4.2 Multiple direct and indirect processes of resource
availability driving plant community species composition
Our SEMs further disentangled the
multiple
direct and indirect processes by which species diversity was influenced
by the availability of each variable. Species diversity and abundance
were mainly controlled by soil TN and soil moisture in shrubland (Fig.
5a, b, Fig. 6a, b, Table A2). Similar results have been found in karst
region, where plant community structure was strongly influenced by soil
TN during the early successional stages (Zhang et al. 2015; Li et al.
2017). In fragmented karst shrubland, a large amount of exposed bedrock
and an extremely shallow soil depth lead to a shortage of soil
resources, therefore, soil is highly TN deficient, which significant
influences species diversity and abundance. Additionally, the soil TN is
mainly originated from the slow release of soil organic matter (Jobbagy
and Jackson 2000). However, the severe water loss along stone crevices
and lots of water evaporation result in temporary droughts in karst soil
patches, especially in shallow soil patches (Wang et al. 2003), which
greatly affects the release processes of soil organic matter, further
exacerbates the soil nutrient deficiency (Moyano et al. 2013). Thus,
soil moisture had positive indirect effects on both shrub abundance and
species diversity through affecting soil TN (Fig. 5a, b, Fig. 6a, b,
Table A2). Furthermore, soil depth had a significant positive effect on
species diversity due mostly to directly affecting plant survival space
and soil resource
level.
As a whole, the soil nutrient level of woodlands higher than that of
shrublands because a large number of litters were accumulated in karst
woodland (Zhang and Pan 2012; Islam et al. 2015), therefore woodland
could provide enough nutrients in terms of shrub survival and growth,
and soil nutrient level of woodlands did not limit the increase of shrub
abundance. Soil depth was the significant predictor of shrub abundance
in woodland, which was attributed to the fact that shallow soil always
exposes a large amount of bedrock surface, but deeper soil provides more
growing space for shrubs (Lundholm 2010). In contrast, species diversity
was mainly positively influenced by soil TK, and negatively influenced
by soil depth (Fig. 5d, Fig. 6d, Table A3), which demonstrated that
plant species tend to occur and grow in
shallow soils with high nutrient
content. This pattern of shrub diversity in woodland was probably
because other biological factors dominate the spatial distribution of
different species, as well as was supported by the fact that shallow
limestone soil is rich in organic matter, nutrients (N, P and K), and Ca
(Zhang et al. 2010).
For the tree layer of woodland, soil depth and pH had significant
positive and negative effect respectively on abundance and species
diversity, which indicated that the deep soil with a high nutrient level
and growing space, as well as low acidity, was favorable for the growth
and survival of high-biomass wood species. Additionally, soil Mg had a
significant negative influence on abundance, while the negative effect
originated mostly from the indirect affecting soil pH. The result
further suggested that soil pH is an important determinant for shaping
tree abundance in the forest dominated by Pinus massoniana L.,
possibly because P. massoniana prefers slightly acidic soil
rather than alkaline limestone soil. However, the species diversity was
mainly negatively influenced by soil nutrients (TN and TK) was because
of the cause that the significant negative relationship between soil
nutrient content and soil depth in karst woodlands.
Conclusions
Our study provided novel insights to quantify and compare the effects of
resource availability and heterogeneity on plant community
characteristics in the most fragile karst landscapes in southwestern
China. 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,
thereby tending to support the resource availability hypothesis.
Thus,
the resource availability of
shrubland
and woodland should be more important for protecting and restoring
objects than resource heterogeneity, especially in highly fragmented
shrubland. Our SEMs
further
demonstrated that the multiple direct and indirect processes of resource
availability determined karst species diversity and abundance
simultaneously, whereas the multiple pathways were different in
different vegetation types, emphasizing that steering community
restoration and reconstruction also have to take into account multiple
pathways of resource availability for structuring different karst
community types.
Supplementary
material: Appendix A.
Acknowledgements
This study was funded by the
National
Key R&D Program of China (2016YFC0502304). We thank Xinjia He, Zhaojun
Wei, Xiaoxian Lu and Lin Peng who helped with materials collecting and
field work at Youyang, Yinjiang and Beibei.