# Cocoa tree performance and yield are affected by seasonal rainfall reduction

Lucette Adet, Danaë M. A. Rozendaal, Pieter A. Zuidema, Philippe Vaast, Niels P.R. Anten

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Lucette Adet, Danaë M. A. Rozendaal, Pieter A. Zuidema, Philippe Vaast, Niels P.R. Anten. Cocoa tree performance and yield are affected by seasonal rainfall reduction. Agricultural Water Management, 2024, 302, pp.108995. 10.1016/j.agwat.2024.108995. hal-05183135

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# Cocoa tree performance and yield are affected by seasonal rainfall reduction

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Lucette Adet<sup>a, *</sup>, Danaë M.A. Rozendaal<sup>a, b</sup>, Pieter A. Zuidema<sup>c</sup>, Philippe Vaast<sup>d, e</sup>, Niels P.R. Anten<sup>a</sup>

## ARTICLE INFO

Handling Editor - Dr. B.E. Clothier

**Keywords:**

Cocoa production

Water availability

Perennial crop

Potassium application

Ecophysiology

## ABSTRACT

In West Africa, long dry spells are likely to become more frequent and intense as a result of global climate variability, which may significantly impact cocoa tree performance (i.e. morphology, physiology, growth and production), and asks for improved water management. Potassium (K) application may mitigate drought impacts on cocoa, yet knowledge on how shorter, drier wet seasons affect cocoa and on the potential mitigating effect of K application is limited. This study investigated the effects of reduced water availability via shelters and K application on cocoa leaf traits, root growth, reproductive dynamics and yield of cocoa trees in a 6-year old plantation. Two soil moisture levels and two K treatments were considered: a control (no shelter) and sheltered (67 % rainfall reduction) treatment, either with or without 200 kg ha⁻¹ K application. Results indicated rain sheltering significantly decreased soil moisture by 9 %, root interceptions by 50 %, root mass density by ~50 %, stomatal conductance (Gs) by ~60 %, flush intensity by ~70 %, leaf greenness ~48 % and leaf size by ~68 %. While individual pod mass remained unchanged, flower intensity, healthy cherelles, pod, and bean numbers per tree dropped, leading to a dry bean yield reduction from ~2100 kg ha⁻¹ to ~1450 kg ha⁻¹ (~31 %). The beneficial impact of K application was primarily noticeable under control conditions, where it increased Gs, leaf size and greenness. Also, about 35 % of precipitation is intercepted by and directly evaporates from the cocoa canopy and thus never reaches the soil. Future agro-meteorological crop models need to take this into account. Overall, results indicate a strong negative effect of reducing water availability during the wet season on leaf physiology, pod production and cocoa yield. Yields were mostly reduced because of a lower number of pods and much less by an effect on individual pod size. However, K application may not mitigate drought effects and sustaining cocoa yield would require integrated water management.

## 1. Introduction

Cocoa is of great importance for the livelihood of millions of small-holder farmers in West Africa as it serves as a major source of income, contributing to poverty reduction and economic growth (Olwig et al., 2024; Peprah, 2015). It creates economic opportunities for farmers and various actors along the cocoa value chain (Laven and Boomsma, 2012). However, cocoa cultivation faces significant challenges from climate change, leading to less regular rainfall (Schroth et al., 2016). Climate change models predicted declines in rainfall for parts of the West African

cocoa belt, especially in Ivory coast and Ghana (Asante, 2023). Many cocoa-growing areas will face increasingly severe dry seasons (Abdulai et al., 2018; Ajayi and Ilori, 2020; Dai, 2011) as a result of reduced rainfall. In recent years, shifts in the timing of the wet and dry seasons have negatively affected cocoa production resulting to significant socio-economic implications. This year a severe drought followed by heavy rains has drastically reduced cocoa production in Côte d'Ivoire and Ghana, leading to a sharp increase in cocoa prices from $3000/ton to over $10,000/ton (Nanyang Technological University (NTU) 2024; Tabe-Ojong et al., 2024). Shifts in wet and dry seasons will likely reduce

<footnote><sup>a</sup> Centre for Crop Systems Analysis, Wageningen University & Research, P.O. Box 430, Wageningen 6700 AK, the Netherlands</footnote>

<footnote><sup>b</sup> Plant Production Systems Group, Wageningen University & Research, P.O. Box 430, Wageningen 6700 AK, the Netherlands</footnote>

<footnote><sup>c</sup> Forest Ecology and Management Group, Wageningen University, P.O. Box 47, Wageningen AA 6700, the Netherlands</footnote>

<footnote><sup>d</sup> UMR Eco & Sols, Centre de Coopération Internationale en Recherche Agronomique Pour le Développement (CIRAD), Université Montpellier, Montpellier, France</footnote>

<footnote><sup>e</sup> World Agroforestry Centre (ICRAF), Nairobi, Kenya</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: lucetteadet@gmail.com, lucette.adet@wur.nl (L. Adet).</footnote>
suitable cocoa cultivation areas (Abdulai et al., 2018; Bunn et al., 2019), threatening farmers' livelihoods and the sustainability of the chocolate industry (Asante et al., 2017; Peprah, 2015; Ruf and Schroth, 2004). Cocoa trees are sensitive to rainfall variations, leading to reduced performance, yield, and increased vulnerability to diseases and pests (Anning et al., 2022; de Almeida et al., 2016; Mensah et al., 2023). Several studies have examined how variation in water availability impacts cocoa crops. Specifically, these studies have investigated the impact of adding water through irrigation as a potential drought mitigation strategy (Adet et al., 2024a and 2024b; Hutcheon et al., 1973). However, just two studies have explored the effects of reduced rainfall along the production cycle, both in an ever-wet region in Sulawesi (3700 mm annual rainfall; Köhler et al., 2010; Moser et al., 2010). Similar experiments in more arid regions where cocoa is generally grown are needed. In addition, irrigation and rainfall reduction experiments differ in several aspects: in irrigation experiments, soil drought is mitigated, mostly during the dry season and without changing air humidity and temperature (Dai, 2011). In contrast, in rainfall reduction experiments shelters are used to intensify drought stress, usually during the wet season and thus potentially affecting vegetative and reproductive development more strongly.

Cocoa trees exhibit an array of changes in their physiological and morphological traits in response to soil water availability (de Almeida and Valle, 2007; Osorio Zambrano et al., 2021; Zuidema et al., 2005). Drought and water stress have a negative effect on cocoa tree physiology, root dynamics, and yield (Adet et al., 2024a, 2024b; Aikpokpodion et al., 2003; Kummerow et al., 1982; Mommer, 1999; Moser et al., 2010; Niether et al., 2019; Nygren et al., 2013; Omotayo et al., 2018; Rao et al., 2016; Tezara et al., 2016). There remains a critical knowledge gap in understanding the integrated effects of nutrient management, specifically potassium application, under water stress conditions in the context of climate change. This study assesses whether water stress during the wet season can be mitigated by potassium application for mature cocoa trees.

Potassium (K) plays a major role in regulating physiological processes in plants, increasing drought tolerance (Wang et al., 2013). It is an indispensable nutrient for cocoa, among others because it is one of the nutrients that is removed in large amounts from cocoa plantations by harvesting pods (van Vliet et al., 2015). K application has been proposed to mitigate drought effects on cocoa growth and yield (de Almeida and Valle, 2007; Djan et al., 2018), improving water-use efficiency, photosynthesis, and stress tolerance in cocoa seedlings (Kaba et al., 2022). In other crops such as cassava (Chua et al., 2020) and banana (Taulya, 2013), K application has also been shown to mitigate negative effects of drought on crop yield. However, whether these findings also hold for mature trees under field conditions is unknown (Lahive et al., 2019). Previous research in Côte d'Ivoire demonstrated that K application did not mitigate the negative effects of drought on leaf water potential, Gs, and leaf area in mature cocoa trees (Adet et al., 2024a). Building on these findings, this study will now evaluate the impact of K application under water stress conditions during the wet season.

In this study, we assessed the effects of reduced rainfall through rainfall interception and K application individually or interactively on (1) cocoa root traits, (2) leaf physiology and morphology traits, and (3) reproductive dynamics and yield. We also aimed to determine how much precipitation penetrates through the cocoa canopy, and how much is intercepted and directly evaporates without reaching the soil surface. To this end, shelters were used during the rainy season to reduce the amount of rainfall reaching the ground (throughfall and stemflow) under the cocoa trees, and trees were provided either with K fertilizer or not. We hypothesized that reduced rainfall will: (i) decrease root mass density; (ii) lead to lower Gs, lower leaf greenness, smaller leaf area, smaller specific leaf area, and lower flush intensity; and (iii) negatively affect flower intensity and cherelle production, as well as pod, bean production and annual yield. We expect that potassium application will mitigate the effects of reduced rainfall.

## 2. Material and methods

### 2.1. Study area

The experiment was performed at the "Centre National de Recherche Agronomique" (CNRA) research station, situated in Divo, Côte d'Ivoire (5°48' N, 5°18' W) from April 2020 to April 2022. Divo is situated in a forested area of Côte d'Ivoire, known for its favorable climatic conditions for cocoa production (warm temperatures, adequate rainfall, and high humidity). The area has an average annual temperature ranging from 24 – 26.7C with an average rainfall of 1200 mm per year. Rainfall has a bimodal distribution normally with a long wet season from mid-March till late June, a short dry season in July and August, a short wet season from September till November and a major dry season from December to early March (Tosto et al., 2022). However, the rainfall distribution can vary between years.

The experimental design was a randomized block design with six replicates (Fig. S4). The plot consisted of six rows of 24 cocoa trees with two border rows on each side. Within each experimental unit, measurements were conducted on the four central trees. The experiment utilized the 'Mercedes' cocoa hybrid variety, a widely used and improved variety in Côte d'Ivoire known for its high yield (Eskes, 2011) and relative drought tolerance (personal observation). Mature six-year-old trees, arranged in a 3 × 2.5 m planting design (~1320 plants ha⁻¹), were used, with half of the plot subjected to a rainfall reduction treatment under shelter.

Trees were not fertilized prior to the start of the experiment. For all nutrients, national recommendations (Ruf, 2015) were followed to establish fertilizer rates. All the trees received 156 kg ha⁻¹ year⁻¹ of Di-Ammonium Phosphate (DAP) and 487.5 kg ha⁻¹ year⁻¹ of Nitrabor; in total, which amounted to 174 kg N ha⁻¹ and 72 kg P ha⁻¹. Nitrabor not only provides 15.4 % nitrogen (N), 14.1 % Nitrate (NO3), 1.3 % Ammonium (NH4) but also includes 25.9 % calcium oxide (CaO) and 0.3 % boron (B). The potassium application treatment was randomly assigned to the experimental units.

Potassium application was applied using potassium chloride (KCl) at rates of 0 and 206.7 kg per hectare per year, corresponding to 0 and 124 kg of K per hectare. These treatments are referred to as 'without K' and 'with K,' respectively. The fertilizers were applied at a distance of 40 cm around each tree into three equal applications in April, July, and September. The first dose of fertilizer was applied prior to the start of the rainfall reduction treatment in May to provide the necessary nutrients at the onset of the induced dry months. The treatment was implemented as soon as the first rains started after the rain shelters were re-installed. The border tree lines were included to prevent mutual interference between plots and treatments, ensuring that potential effects or interactions between adjacent plots are minimized.

### 2.2. Rainfall reduction treatment

The rainfall reduction treatment reduced the amount of direct rainfall to experimental plots to investigate its effects on cocoa tree responses. To this end, we used shelters of transparent plastic to intercept and decrease the rainfall reaching the ground to mimic water deficit conditions. They were positioned in the middle of the month with distinctly higher rainfall during each wet season. During initial installation, sheltering was delayed by two months. The shelters were removed when the rains stopped. Shelters were installed for 14 months (May to Jul and Sep to Nov 2020; Jan to April and Aug to Sep 2021 and Mar to Apr 2022). Nine of these received over 100 mm of rainfall and five less than 100 mm (see Fig. S1). The months during which the shelters were installed varied across years due to differences in the timing of rainfall.

Plastic shelter bands with a width of 2 m were used to cover the soil surface during the abovementioned months. With a standardized setup, careful positioning, and regular maintenance, the shelters were checked
to ensure uniformity in rainfall reduction. The bands were placed horizontally (48 m long) between tree rows (12 in total) and intercepted 67 % of the throughfall during the 14 months when shelters were on the field (Table S2). The width of the band was constant resulting in a total coverage area of 1152 m² (48 m × 2 m × 12).

The rainwater, intercepted by the plastic bands, was diverted to trenches outside the experimental plot, at the lower side of the slope to prevent water reflux. The volumetric soil water content was measured monthly at 20 cm depth throughout the experiment at the base of all selected sheltered and unsheltered trees, using a FieldScout TDR 350 handheld device.

## 2.3. Rainfall partitioning

We partitioned rainfall into its components: the part that penetrates the canopy directly (throughfall), that runs along the stem (stemflow), and the part that was intercepted by the canopy and then evaporated without reaching the soil (interception loss) (Ward and Robinson, 2000). From an agro-meteorological perspective, this intercepted water contributes to the total amount of evapotranspiration having a cooling and humidifying effect on the cocoa canopy and a turbulence effect on the air above the canopy, but does not contribute to the water supply of the plant. In our study, data on stemflow and throughfall were collected from 42 sheltered trees during the months when the shelters were in place, from which the interception loss was subsequently calculated by subtracting the sum of stemflow and throughfall from the rainfall. To quantify throughfall, water was collected in the plots where shelters had been placed, using 5 litre-capacity polyethylene bottles with an opening of 95 cm² to capture the rainfall that passed through the trees' crown. Seventy-two bottles were installed beneath the canopy at regular intervals, at 0.5 m, 1.5 m and 2.5 m, from the trees in a straight line within each experimental unit (Mensah et al., 2023). The bottles were put at 0.25 m above the ground and thus above the litter layer as well, to prevent water droplets and soil particles splashing in. Stemflow water was collected using plastic petticoat-type gauges wrapped around the trees, as a conduit to channel the rainfall from the trunk to the 25 litres collecting tank. Stemflow was converted from liters per tree (L tree⁻¹) to mm (i.e., L m⁻²) by taking the projected crown area of the trees, which we assumed to be equal to the area available to each tree which when trees are spaced 3 ×2.5 m is 7.5 m². The local rain gauge of the weather station installed at the site served as a rainfall reference. The formulas used to derive interception loss and water availability are included in Supplementary Material Table S2.

## 2.4. Root measurements

Three soil cores per tree were extracted to measure root mass density in a cylinder volume of 441 cm³ (a depth of 20 cm and a width of 5.3 cm), at 20 cm distance from the trees. When the auger hit a big woody root the samples were discarded and resampling was done close by. This sampling procedure may have caused us to miss some of the thicker roots. Then, the roots were separated from the soil with a 2 mm sieve while rinsing with running water, followed by a wash with distilled water to eliminate any residual debris. After washing, the roots were directly weighed to obtain their fresh mass per unit of soil volume and then placed in an oven at 70 °C for a period of 48 hours. After drying, the roots were reweighed again to obtain their dry mass and calculate the root dry mass per unit of soil volume referred to as root mass density onward.

Soil trenches were dug to a depth of 80 cm to examine the root system at different depths. We restricted the analysis to top 60 cm as a pilot trench on cocoa next to our fields showed no significant roots below that depth. The trenches were 60 × 80 cm, with one of the walls located at 20 cm from the nearest tree and approximately 2 m from the next nearest tree. A grid divided into small squares, each with an area of 25 cm², was affixed vertically to the trench walls and aligned

perpendicular to the trees being studied (Tardieu and Manichon, 1986; van Noordwijk et al., 2000). While visually inspecting the soil profile within each grid square (5 cm), the number of root interceptions was recorded as a proxy of root length (Tennant, 1975). This approach is a modification of the Tennant method as it was applied on a soil profile instead of on washed root samples, hence we could not estimate root length itself.

## 2.5. Leaf greenness

The leaf greenness was measured using a SPAD-502 portable chlorophyll meter (Konica Minolta Inc., Tokyo, Japan), which provides a proxy for leaf chlorophyll content. Randomly selected mature and fully expanded leaves from the second-youngest flush were chosen for the measurements. The SPAD meter readings were taken at the midrib of each selected leaf on both leaf sides. Measurements were taken on five unshaded leaves, five middle canopy leaves, and five lower canopy leaves per tree in each treatment within each block. SPAD measurements were taken every month throughout the experimental period.

## 2.6. Stomatal conductance

Gas exchange measurements were performed using a portable SC-1 Steady State Leaf Porometer (Decagon Devices, Pullman, WA, United States). Stomatal conductance (Gs) was measured on the interveinal areolae at the mid-lamina of the abaxial (lower) surface of each leaf. The leaves were intact, green, healthy, mature, exposed to sunlight and without signs of disease or damage. The leaf was allowed to equilibrate for a short period of approximately 30–120 seconds. The reading was recorded once the equilibration period was completed. The leaf sample readings that took longer than 180 seconds to equilibrate were discarded. The Gs measurements were taken on the four central trees between 7:00 am and 9:30 am during both dry and wet seasons. This timeframe was chosen to conduct measurements before the significant decrease in relative humidity and increase in heat. In the wet season, measurements were done on days without rain.

## 2.7. Leaf size measurements

Leaves from three canopy layers (high, mid, low) were randomly sampled during each season (long/short dry/wet seasons) to measure leaf fresh weight, leaf dry weight, and leaf area. Leaf pictures or scans were first taken and then analysed using the image analysis software ImageJ to derive leaf area. Leaf fresh weight was determined, and then leaves were oven-dried at 65°C for 48 hours. The data for the four monitored trees were averaged per tree per plot. Specific leaf area (SLA) was calculated as leaf area (cm²) / leaf dry weight (g⁻¹).

## 2.8. Monitoring cocoa pod development and yield

Four central trees within each experimental unit were selected to monitor pod development and yield during two years. The number of flowers produced in the cushions to characterize flowering dynamics was monitored monthly. A scoring system from 0 to 4 was used to rate the intensity of flowering for each tree. Specifically, a score of '0' indicated the absence of flowers; '1' meant that less than 25 % of the tree's branches and canopy were covered in flowers; '2' indicated that flowers were present on 25–50 % of the tree's branches and canopy; '3' indicated that 50–80 % of the tree was covered with flowers; and '4' corresponded to a coverage of 80–100 % with flowers.

Cocoa pod development was monitored monthly during both minor and major harvest periods, focusing on three pod categories per tree: healthy cherelles, wilted cherelles, and ripe pods, with cherelles indicating young pods in the early stages of development. The number of beans inside each sampled pod (6 pods per tree) was counted, the weight of individual sampled pods and the number of beans per pod were
estimated. Finally, the annual yield per tree was calculated by multi-
plying the number of pods per tree with the beans per pod and the
average individual bean weight (1.5 g).

## 2.9. Statistical analysis

Linear mixed-effects models evaluated the effect of the rainfall reduction treatment, potassium application and their interaction (fixed effects), on cocoa root characteristics, leaf physiology, leaf morphology, production and reproduction as response variables. A random intercept was included, either per tree or per leaf (for leaf traits). Additionally, we used a linear mixed-effects model analysis to evaluate the effect of the rainfall reduction treatment on the seasonal soil volumetric water content. The model included the rainfall reduction treatment, the season and their interaction as fixed effects and a random intercept for each individual measurement location to test whether soil water content depended on shelter treatment and/or seasonality. For each response variable, a model comparison was performed, comparing models with all combinations of fixed effects based on the Akaike Information Criterion (AIC). Models that differed <2 AIC units were regarded to perform equally well (Akaike, 1974; Burnham and Anderson, 2002), and in the case of multiple best models, the model with the least fixed effects was selected. Bootstrapped confidence intervals were calculated to assess significance of the predictors. PostHoc tests were only performed when there was a significant interaction between the shelter and potassium treatment. We determined the proportion of variance explained by the fixed effects in the model, the marginal R-squared marginal (R²m), and the conditional R-squared (R²c) accounting for both fixed and random effects. Statistical analyses were performed using R software version 4.2.2 (R Core Team, 2022); linear mixed-effects models were performed using the glmmTMB package (Brooks et al., 2017).

# 3. Results

## 3.1. Effects of treatments on rainfall components and soil water availability

Annual rainfall varied from 1317 mm in 2020 to 1118 mm in 2021, exhibiting notable monthly variability throughout the study period. Peak rainfall was recorded in June and October 2020, March and October 2021, and April 2022 (Fig. S1D). Relatively large volumes of throughfall were observed during July and October 2020, as well as in March 2021 (Fig. S1A), while significant amounts of stemflow were recorded in June and October 2020, and in March and September 2021 (Fig. S1B). On average, monthly throughfall accounted for 62 % of the monthly rainfall, ranging from 22 % to 91 %, while stemflow accounted for 3 %, ranging from 0 % to 8 %. Consequently, approximately 65 % of the rainfall reached the soil, primarily through throughfall, with the remaining 35 % directly evaporating from the canopy.

In the control plots, water availability for cocoa trees was approxi-
mated as the combined total of throughfall and stemflow (Table S2).
Rainfall was recorded during specific monitoring periods across the
experiment years. In 2020, the average rainfall during these periods was
147 mm per month, resulting in a rainfall accumulation of 883 mm. The
year 2021 showed a decrease, with a monthly average of 103 mm of
rainfall, and a total of 616 mm. In 2022, data was collected for only two
months (March-April); during this period, the monthly average rainfall
recorded was 106 mm, amounting to a total of 211 mm. Interception loss
was significantly higher in shelter conditions compared to the control by
~36 %, which reduced the amount of rainfall that reached the soil
(Table S1). Rainfall retained by the canopy and subsequent loss signif-
icantly influenced the amount of rain water reaching the soil, particu-
larly during periods of low rainfall. Across all years, the values of rainfall
reaching the soil, i.e. water available for plant uptake, were notably
lower in sheltered plots (RRS) compared to control plots (RRC). Lower
RRC values were observed during May, June, and October 2020, as well

as March and April 2021 indicating higher canopy interception.
Conversely, higher RRS values were recorded during the same periods,
indicating greater rainfall reaching the soil surface in the sheltered plots
(Table S1).

The measurement of rainfall distribution is important for understanding the eco-physiological impact on cocoa development. In 2020, the recorded throughfall and stemflow during monitored periods were 489 mm and 17 mm, respectively. Notably, the interception loss was significantly higher in sheltered plots at 722 mm, compared to 394 mm in control plots. This resulted in 507 mm of rainfall reaching the soil (RRC) in control conditions, while only 177 mm (RRS) did so in sheltered conditions. The following year, 2021, mirrored this pattern with total throughfall and stemflow at 446 mm and 24 mm, and interception losses at 170 mm for control plots versus 469 mm for sheltered plots. Consequently, 467 mm of rainfall reached the soil in control plots, in contrast to 170 mm in sheltered plots indicating the substantial impact of drought (shelters) on water availability to cocoa plants. In 2022, the total throughfall and stemflow were 101 mm and 3 mm respectively. Interception loss amounted to 111 mm for control plots, with sheltered plots incurring a greater loss of 178 mm. This led to a reduced RRS of 36 mm, compared to an RRC of 104 mm, further demonstrating the significant influence of shelter on water dynamics within the plots (Table S1; Fig. S1).

In 2020, 20 % of the rainfall recorded during the monitored months reached the soil in the sheltered plots (RRS), while a significantly higher 57 % was observed in the control plots (RRC). In 2021, RRS was 28 % of the total monitored rainfall, indicating that slightly more rainfall reached the soil in sheltered conditions compared to 2020. The RRC for this year was higher at 76 %. In 2022, only 17 % of rainfall reached the sheltered soil, and 49 % reached the control plots (Table S1).

The shelter treatment decreased soil volumetric water content in both dry and wet seasons (Table 1, Fig. S1C; Fig. S2). During the wet seasons, average soil water content was 20.0 % in the sheltered plot, compared to 29.1 % in the control plot. During the dry seasons, a low average soil water content of 16.7 % was observed in the sheltered plot, while the control plot exhibited a much higher average soil water content of 25.7 % (Fig. S1C; Fig. S2).

## 3.2. Treatment effects on cocoa tree functioning

There was a significant shelter by K interactive effect on root dry mass density (Table 1). Control trees had greater root mass density of about 50 % compared to control trees with K and with sheltered trees with or without K (Fig. 1A). Control trees also had significantly more root interceptions compared to the sheltered trees, but the effect of K was not significant (Table 1; Fig. 1B). The negative effect of the sheltering treatment on root interceptions was most apparent in the shallow soil layers, from 0 to 10, 10–20, and 20–30 cm depth, as the root density per 25 cm² in the control trees was greater than in the sheltered trees. This difference disappeared in deeper soil layers from 30 to 40, 40–50, and 50–60 cm depth (Fig. S3).

The shelter treatment and K application had a significant effect on cocoa leaf physiology and morphology (Table 1). Sheltering significantly reduced Gs by ~60 % while K application positively affected Gs by 10 %. The Gs response to K tended to be stronger in the control trees compared to sheltered trees (Fig. 2A; Fig. S5), but the K x shelter interaction was not significant (Table 1). Flushing intensity was significantly and negatively affected by the shelter treatment (Table 1; Fig. 2B), but K did not significantly affect flushing intensity (Table 1). Cocoa leaf greenness was significantly affected by the shelter treatment, K application and their interaction (Table 1). Leaf greenness was lowered by ~48 % in the sheltered than in control trees. When K was applied to the control trees, leaf greenness increased (Fig. 2C; Fig. S5). However, K appeared to have a negative effect in the sheltered treatment. There were significant effects of the shelter treatment, K application, as well as their interaction on individual leaf area (Table 1;
<caption>**Table 1**</caption>

Results of mixed-effect models that assessed effects of rainfall reduction (Rain), potassium application (Pot) and their interaction (indicated with ":") on belowground characteristics (for soil water content, the effect of season was evaluated instead of potassium application), leaf physiology, production and reproduction. ** indicates significant predictors. The marginal (R²m) and conditional (R²c) R squared are indicated.

<table><thead><tr><th>Variables</th><th></th><th>Units</th><th>Predictors</th><th></th><th>R<sup>2</sup>m</th><th>R<sup>2</sup>c</th></tr></thead><tbody><tr><td rowspan="3">Belowground</td><td>Soil Volumetric Water Content</td><td>%</td><td>Rain**</td><td>Seas**</td><td>0.20</td><td>0.32</td></tr><tr><td>Root mass Density</td><td>g cm<sup>-3</sup></td><td>Rain**</td><td>Pot**</td><td>0.05</td><td>0.07</td></tr><tr><td>Root interceptions</td><td>cm</td><td>Rain**</td><td>-</td><td>0.02</td><td>0.62</td></tr><tr><td rowspan="6">Leaf Physiology</td><td>Stomatal Conductance</td><td>mmol m<sup>-2</sup> s<sup>-1</sup></td><td>Rain**</td><td>Pot**</td><td>0.17</td><td>0.32</td></tr><tr><td>Flush Intensity</td><td>-</td><td>Rain**</td><td>-</td><td>0.26</td><td>0.33</td></tr><tr><td>Leaf Greenness</td><td>-</td><td>Rain**</td><td>Pot**</td><td>0.30</td><td>0.35</td></tr><tr><td>Leaf Area</td><td>cm<sup>2</sup></td><td>Rain**</td><td>Pot**</td><td>0.37</td><td>0.42</td></tr><tr><td>Specific Leaf Area</td><td>cm<sup>2</sup> g<sup>-1</sup></td><td>Rain**</td><td>Pot</td><td>0.19</td><td>0.24</td></tr><tr><td>Flowering Intensity</td><td>-</td><td>Rain**</td><td>Pot**</td><td>0.03</td><td>0.16</td></tr><tr><td rowspan="3">Reproduction</td><td>Healthy Cherelles</td><td>-</td><td>Rain**</td><td>Pot**</td><td>0.01</td><td>0.16</td></tr><tr><td>Wilted Cherelles</td><td>-</td><td>Rain</td><td>-</td><td>0.005</td><td>0.19</td></tr><tr><td>Annual Yield</td><td>kg ha<sup>-1</sup></td><td>Rain**</td><td>Pot**</td><td>0.11</td><td>0.31</td></tr><tr><td rowspan="3">Production</td><td>Pod Number</td><td>-</td><td>Rain**</td><td>Pot</td><td>0.03</td><td>0.18</td></tr><tr><td>Pod Mass</td><td>g</td><td>Rain</td><td>-</td><td>0.01</td><td>0.12</td></tr><tr><td>Pod Bean Number</td><td>-</td><td>Rain**</td><td>Pot**</td><td>0.21</td><td>0.44</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Effects of shelters (Control vs Sheltered) and potassium application (With K vs Without K) on root response variables. A- root mass density per unit soil volume; and B- Root interceptions (Control vs Sheltered) and potassium application (With K vs Without K). Values represent means ± standard error. In case of a significant difference between the shelter treatment and potassium application, posthoc tests were performed, with different letters above the bars indicating a significant difference at *P*<0.05.</figcaption></figure>

Fig. 2D; Fig. S6). The presence of the shelters had a negative effect on leaf area, indicating that cocoa trees under sheltered conditions had smaller leaves (~68% decrease). The K effect was small, with an increase in leaf area in control trees and a decrease in sheltered trees (Fig. 2D). The shelter treatment significantly and negatively affected the specific leaf area (SLA), with lower SLA for sheltered trees and no significant effect of K (Table 1; Fig. 2E; Fig. S6).

### 3.3. Treatment effects on cocoa reproduction and yield

The shelter treatment and K application individually had a significant effect on reproduction (Fig. 3, Table 1). Flowering intensity, as indicated by scores (1–5) for the number of cushions with flowers per tree, was significantly influenced by the shelter treatment, K application, and their interaction. Trees in the control treatment produced more flowers (~65% higher score) than sheltered trees, when not fertilized

with potassium (Fig. 3A). The shelter treatment and K application significantly affected the number of healthy cherelles (Table 1). Control trees had a higher number of healthy cherelles (~55%) than sheltered trees (Fig. 3B). Interestingly, K application tended to have a negative effect on the number of healthy cherelles in the sheltered trees, but a weak, positive effect in the control trees. There was no effect of either rainfall reduction or K application on the occurrence of cherelle wilt (Table 1; Fig. 3C).

The annual dry bean yield per tree was significantly reduced by the shelter treatment (Table 1, Fig. 4A). Overall, the yield declined from ~1.6–1.1 kg tree⁻¹ due to sheltering, which, given the density of 1320 trees per ha, would convert into a reduction of roughly 2100–1450 kg ha⁻¹. The number of pods per tree was significantly reduced in the sheltered trees (Table 1; Figs. 4A and 4B). Contrary to the sheltering treatment, K application did not significantly affect the number of pods. There were no effects of either sheltering or K treatment
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Effects of shelters (control vs sheltered) and potassium application (With K vs Without K) on leaf traits: A- stomatal conductance, B- flush intensity scores (the number of new flushes), C- leaf greenness (SPAD values), D- leaf area and E- specific leaf area. Values represent means ± standard error. In case of a significant interaction between the shelter treatment and potassium application, posthoc tests were performed, with different letters above the bars indicating a significant difference at *P*<0.05.</figcaption></figure>

on individual pod mass. By contrast, the number of beans per pod was significantly affected by the shelter treatment, K application, and their interaction (Table 1). Sheltered trees had significantly fewer beans per pod (Table 1 and Fig. 4D). Conversely, K application did not lead to an increase in the number of beans per pod in sheltered trees resulting in greater yield (Fig. 4D). Sheltered trees generally had lower bean yield, due to a lower number of pods and generally fewer beans per pod.

# 4. Discussion

This study investigated the effects of reduced soil water availability by intercepting throughfall, K application and their interaction on cocoa trees. Reduced water availability led to decreased root interceptions and root mass density, with K increasing the latter effect. Shelters also reduced stomatal conductance (Gs), leaf characteristics, and yield, while K application tended to have a positive effect on yield. These findings highlight the negative effects of reduced soil water on cocoa traits and suggest a potential role for K in enhancing yield, although its mitigating effect was not consistently demonstrated.

## 4.1. *Rainfall reduction reduced soil water availability*

Our results show that about 65 % of the rainfall reaches the soil directly through throughfall and stemflow, with the remaining 35 % intercepted by the canopy. This finding is consistent with results for forest ecosystems (Levia and Frost, 2006; Limousin et al., 2008; Mair and Fares, 2010), but is much higher than the 11 % reported by Dawoe et al. (2018), and 24 % by Opakunle (1989) for cocoa plantations. These differences may result from variation in daily rainfall patterns, evaporation rates, and canopy characteristics (Carlyle-Moses and Gash, 2011; Crockford and Richardson, 2000; Imbach et al., 1989; Opakunle, 1989; Ufoegbune et al., 2010). Interception is commonly modelled based on LAI and leaf properties (thickness of water film before drips fall of a leaf tip; Gash et al., 1995) but evaporation rates from forest canopies during precipitation can be high, making the reduction in throughfall plus stemflow relative to rainfall larger than what is stored in the canopy at the end of the rainfall event (Carlyle-Moses and Gash, 2011). However, evaporation of intercepted rainfall will increase air humidity and decrease air temperature and evapotranspiration, in terms of the tree's water balance, at least partially compensating for the reduced water
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Effects of shelters (control vs sheltered) and potassium application (With K vs Without K) on reproduction. A- flowering intensity score per tree (mean score for the number of flower cushions/tree). B- average number of healthy cherelles produced per tree. C- the fraction of wilted cherelles per tree. Values represent means ± standard error. In case of a significant interaction between the shelter treatment and potassium application, posthoc tests were performed, with different letters above the bars indicating a significant difference at *P*<0.05.</figcaption></figure>

capture by the soil (Herbst et al., 2008). For instance, denser canopies or rainfall being distributed over light (but more frequent) showers could lead to relatively more rainfall evaporating after being intercepted by the canopy. This highlights the significant impact of canopy interception on soil water availability, and the importance of considering interception losses in cocoa cultivation water management strategies, particularly in regions prone to drought.

Variation in the fraction of rainfall reaching the soil between different sites also has implications for crop growth modelling of yield responses to rainfall and associated decision support for irrigation or for other types of agro-meteorological models of cocoa-based systems. Most crop growth models, including the CASE2 crop growth model for cocoa (Zuidema et al., 2003), assume that all rain reaches the soil (using daily rainfall as model input). Our results and those of others (Augusto, De Miranda, 1994; Dawoe et al., 2018; Niether et al., 2018), challenge this assumption by demonstrating that not all rainfall directly infiltrates the soil due to interception by the canopy. Note that some process-based agroforestry models like HYPAR, WaNuLCAS and Hi-sAfe (Dupraz et al., 2019; Luedeling et al., 2016) do consider canopy rainfall interception. This interception leads to a portion of the rainfall being retained in the canopy or evaporating before it reaches the soil. As a result, the models that assume all rainfall reaches the soil may overestimate the amount of water available for cocoa cultivation. Effective canopy management, such as pruning to optimize light and rainfall penetration, can help mitigate the negative effects of rainfall reduction.

The shelter treatment not only reduced soil moisture (Fig. S2) during the treatment period but also appeared to have an enduring effect, with soil moisture levels remaining lower than in the control treatment even when the shelters were no longer in use (Fig. S1C). This can be explained by the fact that months without shelters tended to have lower rainfall, thus prolonging the process of rewetting of the soil i.e. water absorption, infiltration, and percolation, after removal of shelters. This 'carryover' effect indicates that short-term changes in rainfall patterns can have prolonged consequences on soil conditions (Niether et al., 2017).

## 4.2. Rainfall reduction affects cocoa tree performance

Cocoa trees have a shallow rooting system (Niether et al., 2019), a characteristic which was confirmed by our results (Fig. S3). Roots were concentrated in the top 40 cm of the soil which is consistent with results found by Schwendenmann et al. (2010). This trait has been related to the relatively low drought tolerance of cocoa as it limits the tree's ability to access deeper, wetter soil layers during dry periods (Kummerow et al., 1982; Moser et al., 2010; Nygren et al., 2013). However, we stress that in our study the number of root interceptions should be regarded purely as a proxy to qualitatively compare root distributions between treatments as it may have underestimated real root lengths due to the modified Tennant approach used.

While deeper root systems could enhance drought tolerance in cocoa (Moser et al., 2010; Smith and de Smet, 2012), there has so far been little exploration of genotypic variation in rooting depth in cocoa (van Noordwijk et al., 2021). In addition, the negative effect of the shelter treatment on root length in the first 40 cm in the soil disappeared in deeper soil layers, which may be due to drought-stimulated root growth in deeper layers which might still have residual moisture. Designing cocoa systems should include strategies to enhance rooting depth and density, such as selecting genotypes with deeper rooting systems.

Sheltering reduced the number of root interceptions and root mass density, probably impacting effective soil resource exploration (dos Santos et al., 2014, dos Santos et al., 2016; Moser et al., 2010; Pinheiro et al., 2005). This is in contrast with results for tomato revealing that less soil water induced the development of more roots (Ahmad and Li, 2021). To our knowledge, no study in cocoa reported on this difference. Typically, plants respond to low soil water by investing more in root production at the expense of shoot and reproductive growth (Comas et al., 2013; Génard et al., 2008; Maguire and Kobe, 2015), and inhibited photosynthate transportation to roots (Hasibeder et al., 2015).

While reduced soil moisture normally triggers adaptive responses in plants like xylem hydraulic signaling and abscisic acid (ABA) production (Liu et al., 2005; Pirasteh-Anosheh et al., 2016), the sheltering treatment may have disrupted these responses by further decreasing soil moisture
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Effects of shelters (control vs sheltered) and potassium application (With K vs Without K) on cocoa production variables. A- annual yield per tree. B- number of pods produced per tree. C- average pod mass. D- number of beans per pod. Values represent means ± standard error. In case of a significant interaction between the shelter treatment and potassium application, posthoc tests were performed, with different letters above the bars indicating a significant difference at *P*<0.05.</figcaption></figure>

levels, making it more difficult for the plant to cope with the stress (Liu et al., 2005).

Our results also showed significant, negative effects of rainfall reduction on Gs, leaf size and leaf greenness, all of which reduce tree photosynthesis. However, Adet et al., (2024a) found a reduction in Gs and sap flux rate (transpiration) which may have led to reduced photosynthesis efficiency. Thus more CO₂ is taken up at the leaf surface per unit of water transpired leading to a higher photosynthetic water-use efficiency (Balasimha et al., 1991; Mensah et al., 2023). K application positively affected Gs, supporting the notion that it plays an important role in stomatal regulation in cocoa seedlings (Anokye et al., 2021). In a complementary study (Adet et al., 2024a), K application was found to increase stem sap flow indicating higher rates of transpiration which could have been mediated by higher Gs.

We found that the reduction in rainfall decreased cocoa leaf greenness and size, potentially indicating drought-induced inhibition of leaf expansion (Kacou Antoine Alban et al., 2016; Fanizza et al., 1991; Lahive et al., 2019; Rolando et al., 2015). The reduction in leaf greenness suggests lower chlorophyll synthesis or increased chlorophyll degradation, impacting gas exchange, light absorption, and

photosynthetic rates (Mensah et al., 2023; Monteoliva et al., 2021). Sheltered trees exhibited lower SLA values indicating thicker and/or denser leaves, possibly adapted to reduced rainfall conditions (Salazar et al., 2018). Furthermore, sheltering resulted in a lower leaf flush intensity, potentially reflecting reduced assimilate availability and active responses to minimize water loss. Taken together, these responses may reduce whole-plant carbon gain (Carr and Lockwood, 2011; Daymond et al., 2002).

### 4.3. Rainfall reduction and cocoa yield

The rainfall reduction treatment negatively impacted several reproductive and yield associated traits including flowering intensity, the number of healthy cherelles, pod number, and bean number per pod, resulting in a noticeable yield reduction. Similar results have been reported by Abdulai et al. (2018a) in West Africa, Gateau-Rey et al. (2018) in South America, Moser et al. (2010) and Wuriandani et al. (2018) in Asia. In our study, trees produced on average ~1 kg of dry beans when water availability was reduced compared to ~1.6 kg in control soil water conditions, representing ~31 % reduction in yield. Despite the
reduction, our yields of 1400–3000 kg ha⁻¹ surpass the national average of 400–800 kg ha⁻¹. This suggests that the experimental conditions significantly boosted productivity above typical cocoa cultivation levels occurring on farms. Moser et al. (2010) found 10 % loss in cocoa yield in response to a rainfall reduction of 53 % over 13 months in Sulawesi. This divergence could be due to our study being conducted in a much drier area (1200 mm annual rainfall vs 2844 mm in Moser et al. 2010). Results of simulations with a physiological crop growth model for cocoa showed that water limitation could lead to 50 % loss in simulated cocoa bean yield, predominantly because of variation in dry season duration (less than 50 mm of rain during the two driest months) as shown by Zuidema et al. (2005). Our study, which decreased soil water availability during the wet season, shows that variation in wet season rainfall may also significantly affect yield. Similarly, the El Niño-Southern Oscillation (ENSO)-related reduction in wet season precipitation caused a 62 % loss of cocoa production in Central Sulawesi, Indonesia (Keil et al., 2008). The variability in cocoa yield responses to rainfall pattern alterations may be linked to differences in the proportion of rainfall effectively reaching the soil.

Sheltering-induced yield reductions were associated with a tendency towards reduction in flowering intensity, and with the production of fewer cherelles and mature pods, which, in turn, also had fewer beans per pod when K was not applied. Reductions in flowering were also reported in field-grown cocoa trees as a result of low soil moisture by Sale (1970), and Schwendenmann et al. (2010). In fact, the peak in flowering in most cocoa growing areas coincides with a peak in rainfall (Young, 1983) with increased flowering intensity during the rainy season and a decline in the dry season (Adjaloo et al., 2012). The effect of water availability on yield is strongly influenced by flowering, the number of cherelles, and pod number, rather than pod mass. This indicates that water availability during the flowering stage is important for determining the number of beans that will set and their distribution per pod. To maximize yield, soil water availability must be guaranteed during the flowering stage when the potential yield is being established for the season, for example through irrigation in drier areas.

Our results show no significant effect of either the shelter or K treatment on cherelle wilt. However, Adet et al. (2024b) documented a contrasting effect, where withholding dry-season irrigation resulted in a reduction of cherelle wilt in mature cocoa trees. Differences could be related to the timing of water deficit application. In the presence of mature or older pods, cocoa trees may prioritize directing their resources towards maintaining the existing pods rather than supporting the growth of new cherelles (Adjaloo et al., 2012; Goudsmit et al., 2023). This preference agrees with the carbon allocation theory proposed by Génard et al. (2008), suggesting that trees allocate resources strategically, potentially resulting in an increased cherelle wilt due to the conservation of energy and resources to sustain mature pods (Lass and Wood, 1985; Valle et al., 1990). When drought reduces the overall number of pods, it may also reduce competition between older and younger pods and thus actually reduce cherelle wilt, as may have happened in Adet et al. (2024b). The shelter-induced water limitation in the current study occurred during the wet season, a time when most of the pods were still relatively young with little competition between older and younger pods. Cherelle wilt may have been mediated by an overall lack of assimilates and their partitioning to reproductive organs in sheltered trees (Adjaloo et al., 2012; Valle et al., 1990). As noted above, this reduction in photosynthesis was the likely result of reduced stomatal closure, leaf greenness, leaf size and flushing intensity (Baligar et al., 2017; Feller and Vaseva, 2014; Mensah et al., 2023). The variation in yield appeared to be related to the variation in pod number and the number of beans per pod. Nonetheless, this relation is less pronounced in the current study compared to the more significant variation observed in previous studies (Adet et al., 2024b), suggesting that the use of pod number as yield proxy may work better under some conditions than others.

### 4.4. Interactive effects of potassium and soil water availability on cocoa performance

K application has been shown to enhance drought stress tolerance of cocoa seedlings (de Almeida and Valle, 2007; Djan et al., 2018; Kaba et al., 2022). When soil water availability is low, an adequate supply of K was found to help plants maintaining various physiological processes in these studies. In our present study, K application had a positive effect, in both control and sheltered conditions on root interceptions, Gs, flush intensity, and pod number, and tended to have a positive effect on yield (Fig. 2 – Fig. 4; Fig. S2; Fig. S3). However, these effects tended to be stronger in the control than in the sheltered trees while some effects, such as the positive effect of K application on root mass density, were only apparent in the control trees. In a previous study, the positive effect of K application on yield was only apparent under high water availability and not under dry conditions (Adet et al., 2024b). Also, contrary to our hypothesis, we found no indication of K being able to mitigate the negative effects of the rainfall reduction on Gs in cocoa trees (i.e., in the sense of these negative effects being smaller in K-fertilized trees). This is consistent with Adet et al. (2024a), who found that K application did not mitigate the negative effects of withholding dry-season irrigation on leaf functioning. Together these findings indicate that K efficiency relies on adequate soil water availability and timing and that it may be context-dependent. However, its specific role as a drought-mitigating strategy in mature cocoa trees merits further investigation.

## 5. Conclusion

The reduction in water availability during the wet season induced by the shelters negatively affected cocoa root growth, leaf physiology, morphology, reproduction and cocoa yield. K application significantly increased yield, but did not fully mitigate the negative effects of drought on tree performance. These results emphasize the potential implications of increased variation and possible reduction in wet-season rainfall volume associated with climate change for cocoa yield. They thus highlight the need for water management strategies to mitigate current and future soil water deficit, and to adopt climate-smart practices to enhance cocoa yield resilience in water deficit conditions.

### CRediT authorship contribution statement

**Pieter Zuidema:** Writing – review & editing, Validation, Supervision. **Philippe Vaast:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Data curation. **Lucette Adet:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Danaë Rozendaal:** Writing – review & editing, Visualization, Validation, Supervision, Methodology. **Niels Anten:** Writing – review & editing, Validation, Supervision, Funding acquisition.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data Availability

Data will be made available on request.

### Acknowledgements

This research was conducted within the framework of the CocoaSoils program (www.CocoaSoils.org), funded by the Norwegian Agency for Development Cooperation (NORAD), Grant number RAF-17/0009-
CocoaSoils. We thank Deo Gratias Hougni for his support in the statistical analysis, and scientific advice. We thank also Kouamé Joachim, Silué Kalourogo Brahima and Milogo Narcisse at CNRA site for their contribution to the data collection.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.agwat.2024.108995.

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