3.2 The relative importance of resource availability and
heterogeneity in plant abundance
The total variation in community abundance explained by
both resource availability and
heterogeneity was 67%
for
shrubland and 64% and 63% for shrub layer and tree layer in woodland
(Fig. 4). The shared fraction between resource availability and
heterogeneity explained a large proportion of the variation in community
abundance (48.4%, P =0.002 in shrubland; 33.7%,P =0.004
and 30.7%, P =0.004 in the respective tree layer and shrub layer
in
woodland).
Abundance was also largely explained by the unique contribution of
resource availability (16.2%, P =0.003 in shrubland; 19.5%,P <0.001 and 31.8%, P <0.001 in the
respective tree layer and shrub layer in woodland), but
the
unique contribution of resource heterogeneity was marginal (2.8%,P =0.006 in shrubland; 0.9%, P <0.001 and 0.2%,P <0.001 in the respective tree layer and shrub layer in
woodland) (Fig. 4).
3.3
Multiple direct and indirect effects of resource availability on species
diversity and abundance
To further disentangle the complex relationship between resource
availability and plant community characteristics, SEMs were carried out
for each vegetation type. Our SEMs (Fig. 5a, b, c, d, e, f) produced
good fits with our data for community abundance and species diversity in
shrubland and woodland (Table 3). Therefore, these models successfully
assessed the extent to which factors influenced species diversity and
abundance through direct and indirect pathways.
In shrubland, the standardized effect of environmental factors derived
from SEMs showed that soil TN had positive direct effects on both shrub
abundance (standardized coefficient, λ=0.67) and species diversity
(λ=0.77). Soil moisture had positive indirect effects on both shrub
abundance (λ=0.67) and species diversity (λ=0.72) via soil TN.
Therefore,
the soil TN and soil moisture are the strongest predictor of shrub
abundance and species diversity. Additionally, soil depth had a
significantly positive effect on species diversity by direct and
indirect effects (Fig. 5a, b, Fig. 6a, b, Table A2).
For the shrub layer in woodland, soil depth had positive indirect effect
(λ=0.37), and soil pH had negative indirect effects (λ=-0.24) on plant
community abundance through TN and TK, Additionally, soil nutrient
factors all had negative direct effects (TN: λ=-0.48; TK: λ=-0.36) on
abundance (Fig. 5c, Fig. 6c, Table A3). In contrast, soil depth had
negative indirect effect (λ=-0.18), and soil pH had positive indirect
effects (λ=0.21) on species diversity. Species diversity was mainly
positively explained by soil TK (λ=0.41) (Fig. 5d, Fig. 6d, Table A3).
For the tree layer in woodland, plant community abundance was mainly
influenced by soil ρb (λ=0.73), followed by soil pH (λ=-0.61), Mg
(λ=-0.51)
and
depth (λ=0.41), among which the negative effect of soil Mg on abundance
was through indirectly effecting soil pH. The direct effect of soil
depth on abundance was non-significant, but soil depth had positive
indirect effect on abundance via ρb (Fig. 5e,
Fig. 6e, Table A3). Overall species
diversity was primarily driven by soil TN (λ=-0.80), depth (λ=0.61), TK
(λ=-0.49) and pH (λ=-0.42). Soil depth and soil pH indirectly influenced
species diversity via soil nutrient factors, and soil TN and TK had
direct negative effect on species diversity (Fig. 5f, Fig. 6f, Table
A3).
Discussion