1. Introduction
Community assembly mechanisms have always been a topic of ecological
research, and natural communities are generally believed to be
structured by a set of processes (Chase et al., 2014; Levine et al.,
2017; Wang et al, 2021). Niche theory-based determinative
processes , including the influence of the abiotic environment on
fitness (Wang et al, 2021; e.g. habitat filtering) and biotic
interactions, in particular interspecific competition (Leibold, 1998),
and neutral theory-based stochastic processes , including spatial
dispersal limitation, demographic stochasticity, and ecological drift
(Hubbell, 2005; Zhou & Zhang, 2008; Chase & Myers, 2011) have been
regarded as two primary ecological mechanisms driving community assembly
(Li et al., 2019). The relative importance of these processes tends to
vary among ecosystems (Liu et al., 2013; Jiang et al., 2018), spatial
scales (Zhang et al., 2021), community succession (Csecserits et al.,
2021) and even in different environments, especially extreme
environments (Wang et al., 2021). For example, deterministic processes
may play a greater role than stochastic processes in adverse
environments (Chase & Myers, 2011). Interspecific interactions and
density-dependent mechanisms should be strongest at the neighborhood
scale where individual organisms interact, and environmental filtering
should be stronger than interspecific interactions at the habitat scale
(Cavender-Bares et al., 2009; Purschke et al., 2017).
Plant functional traits are usually used as proxies to determine whether
different tree species have different ecological strategies for resource
capture and reproduction (McGill et al., 2006; Baraloto et al., 2012;
Adler et al., 2013; Liu et al., 2020). Analyses of the distribution of
trait values within communities yield insights of the ecological
processes constraining their assembly (Kraft et al., 2008; Paine et al.,
2011). If the niches of two species overlap, it is generally expected
that the two species are similar in a range of functional traits, and
vice versa (Westoby & Wright, 2006). Based on competition theory,
higher similarity in functional traits for a community could lead to an
increased intensity of interactions among neighboring individuals
(Uriarte et al., 2004; Paine et al., 2011; Funk et al., 2016).
Consequently, communities with scattered trait values are primarily
shaped by niche differentiation, whereas environmental filtering is the
dominant process shaping ecological communities when trait value range
is narrower than predicted (Paine et al., 2011). Thus, based on the fact
that functional traits could represent the key aspects of physiology,
investigating the variation in functional traits at the species level
(i.e., intraspecific and interspecific variation)
and at the community level could be beneficial for a deeper
understanding of how physiological processes shape the assembly of
ecological communities. Plant leaves are critical organs for the
exchange of matter and energy with the photosynthetic organs of plants,
and several biological processes such as plant growth, survival,
reproduction, and ecosystem function are fully influenced by leaf
parameters (e.g., leaf area, length, and dry mass) (Surya et al., 2020).
Leaf functional traits are sensitive to changes in environmental
factors. They can adjust resource utilization strategies to adapt to
different habitats through trade-offs of various traits, which can
reflect the driving mechanisms of the environment on community assembly
(Wright et al., 2004; Tian et al., 2016; Zhang et al., 2020).
Although functional traits could provide a way to infer prevailing
ecological process information based on morphological, physiological,
and ecological characteristics, plant functional traits are not only
affected by environmental factors but also by species evolution history
(Swenson, 2013). Species coexisting in the same habitat might be
relatives sharing common functional traits influenced by evolutionarily
conserved or perhaps distant relatives adopting convergent traits to
adapt to the habitat (Cavender-Bares et al., 2009). Thus, phylogeny and
functional traits do not necessarily present similar information and
patterns (Cadotte et al., 2019), and testing the phylogenetic signals of
functional traits is a necessary key step to more accurately infer the
mechanism of community assembly (Baraloto et al., 2012; Cheng et al.,
2019). Phylogeny is an indirect estimation of ecological similarity
based on species affinity and an estimation of the impact of historical
factors on the existing community (Swenson et al., 2013; Jiang et al.,
2018). Therefore, the combined analysis of phylogeny and functional
traits can not only reveal the impact of community evolutionary history
and functional traits simultaneously on the current community ecological
process (Webb et al., 2002; Zhou et al., 2021), but also contribute to
revealing the ecological processes responsible for evolution and
functional assembly (Zhou et al., 2021). In other words, combined
trait-based and phylogenetic-based approaches is a powerful way to
detect community assembly processes (Kraft & Ackerly, 2010; Gianuca et
al., 2017; Li et al., 2019).
Subtropical region of China holds the largest evergreen broadleaved
forest in the world and harbors abundant seed plants and endemic species
(Xu et al., 2017), which play an important role in biodiversity
protection and carbon balance. However, due to the prolonged and
frequent anthropogenic interferences, vegetation degradation is severe,
and ecological problems are prominent in this area. Research based on
the typical community structure and ecological processes in zonal
vegetation has become an important means of vegetation restoration and
reconstruction (Zhao et al., 2015; Ouyang et al., 2016; Zhang et al.,
2020). As one of the typical types of evergreen broad-leaved forests in
the subtropical China, Lithocarpus glaber–Cyclobalanopsis glaucaforests exhibit high species diversity, stable community structure, and
high ecosystem function and service values (Zhao et al., 2015). To study
the assembly processes and mechanisms, we carried out a series of
studies focusing on species composition, spatial patterns, and effects
of topographic and soil factors on woody species assembly in this
long-term monitoring community. We found that aggregation was the major
spatial pattern and environment (topographic and soil factors) explained
28.10% of the species assembly (Zhao et al., 2015), and it is apparent
that species-based approaches to understand community assembly are
limited. Here, we re-examined this issue by integrating functional and
phylogeny-based approaches to explore the community assembly processes.
The degree to which patterns of functional traits and phylogenetic
dispersion may be easily explained by the abiotic environment and
spatial relationship has been also discussed. We propose that a variance
partitioning approach can be applied to simultaneously address these
challenges (Durate et al., 2013; Zhang et al., 2021). The degree to
which the abiotic environment, dispersal limitation, and/or their joint
effects affect trait dispersion should be determined by partitioning
variance in trait dispersion into pure environmental, spatial and joint
effects. We also measured 9 leaf functional traits and phylogenetic data
of 18 dominant woody species with important value >1.00%
and accurate topographic and edaphic data sets to address the following:
(i) To clarify the variation and trade-off relationship of leaf
functional traits; (ii) To explore the effects of phylogeny and
environment on trait variation; (iii) To disentangle the relative
importance of niche and neutral processes shaping community assembly at
a fine scale.