Abstract
Resource
availability and heterogeneity are recognized as two essential
environmental aspects to determine species
diversity
and community abundance. However, how resource availability and
heterogeneity determine species diversity and community abundance in
highly heterogeneous and most fragile karst landscapes is largely
unknown. We examined the effects of resource availability and
heterogeneity on plant community
composition
and
quantified
their relative contribution by variation partitioning. Then, a
structural equation model (SEM) was used to further disentangle the
multiple
direct
and indirect effects of
resource
availability on plant community
composition.
Species diversity was significantly influenced by the resource
availability in
shrubland
and woodland but not by the heterogeneity in woodland. Abundance was
significantly affected by both resource availability and heterogeneity,
whereas variation partitioning results showed that resource availability
explained the majority of the variance in abundance, and the
contribution of resource heterogeneity was marginal. These results
indicated that
resource
availability plays a more important role in determining karst plant
community composition than resource heterogeneity. Our SEMs further
found that the multiple direct and indirect processes of resource
availability in determining karst species diversity and abundance were
different
in different vegetation types. Resource availability and heterogeneity
both played a certain role in determining karst plant community
composition, while the importance of resource availability far exceeded
resource heterogeneity. We propose that steering community restoration
and reconstruction should be highly dependent on resource availability,
and
multiple direct and indirect pathways of resource availability for
structuring karst plant communities need to be taken into account.
Keywords community abundance, karst shrubland and woodland,
resource availability, resource heterogeneity, species diversity
Introduction
Understanding the mechanisms that maintain vegetation is an essential
ecological goal for the sustainable development of fragile ecosystem.
Two general ecologically-based hypotheses related to resources have been
developed to explain plant community composition at the local scale. The
resource heterogeneity hypothesis
suggests that species diversity is a function of heterogeneity in
resources because of species specialization in heterogenetic resource
gradients (Ricklefs 1977; Huston 1979; Tews et al. 2004; Scott and Baer
2019). Resource heterogeneity generally increases niche diversity and
provides opportunities for speciation events, allowing for increasing
species coexistence
(Rosenzweig
1995, Silvertown 2004; Do Carmo 2016; Feeser 2018). But this is not true
in small spatial patches, where there is a nonsignificant or even
negative heterogeneity-diversity relationship (Wijesinghe et al. 2005;
Tamme et al. 2010; Gazol et al. 2013). In contrast, the resource
availability hypothesis does not necessarily rely on assumptions about
species specializations and proposes that the average level of limiting
resources should govern species coexistence
(Wright
1983; Stevens and Carson 2002; Bakker et al. 2003; Wassen et al. 2005;
Kumar
et
al. 2018). In general, under a low resource availability, the species
diversity and abundance of a plant community are lower because only a
few species possibly survive and grow under such harsh environments
(Désilets and Houle 2005; Comita et al. 2007).
Recent evidence has indicated that the relative importance of resource
availability and heterogeneity in influencing species composition is
different in different plant communities. For instance, Bartels and Chen
(2010) demonstrated that resource availability
drives
species diversity in both young and mature stands of forest ecosystems,
whereas resource heterogeneity dominates in old-growth stands. Shirima
et al. (2016) suggested that the mean soil nutrient availability
explains considerable variations in tree species richness in moist
forests, while vertical soil nutrient heterogeneity is a predictor of
tree species richness in miombo woodlands. In fact, species diversity
and abundance are scarcely the results of a single factor and direct
process, with respect to either resource availability or resource
heterogeneity
(Whittaker et al. 2001). Soil depth varies with topographic position,
which influences species diversity (Baer et al. 2005). Soil acidity has
an indirect effect on plant community composition via its effects on the
availability of key mineral nutrients (Chen et al. 2004; Zhu et al.
2013). Soil nitrogen increases with
stand
development due to increased nitrogen fixation with stand age
(Hume
et al. 2016). Therefore, there is a suite of direct and indirect
environmental factors that simultaneously influence the species
diversity and abundance of karst plant communities, but their direct and
indirect relationships remain poorly understood.
Karst is an important component of global terrestrial ecosystems with
the most fragile ecological environments (Huang and Cai 2007; Ford and
Williams 2007). Fragmentation of karst landscapes is much higher than
other terrestrial ecosystems, where a shallow and discontinuous soil
layer with exposed rocks forms various habitats, such as rocky fissures,
rocky gullies, soil faces, rocky faces and rocky pores (Zhang et al.
2007; Nie et al. 2011; Chen et al. 2016). The broken terrain
dramatically increases the spatial variability of other environmental
variables, notably the physical and chemical
properties
of the soil (Zhong et al. 2014; Toure et al. 2015). Additionally,
previous researches have demonstrated that the spatial distribution of
soil resource varied obviously in karst region (Tateno and Takeda 2003;
Zhang et al. 2006). Therefore, the high edaphic heterogeneity provides
numerous ecological niches for
plant
diversification and speciation (Wang
et
al. 2014), species diversity and abundance of karst plant community are
expected to be determined by resource heterogeneity. Nevertheless, the
soil resource shortage also is a remarkable feature of karst landscapes
owing to slow soil formation, a shallow soil layer and severe soil
erosion, so the species diversity and abundance of karst plant community
are likely to be strongly influenced by the availability of one or more
limiting resource. As consequence, karst landscapes not only represent
limited soil resource availability but also exhibit significant soil
resource
heterogeneity,
the effect of resource availability and heterogeneity on karst species
diversity must be taken into consideration simultaneously. In prior
studies of karst ecosystem, the fact that some resource availability
(e.g. Soil depth, P availability, K availability) limits species
diversity and abundance has been widely demonstrated (Crowther 1982;
Zhang et al. 2013; Toure et al. 2015; Liu et al. 2020). However, few
studies have examined the effects of small-scale resource heterogeneity
on karst plant community
species
composition; and no study has examined the role of resource availability
and heterogeneity
in karst plant species diversity and
abundance.
Here, we chose six sites typical in karst environment in southwestern
China to quantify and compare the effects of resource availability and
heterogeneity on species diversity and abundance. We then further
disentangled the direct and indirect effects of resource availability on
karst plant community composition by SEM. We addressed the following
questions: (1) How do
resource
availability and heterogeneity affect both species diversity and
abundance in the highly heterogeneous and most fragile karst mountains?
(2)
What is the relative importance of
resource
availability and heterogeneity for shaping karst
plant
community
species
composition? (3) How is karst plant community species composition driven
by multiple
direct
and indirect pathways of resource availability?
Materials and Methods