Effects Of Soil Depth On Plant Community Structure And Biomass In Karst Region

Karst is an important component of global terrestrial ecosystems.Karst environment is very harsh because of special geographical characteristic and frequent human interference,especially the extremely lack of soil resource,which limits the sustainable development of karst region.With the development of vegetation recovery and reconstruction in karst region,it is an urgent task to understanding the mechanisms of maintain vegetation in karst region.Community structure and biomass are the two important aspects for vegetation maintenance,and both have a large response to environmental change.This study chose typical karst shrublands and woodlands in southwest China as the research object,using the community investigation,plant leaf and soil sample collection,null models,geostatistics,variation partitioning,structural equation models(SEM)and so on,gradually explored the effects of environment filtering and soil resources availability on species coexistence of karst plant community,and further studied the effect of soil depths on the community structure and biomass in karst region.Moreover,two corresponding simulated experiments were conducted using the artificial herbaceous community with seeds from a karst species pool to investigate the influence of soil depths on species diversity and biomass of karst plant community,and further discussed the influence mechanism of species diversity on community biomass among different soil depths.The main results were as follows:(1)On the whole,the leaf dry matter content(LDMC),specific leaf area(SLA),leaf carbon content(LCC),leaf nitrogen content(LNC),leaf phosphorus content(LPC)and leaf potassium content(LKC)in karst shrubland and woodland community exhibited trait convergence pattern,suggesting that environment filtering played a major role in the two deterministic processes(i.e.environment filtering and limiting similarity)of karst community assembly.In addition,we found that the standardized effect sizes(SES)of leaf functional traits in karst shrubland and woodland community had an increasing trend gradually with increasing soil depth.The SES of LDMC,SLA and LPC in karst woodland community increased significantly along the soil depth gradient,indicating that with the increasing of soil depth,the influence of environment filtering on karst community assembly gradually weakened.(2)Next,the effects of karst soil resource availability and heterogeneity on species diversity were considered simultaneously at small scale.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 assembly than resource heterogeneity.(3)The complex multivariable relationships between each variable availability and plant community characteristics were further disentangle by the fit structural equation model(SEM),which showed that the multiple direct and indirect processes of each variable availability in determining karst plant species diversity and abundance were different in different vegetation types.Species diversity and abundance were mainly controlled by both soil total nitrogen(TN)and soil moisture(W)in karst shrubland,and soil depth had a significantly positive effect on species diversity by direct and indirect processes.In terms of shrub layer in woodland,soil depth had an indirect positive effect on abundance by soil TN and total potassium(TK),and soil nutrient(TN and TK)had a direct negative effect on abundance,by contrast,species diversity be negatively influenced by soil depth,and be positively influenced by TK.For the tree layer in woodland,soil depth and p H had significant positive and negative effect respectively on abundance and species diversity.Overall,soil depth is an important determinant for shaping species diversity and abundance of karst woodland and shrubland community.(4)The map of vegetation spatial distribution in different soil depth showed that the soil depth of karst shrubland was lower than that of woodland as a whole,the spatial distribution of vegetation at different levels of soil depth was obviously different.In addition,the non-metric multidimensional scaling(NMDS)showed that the species composition of different levels of soil depth had significant differences in karst shrubland community and the shrub layer and tree layer of woodland community.The dominant species of shrubland community(Mallotus repandus,Rhus chinensis,Lespedeza bicolor and Pyracantha fortuneana)mainly distributed in shallow soil,and the mean values of abundance of Mallotus repandus and Lespedeza bicolor were highest at soil depths less than 5cm.Pinus massoniana had absolute dominance at all levels of soil depth in karst woodland,and its average abundance increased with the increasing of soil depth.In the shrub layer of woodland,Myrsine Africana was the most abundant in all levels of soil depth and tended to distribute in deeper soils.The effects of soil depth on species diversity and biomass were different in different plant communities,with the increasing of soil depth,species diversity of shrubland community increased significantly;species diversity and biomass of the tree layer of woodland community increased significantly,but biomass of the shrub layer of woodland community decreased significantly.Furthermore,the synergistic effect of soil depth and species diversity on biomass of shrubland community was positive,plant community characteristics(species diversity and abundance)had a direct positive effect on shrubland biomass,and soil depth had a positive effect on shrubland biomass through affecting species diversity.However,the stratified biomass of karst woodland communities had different responses to both species diversity and soil depth.The soil depth had an indirect positive effect on community biomass by abundance,and abundance had a direct positive effect on community biomass in the tree layer of woodland;under the comprehensive influence of the tree biomass and the spatial distribution of dominant shrubs,community biomass had a siginificant negative relationship with soil depth in the shrub layer of woodland.(5)Also,considering a phenomenon known as the funnel effect that karst rocky outcrops divert water into the surrounding soil patches,an artificial herbaceous community from a karst species pool was established to further evaluate how the species composition and biomass of karst herbaceous communities respond to soil depth and the funnel effect.There were no significant differences in community abundance,coverage,or PAR canopy capture rate among containers with different soil depths when seedling establishment was complete.Three months after the experimental treatment,all artificial communities were dominated by two grasses,Miscanthus sinensis and Lolium multiflorum.Plant abundance increased significantly with increasing soil depth,mainly owing to an increase in the individual numbers of the dominant species.The colonization frequency of most forb species was lower in shallow soil than in deep soil,but Miscanthus sinensis,Lolium multiflorum and Setaria viridis survived in every plant community in three depth treatments.The result indicates that species colonization in karst herbaceous communities along the soil-depth gradient is non-random,with differences in soil depth driving the colonization of forbs and having no effect on the dominant graminoids.The community coverage,PAR canopy capture rate,and shoot,root and total biomass increased significantly with increasing soil depth.Funnel size and the soil depth × funnel size interaction significantly affected root biomass and root/shoot ratio,however,there was no effect of funnel size or the soil depth × funnel size interaction on community abundance,species richness and diversity,coverage,PAR canopy capture rate,shoot biomass,or total biomass.(6)To understand further the influence mechanism of species diversity on biomass among different soil depths,we also established artificial herbaceous communities with seeds from a karst species pool.The linear mixed models revealed that,the fixed effects,including soil depths and the species diversity and their interactions,all had significant effect on karst herbaceous community biomass.The random effects,the species composition also affected significantly herbaceous community biomass,but the abundance had no significant impact on herbaceous community biomass.The species diversity had a positive effect on herbaceous community biomass among different soil depths,and the influence degree was the most obvious in medium soil.The net diversity effects(species diversity ? 2)were greater than 0 in all multi-species community,and the complementary effect and sampling effect both positively affected the herbaceous community biomass in different degrees.As soil depths and species diversity increasing,the net diversity effect,complementary effect and sampling effect all significantly increased in multi-species communities.The influence of complementary effect on herbaceous community biomass was greater than that of sampling effect among different soil depth and species diversity.Meanwhile,with increasing soil depth,the proportion of complementary effect in net diversity effects increased,while the proportion of sampling effect declined gradually.(7)Furthermore,the soil depth affected the karst species coexistence by driving the changes of leaf functional traits.Both at the species and community level,the vegetation in karst region had higher LDMC and LCC,and lower SLA LNC LPC and LKC compared with the vegetation in non-karst region.There were close correlation among different leaf functional traits in karst shrubland and woodland,LDMC was significantly negative related with SLA,LNC,LPC and LKC,and significant positive correlations were found among SLA,LNC and LPC.Additionally,with the increasing of soil depth,leaf functional traits of shrubland community had the similar variation tendency to that of woodland community,LDMC and LCC both significantly decreased,while SLA significantly increased in karst shrubland and woodland communities.LNC,LPC and LKC also showed an increasing trend,but only LNC of shrubland community significantly increased along the soil depth gradient.In summary,the environment filtering played a major role in karst shrubland and woodland community assembly.Moreover,the importance of soil resource availability far exceeded soil resource heterogeneity in determining karst community composition,especially soil depth,which is highly consistent with the resource availability hypothesis.Field experiment combined with simulated experiments suggested that the community structure,biomass and leaf functional traits were drived by soil depth,and the soil depths combined with species diversity directly and indirectly affected karst community biomass.