Stipa steppes in scantily explored regions of the tibetan Plateau:classification,community characteristics and climatic distribution patterns

Aims As a unique geographical unit of the earth,the tibetan Plateau is extensively covered by various Stipa communities.However,their vegetation features have not been reported systematically till now,especially in some scantily explored regions.In this study,we endeavor to reveal the community types,quantitative charac-teristics and climatic distribution patterns of Stipa steppes in these areas based on primary relevés obtained from fieldwork.Methods We collected a total of 223 plots in 79 study sites in the Changthang Plateau and the Yarlung Zangbo Valley,ranging from 79°E to 91°E.the categories of Stipa formations were identified according to the classification scheme in Vegetation of China and then verified by Nonmetric Multidimensional Scaling.We performed detrended correspondence analysis and detrended canonical correspondence analysis to hunt for the alteration of Stipa communities along the precipitation gradient.Quantitative characteristics including species richness,coverage,biomass as well as importance values(IV)of dominant species were calcu-lated and visualized,respectively.Important Findings Stipa steppes in scantily explored regions of the tibetan Plateau are classified into 11 formations but major formations are rather limited in number.Formation(form.)Stipa purpurea is the most widespread Stipa assemblages not only in scantily explored regions but also across the whole tibetan Plateau.the characteristics of Stipa com-munities,including coverage,species richness,productivity and IV of dominant species,demonstrate the features of typical alpine steppes on the tibetan Plateau.Precipitation proves to be the prime climatic factor controlling the distribution patterns of Stipa assemblages.Form.Stipa subsessiliflora var.basiplumosa and form.Stipa glareosa normally distribute in arid habitats,but rainfall for the former is of greater variance.Form.Stipa roborowskyi and form.Stipa capillacea favor moderately moist environment.Form.Stipa purpurea and form.Stipa roborowskyi can tolerate a fairly broad range of precipitation.

Vegetation classification at the association level under the China Vegetation Classification System: an example of six Stipa steppe formations in China

Aims The latest China Vegetation Classification System(China-VCS)for natural/semi-natural vegetation has eight hierarchical levels:Association<Association-group<Subformation<Formation<Formation-group<Vegetation-subtype<Vegetation-type<Vegetation-type-group.The classification is based on dominant species and their growth forms and has been completed at the formation level.The principal challenge today in Chinese vegetation classification is to develop the China-VCS at levels below the formation in a way that is consistent with current international standards.We explored the following question:how can existing vegetation plot data help develop the China-VCS and improve its compatibility with other international classification systems?Methods We compiled 401 plots having plant cover and/or aboveground biomass measurements collected in six Stipa steppe formations and divided them into those with cover data(299 plots)and/or biomass data(283 plots).We applied a combination of hierarchical clustering and ordination to partition the cover and biomass data sets into formations and constituent associations.We then used supervised noise clustering to improve the classification and to identify the core plots representing each association.Diagnostic species were also identified at both association and formation levels.Finally,we compared the classification results based on cover and biomass data sets and combined these results into a comprehensive classification framework for the six formations.Important Findings Our results using cover data were comparable with those using biomass data at both formation and association levels.Three Stipa formations were classified into associations based on cover data,two based on biomass data and one based on both biomass and cover data.Twenty-seven associations were defined and proposed within the six formations,using cover or biomass data as consistent classification sections(CCSs).Both dominant species in the dominant stratum and diagnostic species from multiple strata of the core plots were used to characterize vegetation types at both formation and association levels,improving the compatibility of our classification with the International Vegetation Classification.Temperature and precipitation were found to be important climatic factors determining the distribution pattern and species composition of Stipa-dominated vegetation.We propose a framework for plotbased vegetation classification in the China-VCS,using our work with Stipa-dominated steppe vegetation as an example.We applied the concept of CCS to make optimal use of available data representing both plant cover and biomass.This study offers a model for developing the China-VCS to the association level in a way that is consistent with current international standards.

Effects of Rest Grazing on Organic Carbon Storage in Stipa grandis Steppe in Inner Mongolia, China

This study was aimed to evaluate the potential effects of rest grazing on organic carbon storage in Stipa grandis steppe of Inner Mongolia, China. Using potassium dichromate heating method, we analyzed the organic carbon storage of plant and soil in Stipa grand＆ steppe after rest grazing for 3, 6, and 9 yr. The results indicated that as the rest grazing ages prolonged, the biomass of aboveground parts, litter and belowground plant parts （roots） of the plant communities all increased, meanwhile the C content of the biomass increased with the rest grazing ages prolonging. For RG0, RG3a, RG6a, and RG9a, C storage in aboveground vegetation were 60.7, 76.9, 82.8 and 122.2 g C m2, respectively; C storage of litter were 5.1, 5.8, 20.4 and 25.5 g C m^-2, respectively; C storage of belowground roots （0-100 cm） were 475.2, 663.0, 1 115.0 and 1 867.3 g C m^-2, respectively; C storage in 0-100 cm soil were 13.97, 15.76, 18.60 and 32.41 kg C m^-2, respectively. As the rest grazing ages prolonged, the organic C storage in plant communities and soil increased. The C storage ofbelowground roots and soil organic C was mainly concentrated in 0-40 cm soil body. The increased soil organic C for RG3a accounted for 89.8% of the increased carbon in vegetation-soil system, 87.2% for RG6a, and 92.6% for RG9a. From the perspective of C sequestration cost, total cost for RG3a, RG6,, and RG9a were 2 903.4, 5 806.8 and 8 710.2 CNY haq, respectively. The cost reduced with the extension of rest grazing ages, 0.15 CNY kg^-1 C for RG3a, 0.11 CNY kg-~ C for RG6a and 0.04 CNY kg℃ for RG9a. From the growth characteristics of grassland plants, the spring was one of the two avoided grazing periods, timely rest grazing could effectively restore and update grassland vegetation, and was beneficial to the sustainable use of grassland. Organic C storage for RG9a was the highest, while the cost of C sequestration was the lowest. Therefore, spring rest grazing should be encouraged because it was proved to be a very efficient grassland use pattern.

Emission characteristics of carbon dioxide in the semiarid Stipa grandis steppe in Inner Mongolia, China

Using the static opaque chamber method, the soil respiration rates （SR） were measured through the continuous experiments in situ in semiarid Stipa grandis steppe in Xilin River Basin of Inner Mongolia, China from June 2001 to June 2003, in parallel, the difference between the SR and the ecosystem respiration rates （TER） were compared. The results indicated that the seasonal variations of the SR and TER were obvious with higher emissions in growing season and a relatively low efflux level in non-growing season, furthermore, the negative effluxes were found in the observation site in winter; the annual CO2 efflux of total ecosystem ranged from 160.5 gC/（m^2·a） to 162.8 gC/（m^2·a） and that of soil ranged from 118.7 gC/（m^2·a） to 152.3 gC/（m^2·a）. The annual SR accounted for about 74.0% to 93.5% of the annual TER, but the results of Analysis of Variance （ANOVA） indicated that the difference between the annual average TER and SR did not reach the significance level of 0.05. The TER was under similar environmental controls as SR, in growing seasons of drought years, the variations of soil moisture at 0-10 cm and 10-20 cm depth could account for 79,1% 95.6% of the changes of the SR and TER, but in non-growing season, more than 75% of the variations of the SR and TER could be explained by the changes of the ground temperature of soil surface layers.

Comparison of CO_(2) Effluxes and Their Driving Factors Between Two Temperate Steppes in Inner Mongolia, China

Soil respiration is a key component of the global terrestrial ecosystem carbon cycle. The static opaque chamber method was used to measure the CO2 effiuxes from soil of a semiarid Aneurolepidium chinense steppe and a Stipa krylovii steppe in the Xilin River Basin of Inner Mongolia, China from March 2002 to December 2004. The results indicated that the soil respiration rates of the semiarid Aneurolepidium chinense steppe and the Stipa krylovii steppe were both relatively high from mid-May to mid-September of each year and remained low during the rest of the year. The minimum value of soil respiration occurred in December or January and negative effiuxes of CO2 appeared for several days during the non-growing season of individual years at the two sampling sites. A high annual variation was found in the two steppes with the coefficients of variance （CV） being over 94%, even high to 131%. The annual sums of soil CO2 effiux of the Aneurolepidium chinense steppe varied between 356.4 gC m^-2 yr^-1 and 408.8 gC m^-2 yr^-1, while those of the Stipa krylovii steppe in the three years were in the range of 110.6 gC m^-2 yr^-1 to 148.6 g Cm^-2 yr^-1. The mean respiration rates of the Aneurolepidium chinense steppe were significantly higher than those of the Stipa krylovii steppe in different statistical periods with the exception of the non-growing season. About 59.9% and 80.6% of the soil respiration variations in both steppes for the whole sampling period were caused by the changes of temperature and soil water content. In the Aneurolepidium chinense steppe, the soil respiration rate has significant or extremely significant positive correlation （r = 0.58 - 0.85, p 〈 0.05 or p 〈 0.01） with air temperature and ground temperature of the topsoil except in 2002; the unique contributions of temperature change to the soil respiration variation of the three years were 53.3%, 81.0% and 58.6%, respectively. But, for the Stipa krylovii steppe in the same time interval, the soil water content （especially that of the 10-20 cm layer） has a greater effect on the change of soil respiration, and the unique contributions of the change of the 10-20 cm soil water content to the variations of soil respiration in 2002 and 2003 were 60.0% and 54.3%, respectively. In 2004, in spite of the higher contribution of temperature than soil water content, the contribution of ground temperature at a depth of 10 cm was only 46.2%, much weaker than that of any single year in the Aneurolepidium chinense steppe.

Differential Responses of Ammonia-oxidizers Communities to Nitrogen and Water Addition in Stipa baicalensis Steppe,Inner Mongolia,Northern China

Atmospheric nitrogen deposition and precipitation as an important phenomenon of global climate change have a great impact on grassland ecosystems. However, little is known about how the soil ammonia-oxidizing microorganisms respond to the both changes. Ammonia oxidization is a crucial step in the soil nitrification and greatly inlfuenced by soil nitrogen availability. We used PCR and DGGE （denaturing gradient gel electrophoresis） approaches to investigate the responses of AOB （ammonia-oxidizing bacteria） 16S rRNA and AOA （ammonia-oxidizing archaea）amoA genes to nitrogen and water input inStipa baicalensis steppe, Inner Mongolia, northern China. After two years of nitrogen and water addition treatment, it was found that PNA （potential nitriifcation activity） was greatly enhanced by lower N fertilization treatment under water addition and higher N fertilization under no-water addition, while it decreased markedly in higher N fertilization under water addition. The community structure of AOB responded more sensitively to N fertilization and water input than AOA, resulting in the significantly decreased diversity in the AOB community along with a higher N fertilizer rate, but an obvious increase in the AOA community, demonstrating the active growth of AOA in higher N fertilization soils. Phylogenetic analysis showed that AOB communities were dominated byNitrosospira clusters3, 4 andNitrososmonas clusters 6 under water addition andNitrosospira culsters 1, 3 and 4 and under no-water addition, while AOA communities were grouped intoCrenarchaeote clusters 1, 2 and 5 under no-water addition and Crenarchaeote clusters 1, 2 and water lineage under water addition. The differences between the two water addition regimes strongly suggest that water input acts as an important role in shifting AOA and AOB communities. Moreover, in contrast to the AOA, the diversity of AOB was negatively correlated with total N, NH4^＋, NO3^- and pH under water addition, implying a signiifcant N fertilization and water effect on shaping AOA and AOB communities. In conclusion, our studies suggested that N fertilization and water addition and their composite effects had signiifcantly changed AOB and AOA communities, meanwhile, AOB and AOA communities could develop a desirable complementary mechanism in response to external changes.

Variation in carbon,nitrogen and phosphorus partitioning between above-and belowground biomass along a precipitation gradient at Tibetan Plateau

Precipitation is a potential factor that significantly affects plant nutrient pools by influencing biomass sizes and nutrient concentrations. However, few studies have explicitly dissected carbon(C), nitrogen(N) and phosphorus(P) pools between above- and belowground biomass at the community level along a precipitation gradient. We conducted a transect(approx. 1300 km long) study of Stipa purpurea community in alpine steppe on the Tibet Plateau of China to test the variation of N pool of aboveground biomass/N pool of belowground biomass(AB/BB N) and P pool of aboveground biomass/P pool of belowground biomass(AB/BB P) along a precipitation gradient. The proportion of aboveground biomass decreased significantly from mesic to drier sites. Along the belt transect, the plant N concentration was relatively stable; thus, AB/BB N increased with moisture due to the major influences by above- and belowground biomass allocation. However, P concentration of aboveground biomass decreased significantly with increasing precipitation and AB/BB P did not vary with aridity because of the offset effect of the P concentration and biomass allocation. Precipitation gradients do decouple the N and P pool of a S. purpurea community along a precipitation gradient in alpine steppe. The decreasing of N:P in aboveground biomass in drier regions may indicate much stronger N limitation in more arid area.