# Interclonal variability in sensitivity to wind breakage: Comparative analysis of the mechanical behaviour of stems of two Hevea clones

Arnauld ENGONGA EDZANG, Rostand Moutou Pitti, Daouda Kouassi, Jean-Luc Hofs, Joseph Gril, Eric Badel

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## To cite this version:

Arnauld ENGONGA EDZANG, Rostand Moutou Pitti, Daouda Kouassi, Jean-Luc Hofs, Joseph Gril, et al.. Interclonal variability in sensitivity to wind breakage: Comparative analysis of the mechanical behaviour of stems of two Hevea clones. Forest Ecology and Management, 2024, 562, pp.121934. 10.1016/j.foreco.2024.121934. hal-04586981

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https://hal.inrae.fr/hal-04586981v1

Submitted on 27 Nov 2024

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# Interclonal variability in sensitivity to wind breakage: Comparative analysis of the mechanical behaviour of stems of two Hevea clones

Arnauld Engonga Edzang<sup>a, b, *</sup>, Rostand Moutou Pitti<sup>b, c</sup>, Daouda Kouassi<sup>d</sup>, Jean-Luc Hofs<sup>d</sup>, Bruno Moulia<sup>a</sup>, Joseph Gril<sup>a, b</sup>, Eric Badel<sup>a, **</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Hevea brasiliensis
Wind
Bending moment
Bending stiffness
Breakage
Biomechanics
Wood

## ABSTRACT

The rubber tree (*Hevea brasiliensis*) is the main source of natural rubber production, with more than 14 million tonnes produced each year. Unfortunately, throughout their economic lifespan on the plantation (between 35 and 40 years), heveas trees are sensitive to wind damage, especially trunk breakage. Over a 30-year period, it is estimated that losses due to the wind breakage of tree stems reach nearly 40%. Industrial hevea plantations are organized in monoclonal plots. Clones are selected based on their productivity, disease resistance, etc., but not for their mechanical behaviour or sensitivity to wind-induced breakage, as breeders lack the appropriate tools to assess these criteria. Field observations highlight variations among clones in their susceptibility to wind breakage, underscoring the increased importance of evaluating mechanical criteria. Among the known causes of these differences in Hevea trees are competition between latex production and growth, crown shape and the presence of tension wood. In this article, we investigated the mechanical behaviour of the stems of two Hevea clones. One, IRCA825, is recognized as very sensitive, while the other, IRCA41, is known for its resistance to wind breakage. We measured biomechanical parameters such as trunk bending stiffness (EI), green wood stiffness (E) and modulus of rupture (MOR). To assess these parameters, we performed bending tests until breakage on standing trees of clonal fields in Ivory Coast, followed by wood characterisation tests in laboratory. A comparative analysis of the bending tests revealed that clones IRC825 and IRCA41 exhibit equivalent trunk bending stiffness. Nevertheless, IRCA41 differs from IRCA825 by having higher green wood stiffness and breaking strength, attributes that mainly explains its higher wind resistance. Comparative analysis of the wood structure indicated that the differences in wood stiffness and breaking strength are mainly attributed to the higher wood density in IRCA41 compared to IRCA825, with no discernible differences in microfibril angle and specific modulus.

## 1. Introduction

The rubber tree (*Hevea brasiliensis*) is a tropical plant native to the moist forests of the Amazon basin that is cultivated for its latex production. It is the only source of natural rubber, which is used in a variety of industries, including the tyre and medical industry (gloves, condoms, etc.). The world production is around 13.8 millions of tonnes/year; the main producers being Thailand (41%), Indonesia (21%), Vietnam (10%) and Ivory Coast (10%) (SIPH, 2024). The world market reaches USD

18.14 billions and could reach USD 22.8 billion in 2029.

Hevea selection is based on several criteria such as growth rate, latex productivity and its physiological indicators, resilience of the exploited stand over time, tolerance to abiotic stresses such as tapping panel dryness (known as dry notch) and tolerance to foliar fungal diseases (Gohet et al., 1996; Déon et al., 2014; Tran et al., 2016). During their economic life in plantations, Heveas plantations are sensitive to wind damage (windthrow and windsnap) (Nicolas, 1990). This factor limits rubber plantations to regions that are not regularly hit by cyclones or

<footnote><sup>a</sup> Université Clermont Auvergne, INRAE, PIAF, Clermont Ferrand 63000, France</footnote>

<footnote><sup>b</sup> Université Clermont Auvergne, CNRS, Clermont Auvergne INP, Institut Pascal, Clermont-Ferrand 63000, France</footnote>

<footnote><sup>c</sup> CENAREST, IRT, Libreville, Gabon</footnote>

<footnote><sup>d</sup> SOGB, SOCFIN, San Pedro, Cote d'Ivoire</footnote>

<footnote>* Corresponding author at: Université Clermont Auvergne, INRAE, PIAF, Clermont Ferrand 63000, France.</footnote>

<footnote>** Corresponding author.
E-mail addresses: arnauld.engonga-edzang@inrae.fr (A. Engonga Edzang), eric.badel@inrae.fr (E. Badel).</footnote>
typhoons. Even in these regions, repeated stem breakages are recorded during episodes of moderate wind intensity (excluding storms). Clément-Demange et al. (1995) report that 30–50% of the initial number of trees can be broken by wind during the life of a plantation. The sensitivity to stem breakage varies from clone to clone (Nicolas, 1990). However, growers and breeders do not have relevant mechanical criteria to assess the sensitivity of trees to wind or to improve their mechanical resistance preventively. Actual climate changes (Vecchi et al., 2019) could make the situation even more critical for hevea plantations.

Knowledge of the biomechanical factors and mechanisms associated with wind-induced tree failure has increased significantly in recent decades (Petty & Swain, 1985; Moore & Gardiner, 2001; Peltola, 2006; Fournier et al., 2013; Gardiner et al., 2016). Experimental and numerical studies have elucidated the contributions of tree architecture, of the root system and the wood material properties, to the biomechanical response to wind (Sellier & Fourcaud, 2009; Yang et al., 2017). The longitudinal distribution of secondary growth has also been studied and was shown to be a central parameter of sensitivity to trunk breakage (Ancelin et al., 2004; Alméras & Fournier, 2009; Dongmo Keumo Jiazet et al., 2022).

To understand the differences in wind breakage sensitivity between Hevea clones, studies have focused on competition between latex production and growth (Gohet et al., 1996; Silpi et al., 2006) or crown shape (Combe & Du Plessix, 1974; Hofmann, 1984; Fourcaud et al., 1999; Milet, 2001). Nicolas and Hofmann suggested that the wood quality, like the presence of tension wood, could explain the differences in wind breakage sensitivity between Hevea clones (Nicolas & Hofmann, 1977). In Heveas plantations, the intensive and repeated latex production induced by tapping strongly affects the biological activity of the tree, particularly primary and secondary growth. Tapping reduces the radial growth by about 50% after only the second week of latex harvesting. Tapping affects the height growth too (Gohet et al., 1996; Silpi et al., 2006). Overall, the growth responses result in a change in the ratio between radial growth and height growth. Crown architecture is also involved in wind breakage of Hevea clones. According to Combe & Du Plessix (1974), breakage-sensitive clones (such as PB 235 and RRIM 600) have a forked and asymmetric crown, strong growth of the primary axis and long, heterogeneous and persistent secondary branches. In contrast, resistant clones (such as GT1 and PB 5/51) show a balanced crown, reduced growth of the primary axis and numerous short, homogeneous and short-lived branches (Combe & Du Plessix, 1974; Hofmann, 1984). A recent complementary study tends to confirm these results (Yun et al., 2019) for PR107 (breakage-sensitive) and CATAS 7–20–59 (breakage-resistant) clones. However, only few studies have investigated the mechanical behaviour of different Hevea clones (Fourcaud et al., 1998; Fourcaud, 2005).

In order to assist growers in selecting wind-resistant clones, this work aims to identify the relevant parameters that explain differences in mechanical behaviour and breakage sensitivity. We focused on the mechanical behaviour of two Hevea clones identified by the planters as showing contrasting sensitivities to wind breakage: one wind-breakage sensitive (WS) clone, IRCA825, and one wind-breakage resistant (WR) clone, IRCA41. We focused on mechanical parameters such as tree bending stiffness, wood material stiffness and breakage-resistant. These parameters were assessed thanks to bending tests on standing trees in plantations based in Ivory Coast. These tests were complemented by laboratory characterizations of the mechanical properties of the wood produced by each clone.

## 2. Materials and methods

### 2.1. In-situ experiments

#### 2.1.1. Selection of experimental blocks and trees

The experiments were performed on a commercial Hevea plantation belonging to the Société des Caoutchouc de Grand-Béréby (SOGB) in

south-west Ivory Coast (Fig. 1). The experimental site is located in a climate zone favourable to Hevea cultivation, with annual rainfall of 1600 mm (from 2013 to 2018) and average monthly variations of 48–581 mm (Perron et al., 2021). It is characterized by a rainfall regime divided into two rainy seasons (a major one from May to July and a minor one from October to November) and two dry seasons (from December to April and from August to September) (Perron et al., 2021). Average annual temperatures from 1961 to 2016 were 26°C (Kouassi et al., 2022).

The experiments were carried out in two blocks of about 45 ha: one block (latitude 4°42'48.8"N, longitude 7°10'36.6"W) for clone IRCA825, planted in 2016; and one block (latitude 4°39'01.6"N, longitude 7°06'41.8"W) for clone IRCA41, planted in 2015. Both blocks showed similar pattern (flat ground, no hill around) in order to ensure they experience the same windy conditions. In each block, the average planting density was 500 trees/ha, with a distance of 2.5 m between two trees and 8 m between two rows of trees. In each block, 20 trees of each clone were selected, spread over an area of approximately 2 ha. IRCA825 was identified by planters as a Wind Sensitive clone (WS), while IRCA41 was known to be Wind Resistant (WR). The selected trees had an average circumference of 50 ±3 cm at 1 m above the ground, which is the minimum circumference required for tapping trees to harvest latex. However, our trees have never been tapped.

#### 2.1.2. Bending test on standing trees

Bending tests on standing trees (Fig. 2) were used to characterize the elastic and fracture behaviour of the trunks of each clone. These tests were carried out in 2022 on 34 trees, 16 WS and 18 WR. These bending tests were carried out using a set of equipment consisting of a cable winch with a capacity of 16000 N (with its steel cable), a synthetic cable (8 mm diameter) with a capacity of 65000 N, a force transducer and strain sensors (PICUS TreeQinetic). A commercial software ensured the automatic data recording at a sampling rate of 5 Hz.

Local deformations on the trunk were measured at different heights ($z= 0.3$ or $0.5$ m, $z= 1.5$ m and $z= 2.5$ m) using strain sensors (elastometers). These were placed on the tensioned side of the bending trunk. The tension cable was fixed in the canopy at a height of between 6 m and 8 m. The applied force was measured by the dynamometer fixed on the tension cable (Fig. 2). Two inclinometers measured the rotation of the cross section, one placed at a height of about 5 m below the point where the force was applied, and the other at the base of the trunk to monitor the rotation of the root-soil system. To prevent uprooting, we fixed a second preloaded cable puller and a dynamometer at the base of the tree (Fig. 2).

#### 2.1.3. Morphometric measurements and mass distribution

The analysis of the bending tests requires several parameters, which are described in detail in (Engonga Edzang et al., 2022). Before each test, the height of the tree, the height of the base of the crown corresponding to the height of the first green branch, the circumference and diameter of the tree at 1 m and the thickness of the bark were measured. Circumference at different heights was measured every 1 m from the base. The mean circumference of the crown, defined as the diameter of the crown projected onto the ground and measured in two perpendicular directions, was evaluated. In addition, we randomly measured biomass distribution for 6 trees (3 for WR and 3 for WS): the mass of the trunk and the branches with their leaves (Fig. 3) were weighed using a load cell (1500 N capacity).

#### 2.1.4. Method for analyzing data from standing bending tests

Stand bending tests provide simultaneous force and strain data (Fig. 4). The raw data were first filtered by a sliding average over a period of 5 seconds in order to reduce the experimental noise.

The force-strain curves (Fig. 4) were then transformed into bending moment-strain curves. The bending moment M was calculated using the measurement data according to the following equation:
<figure><img src="image_4.png" /><figcaption>Fig. 1. a. Map of Ivory Coast indicating the geographical location of the experimental site. SOGB (Société des Caoutchouc de Grand-Béréby) is located in the south-west of Ivory Coast (latitude 4°40'31.638 "N, longitude 7°5'32.151 "W). b. Regular structure of the Heveas plot (IRCA 825).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. Schematic diagram of the fracture test showing the arrangement of the sensors. The elastometers are placed on the tensioned side of the bent stem. The inclinometer at the base of the tree measures the rotation (θrs) of the root-ground system, and the one at around 5 m measures the rotation of the section (θ5m). A second puller is placed at the base of the tree to prevent uprooting.</figcaption></figure>

$$M = F \cos\beta \cdot [z_F - z] + F \sin\beta \cdot [v_F - v(z)] + mg \cdot [v(z_G) - v(z)] \quad (1)$$

where F is the force applied to the tree, β the angle of the cable with the horizontal, m the mass of the crown and the part of the trunk above the strain measurement, g is the gravitational constant; v(z), vF and v(zG) are the horizontal displacement of the section z, of the section zF (where the force is applied) and of the centre of gravity of the crown and upper part of the trunk, respectively, also determined from field measurements (Engonga Edzang et al., 2022). For a section of homogeneous elastic material with Young's modulus E, the strain ε of this section is related to the load by the following relationships:

$$\epsilon = \frac{M}{2EI} \cdot D \quad (2)$$

where the product EI is the bending stiffness of the section, and I =π.D3/D.64 is the squared moment of the section. D// and D are the diameters of the section in the direction parallel and perpendicular to the bending direction, respectively. The bending stiffness refers to the rigidity of the trunk. The maximum longitudinal stress σ at the circumference of the section can be obtained from the previous two equations by the following relationship:

$$\sigma = \frac{M}{2EI} \cdot D \quad (3)$$

Thus, from Eqs. (1) to (3) above, the force-strain curves (Fig. 4) are transformed into bending moment-strain curves and used to extract the bending stiffness EI and the maximum stress at failure.
<figure><img src="image_6.png" /><figcaption>Fig. 3. Measurement of tree biomass, trunk mass and branch (and leaf) mass.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Experimental force-strain curve of a bending test on a standing tree until the stem breaks (blue). Filtering of raw data by a sliding average over a 5-seconds range (green). Strain refers to the recording of the elastometers that provide the relative elongation of the fibers.</figcaption></figure>

## 2.2. Physical and mechanical characterization of wood

### 2.2.1. Sampling

After the standing tree had been broken up, logs approximately 70 cm long were taken from 14 trees (7 trees per clone, randomly selected) at 1 m high. From each log, we sampled one diametral board and two transversal discs. These elements were placed in a hermetic plastic bag, sent to the UMR-PIAF (Clermont-Ferrand, France) and stored in a cold room at around 7°C and 70% relative humidity before the characterization tests.

From each board, we sampled tangential bars of 17 mm × 25 mm × 410 mm (in the radial, tangential and longitudinal directions respectively) for mechanical tests (Fig. 5). Two cubes were sampled at the ends for infradensity (ID) measurements. Among the discs, 6 (3 per clone) were dedicated to microdensity and microfibril angle (MFA) measurements.

### 2.2.2. Dynamic elastic modulus: BING© method

The dynamic modulus of elasticity (E<sub>dyn</sub>) of the bars was evaluated using the BING© method developed by CIRAD (Brancheriau & Paradis,

2007). The method is based on the analysis of the natural frequencies in free vibration, bending or compression, after a one-sided impact (Brancheriau & Bailleres, 2002). The test apparatus consists of an EMU4535 microphone, which picks up the pressure changes induced by the sound waves and converts them into an electrical voltage. The voltage is recorded by a 4224 IEPE picoscope. The sampling frequency was set at 60.1 kHz and the acquisition time was 1.1 s. The system is controlled by 'BING' software. The sample is placed on two flexible supports so that the vibration is as free as possible. Percussion was applied to one end of the sample, in a tangential direction for bending and in a longitudinal direction for compression. The microphone was placed at the other end. Each test was repeated 3 times per sample and the result was the average of these tests.

### 2.2.3. 4-point bending tests

Four-point bending tests were performed to determine strength (modulus of rupture, MOR) using a ZwickRoell testing machine equipped with a 50 kN load cell. This machine was equipped with a video-Xtens 3–300 extensometer, which allowed non-contact measurement of the curvature during the test.

### 2.2.4. Infradensity (ID) and density measurements at ring level

The infradensity is defined as the ratio between the dry mass of a sample and its water-saturated volume:

$$ID = \frac{\text{Dry mass}}{\text{Saturated volume}} \# \qquad (4)$$

To determine the saturated volume, we used the double weighing method using a 0.1 mg precision balance (OHAUS). A container of water was first weighed. Then the sample was immersed without contact with the edges of the container using a needle. The resulting increase in mass gives the volume by applying Archimedes' principle. The dry mass was measured after the sample was dried during 48 hours in an oven at 105°C.

The x-ray microdensity was measured in the Silvatech platform (INRAE, Nancy-France). Strips of 2 mm thickness were taken along 4 radii of a disc starting from the pith (Fig. 5). They were stabilised between 12% and 15% humidity and then scanned with a microtomograph. The X-ray images were analysed using the CERD 2.8 programme (Mothe et al., 1998), which allows the rings to be delineated and the density values of each to be calculated. However, in Hevea trees, as in most tropical species, no ring boundary is clearly visible. So arbitrary rings of 2 mm were drawn. The CERD 2.8 software divided each ring into twenty parts and provided a density value every 100 µm from the
<figure><img src="image_8.png" /><figcaption>Fig. 5. Sampling and flow diagram of the various samples for the determination of mechanical properties (a) and structural characteristics (b, c and d).</figcaption></figure>

pith to the bark.

### 2.2.5. Microfibril angle measurement (MFA)

Radial strips were taken along the R1 and R3 radii, as shown in Fig. 5, and cut from pith to bark with a 5 mm interval. The average of 3 measurements was taken for each sample. MFA measurements were carried out in the Silvatech platform (INRAE, Nancy-France) using an Oxford Diffraction Super Nova diffractometer. The samples were bombarded on the tangential face with a Cu-Kα X-ray beam of wavelength λ = 1.542 Å and spot size of 300 µm in diameter. The results were recorded as diffractograms reflecting the diffraction peaks. The average MFA was correlated with the width of the diffraction peak in the 002 plane of the cellulose according to the expression: MFA = 0.6 T, where T is the half-width of the diffraction peak (Cave, 1997; Zue Ondo et al., 2021).

## 3. Results

### 3.1. Interclonal comparison at tree level

#### 3.1.1. Morphometric comparative analysis

The WS and WR clones had crowns of similar diameter but different height and slenderness (Table 1). The height of the first green branch of the crown was very low for WS compared to WR. Crown height, calculated as the difference between the height of the tree and the first branch height, was higher too for WS than for WR. Both together results in a larger crown area for WS than for WR. In addition, slenderness, defined as the ratio of tree height to diameter at breast height, was significantly higher for WS (99.3) than for WR (93.0).

#### 3.1.2. Bending stiffness (EI) and breaking strength (MOR)

Fig. 6 shows a typical bending moment versus strain curve obtained from bending tests on standing trees. According to Eq. (2), the slope of the linear part is equal to the ratio between the bending stiffness (EI) and

<figure><img src="image_9.png" /><figcaption>Fig. 6. Bending moment vs strain curve during a bending test on a standing tree until the stem breaks. The bending moment is calculated from the force measurements. Mmax corresponds to the maximum bending moment at failure. α is the slope of the linear part of the curve and refers to the bending rigidity of the stem.</figcaption></figure>

<caption>**Table 1**</caption>

Main morphological parameters of IRCA825 (WS) and IRCA41 (WR) trees. Comparison was performed using Student's t-test. m ± sd: mean and standard deviation. p: Student's t-test plus-value; (*): significant difference.

<table><thead><tr><th>Clone</th><th>Age (year)</th><th>Circumference at 1 m (cm)</th><th>Tree height (m)</th><th>Height 1st branch (m)</th><th>Crown height (m)</th><th>Crown diameter (m)</th><th>Slenderness factor</th></tr></thead><tbody><tr><td>IRCA825 (WS)</td><td>5</td><td>56.9 ± 1.5</td><td>17.6 ± 1.0</td><td>3.0 ± 0.5</td><td>14.8 ± 1.1</td><td>7.0 ± 0.9</td><td>99.3 ± 7.0</td></tr><tr><td>IRCA41 (WR)</td><td>6</td><td>56.1 ± 1.7</td><td>16.5 ± 0.7</td><td>4.0 ± 1.1</td><td>12.5 ± 1.2</td><td>7.0 ± 0.8</td><td>93.0 ± 7.2</td></tr><tr><td>p-value</td><td></td><td>p =0.100</td><td>p =0.679</td><td>p &lt;7.04×10<sup>-5</sup> *</td><td>p &lt;2.78×10<sup>-7</sup> *</td><td>p =0.760</td><td>p =0.019*</td></tr></tbody></table>
the half diameter (D//2) of the stem in the bending direction.

Table 2 shows the average bending stiffness of IRCA825 (WS) and IRCA41 (WR) trees measured at 1.5 m height. The lower value with increasing height is explained by the tapering of the trunks shape. The comparison showed that there was no significant difference between the two clones.

From the bending moment data, the maximum stress at the periphery of the trunk at failure was calculated using Eq. (3). The curves in Fig. 7 show the evolution of the maximum stress (MOR) as a function of height for WS and WR trees. For WS trees the MOR increased from 42.2 ±5.9 MPa (z= 0.3 m) to 48.1 ±10.0 MPa (z= 2.5 m) and reached a maximum between 1.5 m and 2 m height. It then decreased with height. The WR trees showed the same trend: the MOR increased from 46.7 ±8.8 MPa (z= 0.3 m) to 60.3 ±13.5 MPa (z= 2.5 m) with a maximum of 66.1 ±12.7 MPa between 1.5 m and 2 m height. These results need to be compared with our observations in the field: in all bending tests, failure occurred between 1.5 m and 2.5 m height for both clones (Fig. 7), with an average height of 1.7 m for WR and 1.9 m for WS. There was therefore a very good agreement between the MOR calculated from the measurements and the break observations in living trees.

We made an inter-clone comparison of the maximum stress around the failure zone (between 1 and 2 m high) in the two clones. Fig. 7 shows that the maximum stress in WR was significantly higher than in WS, regardless of the height of the measurement. We observed significant differences ranging from 23% (at 1.5 m and 2 m height, p= 0.003) to 26% (at 1 m and 2.5 m height, p= 0.011).

## 3.2. Interclonal comparison at the wood and cell-wall scales

### 3.2.1. Wood mechanical properties

To corroborate the observations made at tree level, we tested and compared the mechanical properties measured on wood samples from the two clones in the laboratory. The graph in Fig. 8a shows that the average E<sub>dyn</sub> of WS wood (10.2 ±1.1 GPa) was lower than for WR wood (11.7 ±1.4 GPa). This difference was 14% and significant between the two clones. Comparative analysis of the MOR also enlightened significant differences (Fig. 8b): the average MOR of WS wood (57.4 ±6.8 MPa) was 14% lower than for WR (65.6 ±12.6 MPa). These results therefore indicate that the WR clone produced stiffer (+14%) and stronger (+14%) wood than the WS clone.

### 3.2.2. Comparative analysis at tissue and cell-wall levels

We compared the two clones at the tissue level. In particular, we investigated the radial variation of wood density and the infradensity of the two clones. On the one hand, the Fig. 9a shows that the microdensity in WS increased sharply from pith to bark, indicating that the newly formed wood in this clone was denser than the old wood. On the other hand, the average profile of radial variations in microdensity of WR was very stable; with a very slight decrease from the pith to the bark. In addition, the microdensity in WR was significantly higher than in WS. These results were confirmed by the infradensity measurements (Fig. 9b): the average infradensity of the wood of WS (0.50 ±0.02) was lower (-12%, p=0.0001) than in WR (0.56 ±0.02).

Using modulus of elasticity and infradensity data, we looked for interclonal differences at the parietal scale. We determined the specific modulus E<sub>dyn</sub>/ID, which characterises the intrinsic longitudinal stiffness of the cell wall. Fig. 10a shows no difference (p = 0.354) between the mean specific moduli of the two clones with E<sub>dyn</sub> = 20.5 ±2.3 GPa for WS and E<sub>dyn</sub> = 20.9 ±2.8 GPa for WR. Analysis of the radial MFA profiles showed no difference between the two clones, which showed similar radial trends (Fig. 10b). The profiles were characterised from the pith to the bark by a sharp drop during the first 2 cm, followed by a stabilization up to the bark.

<table><caption>**Table 2**
Mean bending stiffness values for IRCA825 (WS) and IRCA41 (WR) trees. The stem stiffness is measured at different heights of the tree (0.5, 1.5 and 2.5 m). A Student's t-test compares the two clones. p is the p-value of this test.</caption><thead><tr><th rowspan="2">Measuring height (m)</th><th colspan="2">Bending stiffness, EI (kN m<sup>2</sup>)</th><th rowspan="2">p-value</th></tr><tr><th>IRCA825 (WS)</th><th>IRCA41 (WR)</th></tr></thead><tbody><tr><td>0.5</td><td>310.9 ± 63.9</td><td>355.4 ± 95.3</td><td>p = 0.131</td></tr><tr><td>1.5</td><td>232.9 ± 46.9</td><td>221.3 ± 34.7</td><td>p = 0.431</td></tr><tr><td>2.5</td><td>211.1 ± 42.7</td><td>187.4 ± 24.5</td><td>p = 0.064</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Evolution of maximum stress as a function of height for IRCA825 (WS) and IRCA41 (WR) trees. The dots correspond to mean values and the vertical bars illustrate the standard deviations. The two vertical lines (dotted and solid) correspond to the average breaking heights observed in the field respectively for IRCA825 (WS) and IRCA41 (WR).</figcaption></figure>

## 4. Discussion

### 4.1. Interclonal comparison of wind stability of IRCA825 (WS) and IRCA41 (WR) clones

IRCA41 trees (WR) showed a morphology that is more conducive to wind stability than for IRCA825 trees (WS): the WS crowns are less voluminous and their slenderness is lower (Table 1). From a mechanical point of view, this can be explained by two factors: on the one hand, the drag force applied on the canopy increases with the sizes of the crown. On the other hand, this force is directly proportional to the square of the wind speed U. In its simplest form, the drag force is given by (Wessolly, 1991; Fournier et al., 2015; Gardiner et al., 2016):

$$F_Z = \frac{1}{2} \cdot \rho_{\text{air}} \cdot C_d \cdot A \cdot U^2 \quad (5)$$

where ρ<sub>air</sub> is the density of the air and C<sub>d</sub> is the drag coefficient, which includes the density of the foliage. The drag forces thus bring into play the characteristics of the crown through the intercepted crown area A and the drag coefficient C<sub>d</sub>, which represents the resistance behaviour of the crown in the wind.

Furthermore, the greater the lever arm of the drag force, the greater the momentum acting on the trunk. A taller tree crown increases the resulting lever arm and therefore the momentum acting on the trunk.

To calculate a wind safety index, as defined by Fournier et al. (2015), wind speed is a key parameter. However, it is possible to estimate a wind safety index (WSI), for a fixed wind, for clones WS and WR based on the expression proposed by Noyer (2017):
<figure><img src="image_11.png" /><figcaption>Fig. 8. Interclonal comparison of E<sub>dyn</sub> dynamic modulus of elasticity (a) and modulus of rupture MOR (b), measured on green wood samples sampled after tree felling. (*) indicates a significant difference in the two-tailed comparison test. p is the p-value. Bars illustrate the standard error.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. a. Radial evolution of microdensity in IRCA825 (WS) and IRCA41 (WR) trees. Bars illustrate the standard error. b. Interclonal comparison of infradensity values measured on wood samples sampled after tree felling. (*) indicates a significant difference in the two-tailed comparison test. p is the p-value.</figcaption></figure>

$$WSI = \frac{D^3}{R_c \cdot H^2} \qquad (6)$$

where D is the diameter of the tree measured at 1.5 m, R<sub>c</sub> is the crown radius calculated from the measured crown diameter and H is the height of the tree (Table 1). WSI is a parameter that takes into account the dimensions of the trunk diameter, the height of the trees and the spread of the crown, which affects the wind load. The computation of this mechanical safety index indicated that it was 12% lower for WS (4.8 ±0.9)×10<sup>-6</sup> than for WR (5.3 ±0.9)×10<sup>-6</sup> (p = 0.074). Once again, WR showed a structure that seemed to be more wind resistant than WS. This better configuration of WR can be physiologically explained by a different growing strategy that provided a better ratio between diameter allocation and height growth, which stiffened the stem faster than it increased the wind resistance. However, it should be noted that this interclonal difference for the WSI safety index, as for the other morphometric parameters, was measured in a given growth configuration fixed by the date of sampling. This configuration may considerably

vary during ontogeny and may be strongly influenced by silvicultural operations or tapping. This significantly reduces tree growth, with a very significant effect on secondary growth (Silpi et al., 2006). Finally, the impact of tapping on biomass allocation between primary and secondary growth may differ from clone to clone and may lead a modulation of the analysis.

## 4.2. Interclonal difference in the wind resistance of trees

Standing bending tests showed that the bending stiffness (Table 2) of the trunks (EI) of IRCA825 (WS) and WR (WR) were quite similar. This is mainly due to similar trunk diameters, despite the fact that the WS trees were 1 year younger than the WR trees. The bending stiffness of stems characterises their ability to deform the trunk it experiences a bending moment (Niklas, 1992). It depends linearly on the second quadratic moment I, which varies with the diameter in the bending direction raised to the power of three (I ∝ D<sub>II</sub><sup>3</sup>, where D<sub>II</sub> is the diameter in the bending direction). Thus, similar modulus of elasticity and similar
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** a. Interclonal comparison of specific modulus values (Es) measured on wood samples sampled after tree felling. p is the p-value of the two-tailed comparison test. b. Mean profiles of radial variations of microfibril angle in the two clones. Bars illustrate the standard error.</figcaption></figure>

diameters provided the same trunk bending stiffness for the two clones. However, the trees of the two clones differed in their maximum stress that was 23% higher for WR than for WS (Fig. 7). These values show the same order than in Engonga et al., 2023 that reported MOR ranging from 37 to 46 MPa for PB235 and GT1 clones respectively. Similar trends were found in the green wood samples, with E<sub>dyn</sub> and MOR 14% higher in WR than in WS (Fig. 8). These differences in mechanical properties can be explained at the structural level, mainly by the difference in wood density between trees of the two clones (Fig. 9b). Density is usually a good indicator of longitudinal stiffness and wood strength (Guitard & El Amri, 1987; Izekor et al., 2010; Kiaei & Farsi, 2016). This structural information is often used in conjunction with MFA as the key parameter for estimating the longitudinal modulus of elasticity: together an increase in density and a decrease in MFA would result in stiffer, stronger wood. Conversely, the combination of low density and high MFA would result in more flexible wood, i.e. which would have a higher strain at break point (Evans & Elic, 2001; Yamamoto & Kojima, 2002; Yang & Evans, 2003). More generally, previous studies have shown that these two parameters help to understand the mechanical behaviour of trees facing trunk failure (Fournier et al., 2015; Telewski, 2016). In our study, both the results of MFA and specific modulus measurements (Fig. 10) showed no difference in mechanical properties between WS and WR at the parietal scale. Finally, the density of the wood is the key parameter explaining the better resistance to breakage of WR trees.

### 4.3. Growth strategy and resistance to breaking

Wood structure analysis also revealed different growth strategies between the two clones. IRCA825 (WS) differed from IRCA41 (WR) by a strong secondary growth; resulting in equivalent stem circumferences, despite WS trees were one year younger than WR trees. The pattern of variation in wood properties also differed between WR and WS: the newly wood produced by WS was denser, stiffer and stronger than the older wood; while there is no difference between the old and newly wood for WR. Thus, if the current trend would continue, WS could get a favourable profile of radial variation of properties for an improvement of its mechanical behaviour in bending, since the wood formed at the periphery plays a dominant role in the macroscopic behaviour of the trunk. However, the poor quality of the wood of WS, combined with its low density, resulted in poor mechanical behaviour compared to WR trees. This is certainly exacerbated by the tapping process (for latex harvesting) that takes place when the trees reach a circumference of

50 cm at a height of 1 m. Tapping then becomes an additional carbon sink that significantly reduces the secondary growth of trees (Silpi et al., 2006). Tapping thus hinders the strong diameter growth of the WS clone, which would enable it to compensate for the low density of its wood and thus improve the bending stiffness of its trunks.

### 5. Conclusion and perspectives

The primary objectives of this study were twofold: firstly, to compare the mechanical behaviour of two Hevea clones exhibiting contrasting sensitivities to wind breakage (IRCA825, considered sensitive, and IRCA41, considered resistant). Secondly, we aimed to identify relevant structural and mechanical parameters that could elucidate the disparities in mechanical behaviour observed between the two clones.

This work necessitated extensive measurement campaigns on leaving trees; a complex and labor-intensive field task. Several measurements are difficult to get precisely and many assumptions needed to be done during the analysis of the mechanical data; such as the uniformity of the diameter distribution along a trunk, the consistent shape of the crown, among others. Nonetheless, the large number of mechanical tests conducted enabled the identification of differences between clones. The comparative analysis of mechanical behaviour of standing stems showed that IRCA41 trees (WR) exhibited superior overall mechanical behaviour. They displayed higher wood stiffness and higher wood breaking strength compared to IRCA825 (WS) trees. The higher wood density of WR mainly explained its higher properties compared to WS. The combination of lower wood strength in WS trees and larger crown size, which generates higher wind loads, explained their higher sensitivity to breakage compared to WR trees. These results suggest that the wind safety index WSI proposed by Noyer (2017), when combined with wood density measurements, could serve as a reliable indicator of Hevea clones' susceptibility to wind breakage. The WSI provides relevant insights into the tree's morphological structure, while wood density provides an assessment of its wood strength. Monitoring these parameters, especially morphology, over the tree's lifespan- from planting to the end of exploitation- could facilitate the study of natural ontogenic effect and the impacts of silvicultural treatments or tapping.

It's worth noting that investigating wind breakage requires considering the wind-tree interaction, which includes the knowledge of the drag force. Another potential avenue for research could involve recording wind speed profiles within a Hevea tree canopy while simultaneously measuring the deformations induced on the stems. This
experimental approach would enable the accurate determination of the drag coefficient.

Our study demonstrated the relevance of the biomechanical approach in evaluating two clones initially identified by growers for their contrasting susceptibility to breakage. However, we emphasize that these conclusions were drawn from research involving only two clones. Before extrapolating these findings to other clones, such as considering wood density as criterion for clone selection criterion, further studies are necessary to broaden this approach to other clones with varying sensitivities to wind breakage. Regarding density, this expansion could be relatively straightforward, as density measurements can be obtained non-destructively from tree cores.

Other factors play important roles in the stability and mechanical behaviour of trees. Growth stress and the formation of reaction wood, in particular, have a strong influence on the mechanical state of the trunk. For future research, it would be pertinent to investigate the role these factors play. Additionally, trees have a remarkable ability to daily adjust their primary and secondary growth in response to prevailing winds they experience- a biomechanical process known as thigmomorphogenesis. This process likely plays a cruxial role in the mechanical dimensioning of stems. However, only a few studies have investigated the intraspecific variability of this biomechanical process, particularly in Heveas trees. Investigating this aspect would be valuable for providing an additional criterion to planters in their selection process.

Finally, we draw attention to planters that they may have an opportunity to modify their cultural practices. Just as trees require time to adapt their growth to the prevailing wind they experience, we suggest that the planters postpone the tapping process for clones susceptible to breakage, allowing them a longer time to acclimate their growth.

## CRediT authorship contribution statement

**Jean-Luc Hofs:** Resources. **Daouda Kouassi:** Resources. **Arnauld Engonga Edzang:** Writing – original draft, Visualization, Resources, Investigation, Formal analysis. **Bruno Moulia:** Writing – review & editing, Funding acquisition, Conceptualization. **Rostand Moutou Pitti:** Writing – review & editing, Visualization, Methodology. **Eric Badel:** Writing – review & editing, Visualization, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Joseph Gril:** Writing – review & editing, Visualization, Methodology, Investigation, Formal analysis.

## 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 on wind damage affecting cultivated Hevea tree clones in plantations was funded by IFC (Institut Français du Caoutchouc) and its member-companies Michelin, SIPH and Socfin. The authors thanks Stephane Ploquin (PIAF, INRAE) for his help in the experiments preparation and all the technical support provided by the SOGB Company during the experimental campaigns in Ivory Coast.

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