Biomass And Net Primary Productivity In Different Successional Stages Of Karst Vegetation In Maolan, Guizhou Province, SW China

The Karst ecosystem in southwestern China,growing in a harsh environment,is a specific type of ecosystem which easily leads to land degradation and rocky desertification under the disturbance of human activites.The biomass and net primary productivity(NPP) of Karst vegetation is very difficult to measure due mostly to the Karst morphology and rigoerous habitat,especially the belowground biomass. However biomass and NPP are key factors in understanding the ecosystem stability in this region.Here the biomass and NPP of Karst vegetation in each successional stage were estimated in Maolan National Natural Reserve of Guizhou Province,SW China. The investigation of 20 plant communities,harvest of biomass and sampling of tree rings were carried out in April-October 2007 and July-September 2008.The Karst vegetation was divided into five succession stages according to its community physiognomy,dominant species,habitat types,mean diameter at breast height(D), mean tree height,tree density and community coverage:herb community,herb-scrub community,scrub-shrub community,sub-climax community and climax community of evergreen-deciduous broadleaved mixed forests.Meanwhile we divided the Karst communities into rock-community and soil-community based on the soil substrate. The biomass and NPP were then measured and estimated in 10 representative plots.The main results are as follows:1)The species composition is significantly different in each succession stages.In herb community and herb-scrub community,the main species are the pioneers such as Rhus chinensis,Glochidion puberum,Imperta cylindrical var.major,Eulalia speciosa, and Ptcridium revolutum.In scrub-shrub community,the dominants are some transitional species between herb and forest communities,such as Lindera communis, Nandina domestica and Mahonia duclouxiana.In sub-climax community and climax community of evergreen-deciduous broadleaved mixed forests,the main trees are Swida parviflora,Swida austrosinensis,Clausena dunniana,Platycarya longipes,Ilex ficoidea and Castanopsis carlesii var.spinulosa.2)The forward succession of Karst vegetation significantly increases(p＜0.001) its aboveground biomass from 4.76 t/hm² in rock herb community(Pteridium revolutum-Inula cappa) to 144.66 t/hm² in soil climax evergreen-deciduous broadleaved mixed forest(Clausena dunniana-Platycarya longipes).3)The forward succession of Karst vegetation significantly increases(p＜0.001) its belowground biomass from 2.63 t/hm² in rock herb community(Pteridium revolutum-Inula cappa) to 58.15 t/hm² in rock climax evergreen-deciduous broadleaved mixed forest(Clausena dunniana-Platycarya longipes).4)The coarse root biomass in rock community is higher than in soil community; however in the same succession stage,the differences are not significant(p＞0.05). The fine root biomass in soil community is significantly higher than in rock community in the same successional stage(p＜0.05).5)The distribution of root biomass in soil communities concentrates in the soil layer from surface to the depth of 10 cm,accounting for 98.9%,98.4%,73.9%,47.4% and 97.9%of total belowground biomass from soil herb community to soil climax community,respectively.6)The belowground carbon stock in soil community is significantly higher than in rock community in the same stage.The highest belowground carbon stock(223.63 tC/hm²) appears in the soil community of scrub-shrub stage.7)The forward succession increases the net primary productivity(NPP) of rock community from 2.73 t·hm^-2·a^-1 in rock herb community to 13.58 t·hm^-2·a^-1 in rock climax evergreen-deciduous broadleaved mixed forest,but the difference are not significant.NPP is mainly allocated to leaves and fine roots,accounting for over 60% of total NPP.The highest value(76.09%) appears at the stage of soil sub-climax community(Swida parviflora-Lindera communis).