4.1 Resource availability plays a large role for shaping plant
community species composition
Our findings highlight that species diversity and abundance were both
significantly associated with resource availability in karst ecosystem
(Fig. 2a, b, c, d), which completely supports the resource availability
hypothesis. Similar results have proven that the availability of single
variables (soil organic matter, phosphorus, nitrogen, etc.) was closely
related to species diversity (Ou et al. 2014), but our study further
focused on the effect of integrated environmental variables on species
diversity. It is important to note that the total amount of resources in
karst regions is very limited because of the slow and patchy development
of nutrient-poor soil over exposed rock (Kavouri et al. 2011). Given
these resource limitations, species diversity and abundance
significantly increased with increasing resource
availability
in shrubland and the tree layer of woodland. Broadly speaking, karst
woodland with better soil environmental conditions could provide enough
nutrients in terms of shrub growth (Asensio et al. 2013), therefore the
negative resource availability-species diversity relationship was most
likely because dense overstory canopy intercept available light (Neufeld
and Young 2014; Zhang et al. 2017).
Meanwhile, plant community abundance significantly increased with
increasing resource heterogeneity in the shrub layer and tree layer of
karst woodland, corroborating the findings in other ecosystems (Tilman
1982; Silvertown 2004; Stein et al. 2014). This is because karst
woodland with low fragmentation degree have a relatively mild soil
environment for plant survival and growth. Increasing heterogeneity
provides enough nutrients and more niches to seed germination and plant
growth, allowing for increased community abundance in woodland.
Therefore, the significant relationship between resource heterogeneity
and community abundance also favour the resource heterogeneity
hypothesis. Given that plant community abundance in karst region was
significantly associated with resource availability, as well as resource
heterogeneity, which suggested that resource availability and
heterogeneity as two important drivers of community abundance is not
mutually exclusive in karst shrubland and woodland. Our variation
partitioning further elucidate the relative importance of resource
availability and heterogeneity in determining community abundance. It is
clear that a large proportion of variation in community abundance was
explained by unique resource availability, with only a small amount
explained by unique resource heterogeneity (Fig. 4), demonstrating that
resource availability had a much higher explanatory power for the
variation in community abundance compared with resource heterogeneity.
Species diversities of tree and shrub layer of woodland were not related
with resource heterogeneity fully confirmed that resource availability
was more significant predictor of species diversity rather than resource
heterogeneity. This finding also corresponds with those from other
ecosystems, where species diversity appeared to be determined by
resource availability rather than resource heterogeneity (Baer et al.
2004; Lundholm 2010). The degree of rock fragmentation in karst
shrubland is far higher than that in karst woodland (Liu et al. 2019),
thereby plants are likely to face resource deficiency in the highly
heterogeneous karst shrubland. Additionally, plant individuals cannot
survive when the soil patch size is relatively smaller than the plant
root size (Lundholm 2009; Gazol et al. 2013; Tamme et al. 2016; Schuler
et al, 2017). To the extent that the size of the root system of
individual plants is more likely to exceed the soil patch size with
increasing environmental heterogeneity in karst shrubland, so that
plants located in unfavorable soil patches face an increased risk of
mortality (Laanisto et al. 2013). As a consequence, species diversity
and abundance significantly decreased with increasing resource
heterogeneity mainly because resource availability limits survival and
growth of plant individuals in the highly heterogeneous karst shrubland,
while increasing resource heterogeneity does not work when soil and
space resource is extreme shortage in karst shrubland.