4.1 Variation in leaf functional traits and trade-off
relationships
We found that the mean values of nine leaf functional traits of 18
dominant tree species in this community were within the ranges of global
leaf trait variation (Chen et al., 2016; Zhao et al., 2020), and it was
moderate in intraspecific and community-level variations, while
significant in interspecific variation, indicating that accurate
measurements of multi-source variation of functional traits were
significant for deep understanding of the processes of community
assembly (Westerband et al., 2021). Compared to that in other vegetation
types (Li et al., 2017; Liu et al., 2020; Wang et al., 2021), the
intraspecific variation in this community was at a lower level and
supported the “a spatial trait variance partitioning hypothesis,” that
is, due to the limitation of environmental heterogeneity and individual
number, the importance of intraspecific variation was less at the fine
scale.
The functional traits of plant leaves are determined by both genetic and
environmental factors. They usually show niche differentiation and
divergence of ecological strategies through intraspecific variation so
as to reduce the intensity of competition (Kang et al., 2017) and to
adapt to a broad environmental gradient. In this community, the
intraspecific variation in leaf chemical characteristics (LC, LN, LP,
and LC:LN) of dominant tree species was higher than that of structural
characteristics (LA, SLA, and LDMC). However, the interspecific
variations showed the opposite trend except for the related-LP traits,
indicating that the variation in leaf chemical traits had higher
plasticity in coping with local environmental changes. Zhao (2015) found
that the soil and topographic variables in the community had moderate
spatial variation, which had significant effects on the species
composition and spatial distribution of the community. And as we all
know, the availability of nitrogen and phosphorus in soil is the main
influencing factor influencing LN and LP contents, respectively (Li et
al., 2017). Therefore, to adapt to more complex environmental
conditions, dominant species increase their distribution space along the
environmental gradient by adjusting the plasticity of leaf chemical
traits. Compared to chemical traits, leaf structural traits are mainly
limited by genetic factors and are relatively stable and closely related
to life forms and species type; therefore, interspecific variation is
larger (Ackerly & Reich, 1999). In addition, it greatly differs for
species at different phylogenetic stages in leaf size, leaf life span,
LN content and photosynthetic ability. Eighteen dominant species in this
community belonging to 10 families, including Fagaceae, Theaceae,
Anacardiaceae, Pinaceae, and Taxodiaceae (Table 1), exhibited
significantly different phylogeny background and thus may help them
coexist through the coordination of functional traits. In addition,
needle-leaf trees are generally thought to be better adapted to cold or
nutrient-poor environments than broadleaf trees (Liu et al., 2020; Liu
et al., 2021). With an increase in water stress, plants tend to exhibit
xerophytic leaves with a thicker leaf structure (Guerful et al., 2009;
Werden et al., 2017). Similarly, our results showed that coniferous
species, especially P. massoniana had the highest LT and the
lowest LA and SLA in this community, which may be because it was the
first pioneer tree species to enter this community; and in order to
resist the arid and barren environment, they possessed leaf functional
traits such as thicker leaves suitable for storing water and lower LA
and SLA to reduce water loss through transpiration. This result was
consistent with those of previous studies that showed larger SLA and
thinner leaves of broad-leaved trees than those of coniferous trees
(Tian et al., 2016). Furthermore, the LN content of evergreen species
was significantly lower than that of deciduous species because the
generation cost of leaves was related to seasonal variation; leading to
different adaptive strategies of evergreen and deciduous trees based on
the variation of traits to adverse environments (Liu et al., 2021). In
summary, the variations in leaf functional traits in L. glaber–C.
glauca evergreen broad-leaved forest community were mainly attributed
to the life form and interspecific variation, which was significantly
affected by genetic background and taxon, and provided an important
prerequisite for community assembly and species coexistence.
Meanwhile, there was a correlation between leaf structure and chemical
traits in the community. SLA, LDMC, and LN all reflect adaptation
strategies to the environment (Wright et al., 2004; Xun et al., 2020),
and there was a significant positive correlation between SLA and LN,
reflecting photosynthetic capacity and nutrient turnover at the species
and community levels. SLA significantly correlated with LT, LDMC, and LC
negatively, while no correlation was observed between LDMC and LT (Table
4 and 5), indicating that LT and LDMC affected SLA in different ways in
this community. Simultaneously, the construction of a leaf defense
structure requires a large amount of photosynthate, and LC is usually
used to compensate for consumption during development (Schulze et al.,
1994). Therefore, LC increased with an increase in LDMC and LN. In
addition, light is an important factor affecting SLA (Wyka et al.,
2012), and LDMC is closely related to water (Saura-Mas et al., 2009).
The results showed that LDMC had lower interspecific variation and did
not correlate with the chemical traits (except with LC), indicating that
subtropical evergreen broad-leaved forests had sufficient hydrothermal
conditions and that the major factor affecting community assembly should
be light rather than water.
4.2 Phylogenetic effects on leaf functional
traits
The evolution is close to Brownian motion; that is, species with similar
phylogenetic positions have similar characteristics and have certain
evolutionary conservation. Species with similar functional traits are
often phylogenetically similar (Losos, 2008). When a strong phylogenetic
signal is detected in functional traits, environmental filters are
probably selected for phylogenetically close species, causing
phylogenetic clustering (Amaral et al., 2021). Here, only a number of
leaf structural traits (LA and LT) showed strong and significant
phylogenetic signals, indicating that LA and LT were closely related to
phylogenetic history and showed strong phylogenetic conservation; that
is, the more phylogenetically close species were more similar to LA and
LT. All the dominant tree species had considerably high values of LA,
except the deciduous tree species (Q. fabri and A. kurziivar. Kurzii ). All evergreen tree species belonging to the same
family and genus had more similar traits, especially for the two most
dominant species (L. glaber and C. glauca ) in the
community, both belonging to Fagaceae (Table 2 and Fig. 1).
The distributions of chemical traits (K < 1, p> 0.05) (Fig. 1) were not consistent with the phylogenetic
relationships, indicating that the phylogenetic signals of these traits
were random or divergent. Therefore, compared to the leaf chemical
traits, the formation and development of structural traits were more
affected by genetic differences, which is consistent with the results of
Cao et al. (2013). In other words, the phylogenetic signal test based on
functional traits showed a lack of consistency between the leaf
functional trait patterns and phylogenetic patterns in this community,
and no specific trend or relationship between them was observed. The
phylogenetic relationships of L. glaber–C. glauca evergreen
broad-leaved forest community were inconsistent with the changes in
functional traits with the historical processes. This observation was
supported by the work of Cheng et al. (2019) on the construction
mechanism of tropical cloud forest communities.
Numerous studies have indicated that phylogeny has a significant effect
on the functional trait composition and that the relationships among
traits are generally weakened after removing phylogeny (Cadotte et al.,
2019; Wang et al., 2020; Liu et al., 2021). This study also found that
the traits of coexisting species in the community had a phylogenetic
structure; however, only a few leaf traits (LA and LT) showed strongly
phylogenetic signals. But the relationship among traits after removing
the influence of the phylogeny remained a little changed at community
level or even significantly enhanced at species level. This indicated
that in the local community, environment had a greater impact on the
spatial distribution of individuals, and plants follow the “realism”
strategy to adapt to the environment, meaning, plants would adjust these
trait-off relationships according to their habitats to archive the best
survival state, which was less limited by the evolutionary history. It
was clear that the functional community structure was not consistent
with the phylogenetic structure. Combined analysis of phylogenetic and
functional trait structures will more accurately infer the main
ecological processes driving species coexistence.
4.3 Community assembly mechanisms of integrated phylogenetic
leaf functional
traits
Many studies have attempted to distinguish between determinative and
stochastic processes by partitioning the variation of species
composition into environmental and spatial components (Legendre et al.,
2009; Chang et al., 2013; Qiao et al., 2015). However, species niches
are determined by their functional traits, which further influence their
distribution along environmental gradients (McGill et al., 2006). Thus,
the effects of determinative processes on community assembly are
expected to be underestimated based on species identity, which does not
consider the functional properties of species. However, we found that
functional traits (12.35%) did not improve the interpretation rate of
niche-based processes by considering only species identity (28.10%,
Zhao et al., 2015) and phylogenetic structure (29.80%) (Fig. 4). This
observation was similar to that by Jiang et al. (2018) for temperate
deciduous broad-leaved Korean pine forests, in which functional traits
could not better reveal ecological processes compared with that by
species.
However, integrating functional traits with phylogeny can greatly
improve the ability to infer determinative and stochastic processes, and
trait- and phylogenetic-based approaches are powerful ways to detect
community assembly processes (Li et al., 2019; Amaral et al., 2021). We
found that the interpretation rate of community assembly based on
functional traits (63.38%) was higher than that based on phylogeny
(47.96 %). However, the pure space variable could significantly explain
higher functional traits than that by environmental variables (Fig. 4),
indicating that the neutral stochastic process played a leading role in
the construction of community functional traits. Compared to the
community functional trait composition, the environment had a greater
contribution to the spatial variation in the phylogenetic structure
(Fig. 4). This indicates that the phylogenetic structure of the
community was aggregated (mainly affected by habitat filtration);
however, leaf functional trait composition showed a dispersion pattern
(mainly affected by stochastic processes). It also verified the opinion
that species identity is a more holistic concept and could better depict
multiple traits of a plant, while leaf functional traits could depict
one or certain facets of a plant (Jiang et al., 2018). At the same time,
among all edaphic and environmental variables, only altitude, aspect,
and TK reflecting light and water utilization of plants had a
significant effect on community functional composition (Fig. 3), which
was related to the fact that leaves were key organs of plant
photosynthesis and mainly exercise the functions of photosynthesis and
nutrient turnover. Therefore, we need to consider more functional traits
(such as height and wood density) to provide more detailed information
to improve the interpretation rate of community functional trait
composition.