4.4. The effects of soil type, fertility, and nutrient
availability on conifers richness
Our findings support the assumption that site-specific habitat factors,
e.g. topography and soil, determine the pattern of species richness
besides climate at the regional scale. The direction and strength of the
determinants vary among taxa indicating a functional relationship
between site conditions and the respective plant community (Zellwegeret al. 2015). Climate and soil types have a significant role in
shaping plant richness patterns along the elevation gradient in Eastern
Himalaya (Sharma et al. 2019), and this supports our finding that
soil variables were the second-strongest predictors after topographic
heterogeneity explaining the variance in all richness groups (see Fig.
2). Furthermore, Sharma et al. (2019), found similar findings
that soil type was the second strong predictor and explained 36.4%
deviance in the species richness pattern. Soil texture type and
nutrient availability explained
significant contributions, and similar findings were found in boreal
conifer forests in Europe (Germany) and North America (Canada)
(Schmiedinger et al. 2012). In detail, among the soil-type
predictors, explanatory sand variable showed the highest significant
positive contribution to all richness groups and this finding explaining
the importance of sandy soils in the presence of high carbon which leads
to an increase of the water-holding capacity, so soil aggregate
stability increased with a greater number of macropores. The improvement
of macropores and soil stability will increase soil infiltration
capacity and enhancement of root penetration and subsequent boosting the
growth of seedlings of various species. Therefore, soil type is an
important factor, and ecological tool affecting vegetation dynamics and
contributing to the sustainable management of conifer forests (Steinet al. 2014; Quesada & Lloyd 2016; Yao et al. 2019). In
comparison to climatic water predictors, soil water availability
explained and contributed more (see Table 1), and this result supports
the finding that variables of soil water balance performed better than
that of climatic water in the tree species distribution modelling
(Piedallu et al. 2013). Furthermore, previous studies highlighted
the positive influence of water and the importance of soil moisture
availability to habitat quality and plant communities; where the
abundance of soil water causes more activation of the weathering
processes and produces more nutrients for plants (Hjort et al.2015; Moeslund et al. 2013).
CEC is an ecological indicator of soil fertility, and highly explained
the variation in plant species richness in China (Zhang et al.2016) supporting our results that soil-fertility indicators (carbon and
CEC) are main relevant drivers of the conifer richness pattern, and this
is because soil fertility has a direct and indirect effects on the plant
species growth, potential distribution, and thereby species composition
and richness dynamics (Mod et al. 2016; Quesada & Lloyd 2016;
van der Sande et al. 2018; Ali et al. 2019). Furthermore,
several recent studies of biodiversity-ecosystem functioning supported
our finding of the significant positive contribution of the available
nitrogen and phosphorus in the variation of conifer richness pattern and
suggested that abundance of soil biota increases the decomposition due
to high availability of soil nitrogen and phosphorus. The diverse forms
of such nutrients may promote plant diversity through partitioning of
resources when species differ in their preference for different forms of
these nutrients and subsequent rapid forest regeneration (Lalibertéet al. 2014; van der Putten et al. 2016).
Available soil phosphorus didn’t have a significant correlation with
other soil variables (see Table S4, Appendix A) indicating to the
significant and independent role of phosphorus in the soil spatial
heterogeneity, forest ecosystem functioning and thereby community
dynamics (Laliberté et al. 2014; Xu et al. 2016). This
finding strongly supports the suggestion that soil fertility, especially
phosphorus, increases species richness and forest productivity (van der
Sande et al. 2018). Furthermore, our findings supports our soil
nutrient-fertility hypothesis which could be used as evidence for the
mutual interaction between species richness and ecosystem functions
linked to carbon and nitrogen cycling, which sustain soil fertility and
carbon sequestration (Maestre et al. 2012; Laliberté et
al. 2014; Quesada & Lloyd 2016; Liu et al. 2018a). Also, soil
carbon and nitrogen stocks increases the recovery of conifer species
richness of the secondary forests at high elevations, and this finding
strongly support our significant positive explanatory power of carbon
stock on the richness pattern implying that global warming might
decrease the soil carbon and nitrogen stocks at high elevations and thus
may negatively affect the recovery and dynamics of conifer richness (van
der Putten et al. 2016; Tashi et al. 2016; Chen et
al. 2018; Liu et al. 2019).