DISCUSSION
In this study we presented the first systematic quantification of major plant functional traits for palms, other monocots, dicots and gymnosperms and analyzed their relationships to leaf size, plant height and seed mass. Phylogenetic signal in functional traits of all plant clades distributed unimodally, except for stem height in the palm clade (Herben et al., 2008). Weak phylogenetic signal of stem height of palms indicates that phylogeny fails to constrain plant height across palm species, reflecting the uniqueness of functional traits of palms across the world (Barrett et al., 2019; Ma et al, 2015; Sylvester & Avalos, 2013). There has been growing consensus that strong phylogenetic signal across species is due to the similar traits of phylogenetically closely related species (Aizen et al., 2015; Fuzessy et al., 2021). The strong phylogenetic signal observed in plant height, leaf size and seed mass essentially reflects the four dominant phylogenetic groups that differ in major functional traits (Collyer et al., 2021; Zheng et al., 2017).
Much progress has been made in recent years outlining the LHS strategy scheme at species level (Klimešová et al., 2015; Westoby & Wright, 2006), but relatively little attention has been given to functional trait patterns across plant clades, especially at genus level. Although trait relationships will become weak when species groupings are merged (Falster & Westoby, 2005), our results provide evidence that seed mass, even at genus level, is positively and closely correlated to plant height and leaf area across plant species within the clade of dicots and other monocots, suggesting the covariations between functional traits among plant species (Falster et al., 2018; Moles et al., 2005). The close correlations between functional traits in these two plant clades support the notion that LHS scheme is unable to describe the major variation of plant traits of dicots and other monocots. Although broad leaves have evolved in gymnosperms, we failed to detect any correlation between seed mass, leaf area and plant height among the clade gymnosperms, implying that the LHS scheme appears to hold for understanding the trait spectra of gymnosperms (Cornelissen, 1999). However, palms appear to be unique phylogenetic group because leaf size rather than plant height is consistently and positively correlated with seed mass (Göldel et al., 2015), suggesting that leaf size is a key driver of variation of seed mass in the palm clade (Wright et al., 2004). Our partial R2 for the logistic regression model provided further evidence that leaf size (i.e., blade length) rather than stem height explained a majority of variation in seed mass across palm species, while plant height contributed more to variation in seed mass than leaf size across species within dicots and other monocots. Moreover, the PCA ordinated identified palms, other monocots, dicots, and gymnosperms in the respective location in a multivariate trait space. Trait correlations within each clade are considered to be caused by divergent patterns of correlated evolution of traits that inherited by its descendant lineages (Messier et al., 2010; Westoby et al., 2002). Therefore, the findings of the current study show that plant species exhibit unique strategy scheme within each clade but variety of ecological strategy schemes across the different plant clades, i.e., palms, other monocots, dicots and gymnosperms.
Seed mass is an important ecological character affecting many aspects of plant ecology (Moles et al., 2005), because it variation can span 10 orders of magnitude across plant species (Rees & Venable, 2007). It is generally believed that seed mass has been the representative of dispersal ability, competitiveness and survival (Zhang et al., 2020), imposing great impacts on plant regeneration strategies and diversity of community. We showed that plant height scales positively with seed mass both in the clades of dicots and other monocots, which is consistent with previous meta-analyses of functional traits showing that plant size and seed mass pattern positively (Díaz et al., 2015; Moles et al., 2005; Pierce et al. 2014). However, a positive relationship between leaf size and seed mass observed in dicots and other monocots is contrary to previous studies demonstrating that seed size does not scale consistently with leaf size (Cornelissen, 1999; Wright et al., 2007). Quantifying plant traits at genus level among different clades or large dataset of species involved in our study may partially explain this discrepancy. By comparing palms with other monocots, however we found that leaf size rather than plant height appears to be a consistent function of variation in seed mass (Santini et al., 2017; Winkel et al., 2001). As the main organ of photosynthesis in plants (Price et al., 2014), the limited numbers of large leaves of palms contribute a lot to seed development (Givnish, 1987; Onstein et al., 2017). The correlation of leaf size with seed mass may reflect strong natural selection for shade tolerance in understory palms (Göldel et al., 2015; Ma et al., 2015). In our study, there is a lack of close correlation of seed mass with plant height and leaf size in the extant gymnosperms, implying that variation in seed size of gymnosperms is mainly structured by dispersal syndrome and cone morphology (Leslie et al., 2017). Another possible explanation can be that extant gymnosperms exhibit a narrow range of seed sizes and lack very small seeds (Moles et al., 2005).
In conclusion, our meta-analysis provides strong support to our prediction that the LHS strategy scheme does not consistently identify plant functional trait patterns across plant clades. However, LHS scheme captures a substantial part of the same spectra of strategy variation within each plant clade. The findings of our study provide important insights into better understanding seed mass correlations with plant height and leaf size across plant clades. These findings also add to our knowledge on the evolution and variation of plant functional traits, which are important in shaping plant life history strategies.