Discussion
By conducting a meta-analysis of studies of 1071 plant species, our
results suggest phylogenetic conservatism for the majority of plant
traits tested in this study, i.e., seed mass, plant height, leaf area,
genome size, and leaf N, as observed in previous studies (Moles et al.,
2005; Swenson & Enquist, 2009; Davies et al., 2013; Wang et al., 2022).
This is most likely due to the similar traits of phylogenetically
closely related species rather than the similarity of traits at higher
taxonomic levels. Apart from phylogenetic signal of these traits, we
showed that growth form of the 1071 species was not randomly distributed
but followed a Brownian model of trait evolution, indicating that the
closely related species are more likely to have similar growth form than
might be expected by chance (Kerkhoff et al., 2006). The phylogenetic
signal carried by plant growth form may potentially explain the
differences in seed mass, plant height, and leaf N between the woody and
non-woody plant species.
Previous studies have looked at the patterns of associations of seed
mass with other plant traits (Westoby, 1998; Guo et al., 2010; Santini
et al., 2017), however, few studies have incorporated phylogenetic
affiliations in gauging the correlations between seed mass and several
attributes. By controlling phylogeny, plant height appears to be a
reliable predictor of seed mass across species based on the results of
PGLMM. Although dispersal mode and growth form may modify the pattern of
association of plant height with seed mass (Thompson & Rabinowitz,
1989; Leishman & Westoby, 1994c; Leishman et al., 1995), we found
consistent positive correlation between seed mass and plant height. The
positive relationship between seed mass and plant height may facilitate
long-distance seed dispersal because seed dispersal distance is more
strongly correlated with plant height than with seed mass (Thomson et
al., 2011).
Plant height is also closely related to leaf area (Falster & Westoby,
2003), therefore positive relationships between leaf area and plant
height are likely to lead to a positive correlation between seed mass
and leaf area. In our study, there were also consistent and positive
correlations between seed mass and leaf area, suggesting that the
correlations between leaf area and seed mass are conserved across
life-forms. These patterns accord well with independently gathered data
on the relationship between seed mass and leaf area both in the woody
and annual species (Senn et al., 1992; Niinemets & Kull, 1994;
Cornelissen, 1999; Santini et al., 2017). The positive relationship
between seed mass and leaf area may reflect the great contribution of
photosynthetic capacity and nitrogen content of leaves.
The role of the relationship between genome size and seed mass has
gained much less attention over the two decades (Moles et al., 2005a, b;
Beaulieu et al., 2007). Despite several studies that found a quadratic
relationship between genome size and seed mass (Beaulieu et al., 2007;
Knight & Beaulieu, 2008; Krahulcová et al. 2017), our GLM and PGLMM
models showed positive association between genome size and seed mass
across 1017 species. This discrepancy is largely due to the fact that
previous studies failed to incorporate phylogeny into this
consideration. To understand the forces shaping the evolution of seed
mass, we will also need to consider other plant traits, such as leaf N
and growth form. Without controlling phylogeny, seed size was associated
with growth form and woody plants tended to have larger seeds than
smaller herbaceous plants, possibly due to the larger height of woody
plants than of herbaceous species (Jurado et al., 1991). However,
incorporating phylogenetic affiliations into the model failed to detect
the clear association between seed mass and growth form across the plant
species, indicating that phylogeny conceals the effect of growth form on
the variation in seed mass. This finding may not be in agreement with
the observation that variations in seed mass are consistently associated
with those in growth form (Mole et al., 2005).
Despite the strong phylogenetic signal in several plant traits, our
meta-analysis successfully teased apart the relative contributions of
phylogeny, plant height, leaf area, genome size, leaf N and growth form
on explaining variations in seed mass across the plant species. We first
showed that phylogeny had much more power to explain variations in seed
mass than did other plant traits, whereas plant height, leaf area and
genome size only explained the minority of these variations although the
leaf-height-seed (LHS) scheme states that plant height and leaf area are
closely correlated with seed mass (Westoby, 1998). Growth form and leaf
N explained little variation in seed mass, reflecting the main effect of
phylogeny on affecting seed mass variation. Therefore, our study
suggests that divergences in seed mass have been more closely correlated
with phylogeny than with divergences in other plant traits. If this
pattern holds equally for plants of different taxa, investigation on the
correlations between plant traits should not ignore the contribution of
phylogeny.
Together, our results indicate that although there is a significant
phylogenetic signal in plant traits (seed mass, plant height, leaf area,
genome size, leaf N and growth form), phylogeny appears to explain
variations in seed mass better than these explanatory variables. This
implies that phylogeny has acted as one of the most important factors
that govern the variations in seed mass across plant species. Our work
is expected to open the door to further investigate the contributions of
phylogeny and explanatory attributes to the variation in given plant
traits.