Discussion
Across a broad taxonomic and geographic range of plant species in SW
China, plant growth form and climate region had little power to explain
whether a plant species had fleshy or dry fruit. In contrast, fruit type
exhibited strong phylogenetic conservatism. This conservatism was
distributed over the entire phylogeny; in other words, broad
phylogenetic relationships established at the base of the phylogeny, as
well as recent phylogenetic relationships towards the tips of the
phylogeny, all contributed to the strong phylogenetic conservatism of
fruit type.
Fleshy fruit pulp has been hypothesized to evolve as a defense against
seed predation (Mack 2000), and fleshy fruit are secondarily associated
with endozoochory to promote seed dispersal (Howe & Smallwood 1982;
Onstein et al. 2017). Therefore, weak phylogenetic conservatism
of fruit type might be expected because of the strong selection from
seed predators and frugivore seed dispersers. In contrast to this
expectation, a strong phylogenetic signal of fruit type was detected in
this study which is consistent with some previous studies that focused
at different scales of both geography and taxa (Herrera 2002; Chenet al. 2017). Fleming and Kress (2011) reported that the first
appearance of the core angiosperm families producing fleshy fruits were
earlier than the first appearance of the families of their major fruit
eating animals. Therefore, the phylogeny signal residing towards the
base of the phylogeny might be independent of coevolution with
frugivores, and instead the later-appearing frugivores evolved to take
advantage of the preexisting fruit types. Other fruit/seed traits, such
as seed size, fruit size and fruit color appear to evolve predominately
under the selection pressure of fruit-eating animals; these traits also
show phylogenetic signal, which implies that selection may generate
phylogenetic signal if related species are under the same selective
pressures (Chen et al. 2004; Moles et al. 2005). In our
study, the majority of families (67.8%) exclusively produce fleshy or
dry fruits, with shifts between dry and fleshy fruit occurring in the
remaining families.
We had expected both plant growth form and climate region to explain
much of the variation in fruit type. Our analyses suggest that the
reason they do not provide much explanation differs between the two
factors. Like fruit type, growth form itself shows strong phylogenetic
conservatism that is distributed over the phylogeny (Fig. 3). When
phylogeny is not included, growth form explains 22.8% of the variation
in fruit type, while this drops to 1.5% when phylogeny is included
(Table 2). As another useful comparison, the partial
R2lik for growth form is 1.7% when
the full phylogeny is used, but this increases to 25.1% when the
0.67-threshold phylogeny is used which removes the phylogeny information
in the top 1/3 of the phylogeny. These two patterns suggest that fruit
type and growth form are evolving largely independently, and their
apparent association in the raw data – with 55.6% of the wood
species and only 9.5% of the herbaceous species having fleshy fruit
– is due largely to the phylogenetic conservatism of both traits.
In contrast to growth form, the climate region in which species occur
does not show strong conservatism. What conservatism tropical and
temperate climate regions have occurs towards the base of the phylogeny,
implying that there are major clades being associated with tropical and
temperate regions, but at finer scales towards the tips of the
phylogeny, the role of phylogeny diminishes (Fig. 3). Thus, the
explanation for climate region having little effect on fruit type is
that climate region is relatively phylogenetically labile, changing up
the phylogeny more rapidly than fruit type, especially towards the tips.
In contrast to our study, previous studies have shown effects of climate
region (or environmental factors in general) on fruit type (Willsonet al. 1989; Bolmgren & Eriksson 2005; Chen et al. 2017;
Zhao et al. 2018). There are several possible explanations for
our differing results. We confined our study to plants occurring in SW
China, a region known for its biogeographic and
topographic/environmental diversity (Zhu 2012). Because tropical,
subtropical, and temperate climate regions are in close proximity, we
suspect that dispersal of species among regions is rapid on the
phylogenetic time scales represented by our study plants. This suggests
that dispersal and local speciation may be responsible for the lack of
phylogenetic conservatism of those traits that determine the climate
region in which species live. Thus, the traits that determine success in
a climate region are evolutionarily labile, and the trait of fruit type
plays at most a minor role in determining success in climate regions.
There may also be methodological explanations for why our results differ
from many previous studies, that have investigated phylogenetic patterns
for traits using different taxonomic levels, for example in nested
ANOVAs (Chen et al. 2004; Valverde-Barrantes et al. 2017).
Our approach allows a finer-scale assessment of the role of phylogeny.
For example, using the 0.67-threshold phylogeny, in which phylogenetic
information above roughly the family level is removed, growth form was a
moderately strong predictor of fruit type, explaining 25.1% of the
variance. However, using the full phylogeny, this dropped to 1.7%.
Therefore, if nested ANOVAs did not account for the fine relationships
towards the tips of the phylogeny, then they would miss the full effects
of phylogenetic conservatism.
Exploring the drivers of the geographic variation in plant functional
traits, especially along latitude or/and altitude gradients, has long
attracted attention in eco-biogeography (Reich & Oleksyn 2004; Moleset al. 2007; Moles et al. 2009; Diaz et al. 2016;
Valverde-Barrantes et al. 2017). Several models have been
introduced to account for phylogenetic non-independence among species,
although they have not quantified the contribution of phylogeny itself
to trait variation (Swenson & Enquist 2007; Chen et al. 2017).
However, quantifying the effect of phylogeny in comparison to other
factors is useful (Valverde-Barrantes et al. 2017; this study).
Partial R2s can disentangle the relative contributions
of both predictor variables (climate region and growth form in this
study) and covariances (phylogeny and other possible random effects).
Combined with the findings from recent empirical studies on other
important plant functional traits (Moles et al. 2005; Swenson &
Enquist 2007; Valverde-Barrantes et al. 2017), our results
suggest that methods using the full phylogeny rather than earlier
methods such as nested ANOVAs have more power to explore the
phylogenetic patterns of functional traits.