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Based on the vegetation inventory data,we investigated the density,storage and spatial characteristics and dynamics of vegetation carbon in Northwest Guangxi,China from 2005 to 2010,using biomass expansion factors and spatial analysis,respectively.The results showed that the total storage of vegetation carbon increased from 4.19 × 104 t to 4.27 × 104 t,about 1.84% per year.Similarly,the density of vegetation carbon also increased from 29.04t/hm2 to 29.57 t/hm2.Vegetation carbon density was highest(40 t/hm2) in National Nature Reserve.From 2005 to 2010,vegetation carbon density in the previously non-vegetated land increased significantly through artificial seeding,direct seeding,aerial seeding or natural germination.Vegetation carbon density of revegetated land through Conversion of Cropland into Forestland Program was also increased by 3.00 t/hm2.Vegetation carbon density from all types of forest stand was increased.With regards to the spatial distribution of carbon storage and density,the density was higher in the west,north,but lower in the middle-east,south.The average carbon density was more than 40 t/hm2 in the west but lower than 25 t/hm2 in the east and middle.The carbon storage and density in the karst areas and non-karst areas increased.Furthermore,vegetation carbon density in seven counties had increased to higher grade.Our study indicated that the carbon storage and density of vegetation in the study area was increased significantly and ecosystem conditions was improved through the application of policies of karst rocky desertification control,such as ecological migration and Conversion of Cropland into Forestland Program.

Karst ecosystem is very fragile and sensitive to environmental change.China has the largest karst area(344.3×104 km2) with the widest distribution and highest landform diversity in the world,mainly or primarily concentrating in the southwest(about 55×104 km2).The karst region of southwest China is geology-controlled eco-environment,and the basic characteristics include the shortage of surface water and soil resources,low vegetation coverage and high diversity of microhabitats.Due to special geological background,intensive karst process and recent irrational land uses,both vegetation destruction and soil and water loss are increasingly serious in karst region of southwest China.This results in karst desertification,a process of land degradation involving extensive exposure of basement rocks and drastic decrease in soil productivity.Karst desertification has been leading to the concentrated poverty population and the lack of cultivated land.The karst region of southwest China,the crucial area for the West Development in China,is easy to plunge into a vicious circle between poverty and environmental degradation.Therefore,how to rehabilitate vegetation effectively and to utilize soil and water resources harmoniously are the primary problems for the comprehensive control of karst desertification in this region.Although rainfall is abundant,surface water is leaked heavily and groundwater is buried deeply due to dual hydrological structure in karst region of southwest China.Karst drought is serious and water deficit is still the primary factor for influencing vegetation restoration due to discontinuous shallow soils and low soil storage capacity.Thus,study on spatio-temporal heterogeneity of water and plant adaptation mechanisms is very important for vegetation restoration in karst region of southwest China.Water movement in karst region is significantly different from other kinds of region because of special geological background,complex landform and high heterogeneous habitat,and the effects of lithology,topography,the soil-rock distribution and vegetation should be considered altogether.Although many advances have been achieved on hillslope rainfall infiltration and runoff generation,spatial variability of soil water,adaptation of plants to water stress in different habitats,the current researches can not illustrate the difference of water status in various soil-rock environments and the corresponding adaptation mechanisms in karst regions.This heavily affects the achievement of vegetation restoration and reconstruction.In this paper,on the basis of the simple introduction to the characteristics of soil-rock environment,the research advances on rainfall infiltration and runoff generation,spatio-temporal variability of soil water,plant water sources and its adaptation mechanisms were summarized in karst regions.The further study should attach importance to investigate the relationship between water movement in soils and fractured rocks and plant water utilization,and to survey the effects of various environmental factors on eco-hydrological processes in the atmosphere-plant-soil-rock system.In order to reveal the interactive mechanism of vegetation and hydrological processes at hilllsope and catchment scales in karst areas,the multi-disciplinary research methods and techniques,including soil physics,eco-hydrology,plant physiology and karstology,should be applied to discuss the hydrological regulation function of epikarst and its primary influencing factors.These studies would provide scientific basis for optimal disposition of plant community with water-conserving function in karst region of southwest China.

Soil microbial life in vivo is a major component of soils throughout the whole process of soil development.The living environment for soil microorganisms is a very sensitive early-warning indicator of change in soil ecosystems.Using slope cropland as a contrast,we investigated the main soil microbial populations,microbial biomass carbon(MBC),nitrogen(MBN),and phosphorus(MBP) and their fractal characteristic model,as well as soil enzyme activities under seven grain-for-green patterns(cultivation of Zenia insignis,Toona sinensis,Castanea mollissima,Citrus reticulate,Zenia insignis + Guimu No.1 forage,Guimu No.1 forage,and abandoned cropland) in depressions between karst hills.The main soil microbial populations differed under different grain-for-green patterns.Actinomycetes were 71.28% to 96.44% of the microbial population in the slope cropland,Zenia insignis + Guimu No.1,Guimu No.1,and abandoned cropland.Differences in their populations covered up to three orders of magnitude.Bacteria dominated in the other four soils,being 55.24% to 71.79% of the microbial population,but fungi were less than 1%.Bacteria were highest under Guimu No.1 and abandoned cropland,and were lowest in the slope cropland.Fungi populations were in the order: abandoned cropland Zenia insignis Guimu No.1 Citrus reticulate Castanea mollissima slope cropland Toona sinensis Zenia insignis + Guimu No.1,with significant differences between them.All the grain-for-green patterns except Zenia insignis increased total soil microbial populations.The differences between Guimu No.1,abandoned cropland,and Zenia insignis + Guimu No.1 were significant(P0.05),and were significantly higher than the others.Microbial biomass carbon(MBC) in the all grain-for-green patterns except Zenia insignis was significantly higher than in the slope cropland,and was highest in the abandoned cropland.Microbial biomass nitrogen(MBN) varied within a small range,showing a downward trend.Differences in MBN between cultivation of Zenia insignis + Guimu,Citrus reticulata,Zenia insignis and slope cropland were significant(P0.05).Microbial biomass phosphorus(MBP) varied widely in Toona sinensis,Castanea mollissima,abandoned cropland and Guimu No.1 but was significantly higher than in slope cropland.MBC was well correlated with bacteria(D=-4.07,R=0.81,P0.01),followed by the correlation between MBC and actinomycetes(D=3.82,R=0.44,P0.05) and that between MBN and fungi(D=0.58,R=0.61,P0.01).However,there was no fractal relationship between the other microbial biomass and microbial populations.Compared with the slope cropland,all the grain-for-green patterns except Guimu No.1 forage significantly increased the activities of soil sucrase,urease and phosphatase,suggesting an increase in carbon and nitrogen cycling.Under different grain-for-green patterns,the activity of soil sucrase was highest under Toona sinensis,reaching(29.10±7.73) mg g-1 d-1,while the activity of soil urease was highest in the abandoned cropland,reaching(540.7±34.77) μg g-1 d-1,and the activity of soil phosphatase was highest under Castanea mollissima,reaching(991.1±76.28) μg g-1 d-1.All the grain-for-green patterns improved soil microbial properties,suggesting important benefits arising from fragile ecosystem restoration and reconstruction in depressions between karst hills.

Soil microbe is the most important decomposer in forest ecosystems,and plays a key role in material recycling and energy conversion.In this study,the spatial patterns of soil microbial biomass in typical Karst peak-cluster depressions were analyzed through both geo-statistical and traditional statistical methods.The results showed that large variation existed in soil microbial biomass.Soil microbial biomass carbon(Cmic),soil microbial biomass nitrogen(Nmic),and soil microbial biomass phosphorus(Pmic) varied from 44.29—5209.63,20.91—1894.37,0.34—77.06 mg/kg,respectively.The Cmic was significantly positively correlated with Nmic.The value of Cmic/Nmic is 4.78,which obviously lower than that in other ecosystems.Semivariance analysis revealed that the Gaussian model fitted best for Cmic and Nmic,the Spherical model fitted best for Pmic,and the Exponential model fitted best for Cmic/Nmic.The SH%(C/(C+C0)×100%) of soil microbial biomass exhibited moderate spatial autocorrelation,ranging from 25% to 75%.Therefore,the spatial patterns were affected by both random and structure factors.The autocorrelation distance of Cmic and Nmic were about 50 m.Moran′s I decreased gradually with the increase of separation distance,and showed a statistically negative correlation.Moran′s I of Pmic increased when the separation distance was larger than 70 m,which indicated positive correlation.Moran′s I of Cmic/Nmic was lower than others,ranging from-0.2 to 0.2 m.The spatial distribution of Cmic and Nmic were similar,and showed a convex-type distribution in which the contents were high in the middle and low on the two sides.In contrast,The Pmic was distributed with a pattern of high in the upper slope and depressions.The content of Cmic/Nmic showed a concave-type distribution with many small patches.We conclude that soil microbes exist in a certain spatial pattern,and that its contents sharply reduced after disturbing.As a result,the protection of Karst primary ecosystems against human disturbing should be strengthened.

Forests form the main body of terrestrial ecosystems and play an important role in mitigating global climate change through carbon dioxide sequestration in forest biomass. Biomass and productivity are essential characteristics of forestecosystem structure and function,and knowledge of both are essential for the study of carbon sink function and carbon sequestration. Cunninghamia lanceolata,Pinus massoniana,and Eucalyptus urophylla × Eucalyptus grandis are the primary plantation trees used in southern China,accounting for 90% of the plantation area in ten southern provinces. However,few studies have analyzed the dynamics of forest biomass and productivity of these forest types. Forty-five 20 m × 50 m research plots were established and analyzed using three dominant plantation species( C lanceolata,P. massoniana,E. urophylla ×E. grandis) in southern China with the goal of improving our understanding the vertical structure of each community,the distribution of biomass to the various organs,and gaining a better understanding of forest biomass and productivity of differently aged stands. Each plantation type was studied using five age classes: young,middle-aged,near-mature,mature,and over-mature forest,with three replicates of each. Regression models relating tree biomass to diameter at breast height( DBH 1.3 m) were developed to estimate tree biomass. Other biomass components were sampled in quadrats within the same forty-five research plots: shrubs( three 4 m2quadrats),herbs( three 1 m2quadrats),and litterfall( three 1 m2quadrats). This method was used for estimating biomass,productivity,and biomass allocation rates to various organs in the three main plantations types; we also analyzed the relationships between stand age and biomass productivity to determine the biomass allocation rates to various organs. Results showed that a significant relationship exists between stand biomass and stand productivity of the three main plantation types as they relate to stand age,with a high degree of fitting in growth models. The total biomass of C. lanceolata,P. massoniana,and E. urophylla × E.grandis plantations increased with stand age and the biomass of mature forest was 192.30,191.53,105.77 Mg /ha,respectively. Living biomass made up 95.76%—98.39%,75.01%—99.14%,85.60%—97.61% of these three plantation types,respectively. Most of the biomass of the three main plantation types was concentrated in the tree layer and biomass increased with stand age for all three types. The biomass of other layers accounted for only a small percentage of total biomass,and generally litterfall herb shrub biomass. The trunk biomass occupied the greatest proportion of the tree layer; for C. lanceolata,P. massoniana,and E.urophylla × E. grandis truck biomass made up 54. 89%—75. 97%,49. 93%—83. 10%,and 51. 07%—98. 48% of total biomass,respectively,and trunk biomass increased with stand age. Root biomass made up the second largest percent of tree biomass and the percentages of both branch and leaf biomass were lower than root biomass and decreased with stand age.Shrub branches made up the majority of shrub biomass; both the above-and belowground biomass allocation of herbs did not exhibit an obvious trend or pattern of change. Compared with other forest types,the total biomass of C. lanceolata plantations and P. massoniana plantations were both greater than most and the total biomass of E. urophylla × E. grandis was lower than most,but the level of productivity of all three plantation types was high,or 12.37,8.98,21.10 Mg hm-2a-1,respectively. All three are fast growing forest tree species with higher photosynthetic efficiency and potential carbon sequestration than other forest plantation species in South China.