INTRODUCTION
Plant functional traits (i.e., plant height, leaf area and seed size) have been considered as potentially powerful indicators of the ecological processes of species, which can also be used as indicators or reference for the maximum information of plant growth and resource utilization strategies (Adler et al., 2013; Kooyma et al., 2010; Navarro et al., 2010; Wilson et al., 1999). Plant traits have become a core attribute to determine plant strategies and then to understand and predict the evolution, distribution as well as ecological strategies of plant species at the scale of population, community and ecosystem (Chapin et al., 2000; Gaudet & Keddy, 1988; Kunstler et al., 2015), because they directly affect the basic behavior and function of plants, and reflect the survival strategies formed by plants adapting to environmental changes (Ackerly & Cornwell, 2007). Plant strategies can be quantified by measuring various functional characteristics that affect plant fitness and ecological processes (Lavorel & Garnier, 2002). Westoby chose the plant ecological strategy scheme (LHS), i.e., the use of three functional traits (Westoby, 1998), specific leaf area (SLA), plant canopy height and seed mass as representing three fundamental and relatively independent axes of a plant’s ecological strategy to classify plants according to meaningful axes of plant specialization (Díaz et al., 2015; Laughlin et al., 2010; Vendramini et al., 2002). Later on, ecologists have carried out a number of studies on the relationship between plant height, leaf area and seed mass (Lavergne et al., 2003; Wang et al., 2014; Wolf et al., 2022), and found consistent relationship between plant traits, which further improves our understanding of plant adaptation strategies (Koch et al., 2004).
Plant traits represent an outcome of evolutionary processes, therefore its distribution reliably reflects their evolutionary history and phylogenetic constrains (Larson et al., 2020; Reinhart et al., 2012; Wang et al., 2008). Phylogenetically related species share a common evolutionary history and may therefore have similar traits (Ibanez et al., 2016; Losos, 2008). Although the whole point of the scheme is that the LHS variables are not necessarily correlated with each other, much of the literature has provided evidence that the correlated evolutionary divergence of traits has led to trait correlations across plant species (Gingerich, 1974; Revell et al., 2008; Xia et al., 2022), such as correlation between leaf area and seed mass (Laughlin et al., 2010)12. McCarthy et al. (2007) and Reich et al. (2014) have shown that woody gymnosperms invest relatively more in leaves than woody angiosperms. Poorter et al. (2012) provided further evidence that herbaceous monocots have lower leaf mass fractions than herbaceous eudicots because dicots invest relatively more than monocots in leaves. Moreover, Damour et al. (2016) showed that dicots had higher seed mass than monocots. Differing from the other monocots (Raven, 1988; Tomlinson, 2006), palms build their tall primary stature and exhibit unique features such as leaf development and anatomical characteristics, and possibly the correlation of seed mass with leaf area and plant height (Cámara-Leret et al., 2017; Moore, 2003; Sampaio & Scariot, 2008). Therefore, it remains debatable if plant species from different clades will follow a specific LHS scheme at a higher classification level (e.g., genus), though variety of ecological strategy schemes have been proposed across plant species.
An important goal of plant ecology is to separate the key dimensions of ecological variations across species and then to understand how and why they function and vary between species. For example, the widely used LHS scheme of Westoby propose that each dimension of LHS vary widely between species at any given level of the other two, but it is not sufficient to describe the main axes of trait variation of temperate woody species (Westoby, 1998). Therefore, investigating the correlation of trait characters in different plant clades will provide a sound basis further our understanding of the evolution of functional characters among plants (Pierce et al., 2014; Reich et al., 1999; Tjoelker et al., 2005). However, to our knowledge, no study has so far investigated plant trait variation across different clades, especially using large datasets in the context of LHS scheme and phylogeny.
Consequently, it is still highly uncertain whether traits of different plant clades will fit a specific LHS scheme. Or, do all plant species within a specific clade support a plant ecological strategy scheme (LHS)? Although several authors have investigated LHS scheme within each clade such as palms, angiosperms, gymnosperms, annuals, perennials, herbaceous or woody plants (Cámara-Leret et al., 2017; Falster & Westoby, 2005; Kawai & Okada, 2020; Laughlin et al., 2010), but to our knowledge no study has so far used large datasets to investigate correlations of plant traits across different plant clades (i.e., palms, other monocots, dicots, and gymnosperms). We addressed these questions by conducting a meta-analysis of functional traits (plant height, leaf size, and seed mass) from four plant clades, i.e., palms, other monocots, dicots and gymnosperms with contrasting growth forms. The primary aim of the current study was to understand if and how the different plant clades are coordinated along the plant ecological strategy scheme. Specifically, we first used phylogenetic generalized linear mixed models (PGLMM) and partial R2lik logistic regression model, to explore how potential forces drive variation in plant traits between different clades, so as to better understand evolution and correlation between functional traits among sets of plant species (Zheng et al., 2009). We expected that each clade of plant species will share the same plant ecological strategy scheme, while LSH scheme would differ across different plant clades.