Abstract :
Understanding community restoration state and the corresponding assembly
mechanism is helpful to assess the restoration measures and predict
community dynamics. We collected plots by shrub cover (low, medium and
high) in three Caragana mircophylla shrub-encroached grasslands
which were fenced since 1979, 1983, and 2003 (fencing duration) in the
northern China, to explore the effect of fencing duration and shrub
cover on the community restoration by vegetation investigation and
phylogenetic approach. There were significant differences in community
composition among different fencing duration or shrub cover treatments.
Species richness in the site of fencing since 1979 or in the plots of
high shrub cover was relatively higher than that in any other sites or
plots. By phylogenetic analysis, functional traits were phylogenetically
convergent. Based on the standardized effect sizes of mean pairwise
distance (SESMPD) ranged from -1.96 to 1.96 in six out
of nine plots, which suggested that stochastic processes dominated
community assembly. SESMPD were lower than -1.96 in the
rest three plots which indicated that competitive exclusion drove
community assembly. These results indicated that the increase of fencing
duration or shrub cover could enhance competitive exclusion. The present
findings highlighted the importance of shrub in influencing the
community composition and community assembly, supporting that
shrub-encroached grassland is another stable state in the semi-arid
northern China. Therefore, it is essential to distinct shrub-encroached
grassland from degraded grasslands when formulating relevant
conservation and management measures in the semi-arid regions.
Key words : Fencing duration; Shrub cover; Community assembly;
Phylogenetic patern; Shrub-encroached grassland1 INTRODUCTION
Understanding the mechanisms underlying community assembly and
predicting community dynamics has long been a central problem in ecology
(Kraft et al., 2007; Dong et al., 2019), especially in the context of
restoration communities (Jones et al., 2019). Proposed mechanisms range
from niche-based assembly rules (Diamond 1975) to neutral assembly
(Hubbell, 2001). Early workers relied on taxonomic ranks (e.g.,
species-to-genus
ratios;
Elton 1946) to infer ecological processes that structure communities.
Recently, the availability of molecular phylogenies has provided a new
method for more accurately measuring evolutionary relatedness, and has
led to a rapidly growing body of research in this field (Weeb, 2002). A
local community can be considered as a subset of a regional pool.
Processes contribute to local community assembly (Karft et al., 2007;
Liu et al., 2018), including environmental filtering, competitive
exclusion and stochastic processes, and outcomes of these processes may
be reflected in patterns of relatedness within a community (Cornwell et
al., 2006).
Recently,
assessing phylogenetic conservatism, i.e., assessing relationships
between phylogenetic relatedness and ecological similarities, is usually
done before give a conclusion by phylogenetic approach, because more and
more ecologists found that ecological traits are not always conserved in
evolution but convergent sometimes (Cavender-Bares et al., 2004; Liu et
al., 2018). Phylogentic conservatism indicated that the closely related
species tend to be similar of ecological and functional traits
(conserved traits). Under this condition, phylogenetic clustering
pattern suggests environmental filtering, phylogenetic overdispersal
indicates competitive exclusion, and phylogenetic random pattern implies
neutral processes dominating the community assembly (Webb et al. 2002;
Kembel & Hubbell, 2006; Liu et al., 2018). However, when ecological
traits are convergent, phylogenetic clustering suggests competitive
exclusion, and phylogenetic overdispersal indicates
environmental filtering driving the
community assembly processes (Karft et al., 2007; Liu et al.,
2018).
Due to global changes, large areas of terrestrial communities are
degrading, especially in arid and semi-arid regions (Eldridge et al.,
2011; Chen et al., 2014; Dong et al., 2019). For example, in the
semi-arid Inner Mongolia Steppe in the northern China, large areas of
native climax communities, Stipa grandis – Leymus
chinensis communities, degraded or even thicketized because of drought
caused by climate change and over-grazing by cattle or/and sheep, with
the loss of plant diversity and the decline of ecosystem functioning
(Peng et al., 2013; Dong et al., 2019). In response, a number of
restoration practices were carried out to restore the semi-arid
grassland vegetation, and fencing is regarded as an effective management
practice (Wu et al., 2009).
Therefore,
many researches have been focused on the restoration processes (or
secondary succession) of different communities, including forest
(Muscarella et al., 2015; Liu et al., 2018; Pinho et al., 2018),
wetlands (Lozada-Gobilard et al., 2019), grasslands (Conradi et al.,
2017; Barber et al., 2019; Dong et al., 2019) and abandoned agricultural
fields (Dinnage, 2009).
Secondary
succession is community assembly in action with changes in species
richness as well as community composition, and therefore, it offers a
chance to test the relative importance of different assembly processes
along restoration duration and its consequences on the structure and
functioning of plant communities (Pinho et al., 2018). As for the
secondary succession of non-shrub-encroached degraded grasslands, both
arrivals of propagules from outside and germinations from soil seed bank
are very important at the the beginning of restoration, thereby, both
random processes (arrival) and environmental filtering (conditions for
germination) are expected to dominate the community assembly during the
early stage of succession (Meiners et al., 2015; Marteinsdóttir et al.,
2018). As successions proceed, competitive exclusion increasingly
influences community assembly due to the improvement of abiotic
conditions (Chen and Tang, 2016) and interspecific competition (Purschke
et al., 2013; Dong et al., 2019). Consequently, the community gradually
tends to be relatively stable (Zhang et al., 2019). However, there is
still a lack of research on the community assmbly mechanisms in
secondary succession of shrub-encroached grassland considering the
significant differences between non-shrub-encroached degraded grassland
and shrub-encroached grassland. Although some reseachers pointed out
that shrubs can facilitate the establishment of surrounding herbaceous
communities (Koyama et al., 2015) by reducing abiotic stress (Maestre et
al., 2009; Soliveres and Eldridge, 2014), many studies comfirmed that
shrub encroachment influenced the cycling of nutrient such as nitrogen,
carbon and surface water and energy flow (Peng et al., 2013), causing
grassland patches or degradation (Xiong et al., 2004; Zhou et al., 2018;
Pierce et al., 2019).
For shrub-encroached grassland, shrub formed different cover at the
landscape scale along the degree of shrub encroachment (Eldridge et al.,
2011; Peng et al., 2013; Soliveres and Eldridge, 2014). On one hand, the
increase of shrub cover across entire landscapes may affect the
heterogeneity of soil nutrients and light availability, provide more
niche and promote plant differentiation (Breshears, 2006; Soliveres and
Eldridge, 2014). On the other hand, microclimatic conditions (Soliveres
et al., 2012; Zhou et al., 2018) affected by shrubs could act as
environmental filters for plant communities. So far, no research has
explored the impact of shrub encroachment on the assembly of plant
community in shrub-encroached grasslands although some studies have
shown that shrub encorachment has changed the plant composition,
community structure and abiotic environment of local habitat (Eldridge
et al., 2011; Peng et al., 2013).
In this study, we chose communities treated by different fencing
duration and shrub cover in the Caragana mircophyllashrub-encroached grassland in the northern China, and used phylogenetic
distances to assess assembly mechanisms along fencing duration and shrub
cover gradient. We aimed to answer the following questions: 1) did the
increase in fencing duration or shrub cover promote the restoration of
herbaceous communities? 2) were key traits of species in the community
phylogenetically conserved? and 3) how did fencing duration and shrub
cover affect phylogenetic structure of herbaceous community and which
process(es) played important roles in affecting community assembly.