# Light tolerance-related tree growth strategies and their impacts on key functional tree traits: A review

Estelle Nkene Mezui, Loïc Brancheriau, Samuel Ikogou, Rostand Moutou Pitti

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## To cite this version:

Estelle Nkene Mezui, Loïc Brancheriau, Samuel Ikogou, Rostand Moutou Pitti. Light tolerance-related tree growth strategies and their impacts on key functional tree traits: A review. Bioresources, 2024, 19 (4), pp.9946-9963. 10.15376/biores.19.4.NkeneMezui. hal-05182460

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# Light Tolerance-related Tree Growth Strategies and their Impacts on Key Functional Tree Traits: A Review

Estelle Nkene Mezui,<sup>a,b,c</sup> Loic Brancheriau,<sup>b,*</sup> Samuel Ikogou,<sup>c,d</sup> and Rostand Moutou Pitti <sup>a,c</sup>

Light is essential for tree growth and development, yet some species are able, or prefer, to tolerate shady conditions without adversely affecting their development. This ecological preference prompts different strategies in these species, often involving a trade-off between performance and safety in terms of mechanical support, conductivity, and pest and disease protection. Studies on the ecological strategies of trees and their associated functional traits are essential to gain further insight into biodiversity and the processes that shape wood quality from the standpoint of technological properties useful to humans. This literature review showcases current links between ecological preferences and the main functional traits of tree species, with particular emphasis on wood properties.

DOI: 10.15376/biores.19.4.NkeneMezui

Keywords: Ecological strategies; Functional traits; Shade tolerance; Wood properties

Contact information: a: Clermont Auvergne University, CNRS, Clermont Auvergne INP, Institut Pascal, F-63000 Clermont-Ferrand, France; b: CIRAD, UPR BioWooEB, University of Montpellier, 34398 Montpellier, France; c: LaReVaBois, Wood Research and Development Laboratory, 3960 Libreville, Gabon, France; d: Ecole Polytechnique de Masuku (EPM-USTM), B.P. 941 Franceville, Gabon, France;

* Corresponding author: loic分支器iau@cirad.fr

## INTRODUCTION

Trees growing in forest stands compete with neighbouring trees for light, water, and nutrients. This competition affects their growth and the overall functioning of forest ecosystems in terms of biodiversity, carbon sequestration, water regulation through transpiration and oxygen production (Keleş 2019; Imbert et al. 2021). Forests are vital for the biosphere, but also for the diverse range of human applications of wood. During their growth, trees adopt various strategies necessary for their survival at each developmental stage (González-Melo 2022), depending on the species and its surrounding environment. The fundamental biological lifecycle processes (establishment, development, and reproduction) require the uptake of vital resources (energy and elements transfer) and adaptation to prevailing environmental conditions and variations (droughts and storms for instance). Variability in the morphological and physiological characteristics, or so-called functional traits, reflects the diversity in the ways trees perform in carrying out these biological processes. The main functional traits include wood density, specific leaf area (ratio between the surface area and unit dry mass of a leaf), maximum height, photosynthetic capacity, water potential and sap flow (raw and elaborated sap) (Wright et al. 2004; Kunstler et al. 2016; Cachinero-Vivar et al. 2024). Wood density is a functional trait linked to fiber characteristics in hardwoods and tracheids in softwoods. It is used as a determinant of wood quality and varies greatly between species due to their different
porosities. Higher values produce stronger woods, while lower density woods are more suitable for pulp manufacture. Furthermore, hardwoods generally have a higher density than softwoods (Saranpää 2003; Trouy 2015). Specific leaf area reflects leaf thickness. It is a parameter positively linked to the plant's photosynthetic capacity. It can be used to estimate a plant's reproductive strategy based on light and humidity levels, among other factors (Wright *et al.* 2004; Milla *et*. 2008). Photosynthetic capacity is the bioenergetic process that enables plants to synthesize organic matter using light energy, water and carbon dioxide (Wright *et al*. 2004). Water potential corresponds to the energetic state of water in a cell or leaf, for example. It represents the resultant energy of osmotic, turgor and capillary forces, among others (Cachinero-Vivar *et al.* 2024). These performances are assessed in terms of the values of the functional traits related to each function (Violle *et al.* 2007) to ensure the tree's survival, which involves conductivity, support, and pest and disease protection (fungi, insects).

There may be trade-offs between functional traits depending on environmental conditions and ecological preferences (or ecological groups) of the species (Poorter and Kitajima 2007). The ecological strategy of a species is defined by the trade-offs and values of the diverse functional traits. These strategies must be studied to enhance the overall understanding of biodiversity and related ontogenetic mechanisms, and to gain insight into the variability in the properties of the wood produced (wood quality) for various human uses (construction, furniture, packaging, paper, and energy). These strategies have been extensively studied in temperate species (Zimmermann and Brown 1971; Loehle 1988; Kolb *et al.* 1990; Del Tredici 2001; Brym *et al.* 2011; Adler *et al.* 2014; Bianchi *et al.* 2021; Cheng *et al.* 2022; Han *et al.* 2022). Conversely, little is known on the trends in tropical species (Lehnebach 2015; Levionnois 2019; Zue Ondo *et al.* 2021; González-Melo 2022; Han *et al.* 2022; Schmitt *et al.* 2022; Browne *et al.* 2023; Conti *et al.* 2023; Liu *et al.* 2023; Werden *et al.* 2023; Grady *et al.* 2024; Huanca-Nunez *et al.* 2024; Zhang *et al.* 2024), despite the high species diversity hosted in these areas. The aim of this literature review is to highlight current links between ecological preferences and the main functional traits of tree species, while focusing particularly on wood properties. The article is divided into four parts, with the concept of ecological groups described in the first part. Secondly, the links between functional characteristics and ecological groups are examined in terms of mechanical support in the next part, then in terms of hydraulic conductivity in the third part and to end up, in terms of protection against pathogens in the fourth and final part.

LIGHT DEPENDENCY AND ECOLOGICAL GROUPS

Trees are living organisms that require energy for their growth and survival (Rüger *et al.* 2011). Light in forests is highly stratified vertically while being horizontally heterogeneous, largely due to the forest gap dynamics. Light penetrates the forest and disperses from the top to the bottom of the canopy along different gradients (Niinemets and Valladares 2004; Demmig-Adams *et al.* 2017). Trees then use this light for photosynthesis (Fig. 1). Some tree species are able to survive even when there is very little available light available to them, whereas others may tolerate a certain extent of shade at some point during their lifespans without it impairing their development. These so-called understory tree species use a lower light gradient to carry out what is known as minimal photosynthesis (Demmig-Adams *et al.* 2017). Several authors have suggested characterising individuals of tree species based on their average light environment (King
1991; Clark and Clark 1992; Bongers and Sterck 1998; Sterck and Bongers 1998). The need for light is believed to be a key factor in determining species' habitat specialization (Mąderek et al. 2017; Schmitt et al. 2022). Tree species may therefore be classified into different ecological groups according to their light dependence.

A seed and seedling ecology hypothesis was put forward by Swaine and Whitmore (1988) and supported by authors such as Canham (1989) and Hubbell et al. (1999). Under this hypothesis, these authors claim that: “A species cannot be classified in a group without knowing the ecology of its seeds and seedlings,” and they suggest classifying species in two groups, i.e., pioneer and non-pioneer (or climax) species. Pioneers are species whose seeds and seedlings require full sunlight to become established and grow and their plants cannot survive under forest shade at any growth stage (Table 1) (Swaine and Whitmore, 1988; Whitmore 1989). Consequently, they are light-demanding plants. Otherwise, non-pioneer or climax species are those whose seeds and seedlings can become established and grow under forest shade throughout part or all of their lifespans. Their seedlings are thus commonly found underneath a forest canopy (Swaine and Whitmore 1988). Seeds of these non-pioneer species are larger, thereby offering the seedlings more abundant reserves necessary for their growth (Davies and Ashton 1999; Kitajima 2002; Poorter and Rose 2005).

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Comparison of photosynthesis between heliophilous and shade-tolerant species. Photosynthetic light-response ($A_{net}$) measurements for shade-tolerant and shade-intolerant species grown in ambient or elevated CO$_2$ (+300 µmol CO$_2$ mol$^{-1}$). (adapted and redrawn from Sefcik et al. 2006)</figcaption></figure>

The classification into pioneer and non-pioneer groups has since evolved, and most authors propose more nuanced categories of ecological groups. In the study by Blanc *et al.* (2003), the authors classify species in three groups: heliophilous (light-demanding), sciaphilous (shade-tolerant), and hemi-heliophilous (semi-shade-tolerant). The heliophilous trait is easier to distinguish than the sciaphilous trait (Blanc *et al.* 2003). Heliophilous plants require a substantial amount of light to grow. Species that can only grow in the shade are referred to as sciaphilous (Schmitt *et al.* 2022). There is also an intermediate group of so-called hemi-tolerant species. Plants of these species are able to start their life cycle in the undergrowth, but they will only be able to thrive if there is an opening in the forest canopy (Blanc *et al.* 2003). Other authors (Souza *et al.* 2014; Blundo *et al.* 2015; Shen and Cao 2017; Han *et al.* 2022; Wen *et al.* 2023; Jiang *et al.* 2024; Peng
et al. 2024) have proposed various classifications based on morphological traits, growth rates, leaf characteristics, and wood density, among others. These suggest divisions into two, three, or even seven ecological groups.

<table><caption>**Table 1. Character of Pioneer Tree Species in Tropical Rainforests**</caption><tr><td>i</td><td>Seeds germinate only in canopy spaces that are open to the sky and receive full sunlight.</td></tr><tr><td>ii</td><td>Plants cannot survive in the shade - young plants are never found under a closed forest canopy.</td></tr><tr><td>iii</td><td>Seeds are small and produced abundantly and more or less continuously.</td></tr><tr><td>iv</td><td>Seeds are produced early in life.</td></tr><tr><td>v</td><td>Seeds are dispersed by animals or the wind.</td></tr><tr><td>vi</td><td>Dormant seeds are generally abundant in forest soils (especially species with fleshy fruits); orthodox seeds (not known recalcitrant species).</td></tr><tr><td>vii</td><td>High rate of carbon fixation by seedlings; high compensation point</td></tr><tr><td>viii</td><td>Rapid growth in height</td></tr><tr><td>ix</td><td>Indeterminate growth without resting buds</td></tr><tr><td>x</td><td>Relatively few branches</td></tr><tr><td>xi</td><td>Short-lived leaves</td></tr><tr><td>xii</td><td>Shallow rooting</td></tr><tr><td>xiii</td><td>Generally pale, low density, non-siliceous wood</td></tr><tr><td>xiv</td><td>Leaves are susceptible to herbivory, sometimes with little chemical defence.</td></tr><tr><td>xv</td><td>Wide ecological range; phenotypically plastic</td></tr><tr><td>xvi</td><td>Often ephemeral</td></tr></table>

Not all pioneers have all the characters iii to xvi (adapted from Swaine and Whitmore 1988)

Heliophilous species grow faster and reach higher maximum heights (King 1991; Coomes and Grubb 1998; Veneklaas and Poorter 1998; Morel et al. 2018; Schmitt et al. 2022). These so-called canopy species allocate more resources to trunk rather than branch growth, which enables them to rapidly grow in height and compete for light (Poorter et al. 2012). Light competition is therefore beneficial for height growth (Montgomery and Chazdon 2002; Koch et al. 2004; Rüger et al. 2011, 2012). The high growth rate of these species in forest gaps is also favoured by their less developed, short-lived, and physiologically active leaves (Poorter and Bongers 2006; Adler et al. 2014). The findings of Tenius Ribeiro et al. (2019) show that pioneer (i.e., heliophilous) species tend to dominate in nutrient-enriched soils. Soil fertility therefore has an impact on the abundance of this ecological tree group (Xing et al. 2022; Ye et al. 2022).

For sciaphilous (shade-tolerant) species, survival rather than height growth for light is a crucial issue (Montgomery and Chazdon 2002). These species are characterised by their slow height growth (Schmitt et al. 2022) and high radial growth (broader trunk diameter) (Janse-Ten Klooster et al. 2007; Poorter et al. 2012). The high survival rate of these species in the forest understorey is facilitated by the formation of long-lived leaves and larger crowns (Poorter and Bongers 2006; Janse-Ten Klooster et al. 2007; Adler et al. 2014). Broader crowns improve light capture, which contributes to the trees' shade tolerance (Valladares and Niinemets 2008; Niinemets 2010; Poorter et al. 2012).
MECHANICAL SUPPORT FUNCTION

Mechanical support refers to all the mechanisms that enable a tree to support its own mass, maintain its vertical stance through its motricity (Martone *et al.* 2010; Trouy, 2015), while preventing damage from factors such as wind (Fournier *et al.* 2013; Trouy, 2015). The relationship between the tree diameter growth and the formed wood density depends on the species and whether it is a softwood or a hardwood (González-Melo 2022). Density variation within a growth ring is very important in softwoods. This is linked to seasonal changes and the formation of the final wood (Saranpää 2003).

Wood density is a crucial biomechanical feature because it is generally linked to all the other wood mechanical properties (Niklas and Spatz 2010; De Borst et al. 2012). Shade-tolerant species are denser (Lawton 1984; Thomas 1996; Muller-Landau 2004; Osunkoya et al. 2007; Adler et al. 2014) and hence more rigid than light-demanding species. These shade-tolerant species have optimal mechanical properties that provide more effective mechanical support (Table 2). Because of this higher density, shade-tolerant species have a higher survival rate than light-demanding species (Román-Dañobeytia et al. 2021). Wood density variations are often regarded as a trade-off for rapid competitive growth at the expense of wood quality, thereby reducing the trees' safety. However, the biomechanical safety features (bending stiffness, buckling resistance, breakage, or uprooting resistance) involve both tree shape factors and the intrinsic wood properties.

The secondary walls of normal wood consist of the S1, S2, and S3 layers. The S1 layer is made up of several lamellae, within which the cellulose microfibrils are oriented in alternating S- and Z-shaped helical structures (Salmén 2018). The S2 layer is the thickest and richest in cellulose (Salmén 2018). The cellulose fibrils within the S2 layer are almost vertical and all parallel (Salmén 2018). The angle of the microfibrils in this layer is another key parameter that impacts the performance of the mechanical support (Fournier et al. 2013). The S3 layer is also made up of lamellae, like S1. However, the helical texture of the microfibrils is still S-shaped. This layer is mainly made up of lignin and hemicelluloses. An increase in the microfibril angle in the S2 layer reduces the stiffness of the wood and its longitudinal stress resistance (De Borst et al. 2012; Trouy 2015; Maaß et al. 2020; Phan et al. 2024; Wang et al. 2024). This parameter varies according to the tree growth stage. Juvenile wood has a larger microfibril angle and is therefore less stiff than mature wood (Trouy 2015). Other parameters, such as the vessels and lignin content, affect the stiffness of the wood and thereby its mechanical support. A reduction in lignin content would lead to a decline in wood stiffness (Voelker et al. 2011; Özparpucu et al. 2019; Serra-Parareda et al. 2020; Choi et al. 2023). According to Zhang et al. (2023), lignin plays an important role in tree growth. According to the article, lignin deficiency suppresses tree growth through xylem collapse and phloem thickening. As a result, upright plants must biosynthesize lignin to provide mechanical support for secondary cell walls, enabling them to reach great heights. Yadav and Chattopadhyay (2023) support this assertion and add that lignification is an important process that provides the mechanical strength plants need to stand upright (Balk et al. 2023). Otherwise, regarding the vessels, it has been shown that their content, arrangement, and shape may impact the macroscopic wood stiffness (De Borst et al. 2012). There is a clear link between the hydraulic efficiency and mechanical support in relation to the wood physical, chemical, and anatomical properties.
<table><caption>**Table 2.** Mechanical and Physical Properties of Shade-tolerant (n = 20) and Pioneer (n = 10) Rain Forest Tree Species*</caption><thead><tr><th>Properties</th><th>Shade-tolerant</th><th>Pioneer</th><th>p</th></tr></thead><tbody><tr><td>Dry density, vertical stems (kg.m<sup>-3</sup>)</td><td>450.0 (80.0)</td><td>270.0 (130.0)</td><td>***</td></tr><tr><td>Compression strength (N.mm<sup>-2</sup>)</td><td>21.1 (4.5)</td><td>13.3 (8.3)</td><td>*</td></tr><tr><td>Modulus of rupture (N.mm<sup>-2</sup>)</td><td>57.3 (13.5)</td><td>35.8 (24.1)</td><td>**</td></tr><tr><td>Modulus of elasticity (N.mm<sup>-2</sup>)</td><td>5220.0 (1431.0)</td><td>4420.0 (1082.0)</td><td>ns</td></tr></tbody></table>

## CONDUCTIVITY FUNCTION

The vessel size and frequency are linked to the hydraulic conductivity, with larger vessels conducting water more efficiently than narrower ones (Olson and Rosell 2013). However, larger vessels cavitate before narrower vessels (Mauseth and Stevenson 2004). Moreover, the tree height is associated with the vessel size and frequency (Preston *et al.* 2006; Martínez-Cabrera *et al.* 2011). McCulloh *et al.* (2010) found that hydraulic conductivity also varies with the trunk diameter. Although negative correlations have been found between growth-related hydraulic characteristics and wood density (Santiago *et al.* 2004; Hoeber *et al.* 2014), according to Fan *et al.* (2012), anatomical features associated with conductivity are much better descriptors of tree growth rate. Koch *et al.* (2004) and Anfodillo *et al.* (2006) highlighted a link between tree height growth and hydraulic conductivity, with tall trees requiring greater conductivity and therefore larger vessels (Olson *et al.* 2014). Light-demanding species thus have larger vessels than shade-tolerant species (Chave *et al.* 2009; González-Melo 2022). The conduction function is optimal in light-demanding species. The link between vessel size and frequency has been reported by Fan *et al.* (2012); trees with larger diameter vessels have fewer of them. Shade-tolerant species are reported to have narrower and more frequent vessels with a lower hydraulic efficiency (Searson *et al.* 2004; Preston *et al.* 2006; Hacke *et al.* 2006; Pratt *et al.* 2007). Rüger *et al.* (2011) pointed out that little is known about how the growth of species present in species-rich tropical forests differs according to their shade tolerance.

The effects of climatic conditions on tree growth vary between species, and these effects are important for gaining insight into tree stand dynamics (Oboite and Comeau 2021). The different responses of species to drought and frost could mirror the ecological groups to which they belong (Jucker *et al.* 2014; Nourtier *et al.* 2014; Latreille *et al.* 2017). Pfautsch *et al.* (2016) showed that tree height was promoted by the extent of available water. This latter availability would therefore be essential for the growth of light-demanding species. Large air bubbles are more likely to form in large vessels, which means that light-demanding species are more susceptible to freeze-thaw embolism (Pittermann and Sperry 2006; Charra-Vaskou *et al.* 2016). Shade-tolerant species are more drought resistant (Adler *et al.* 2014; Jucker *et al.* 2014), which is facilitated because the vessels are smaller and more numerous, according to Baas *et al.* (2004). Recent studies (Rüger *et al.* 2020; Guillemot *et al.* 2022) have also indicated a natural tendency for slow-growing species to better withstand drought (Fig. 2). In such cases, embolism due to drought stress depends on the pore size in pit primary walls rather than on the vessel size (Cochard and Granier 1999). Levionnois *et al.* (2021) also revealed the low predictive power of vessel

<footnote>*(Adapted from Van Gelder *et al.* 2006); Mean values (± standard deviation) and significance levels (*p*) of *t* test; ns: *p* > 0.05, *: *p* ≤ 0.05, **: *p* ≤ 0.01, ***: *p* ≤ 0.001</footnote>
diameter on resistance to embolism due to drought as compared to the cell wall diameter-thickness ratio.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Correlation between drought tolerance vs. size and growth rate of trees (redrawn from Guillemot *et al.* 2022)</figcaption></figure>

## PROTECTION FUNCTION

The main properties involved in wood durability are the quantity and composition of heartwood extractives (Wink 2010; Martín and López 2023). Extractives are low molecular weight secondary organic compounds (Fig. 3). Secondary metabolites that give the wood its external stress resistance properties are synthesised during the heartwood extractive formation process (Hillis 1971). Such formation and distribution to the heartwood enables it to withstand attacks by fungi and wood-eating insects. This protection enables the heartwood to maintain its strength over time, thus preserving the integrity of the tree. The wood density is also related to its durability (Chave *et al.* 2006; Nock *et al.* 2009). High wood density is associated with enhanced protection against fungal attack (Falster 2006; Larjavaara and Muller-Landau 2010).

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Defence function of plant secondary metabolites (redrawn from Wink 2010)</figcaption></figure>
Most variability in wood durability (including the proportion of heartwood) is genetically controlled (Pâques 2001). Environmental, silvicultural management, and stand site factors have a more limited impact (Harju *et al.* 2001; Taylor *et al.* 2002). The extractive content generally increases from the pith to the outer heartwood, peaking in the heartwood-sapwood transition zone. The extractive content decreases longitudinally with the tree height, whereas it increases as the tree ages (Hillis 1987). The heartwood of species with a long lifespan is more durable (Wheeler *et al.* 1989).

Shade-tolerant species and light-demanding species are both sensitive to external stresses at the juvenile wood stage, yet light-demanding species are more sensitive, given their growth-defence trade-off at this development stage (González-Melo 2022). Tree ageing is linked to tree survival, i.e., a trait of shade-tolerant species. These species will presumably always produce more wood (higher extractive content) than light-demanding species, as confirmed by the findings of various authors (Dudt and Shure 1994; Coley and Barone 1996; Falster 2006; Imaji and Seiwa 2010; Sillett *et al.* 2010; Mäderek *et al.* 2017; González-Melo 2022). This investment in the protective function of shade-tolerant species is reflected, for instance, in their high phenolic compound and tannin contents, as well as their low herbivory levels (Dudt and Shure 1994; Imaji and Seiwa 2010; Mäderek *et al.* 2017). This latter point should nevertheless be considered with caution because studies on heartwood extractive formation products have revealed a range of different strategies regarding this process. In a comparative study of the natural durability of highly durable species from French Guiana, Amusant *et al.* (2014) proposed that there are two types of strategy: extractive quantity vs. extractive quality. These authors suggested that the natural durability of *Vouacapoua americana* is due to its high quantity of extractives, whereas that of *Bocoa prouacensis* is due to the high quality and/or specificity and toxicity of its extractives rather than to their high quantity.

## CONCLUDING REMARKS

Trees - as they grow - fulfil the mechanical support, conduction, and protection functions necessary for their survival. The functional features of trees reflect the different ways in which a specific function can be achieved. A single species cannot be the best performer for all functions and under all growth conditions, which leads us to the trade-off concept. The functional traits of a species reflect its ecological strategy. Previously published studies highlighted the main characteristics of species while sketching a portrait of target species according to their degree of shade tolerance.

Heliophilous species may grow rapidly to reach great heights, while having a slender trunk. These light-demanding species have larger and fewer vessels, which means that their conduction function is efficient and optimal. However, they become more vulnerable to drought-induced cavitation. Sciaphilous species are slow growing with larger trunk diameters (not slender). The crowns are broad and the trees have long-lived leaves, contrary to heliophilous species. These shade-tolerant species have denser and stiffer wood with better mechanical support, which is compatible with their slenderness. Their hydraulic conductivity is, however, less effective given that the vessels are narrower and more numerous. Thus, their xylem is more resistant to cavitation. Sciaphilous species have better pest and disease protection with higher extractive levels alongside their high wood density. Unfortunately, little bibliographical information is available on the protective function.
Overall, minimal research has focused on functional traits associated with the ecological strategies of tropical tree species, despite their great diversity.

The integrative nature of wood density should be highlighted. This technological property appears to be the only feature that dovetails with all tree functions. Wood density is also linked to many of its physical and mechanical properties. Yet the wood density alone cannot explain everything. Rather, trees (and wood) are the result of a complex and extremely refined evolutionary process. These properties, which we have classified as technological, physical, anatomical, chemical, and mechanical, are interdependent. Few studies have jointly focused on all these properties, but gaining greater insight into them would enhance our understanding of different growth strategies and the development of wood quality.

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Article submitted: August 06, 2024; Peer review completed: September 21, 2024;
Revised version received: October 7, 2024; Accepted: October 9, 2024; Published:
October 23, 2024.
DOI: 10.15376/biores.19.4.NkeneMezui