larger
proportion of the biomass to roots through a changed
root: shoot ratio (Comas et al. 2013). Also the rooting
depth may be affected, drought leading to a faster root
growth at depth (Schenk and Jackson 2002b).

Trees in tropical evergreen forests tend to have
shallow roots compared to trees in tropical deciduous
forests (Canadell et al. 1996). In a study of temperate
species, deciduous angiosperm species were found to
have a greater root length per unit root mass (specific
root length, SRL) than evergreen conifer species, this
being due to their ability to build long and thin roots
(Alvarez and Körner 2011). However, in a study of
co-occurring temperate species, leaf habit was found
to have less effect on the root production, being over-
shadowed by influences of soil humidity (Withington
et al. 2021). This suggests that both environmental
and intrinsic factors affect the root growth of the spe-
cies (Dry et al. 2001). In a dry forest in Costa Rica,
Smith-Martin et al. (2020) found that evergreen tree
species had deeper roots than co-occurring deciduous
species. It was suggested that evergreen species need
deeper roots to access water during the dry season.

Unfortunately the rooting patterns of tree species
in the Sahel are highly under-investigated. Previous

<figure><img src="image_2.png" /></figure>
studies have improved our understanding of root development responses to drought, but most of them were carried out on crop species such as maize and millet (Debieu et al. 2018; Faye et al. 2019; Manju et al. 2019). Only few studies have been done on roots of agroforestry tree species, focusing mostly on root biomass and root distribution in relation to soil humidity (Logbo et al. 2013; Marone et al. 2017; Gning et al. 2023). Such studies have confirmed that the general link between humidity and timing of root growth also applies to trees (Logbo et al. 2011 and 2013; Abenavoli et al. 2016; Yandou et al. 2020).

Root systems of a number of important agrofor-
ery tree species thus remain under-investigated, and
little is known about root growth in relation to soil
water content and distribution (Ma et al. 2013), which
could otherwise help to understand the root spa-
tial distribution in the field. Information on biomass
allocation and rooting pattern of juvenile trees under
drought stress is essential to understand not only how
and in which ecosystems they establish, but also how
trees may interact with associated crop species in
agroforestry systems. In the current study, we address
this knowledge gap by comparing root growth of
seedlings of nine tree species with different phenol-
ogy, exposed to different levels of water availability.

Our hypothesis was that root growth of agrofor-
ery tree species differed between deciduous and
evergreen species exposed to different levels of soil
water. In particular, we expected evergreen species to
develop deeper root systems than deciduous species
under drought conditions, given their supposed larger
need for water.

Materials and methods

Study site

The study was conducted at the nursery of Cen-
tre National de Recherches Forestières (CNRF) of
the Institute Sénégalais des Recherches Agricoles
(ISRA) in Dakar, Senegal, located at 14°43'32''(N)
and 17° 26'12''(W). The experiment was performed
from March 2019 to June 2020. The climate is tropi-
cal Sahelo-Sudanian characterized by two seasons,
the rainy season ranging from approximately June
to October and the dry season lasting 7–8 months.
During the experiment, the rainfall (from July 2019

to June 2020) was 485 mm and the average monthly
minimum and maximum air temperatures ranged
from 18.3 and 25.8 °C in February 2019 to 26.2 and
31.1 °C in August 2020, respectively. The soil tem-
peratures in the tubes at 10 cm depth were assessed
with a Hobo Tidbit v.2 water temperature logger
(Onset Computer Corporation, Bourne MA, US) and
monthly averages varied between 24.4 °C (January)
and 32.8 °C (July) (Fig. S1).

Seed sources and pretreatment

Seeds of nine species were procured from the CNRF/
ISRA lab and at the Inspection Regionale des Eaux
et Forêts de Dakar. We included five species that
are considered deciduous in the Sahel (Acacia sen-
egal, Acacia nilotica, Adansonia digitata, Faidherbia
albida, Sterculia setigera) and four species that are
normally evergreen (Balanites aegyptiaca, Anacar-
dium occidentale, Borassus akeassii and Vitellaria
paradoxa) (Table S1), selected for their multiple
uses in providing food, fodder, wood and contribu-
tions to soil conservation in the Sahel (Frederiksen
and Lawesson 1992; Tomlinson et al. 2013; Logbo
et al. 2013). The species were sampled at different
locations across Senegal and are all native to Sene-
gal except A. occidentale, which comes from South
America. Since the tree species were sampled at only
one site per species our data necessarily include a
confoundation of provenance and species effects.

The desiccation sensitive seed of *V. paradoxa* and *B. akeassii* were sown without pretreatment. Seeds of the other species were soaked in a concentrated sulfuric acid solution (95%) for 15 min to 2 h, or in hot water, depending on species, to break seed dormancy. Then, they were washed with tap water during 10–15 min to remove any trace of acid and sowed in petri dishes for pre-germination in the laboratory to synchronize germination of all seeds before sowing. All seeds were sown in polythene bags in March 2019.

Experimental design and sampling

The investigations were conducted under semi-natu-
ral conditions exposed to full sunlight. The trial was
established by digging holes of 2 m depth with an
automated soil auger. PVC tubes with an inner diam-
eter of 12.5 cm and a length of 2 m were inserted into
the hole to a depth of 1.90 m into the natural soil in order to prevent large temperature fluctuations that would have occurred if the tubes were aboveground. The tubes were filled with a homogeneous mixture applied in local nurseries, composed of 1/3 sand and 2/3 natural soil rich in organic material collected in a plantation of *Casuariana equisetifolia*, with a combined density of 0.43 g cm⁻³. The lower ends of the tubes were closed with a plastic lid with small holes to allow drainage. Tubes were watered daily until the soil had sunk and the surface had reached a stable level.

One month after, in July 2019, all seedlings were transplanted into the PVC tubes. Care was taken not to damage the roots during the transplantation process. 225 plants were placed in a randomized complete block design with five blocks containing five seedlings of each of the nine species (45 seedlings per block). Within each block, the different plant species and treatments were fully randomized, and the number of plants was scaled to allow for three sampling periods and two treatments (see below). The plants were grown with 25 cm between plants and 50 cm between rows to avoid competition for light among plants. Until October 2019, all plants received natural rainfall supplemented by watering (1 L tap water) when the top soil was dry.

The first sampling took place on October 15, 2019, to characterize root depth of plants before treatments were installed, using one plant per species per block. Then, the two treatments were applied: Irrigation, where plants were watered every day with 1 L tap water, and drought, where plants used only the remaining soil water until the end of the experiment (meaning no irrigation from October 2019 to June 2020, where the first rains fell). The two treatments were each applied to two plants per species per block.

Eight months after transplanting, the second sampling was made (February 25 to March 10), sampling 90 plants (one plant from each combination of species and treatment in each block). The third and final sampling took place one year after planting (from June 29 to July 18) harvesting the remaining 90 plants (again representing one plant from each combination of species and treatment per block).

## Root measurements

At the first sampling, tubes were dug out to observe the root architecture and to measure the total root

length of each plant. For each plant, the total root length of the taproot was measured with a ruler.

At the second and third sampling, the tubes were dug out and placed on a table. The tubes were cut vertically and the lengths of the roots were measured. The root columns (including soil) were sectioned into 20 cm lengths from the root collar until no root was found. Each sample was put on a plate with a label and two sieves with different mesh sizes (0.25 and 2 mm) were used to recover fine roots. Roots were then separated manually from the soil and organic particles such as dead roots, distinguished by their color and tendency to break. Each sample was placed on a fine-mesh sieve (0.2 mm) with a "root washing pump" to remove any remaining clods of soil from the roots. Roots obtained from the washing activity were wiped and laid on plastic paper, and the fresh weight determined to a precision of 0.01 g using a KERN EMB scale. Then each sample was put in a cooler and transported to the laboratory, where they were stored in a fridge at 6 °C until the scanning procedure.

Coarse (diameter > 1 cm) and medium roots (1 cm > diameter > 2 mm) were sorted and oven-dried over 48 h at 65 °C and weighed. Fine roots (diameter < 2 mm) were scanned with a scanner (Epson perfection V700 Photo, Japan), and images were analyzed using WinRHIZO software (Regent Instruments Inc. Regular version, 2012 b, Canada) to determine fine root length and average diameter. The fine roots were then oven-dried at 65 °C for 48 h to determine dry mass. Specific root length (SRL) was calculated by dividing fine-root length by dry weight and the root length density (RLD) was calculated by dividing fine root length by soil volume. The root: shoot ratio was calculated as root mass divided by shoot mass (stem and leaves).

Before analyses, data for the different soil sections were averaged into larger categories, covering the depths 0–40 cm, 40–80 cm, 80–120 cm, 120–160 and 160–200 cm.

The root growth for each treatment was determined for each combination of block, species and treatment by dividing the change in dry biomass or RLD by the length of the growth period (0–8 months and 8–12 months).

## Aboveground measurements

Every month except March, plant height and basal diameter were measured and the number of leaves
counted. Phenology was observed on all parts from the start of the treatments in September (06–09–2019) until the end of the experiment and was assessed every week, based on nine scores (Table S2).

Predawn leaf water potential (LWP) was assessed three times in December and once in February on detached leaves from 3 to 7 h AM before sunrise, using a Pump-up Chamber Instrument (PMS Instrument Company, Albany, USA) on 120 plants. Except for *B. akeassii* all species were assessed, as this species had few leaves and the instrument was not suitable to measure its LWP. At the second and third sampling, the aboveground biomass (leaves and stems) were put in a paper bag, oven-dried for 72 h at 80 °C and weighed separately.

## Statistical analysis

To analyze the data, we used linear mixed effects models:

$$Y_i = \alpha(\text{time}_i, \text{species}_j, \text{depth}_k, \text{treatment}_i) + A(\text{tree}_m) + B(\text{block}_n) + \text{error}$$

, where α parametrize the fixed effects of Time, Species, Depth, and Treatment, and their interactions, A is a random effect of tree (included for variables measured repeatedly on the same tree such as height and phenology), and B is a random effect of block. Errors were assumed to be independent and following a normal distribution with an expected mean value of zero. Since four-factor interactions were not significant for any of the variables, these were removed from the model (Table 1).

All analyses were performed using the lme4 package in R (2020, version 3.6) and R Studio (version 1.2.5001)).

Model assumptions of variance homogeneity and normality of residuals were evaluated by the residuals and quantile plots. For some variables, a cubic-root transformation (aboveground variables) or a logarithm (root traits) was used to achieve variance homogeneity and a valid model. The ggplot package in R was used to visualize the relationship between treatment, depth, species and time.

To evaluate the hypothesis of differences between evergreen and deciduous species in root traits, biomass allocation and leaf water potential, we extracted least square means for each combination of species and treatment from the model above. Then, for each variable in each treatment, we performed a t-test of

differences between evergreen and deciduous species. These analyses were based on the measurements at month 8, as at that time only few plants had reached the bottom of the tubes.

To estimate relations between traits across species, we performed a series of correlation analyses based on the results from samplings at 8 and 12 months. Correlations between traits were calculated for each treatment at each sampling time, using estimated marginal means for each species from the model above. For this analysis, we converted the 9-point leaf phenology scale to a scale expressing whether the tree had living leaves or not (Table S2) and calculating the mean number of trees having living leaves. To visualize correlations among variables we conducted a principal component analysis (PCA) on the same variables at month 8 separately for the two treatments (Fig. 5).

Finally, we performed canonical correlation analyses of root traits in order to study how species grouped in the multivariate space. Canonical correlation analyses remove collinearity among variables (Chatfield and Collins 1980; Sharma 1996). These analyses were performed separately for each combination of treatment (irrigation, drought) and the two times of sampling (month 8, 12). The variables root biomass, root diameter, SRL and RLD were log transformed and included as dependent variables, while the independent variables were species, blocks and depths. Mean values of the species were plotted against the first two canonical variates together with the canonical directions.

## Results

Seedlings subjected to drought and irrigation, showed strong differences among tree species for almost all variables (Table 1). Furthermore, the numerous interactions demonstrated that treatments had different impacts on the species across time. We observed 58 dead plants in six species at the end of the experiment (Table 2). The evergreen species *A. occidentale*, *V. paradoxa* and *B. akeassii* exhibited the highest mortality rate, while few trees died in the remaining species. This means that the numbers of replicates for these species were reduced compared to species without mortality.
<table><caption>**Table 1** Results of analyses of variance for the nine-tree species for model 1, and tests for differences between deciduous and evergreen species at month 8</caption><thead><tr><th rowspan="2">Variables</th><th colspan="2">Tests from model 1</th><th colspan="2">P-values from tests of differences between deciduous and evergreen species at month 8</th></tr><tr><th>Factors</th><th>P-value</th><th>Irrigated</th><th>Drought</th></tr></thead><tbody><tr><td colspan="5"><strong>Root variables</strong></td></tr><tr><td>Maximum depth<sup>1</sup></td><td>Time*Species*Treatment</td><td>0.0051**</td><td>0.038*</td><td>0.140</td></tr><tr><td>Root biomass<sup>2</sup></td><td>Time*Species*Treatment</td><td>0.0023**</td><td>0.096</td><td>0.023*</td></tr><tr><td>Specific root length<sup>2</sup></td><td>Species*Depth*Treatment</td><td>1.8e-05***</td><td></td><td></td></tr><tr><td>Root length density<sup>2</sup></td><td>Species*Depth*Treatment</td><td>0.013*</td><td>0.640</td><td>0.595</td></tr><tr><td>Average diameter</td><td>Time*Species*Treatment</td><td>0.0007 ***</td><td>0.049*</td><td>0.023*</td></tr><tr><td>Fine root growth</td><td>Species*Depth*Treatment</td><td>0.0017***</td><td></td><td></td></tr><tr><td>RLD growth</td><td>Species*Depth*Treatment</td><td>0.001***</td><td></td><td></td></tr><tr><td>Aboveground variables</td><td>Period*Species*Treatment</td><td>0.0022 **</td><td>0.724</td><td>0.270</td></tr><tr><td>Leaf number<sup>2</sup></td><td>Period*Species</td><td>0.032 *</td><td>0.065</td><td>0.441</td></tr><tr><td>Phenology score</td><td>Period*Depth</td><td>0.010 *</td><td></td><td></td></tr><tr><td>Leaf Water Potential<sup>2</sup></td><td>Period*Treatment</td><td>0.015 *</td><td></td><td></td></tr><tr><td>Height</td><td>Species*Treatment</td><td>2.2e-16 ***</td><td>0.551</td><td>0.875</td></tr><tr><td>Diameter<sup>2</sup></td><td>Species*Treatment</td><td>2.2e-16 ***</td><td></td><td></td></tr><tr><td>Biomass allocation at month 12</td><td>Species*Treatment</td><td>2.2e-16 ***</td><td></td><td></td></tr><tr><td>Leaf biomass<sup>2</sup></td><td>Species*Treatment</td><td>1.2e-05 ***</td><td>0.448</td><td>0.202</td></tr><tr><td>Stem biomass<sup>2</sup></td><td>Species*Treatment</td><td>3.7e-05 ***</td><td>0.114</td><td>0.926</td></tr><tr><td>Root biomass<sup>2</sup></td><td>Species*Treatment</td><td>0.0001 ***</td><td></td><td></td></tr><tr><td>Root:shoot ratio<sup>2</sup></td><td>Species*Treatment</td><td>1.1e-08 ***</td><td>0.006**</td><td>0.055</td></tr><tr><td>Biomass total (Plant mass)<sup>2</sup></td><td>Species*Treatment</td><td>0.0243 *</td><td>0.142</td><td>0.086</td></tr></tbody></table>

¹Log transformation. ²Cubic root transformation

Only the highest order significant interactions from model 1 are shown (see statistics). Asterisks indicate the levels of significance. \*P < 0.05; \*\*P < 0.01; \*\*\*P < 0.001
<table><caption>**Table 2** Leaf water potential (LWP) and mortality (at the end of the experiment) per species and treatment (LWP)</caption><thead><tr><th rowspan="2">Variables</th><th rowspan="2">Months</th><th colspan="3">Species</th><th colspan="3">Evergreen</th><th rowspan="2">V paradoxa</th><th rowspan="2">B. akeassii</th></tr><tr><th>Deciduoud</th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td rowspan="3">LWP(MPa)<br>Irrigation</td><td>Dec</td><td>A. senegal</td><td>A. nilotica</td><td>F. albida</td><td>A. digitata</td><td>S. setigera</td><td>B. aegyptiaca</td><td>A. occidentale</td><td></td></tr><tr><td>Feb</td><td>-0.7 ± 0.4</td><td>-0.9 ± 0.4</td><td>-0.4 ± 0.4</td><td>-0.3 ± 0.4</td><td>-0.3 ± 0.4</td><td>-0.8 ± 0.4</td><td>-0.6 ± 0.4</td><td>-0.4 ± 0.4</td><td>n.a</td></tr><tr><td>Dec</td><td>-1.4 ± 0.5</td><td>-1.4 ± 0.5</td><td>-0.8 ± 0.5</td><td>-0.7 ± 0.5</td><td>-0.4 ± 0.5</td><td>-1.0 ± 0.5</td><td>-0.8 ± 0.5</td><td>-0.5 ± 0.5</td><td>n.a</td></tr><tr><td rowspan="2">Drought</td><td>Dec</td><td>-1.6 ± 0.4</td><td>-2.1 ± 0.4</td><td>-0.7 ± 0.4</td><td>-0.4 ± 0.4</td><td>-0.4 ± 0.4</td><td>-1.1 ± 0.4</td><td>-0.6 ± 0.4</td><td>-0.8 ± 0.5</td><td>n.a</td></tr><tr><td>Feb</td><td>-2.1 ± 0.5</td><td>-2.1 ± 0.5</td><td>-1.0 ± 0.5</td><td>-0.5 ± 0.5</td><td>-0.6 ± 0.5</td><td>-1.9 ± 0.5</td><td>-0.6 ± 0.5</td><td>-1.0 ± 0.5</td><td>n.a</td></tr><tr><td rowspan="3">Mortality(%)<br>Irrigation</td><td></td><td>0</td><td>0</td><td>0</td><td>7</td><td>7</td><td>0</td><td>47</td><td>20</td><td>7</td></tr><tr><td></td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>27</td><td>27</td><td>17</td></tr><tr><td></td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td></tr></tbody></table>

Values represent the least square means ± standard errors (S.E.) of LWP of the species per treatment. n.a.: non-available

## Rooting depth

In all tree species, some individuals grew to the end of the tube, reaching the maximum possible rooting depth of 2 m after 12 months of growth (Figs. 1 and S2). Interactions between Time, Species and Treatment showed that the species had different root development over time (Table 1). Root growth at depth was favored by the drought treatment, and in this treatment, *A. senegal*, *F. albida* and *B. aegyptiaca* reached the maximum depth of the tube already at 8 months after transplanting (Fig. 1). By the end of the experiment at month 12, all species had individuals with roots reaching 2 m depth in the drought treatment, and the experiment can thus not estimate the true maximum rooting depth. Species like *A. digitata*, *S. setigera* and *A. occidentale* had smaller rooting depths under irrigated conditions compared to the *Acacia* species, which showed deeper root growth (Fig. 1 and S2).

T-tests of differences between deciduous and evergreen species showed significant differences in root depth at month 8 for the irrigated plants (Table 1), but contrary to our expectations deciduous species had the deepest roots (Fig. 2).

## Root traits

Root biomass was highest in the upper soil layers, However, fine root growth showed significant interactions between period, species and treatment, and between species, depth and treatment (Table 1, Fig. 3, Fig. S2). From month 0–8, root growth tended to be similar for the irrigated and drought treatments. From month 8–12, as drought stress continued, there were large differences between the two treatments, manifested differently in the species. Drought stressed plants had limited root growth, except in the case of *A. occidentale* where substantial growth was observed below 120 cm. When irrigated, *Acacia* species, *F. albida* and *B. aegyptiaca* displayed large growth in the deeper soil layers, while species such as *A. digitata*, *S. setigera*, *V. paradoxa* and *B. akeassii* had large growth near the surface. Negative growth (net loss of roots) was observed for some species, especially *A. senegal* and *S. setigera* from months 8–12 (Fig. 3, 4). Deep root growth was observed in *B. aegyptiaca* and the leguminous species *A. senegal*, *A. nilotica* and *F. albida* in the irrigated treatment, and
<figure><img src="image_3.png" /><figcaption>**Fig. 1** Average maximum rooting depth of 5 deciduous (blue, solid lines, open symbols) and 4 evergreen (red, dashed lines, closed symbols) tree species across different treatments and harvest times. Drought treatments were initiated immediately</figcaption></figure>

after the harvest at month 4, symbolized by the dashed vertical black line. Note that already at month 8, roots of some species had reached the bottom of the tubes at 2 m, and the full rooting depth can thus not be evaluated

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Test of evergreen (square symbols) and deciduous (circle symbols) across different treatments, drought (red, dashed lines,) and irrigated (blue, solid lines) and harvest times. Error bars denote S.E. (n=5)</figcaption></figure>

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Vertical distribution of fine root growth of 5 deciduous (top) and 4 evergreen (bottom) tree species harvested after 8 months (0–8 months, dashed lines) and 12 months (8–12 months, solid lines) of cultivation under irrigation (blue) and drought (red) treatments. Inserted text in the upper right</figcaption></figure>

of each graph indicates the significance of the highest order
significant interaction from a mixed model analysis within spe-
cies: D (Depth), P (Period: 0–8 or 8–12) and T (Treatment)
correspond to the three factors. Significance levels: * P < 0.05,
** P < 0.01, *** P < 0.001. Error bars indicate ± S.E. (n = 5)

in drought stressed *A. digitata*, *A. occidentale* and *V. paradoxa*.

Root biomass, fine root growth and RLD were cor-
related among species (Fig. 5, Fig. S7-S10). Hence,
the growth of RLD showed a close match with the
root biomass growth, and also showed approximately
the same patterns across depths. These three variables
tended to be higher in deciduous species than in ever-
green species (Fig. 2). The formal t-tests confirmed
this, as there were significant differences between
evergreen and deciduous species for root biomass in
the drought treatment, RLD in both treatments, and
RLD growth in the irrigated treatment (Table 1).

Among species, SRL showed a negative correla-
tion with root diameter at month 8 (Fig. 5, Fig. S7-8),
and was correlated to RLD and RLD growth at month
12 in the irrigated treatment (Fig. S9). As expected,
SRL increased with depth for most species. Effects
of treatments varied among species, with *A. nilotica*
and *S. setigera* having large SRL in the drought treat-
ment, while *F. albida*, *A. digitata* and *B. aegyptiaca*
had larger SRL in the irrigated treatment (Fig. S2).
However, deciduous and evergreen species did not

vary systematically in SRL as the t-tests showed no
significant differences (Table 1, Fig. 2).

Leaf phenology

Frequencies of the nine dominant phenophases varied significantly over time with different patterns among the tree species (Table 1, Fig. 6). In the drought treatment, number of leaves was lower compared to the irrigated treatment for all the tree species except *A. occidentale* and *V. paradoxa*, which seemed to be unaffected by the treatments, and *B. akeassii*, where drought treated plants had a higher final number of leaves. Under irrigation, even the species that we considered to be deciduous kept leaves throughout the experiment, albeit with a large proportion of senescent leaves (Fig. S4). Many trees had fully mature leaves from November to December. After this period, there was a progressive phase of yellowing or loss of leaves, until at the end of the experiment most of the trees showed signs of leaf senescence. This was pronounced in the drought treatment where around
<figure><img src="image_7.png" /><figcaption>**Fig. 4** Vertical distribution of RLD growth of 5 deciduous (top) and 4 evergreen (bottom) tree species harvested after 8 months (0–8 months, dashed lines) and 12 months (8–12 months, solid lines) of cultivation under irrigation (blue) and drought (red) treatments. Inserted text in the upper right</figcaption></figure>

of each graph indicates the significance of the highest order
significant interaction from a mixed model analysis within spe-
cies: D (Depth), P (Period: 0–8 or 8–12) and T (Treatment)
correspond to the three factors. Significance levels: * P < 0.05,
** P < 0.01, *** P < 0.001. Error bars indicate ± S.E. (n = 5)

50% of the trees had brown or no leaves at the end of
the experiment.

Assessed as presence or absence of living leaves,
leaf phenology only showed weak and non-significant
correlations with root traits (Fig. S7-S10).

Leaf water potential (LWP)

Treatments had different effects on LWP in the spe-
cies (Table 1). In the drought treatment, *A. senegal*,
*A. nilotica* and *B. aegyptiaca* showed low LWP of ca.
-2 MPa, with less extreme values observed for the
remaining species. Plants in the irrigation treatment
had higher LWP (Table 2), but especially *A. digitata*,
*S. setigera* and *A. occidentale* showed limited differ-
ences in LWP between treatments (Table 2).

Differences between deciduous and evergreen spe-
cies were not significant (Table 1). However, in the
irrigated treatment, LWP showed a positive correla-
tion with maximum depth, and negative correlations
with root biomass, fine root growth, RLD and RLD
growth. The negative correlations between LWP and
maximum root depth indicates that species with large

and deep root systems also had low water potential.
The same trends, but non-significant, were seen in the
drought treatment (Figs. S7 and S8).

Growth and biomass allocation

The treatments affected height growth of the species
differently as indicated by the significant interactions
between species and treatment (Table 1). The height
tended to increase with time for irrigated plants of
the species *A. senegal*, *A. nilotica*, *A. digitata* and *F.
albida* (Fig. S5). However, for the deciduous species
in the drought treatment, the height increased until
November and then decreased with time due to die-
back of shoots (Fig. S5). For the evergreen species,
there was little or no growth from October to the end
of the experiment in either treatment and only minor
differences between treatments. Results for stem base
diameter were broadly similar to those of the height.
Compared to drought, the irrigation treatments
resulted in increased diameters through time. *A. digi-
tata* and *S. setigera* had a fast stem diameter growth in
<figure><img src="image_8.png" /><figcaption>**Fig. 5** Loading plots from principle component analyses of root variables and leaf numbers for the nine tree species at month 8 for the two treatments. Blue color denote irrigated plants, red denote drought</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6** Mean values of leaf numbers for seedlings of 5 deciduous (upper panels) and 4 evergreen (lower panels) tree species across different treatments and measurement times. Error bars denote S.E. *n*=5. Inserted text in the upper right of each graph indicates the significance of the highest order significant inter-</figcaption></figure>

action from a mixed model analysis within species: D (Date) and T (Treatment) correspond to the two factors. Significance levels: * P <0.05, ** P <0.01, *** P <0.001, ns: non-significant
both treatments whereas *V. paradoxa* tended to grow slowly throughout the experiment (Fig. S6).

Height growth showed a negative correlation with root depth, taller species having deeper roots, but was weakly and positively correlated with root growth and RLD (Figs. S7-S10). Except for a correlation with root diameter at month 12 in the irrigated treatment, plant diameter was not correlated to the other variables.

Seedlings tended to have more leaf, stem and root biomass under the irrigation treatment by comparison to the drought treatment, and the biomass of leaves, stems and roots after 12 months varied significantly with the species and the treatments (Table 1). However, species did not react differently to treatments as indicated by the absence of

interactions between species and treatments. On the other hand, according to the statistical analysis, the root:shoot ratio changed in response to stress, as the treatment effects were significant. Likewise, differences between species were significant, with *S. setigera* having high root:shoot ratios and *A. nilotica*, *A. occidentale* and *B. akeassii* having low ratios (Fig. 7). The total biomass was significantly lower in evergreen than in deciduous species, but only in the irrigation treatment. Especially the N-fixing *A. nilotica* and *A. senegal* were fast growing, while *A. occidentale* and *V. paradoxa* had slow growth. However, the differences between evergreen and deciduous species with respect to root:shoot ratio were non-significant (Table 1).

<figure><img src="image_10.png" /><figcaption>**Fig. 7** Total biomass (a, b) and root:shoot ratio (c, d) based on dry weight of seedlings after 12 months of growth. a) and c) Mean values of five deciduous (left) and four evergreen (right) tree species. b) and d) Mean values of treatments. Inter-</figcaption></figure>

actions between species and treatments were non-significant. Deciduous and evergreen are separated by the dashed vertical black line. Error bars denote S.E. (n=5)
## Canonical variate analysis

Our analysis of canonical variates of root traits, despite being conducted separately for each treatment at the two sampling times (months 8 and 12), showed that the positions of species were rather stable in the two dimensions spanned by the two first canonical variates (Fig. S11). The species were located in clusters with *A. nilotica* and to a lesser extent *A. senegal* being associated with high root biomass, while *F. albida*, *B. aegyptiaca* and to some extent *A. senegal* were associated with high SRL. The remaining species tended to be located in the opposite direction, thus associated with lower root biomass and lower SRL. Although the evergreen species were part of this last group, there was no clear separation between evergreen and deciduous species.

## Discussion

Our hypothesis of evergreen species having deeper root systems than deciduous species was rejected, as deciduous species grew faster at depth and tended to have higher root biomass and root length density than evergreen species.

Smith-Martin et al. (2020) showed that evergreen species had deep roots allowing them to access water, suggesting why these species are able to maintain foliage during the dry seasons. This was not the case for the evergreen species included here as the evergreen species tended to have shallow root systems especially when irrigated (Table 1, Fig. 1).

However, the discrimination between deciduous and evergreen species may not be entirely clear. In the deciduous *Tectona grandis*, deep rooting allows trees to extract water and maintain leaves during drought periods in the rainy season, and at the start of the dry season (Clément et al. 2019). Our results showed that water availability affect both leaf longevity and rooting depth. In the irrigation treatment, species normally considered as deciduous such as *A. nilotica* and *A. digitata* kept green leaves throughout the dry season, whereas *B. aegyptiaca* and *A. occidentale* that were supposed to be evergreen did not always carry leaves when subjected to drought stress. It has previously been shown that e.g. *A. digitata* can change to an evergreen habit when water is sufficient (Korbo et al. 2013). Even *F. albida* seedlings in the

non-irrigated treatment lost leaves during the dry season by contrast with what is commonly observed on adult trees that shed their leaves during the rainy season (Roupsard et al. 1999).

Overall, assuming a connection between leaf habit and rooting depth seems to be too simplistic for the species involved in this study. Instead, root growth may depend on other factors that we discuss below.

## Root growth dynamics and biomass distribution

Tree species in drylands characterized by low and irregular rainfall regimes frequently develop deep root systems, which seems to be a key root trait for optimizing water uptake (Reader et al. 1993). Precipitation and high soil humidity increases the production of fine roots of savannas species (Kulmatiski et al. 2017; Bazié et al. 2019; Freycon et al. 2015). Fast root growth is not uncommon (Laminou Manzo et al. 2009), but is here shown for a range of dry zone agroforestry tree species especially in the group of deciduous species. Plants under continuous irrigation produced more biomass, but the allocation pattern to organs (root: shoot ratio) was higher under drought treatment (Fig. 6).

We found important differences in responses to water treatments among species (Table 1). Root traits were markedly affected by treatments as rooting depth in most species increased in the drought treatment (Fig. 1). Furthermore, the rooting profile changed in species that were shallow rooted under irrigated conditions but deep rooted when subjected to drought (*A. digitata*, *A. occidentale*, *S. setigera* and *V. paradoxa*). This facultative deep rooting may explain the high rates of dry season transpiration observed in *V. paradoxa* (Bazié et al. 2018). The deep rooting profiles found in the non-irrigated treatments may be triggered by the low availability of water near the soil surface (Hopmans and Bristow 2002), and are likely crucial for their ability to grow in the dry zones (Chiatante et al. 2001; Schenk and Jackson 2002a, 2005). Also, Lobgo et al. (2011) showed that tree species grow deep roots to catch the deep water resources in the soil under water deficit. On the other hand, *Acacia* species seemed relatively insensitive to soil moisture in this character, developing a deep root system irrespective of treatments.

SRL appeared to increase in response to drought in *A. nilotica* and *S. setigera* while decreasing in *B.*
aegyptiaca, but was otherwise only showing few effects of treatments. This may be affected by our methodology, as we applied a uniform diameter cutoff across all species, using a traditional 2 mm threshold for fine roots (e.g. Ostonen et al. 2011; Lynch et al 2013; Bayala and Prieto 2020). This is advantageous because of a speedy processing of large amounts of roots. However, the diameter threshold at which roots shift from mainly absorptive to transport roles vary between species (McCormack et al. 2015), meaning that a 2 mm threshold as applied in our study cannot be used to compare the root physiology of the species. For this, detailed studies using order-based or functional classifications of roots may be better, and would be especially interesting in agroforestry systems, where these species often share resources with crop species (McCormack et al. 2015). Since absorptive fine roots tend to have a faster turn-over than fine transport roles, this also plays a role for estimates of carbon turnover and may explain why species with large SRL do not necessarily grow faster than species with thicker roots (Weemstra et al. 2020).

## Root systems and leaf water potentials

The Pearson's correlations and the canonical correlation analysis indicated a co-variation in root biomass and RLD across species. SRL and root diameter were negatively correlated, and otherwise not strongly linked to other parameters (Fig. S7-S10). As appears from the canonical correlation analysis, the species can thus be characterized on the basis of whether they have high/low root biomass and RLD, and high/low SRL (but note the limitations on the SRL mentioned above).

Species with high root biomass were *A. nilotica* and *A. senegal*, and low root biomass was seen in a group of species including *B. akeassii*, *A. occidentale* and *V. paradoxa*. In the scale of SRL, *F. albida*, *B. aegyptiaca*, and *A. senegal* showed high values (and low root diameters), while species such as *B. akeassii* and *A. digitata* scored low.

An alternative interpretation of the data seems to be in relation to the water conservation status of the species. Especially the *Acacia* species and *B. aegyptiaca* tended to show strong reductions in predawn water potential under drought (Table 2) and developed deep, fibrous roots. Correlations indicated that low LWP was associated with deep rooting, and with

high root biomass and RLD (Fig. S7). This may be related to the concept of isohydry vs. anisohydry (e.g. Klein 2014), and we suggest combined measurements of predawn and midday water potentials of these species to achieve a better understanding of their water economy (Yi et al. 2017).

Tolerance of low water potentials is one adaptation to arid conditions, allowing *Acacia* species to keep their stomata open and to maintain carbon assimilation rates despite low soil water content (O'Grady et al. 2009). *Acacia* and *Faidherbia* species have the capacity to fix nitrogen (Njiti and Galiana 1996; Founoune et al. 2002), and therefore may have better access to N and faster root growth. Querejeta et al. (2022) found that N-fixing species in the Sahel (including *A. senegal*, *A. nilotica* and *F. albida*) had lower water potentials than non-N-fixing species. They hypothesized that this was because of a larger accumulation of N-containing osmolytes giving them larger tolerance to desiccation. Combined with larger rooting depths as observed in our study, these species may be at an advantage under dry conditions.

*Adansonia digitata*, *S. setigera* and *A. occidentale*, on the other hand, appeared to be water conserving with high water potentials irrespective of treatments. Conservation of water inside the plant such as is observed in *Adansonia* may be another strategy to survive long dry periods (Chapotin et al. 2006). Thus, the smaller rooting depth in *A. digitata* and *S. setigera* may be related to the ability to store water in their tissues, making it less important to attract water from deeper soil layers (Birnbaum 2012). These deciduous species have hypertrophic root systems (tuber-like roots) as seedlings, allowing them to store water and nutrients in their swollen roots, keeping them alive during dry and unfavorable conditions (Ouédraogo and Thiombiano 2012).

## Practical implications for afforestation and agroforestry programs

Deep roots play a crucial role for water flux in dry ecosystems, and through the mechanism of hydraulic redistribution, trees may transport water from deep and wet soil horizons and to dryer parts of the soil, for example shallow horizons (Dawson 1993; Bayala and Prieto 2020). Thereby, deep-rooted tree species may provide water for associated shallow-rooted species without access to the water table. Such capability
may provide a key role for savanna tree species in agroforestry systems. This study showed deep root development of all species in the non-irrigated treatment (Fig. 1), suggesting that they could potentially redistribute water from deeper soil layers towards the surface, depending on their fine root traits. Hydraulic redistribution was demonstrated in *V. paradoxa* (Bayala et al. 2008) but whether the species studied here differ in their capability for hydraulic redistribution remains to be seen. For example, it can be speculated that water-conserving species would tend to limit diffusion of water into the soil in comparison with species that have a profuse water use. On the other hand, species with low predawn water potentials may dry out the soil to a larger degree and could limit the growth of associated non-perennials. This has a profound influence on the way we should think of agroforestry practices.

A decrease of crop yields under tree canopies was shown for *A. digitata* (Sanou et al. 2012), *S. setigera* (Bakhoum et al. 2001) and *V. paradoxa* (Bayala et al. 2002), suggesting that water conserving species are not especially beneficial for crop species. However, according to the authors, reductions of crop yields may also be related to the effects of their excessive shade, and competition for nutrients. On the other hand, there is evidence of increased crop yields under *F. albida* and *A. senegal* canopies that form common components of agroforestry system in the Sahel (Fadl 2013; Roupsard et al. 2020). These species tended to show depletion of soil water in our experiment, but on the other hand are also known for their ability to fix atmospheric nitrogen, to provide medium shade and for their deep root systems reaching the water table (Dupuy and Dreyfus 1992). However, for *A. nilotica*, which seems to be water depleting, Pandey et al. (2000) observed a lower crop yield under the tree and an increased yield with distance from the tree. Clearly, relationships between rooting architecture of trees, water use and associations with crop species merit additional studies to understand the performance of different species in the agroforestry systems.

## Limits of the methods

Root growth in the study was fast, with some species extending to a depth below 1 m only four months after planting, and 2 m after eight months. The cultivation

of plants in long tubes allowed the monitoring of root morphology, shoot traits and leaf phenology through time, and showed that dug-down PVC tubes may be useful as a technique for high throughput root phenotyping (Shashidhar et al. 2012). In spite of the modification of root geometry, obliging them to grow vertically instead of laterally, this approach might contribute to decipher the response of water stress on the whole root system. Limitations of the approach are actually due to the fast root growth meaning that some plants quickly reached the bottom of the tubes, failing to reveal the maximum of their root growth potential. This may have been influenced by the low density of the mixed organic and sandy soil, giving a porous and well-aerated soil allowing fast root growth. Also, the tubes forced the species to grow vertically, modifying the root architecture of species like *A. digitata*, *S. setigera* and *B. akeassii* that tended to have spiraling and shallow roots. By restricting the root horizontal growth, the volume of soil exploited by the trees has been limited, which could make the responses to drought more severe. Nevertheless, the approach has several advantages over the traditional pot cultivation, because root growth is relatively unrestricted at least in one direction, and temperatures in the tubes more closely resemble the natural soil temperatures (Fig. S1). Still, there is a huge need to expand studies of these important species to larger trees in their natural environments.

## Conclusion

Our data provide an overview on the strategy of root growth for nine rarely studied agroforestry tree species under irrigation and water depletion and show the first results on biomass partitioning between juvenile evergreen and deciduous savanna tree species under semi-natural conditions in the Sahel. Overall, our study indicates that drought leads to reduction of root and shoot biomass and deep root growth, which is a mechanism to tolerate water deficits in arid environments. Deciduous species had faster root growth at depth than evergreen species. The LWP measurements suggested water-conserving behavior of *A. digitata*, *A. occidentale*, *S. setigera* and *V. paradoxa* and water spending behavior of *B. aegyptiaca*, *F. albida* and *Acacia* species. Because they tend to develop deep root systems, the latter group may
have an advantage under long periods of drought. The enhancement of deep root growth under drought stress emphasizes that root plasticity is an important characteristic for plant growth and survival in arid ecosystems. However, further investigations combining physiological measurements with root phenotyping and studies on larger trees are needed to confirm our suggestions about the adaptive patterns of these dry zone species.

**Acknowledgements** We are grateful to Marietou Sankhare for conducting the experiment at ISRA/CNRF when FG was absent. Access to root scanner and WinRHIZO software at LCM laboratory was generously granted by director Dr. Saliou Fall, Alexandre Grondin, Ndiampaly Diouf, Mr. Paul Diouf and colleagues. We thank Dr. Cheikh O. Samb for assistance during the setup of the experiment, and Dr. Bo Markussen for help on statistical analyses. Finally, we thank the editor and the anonymous reviewers for their constructive comments that improved the manuscript considerably. The work was supported by the Islamic Development Bank (IDB), grant no. 600035540 and by the University of Copenhagen.

**Authors' contribution** All authors contributed to the study conception and design. Material preparation and data collection were performed by Fatou Gning, while data analysis was conducted by Fatou Gning in collaboration with Anders Ræbild and Christophe Jourdan. The first draft of the manuscript was written by Fatou Gning. All authors commented on previous versions of the manuscript, and approved the final manuscript.

**Funding** Open access funding provided by Copenhagen University.

**Data Availability** Data raw and semi-processed data supporting the findings of this study will be available from the corresponding author.

## Declarations

**Financial interests** The authors declare they have no financial interests that could influence the conclusions of the paper.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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