3.3 Trait phylogenetic conservatism
All K values of investigated traits were less than 1, ranging from 0.15
to 0.53 (Table 1), which indicated that all traits exhibited low to
intermediate levels of conservatism. In addition, the p values of
five traits (VH, LNC, LCC, LDMC, LT) were less than 0.05.
4 DISCUSSION
Community phylogenetic approach
reveals the relationship of evolutionary processes, regional
environments, and interactions between species, which is important to
understand the mechanisms of species co-occurrence and community
assembly (Cavender-Bares et al., 2009). The present findings
demonstrated that the restoration processes of shrun-encroached
grassland were affected by fencing duration and shrub cover..
The
increase of fencing duration increased the species richness and
phylogenetic diversity, suggesting that longer fencing duration was
conducive to the restoration of community diversity in shrub-encroached
grasslands (Yoshihara et al., 2010; Soliveres & Eldridge, 2014; Koyama
et al., 2015).
Generally,
species richness usually increases first and then decreases in secondary
succession of grazing exclusion (Chen et al., 2014). In this study,
species richness did not decline even in the communities that had been
fenced since 1979. The result of NMDS confirmed the differences in
species composition between fencing durations. Meanwhile, there were
only 19 species in species pool in this study area, far lower than the
number 27-58 in the typical grassland and degraded grassland of this
region (Bai et al., 2000). These results indicated that the community
was still in recovering stage, but the restoration rate was slow. We
also found that the communities with higher fencing duration were more
stable. However, the restoration processes of shrub-encroached grassland
were inconsistent with non-shrub-encroached grassland which could return
to the climax state after about 11-year fencing (Li et al., 1993).
Previous studies have suggested that C. microphylla had higher
water capturing capability than grasses, it will lead to irreversible
stage once C. microphylla dominated the landscape (Peng et al.,
2013), mainly due to that there was a threshold point for transitions
between states (Xiong et al., 2004; D’Odorico et al., 2012). In
addition, the existence of shrub canopy would ameliorate abiotic
environment for the growth of neighbor spceis, enhancing plant richness
and influencing community composition (Maestre et al., 2009; Koyama et
al., 2015). This also explains why the species richness and community
composition of high shrub cover communities were significantly different
from that of low and medium shrub cover community.
Phylogenetic diversity reflects evolutionary history of coexisting
species and clarifies their capacity to adapt to environmental changes
(Alberti et al. 2017).
In
addition, it can represent independent aspects of community structure
(Barber et al., 2019; Jones et al., 2019). Previous studies found that
phylogenetic diversity was positively correlated with species richness
but differed in their temporal or successional patterns (Purschke et
al., 2013). In the present study, the phylogenetic diversity positively
correlated with species richness overall. However, phylogenetic
diversity didn’t increased obviously with species richness in different
succession gradient of shrub cover. This result indicated that most
plant lineages present in the high shrub cover communities also occurred
in the low and medium shrub cover and that they are generally less
speciose in the last two shrub cover communities (Blaimer et al., 2015;
Bares et al., 2019). This also reflected that the impact of shrub cover
on species diversity was not consistent with fencing duration.
In this study, we found that the majority of traits were
phylogenetically convergent, with six out of seven phylogenetic signals
were lower than 0.40. Pipenbaher et al. (2013) found traits were
convergent for most of traits when they tested phylogenetic signal in
the North Adriatic Karst grassland. However, most studies in tropical
and temperate forests demonstraed that traits were phylogenetically
conservatism (Muscarella et al., 2015; Döbert et al., 2017). This
difference may occur, because compared with woody plants, herbaceous
plants tend to have shorter life cycles, more rapid reproduction and
more adaptable to the environmental changes (Donoghue, 2008). The
evolutionary relatedness of ecological traits have an important
influence on the interpretation of community assembly (Liu et al.,
2018). Cavender-Bares et al. (2004) emphasized the importance of
detecting phylogenetic conservatism, and suggested that the results of
testing phylogenetic conservatism should be combined with the observed
phylogenetic dispersion pattern in order to make a strong inference on
the ecological and functional similarity of coexisting species.
The phylogenetic pattern varied widely among different fencing duration
and shrub cover communities. The low shrub cover communities under
fencing since 1979 and high shrub cover communities under fencing since
1983 and 1979 showed evidence of phylogenetic clustering (SES
< -1.96), with co-occurring species significantly more closely
related than would be expected by chance. In contrast, phylogenetic
random dispersion (-1.96 < SES < 1.96) dominated in
low shrub cover communities under fencing since 2003 and 1983, medium
shrub cover under fencing since 2003, 1983 and 1979 and high shrub cover
communities under fencing since 2003 (based on MPD). All of communities
showed a random assembly regarding MNTD, but it showed a trend of
phylogenetic clustering with fencing duration or shrub cover increasing.
When ecological traits evolve convergently, the interpretation of the
community assembly becomes complicated, and its predictive ability
declines (Kraft et al. 2007). Convergent traits can result in clustering
patterns even though competitive exclusion is the dominant process of
community assembly (Webb et al. 2002). With regard to the present
results, clustering in low shrub cover communities under fencing since
1979 and high shrub cover communities under fencing since 1983 and 1979
could be linked to competitive exclusion rather than habitat filtering.
The combination of phylogenetic clustering and convergent trait
evolution seen in our study has also been found in other studies
(Pipenbaher et al., 2013; Purschke et al., 2013; Liu et al., 2018).
Research results from non-shrub-encroached grassland indicate that
competitive exclusion would play more important role in driving the
community assembly with the fencing duration due to the improvement of
abiotic conditions (Wellstein et al., 2014; Conradi et al., 2017; Barber
et al., 2019). Our results indicated that the communities assembly
during the fencing restoration in shrub-encroached grassland was
consistent with that of non-shrub-encroached grassland. In addition, we
aslo found shrub cover significantly affected the community assembly
process. On one hand, previous studies have shown that higher shrub
cover could increase soil fertility (Maestre et al., 2009; Pipenbaher et
al., 2013) and ameliorate abiotic environment(Molina-Montenegro et al.,
2016; Thompson et al., 2017). The decrease in abiotic stress and the
increase in species richness may intensify resource competition among
herbaceous species (Chapin et al.,1994; Purschke et al., 2013). On the
other hand, there may be direct competition between shrubs and
herbaceous species, and higher shrubs have a higher competitive
advantage in light and water resources than herbaceous species (Peng et
al., 2013; Pierce et al., 2019), accelerating competitive ability for
resources. Thus, niche overlap in open space should become significant
and competition should consequently intensify.
More recently, the importance of random colonization and stochasticity
in early community has become evident (Ulrich et al. 2016;
Marteinsdóttir et al., 2018; Liu et al., 2018). Our results found a
random assembly process (-1.96 < SES < 1.96) in six
out of nine communities (Figure 5). Generally, random community assembly
is considered to be strongest in communities that are dominated by
colonization. In arid and semi-arid communities, colonization was often
limited by unfavorable local environmental conditions (Conradi et al.,
2017; Marteinsdóttir et al. 2018), seed availability (Knappová et al.,
2017) or lack of facilitating species (Callaway, 1995). At sites like
Inner Mongolia steppe with low soil nutrient status, limited, seed rain
and low seedling survival (Koyama et al., 2015), a successful
colonization requires a sequence of favorable events. This also proved
that these communities have lower stability than those with longer
fencing duration and higher shrub cover (Figure S2; Table 1).