Understanding the response patterns and potential mechanisms of structure and function in grassland ecosystems to nitrogen (N) enrichment is essential to evaluate ecological impacts of external N input. The muti-level N manipulative experiment offers the possibility to explore the nonlinear response patterns and associated mechanisms of structure and function in grassland ecosystems to additional N input. In this review, we summarized the impacts of additional N inputs on community diversity, carbon (C) and N cycling in grassland ecosystems around the world. Numerous studies illustrated that N enrichment induced the decline of plant species diversity, plant functional diversity and soil bacteria richness in grassland ecosystems, yet the change of fungal diversity was not significant. Above- and below-ground plant productivity showed different responses to N input: aboveground plant productivity exhibited initial increasing and subsequent saturation trends, but root productivity and root:shoot ratio showed downward patterns, and root turnover rate appeared a single-peak pattern of first increasing and then decreasing with the continuous increase of N addition rate. Meanwhile, different C decomposition processes responded variously to N enrichment. Specifically, litter decomposition rates exhibited multiple response of “exponential decrease, liner increase or insignificant change with N addition level”. However, the relationship of soil respiration and CH₄ consumption with N addition was dominated by a single peak trend of increasing at low to medium N levels but declining at high N levels. Likewise, different soil C fractions showed multiple response patterns to N input. N addition generally stimulated soil C storage and particulate organic C accumulation, while the mineral-associated organic C exhibited divergent responses of “increase, unaltered, and decrease” along the N addition gradient. In addition, plant N uptake exhibited initial increasing and subsequent situation trends along N addition gradients, while different soil N transformation processes showed differentiated responses along N addition gradients and the relationship between N₂O emission and N addition rate varied among various grassland ecosystems. An exponential increase of N₂O fluxes with N addition rate was observed in temperate grasslands, while the patterns of first increase and then saturation or linear increase of the N-induced changes in N₂O emissions had been discovered in alpine grasslands. Future studies should focus on the nonlinear responses of rhizosphere processes and phosphorus (P) cycle to external N input, and also explore potential mechanisms from the aspect of multi-dimensional biodiversity changes.

Aims The continuous observation datasets of water, heat, and carbon fluxes measured by the eddy covariance technique are important basis for accurate assessment of regional carbon sequestration and water-holding capacity. However, the rate of gaps in flux datasets is high and common due to various reasons, and different gap-filling methods increase the uncertainties of the related studies. The aim of this study is to introduce and test the applicability of boosted regression trees model (BRT), one of the up-to-date machine learning algorithms, for the gap- filling to flux datasets.

Methods Based on the published valid dataset of water, heat and CO₂ flux, and main environmental factors, including air temperature, atmospheric water vapor pressure, wind speed, solar shortwave radiation, topsoil temperature, and topsoil water content of an alpine Potentilla fruticosa scrubland on the northeastern Qingzang Plateau from 2003 to 2005, the BRT were trained to fill flux data gaps and the results were compared to those corresponding data serials provided by Chinese Flux Observation and Research Network (ChinaFLUX).

Important findings The results showed that the BRT performed well for a large amount of samples (N > 10 000) and the regression slopes of observation data against predicted value were between 1.01 and 1.05 with R² > 0.80. The BRT revealed that the daytime 30-min CO₂ flux (net ecosystem CO₂ exchange, NEE) in the growing season (i.e., May to October) was mainly controlled by solar shortwave radiation and atmospheric vapor pressure, whose relative contributions to NEE variability were up to 74.7%. The topsoil temperature was the determinant for NEE at night during the growing season and the whole day during the non-growing season, and its relative contribution was 68.5%. The 30-min sensible heat flux (H) and latent heat flux (LE) were both linearly related to solar radiation, and their relative contributions were above 58.6%. 30-min flux data gap amount filled by the BRT was significantly less than those by ChinaFLUX. Except for daily net ecosystem CO₂ exchange (p = 0.14), daily gross ecosystem CO₂ exchange (GEE), ecosystem respiration (RES), H, and LE of the BRT were significantly less than those of ChinaFLUX by 17.5%, 21.0%, 2.7%, and 2.2%, respectively. However, there was a reasonable consistency between the daily fluxes of 2003-2005 interpolated by the BRT and by ChinaFLUX due to the small magnitude difference (the regression slopes of the two data series were between 0.95 and 1.17). Except for monthly GEE and RES, monthly NEE, H, and LE of the BRT had no significant difference between the BRT and ChinaFLUX (p > 0.09). Compared with the ChinaFLUX gap-filling method, BRT can simulate the nonlinear relationships between fluxes and environmental factors without complicated mathematical expressions and quantify the relative contribution of environmental factors to the flux data gaps, which is a feasible technique for the integrated analysis of flux data.

Aims The relationships between temporal changes in vegetation growth and climate change tend to be asymmetric. Considering the temporal effects of climate factors on vegetation growth can provide important scientific basis for accurately understanding vegetation-climate relationships and predicting the dynamic responses of vegetation to global climate change. Methods Based on the MODIS normalized difference vegetation index (NDVI), climate, and vegetation type data, this study investigated the temporal effects of climate factors on vegetation growth and the dominant factors influencing vegetation growth on the Qingzang Plateau through establishing four temporal effects equations between climate and vegetation NDVI. Important findings Among the four temporal effects, models considering both time lag and accumulation effects had the highest explanation degree (47%). Compared with model without considering temporal effect, the explanation power of the time lag and accumulation effects on vegetation would increase by 4%-18%. Vegetation dynamics on more than 43% of the Qingzang Plateau was dominated by the combined effects of time lag and accumulation, followed by the area only affected by time accumulation or lag effects, and the area without time effect. The partial correlation coefficient between NDVI and precipitation (0.25 ± 0.56) was overall higher than it between NDVI and temperature (0.08 ± 0.6). The areas dominated by the precipitation were mainly distributed on the northeast and southwest of the Qingzang Plateau with an area ratio of 40.1%, whereas the areas dominated by temperature were largely distributed on the center and southeast of the Qingzang Plateau with an area ratio of 29.7%. These research results can provide basic judgments for the relationships between vegetation growth and climate on the Qingzang Plateau.

Aims β diversity reflects species turnover rate across environmental gradients, and this study attempts to use β diversity to reveal relevant ecological processes underlying the changes in plant community composition along a disturbance gradient induced by small burrowing herbivores.
Methods This study conducted a field survey at Gangcha County in the Qingzang Plateau to determine the effect of disturbance intensities of plateau pika (Ochotona curzoniae) on the β diversity of an alpine meadow measured by Whittaker’s index. Then a variance decomposition was used to measure the contribution of individual species to overall β diversity (SCBD) and single interference sites to overall β diversity (LCBD) according presence- absence community matrix.
Important findings The turnover rate of species in the plant community first increased and then decreased with the increase of the disturbance intensities induced by plateau pika. Species with intermediate site occupancy had a greater contribution to β diversity, in which Agropyron cristatum, Artemisia hedinii and Anemone rivular var. flore-minore were the single-species plants that contributed the most to the β diversity of the study regions. The disturbance plot T₀ with the absence of plateau pika had the greatest contribution to the β diversity of whole study region. LCBD of individual site was negetively related to species richness of that individual site, and had no significant correlation with the disturbed intensity of plateau pika. These results indicate that alpine meadows distributed in high LCBD and Agropyron cristatum, Artemisia hedinii and Anemone rivularis var. flore-minore with high SCBD should be protected to conserve plant diversity when the plateau pikas are present.

Aims Understanding the mode of nutrient limitation on ecosystem net primary production is an important issue of modern ecology. Nutrient availability is a key determinant of ecosystem dynamics, but the relationship between soil resource availability and ecosystem nutrient limitation is still unclear.
Methods A series of nitrogen and phosphorus nutrient addition experiments were set up in four types of alpine grasslands (alpine meadow, alpine meadow-steppe, alpine steppe and alpine desert-steppe) along the precipitation gradient on the Northern Xizang, to systematically study the effects of nitrogen and phosphorus addition on different types of alpine grasslands, and to explore the nitrogen and phosphorus limitation models of different alpine grasslands.
Important findings The results showed that: (1) The effects of nitrogen and phosphorus addition on different alpine grasslands varied. Nitrogen addition significantly increased the aboveground biomass of alpine meadows and alpine meadow grasslands, but had no effect on alpine meadows and alpine desert grasslands. The addition of phosphorus alone had no significant effect on the four alpine grasslands, while the addition of nitrogen and phosphorus had a promoting effect on the aboveground biomass of the four alpine grasslands. (2) With the decrease of precipitation, the nitrogen limitation index of the alpine grasslands gradually decreased from 1.18 to 0.52-0.64, and the nutrient limitation mode transitioned from nitrogen limitation to co-limitation by nitrogen and phosphorus; the phosphorus limitation index was negative in the alpine meadow-steppe and alpine steppe, indicating that phosphorus addition alone has side effects on these two grassland types. These results suggest that alpine meadow is mainly limited by nitrogen availability, and phosphorus addition alone has side effects; the alpine meadow-steppe is between the nitrogen limit and the joint nitrogen and phosphorus limitation, and phosphorus addition alone also has side effects; the alpine steppe is limited by both nitrogen and phosphorus availability, and the addition of phosphorus has side effects; the alpine desert-steppe is jointly limited by nitrogen and phosphorus availability. These results show that nutrient limitation mode transits from nitrogen limitation to nitrogen and phosphorus co-limitation with the decrease of precipitation. This study implies that the impacts of increasing nitrogen deposition under future climate change on different types of alpine grasslands may be different. Additionally, the differences in nitrogen and phosphorus limitation mode should also be taken into consideration when nutrient addition is used to restore different types of degraded alpine grasslands.

Aims As one of the major terrestrial ecosystems of the world, a small fluctuation of grassland soil carbon (C) would affect the carbon cycle of the terrestrial ecosystem and ecosystem multifunctionlity (EMF). The carbon accumulation rate (CAR) of aboveground community well reflects the capacity and efficiency of carbon sequestration in a field from the start to the peak of a growing season. The changes in plant CAR could influence the ability of above- and below-ground community. Currently, the majority of studies have primarily focused on the relationship between community diversity and EMF, while the linkages of CAR with EMF were understudied. We aimed to explore the process and underlying mechanism of how CAR affecting EMF in alpine grassland community. Our results would improve the understanding of EMF maintenance mechanism and provide theoretical support for alpine ecosystem management.
Methods We conducted a field transect survey which consists of a total of 115 sample sites of alpine grasslands on the Qingzang Plateau from July to August 2015. The ecosystem multifunctionality index (M) was calculated from 13 key ecosystem parameters including soil organic carbon content, total nitrogen content, total phosphorus content above- and belowground biomass etc. The normalized difference vegetation index (NDVI, 1982-2013) was adopted to obtain the phenology in 2015. We calculated the CAR value. To explore the underlying mechanism of how CAR affecting EMF, the annual total precipitation and temperature were extracted by the method of thin disk smooth spline interpolation based on observations of meteorological stations from 2011-2015.
Important findings Belowground biomass, soil organic carbon content, total phosphorus content and microbial biomass carbon content had high weighting for CAR (0.58, 0.80, 0.83 and 0.79) and M (1.05, 0.98, 1.02 and 0.97). There was a significantly positive correlation between CAR and M (R² = 0.45, p < 0.01). Our findings suggested that the synergism of plant community and soil elements affected CAR and further regulated EMF under the influences of precipitation and temperature.

Aims Nitrogen use efficiency (NUE) is a key functional trait in plants, which closely relates to ecosystem functions. However, it is still unclear about the regional patterns and affecting factors of plant NUE.
Methods This study quantified leaf and root NUE in 139 grassland plant species and explored their relationships with environmental factors and plant functional groups across 82 sampling sites in Nei Mongol and Qinghai-‌Xizang Plateau.
Important findings 1) We found that leaf NUE (53 g·g ^-1) in meadow steppe was significantly greater than those in alpine meadow (46 g·g ^-1), desert steppe (41 g·g ^-1) and typical steppe (39 g·g ^-1). Root NUE (108 g·g ^-1) in alpine meadow was higher than those in other ecosystems. 2) Leaf NUE was more sensitive to temperature than root NUE, but with increasing drought index they all showed a significant decrease. 3) Leaf and root NUE in forbs were significantly lower than sedges and grasses. In addition, leaf and root NUE of legume were 48% and 60% lower than those of non-legume, respectively. 4) Plant NUE did not show any significant relationship with soil nitrogen content. Overall, there was significant difference between leaf and root NUE in their spatial patterns in the Nei Mongol and Qinghai-Xizang Plateau grasslands. The main impacting factors were plant functional group and drought index. The findings are helpful for better understanding the mechanisms underlying the variation of grassland productivity in China, and also provide more scientific basis for grassland management.

Aims Terrestrial carbon (C), nitrogen (N), phosphorus (P) stoichiometry will reflect the effects of adjustment to local growth conditions as well as species’ replacements. However, it remains unclear about the hierarchical responses of plant C:N:P to P addition at levels of species and functional groups in the N-limited alpine meadow.

Methods A field experiment of P enrichment was conducted in an alpine meadow on the Qinghai-Xizang Plateau during 2009-2013. The stoichiometric patterns of four functional groups (grass, sedge, legume and forb) and five representative species, Elymus nutans (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb), Geranium pylzowianum (upright forb) were investigated in 2013, and the effects of P addition on species dominance and plant biomass were also analyzed.

Important finding Both plant nutrition content and C:N:P varied significantly after five years’ P addition, and the responses were consistent at species- and functional group (exemplar species excluded)-levels in the alpine meadow. P addition had neutral effect on C concentrations of grasses, sedges and forbs at both species- and functional group (exemplar species excluded)-levels. P fertilization increased plant P concentrations and thus decreased C:P and N:P of the four functional groups (exemplar species excluded) and the corresponding species. N concentrations significantly decreased and C:N increased in grasses and sedges after P addition, and the species-level responses were consistent with the functional group (exemplar species excluded) level. P addition significantly increased N contents and decreased C:N in Oxytropis ochrocephala, but had neutral effect on N contents and C:N at the functional group (exemplar species excluded) level of the legumes. While N contents and C:N in forbs responded to P addition differently at species and functional group (exemplar species excluded) levels. In the N-limited alpine meadow, species dominance of grasses increased gradually after P addition due to the increased N and P use efficiencies, while the biomass proportion of forbs decreased because of the lowered nutrition use efficiency.

Aims Alpine shrub-meadows and steppe-meadows are the two dominant vegetation types on the Qinghai-Xizang Plateau, and plays an important role in regional carbon cycling. However, little is known about the temporal-spatial patterns and drivers of CO₂ fluxes in these two ecosystem types.

Methods Based on five years of consecutive eddy covariance measurements (2004-2008) in an eastern alpine shrub-meadow at Haibei and a hinterland alpine steppe-meadow at Damxung, we investigated the seasonal and annual variation of net ecosystem productivity (NEP) and its components, i.e. gross primary productivity (GPP) and ecosystem respiration (R_e).

Important findings The CO₂ fluxes (NEP, GPP and R_e) were larger in the shrub-meadow than in the steppe-meadow during the study period. The shrub-meadow functioned as a carbon sink through the five years, with the mean annual NEP of 70 g C·m ^-2·a ^-1. However, the steppe-meadow acted as a carbon neutral, with mean annual NEP of -5 g C·m ^-2·a ^-1. The CO₂ fluxes of steppe-meadow exhibited large variability due to the inter-annual and seasonal variations in precipitation, ranging from a carbon sink (54 g C·m ^-2·a ^-1) in 2008 to a carbon source (-88 g C·m ^-2·a ^-1) in 2006. The differences in carbon budget between the two alpine ecosystems were firstly attributed to the discrepancy of normalized difference vegetation index (NDVI) because NDVI was the direct factor regulating the seasonal and inter-annual NEP. Secondly, the shrub-meadow had higher carbon use efficiency (CUE), which was substantially determined by annual precipitation (PPT) and NDVI. Our results also indicated that the environmental drivers of CO₂ fluxes were also different between these two alpine ecosystems. The structure equation model analyses showed that air temperature (T_a) determined the seasonal variations of CO₂ fluxes in the shrub-meadow, with NEP and GPP being positively correlated with T_a. By contrast, the seasonal CO₂ fluxes in the steppe-meadow were primarily co-regulated by soil water content (SWC) and T_a, and increased with the increase of SWC and T_a. In addition, the changes of R_e during the growing season in two ecosystems were directly affected by GPP and soil temperature at 5 cm depth (T_s), while R_e during non-growing season were determined by T_s. These results demonstrate that the synergy of soil water and temperature played crucial roles in determining NEP and GPP of the two alpine meadows on the Qinghai-Xizang Plateau.

Aims Net primary production (NPP) is the input to terrestrial ecosystem carbon pool. Climate and land use change affect NPP significantly. Shrublands occupy more than 20% of the terrestrial area of China, and their NPP is comparable to those of the forests. Our objective was to estimate China shrubland NPP from 2001 to 2013, and to analyze its variation and response to climate change.Methods We used a Carnegie-Ames-Stanford Approach (CASA) model to estimate the NPP of six shrubland types in China from 2001 to 2013. Furthermore, we used Theil-Sen slope combined with Mann-kendall test to analyze its spatial variation and a linear regression of one-variable model to analyze its inter- and intra-annual variation. Finally, a multi-factor linear regression model was used to analyze its response to climate change.Important findings We found the annual mean NPP of China shrubland was 281.82 g•m^-2•a^-1. The subtropical evergreen shrubland has the maximum NPP of 420.47 g•m^-2•a^-1, while the high cold desert shrubland has the minimum NPP of 52.65 g•m^-2•a^-1. The countrywide shrublands NPP increased at the rate of 1.23 g•m^-2•a^-1, the relative change rate was 5.99%. The temperate deciduous shrubland NPP increased the fastest with a speed of 3.05 g•m^-2•a^-1 and subalpine evergreen shrubland had a decreasing trend with a speed of -0.73 g•m^-2•a^-1. Moreover, the other four shrublands NPP had a growing trend, only subalpine deciduous shrubland NPP did not change significantly. The response of NPP to climate change of different seasons varies to different shrubland types. In general, the NPP variation was mainly affected by precipitation, and the spring warming also contributed to it. The increase of countrywide shrubland NPP may promote its contribution to the regional ecosystem function.

Aims Little is known about the stoichiometric characteristics of carbon (C), nitrogen (N) and phosphorus (P) in plateau shrubs across China. Sibiraea angustata is a typical and representative shrub species on the eastern Qinghai- Xizang Plateau, and exploring its C, N and P distribution patterns and stoichiometric properties in different organs (including root, shoot, leaf, twig and fruit) would help us better understand the mechanisms of C, N and P cycling and balance in the S. angustata dominated shrub ecosystem.
Methods Sixteen sampling sites were selected on the eastern Qinghai-Xizang Plateau by the stratified sampling method. The height and coverage of the dominant shrubs, latitude, longitude and altitude of the sites were recorded. Three 5 m × 5 m plots were selected at each site. At least 128 biological samples of plant organs of S. angustata were collected and measured, respectively. The C and N concentrations of plant samples were analyzed using an elemental analyzer (2400 II CHNS). The P concentration was analyzed using the molydate/ascorbic acid method after H₂SO₄-H₂O₂ digestion.
Important findings The C, N and P concentrations of different organs followed the order of: shoot (495.07 g·kg^-1) > twig (483.37 g·kg^-1) > fruit (480.35 g·kg^-1) > root (468.47 g·kg^-1) > leaf (466.33 g·kg^-1); leaf (22.27 g·kg^-1) > fruit (19.74 g·kg^-1) > twig (7.98 g·kg^-1) > shoot (4.54 g·kg^-1) > root (4.00 g·kg^-1) and fruit (2.85 g·kg^-1) > leaf (1.92 g·kg^-1) > twig (0.96 g·kg^-1) > root (0.52 g·kg^-1) > shoot (0.45 g·kg^-1), respectively. The ranges of the coefficient of variation (CV) for C, N and P concentrations were 1.71%-4.44%, 14.49%-25.50% and 11.46%-46.15%, respectively. Specifically, the C concentration was relatively high and stable, and the maximum CV values for N and P were found in roots. The N:P value of different organs varied from 7.12-12.41 and the minimum CV for N:P was found in twig, which indicated that N:P in twig had higher internal stability. In addition, correlation analysis indicated that the C concentration was significantly negatively correlated with N and P concentrations and correlation coefficients were -0.407 and -0.342, respectively. However, N concentration had dramatically positive correlation with P concentration and the correlation coefficient was 0.814. These results also could indicate that the C, N and P stoichiometric characteristics in the S. angustata shrub accorded with the homeostatic mechanism and growth rate hypothesis to some extent, the distributions of C, N and P concentrations were closely related to the function of the organs and it should be prudent to use ecological stoichiometric ratios to judge the condition of nutrient limitation at the species level.

AimsThe Zoigê Plateau, as a very important wetland distribution region of China, was the major methane (CH₄) emission center of the Qinghai-Xizang Plateau. The objective of this study is to study the effects of microtopographic changes on CH₄ emission fluxes from five plots across three marshes in the littoral zone of the Zoigê Plateau wetland.
Methods CH₄ emission fluxes were measured in five plots across three marshes in Zoigê Plateau wetland using the closed chamber method and Fast Greenhouse Gas Analyzer from May to October in 2014.
Important findings During the growing season, mean CH₄ emission fluxes from the permanently flooded hollow (P-hollow) and hummock (P-hummock) in the Zoigê Plateau wetland were 68.48 and 40.32 mg·m^-2·h^-1, while mean CH₄ emission fluxes from the seasonally flooded hollow (S-hollow) and hummock (S-hummock) were 2.38 and 0.63 mg·m^-2·h^-1. CH₄ emission fluxes from non-flooded lawn was 3.68 mg·m^-2·h^-1. Mean CH₄ emission fluxes from five plots across three sites was 23.10 mg·m^-2·h^-1, with a standard deviation of 30.28 mg·m^-2·h^-1 and the coefficient of variation was 131%. We also found that there was a significant and positive correlation between mean CH₄ emission fluxes and mean water table depth in the five plots across three sites (R² = 0.919, p < 0.01), indicating that water table depth was controlling the spatial variability of CH₄ emission fluxes from the Zoigê Plateau wetland on microtopography. CH₄ emission fluxes in the P-hollow, P-hummock, and S-hummock showed an obvious seasonal pattern, which was not observed in the lawn and S-hollow. However, CH₄ emission peaks were observed in all the plots during summer and/or autumn, which could be closely related to the water table depth, soil temperature, and the magnitude of litter mass. In addition, we found that the CH₄ emission flux in the P-hollow was much higher than the other four plots in the Zoigê Plateau wetland, suggesting that CH₄ in the P-hollow could be often transported to the surface by ebullition and CH₄ emission from the Zoigê Plateau wetland may be under estimated in the past.

Aims
Our objective was to explore the vegetation carbon storages and their variations in the broad-leaved forests in the alpine region of the Qinghai-Xizang Plateau that includes Qinghai Province and Xizang Autonomous Region.
Methods
Based on forest resource inventory data and field sampling, this paper studied the carbon storage, its sequestration rate, and the potentials in the broad-leaved forests in the alpine region of the Qinghai-Xizang Plateau.
Important findings
The vegetation carbon storage in the broad-leaved forest accounted for 310.70 Tg in 2011, with the highest value in the broad-leaved mixed forest and the lowest in Populus forest among the six broad-leaved forests that include Quercus, Betula, Populus, other hard broad-leaved species, other soft broad-leaved species, and the broadleaved mixed forest. The carbon density of the broad-leaved forest was 89.04 Mg·hm^-2, with the highest value in other hard broad-leaved species forest and the lowest in other soft broad-leaved species forest. The carbon storage and carbon density in different layers of the forests followed a sequence of overstory layer > understory layer > litter layer > grass layer > dead wood layer, which all increased with forest age. In addition, the carbon storage of broad-leaved forest increased from 304.26 Tg in 2001 to 310.70 Tg in 2011. The mean annual carbon sequestration and its rate were 0.64 Tg·a^-1 and 0.19 Mg·hm^-2·a^-1, respectively. The maximum and minimum of the carbon sequestration rate were respectively found in other soft broad-leaved species forest and other hard broad-leaved species forest, with the highest value in the mature forest and the lowest in the young forest. Moreover, the carbon sequestration potential in the tree layer of broad-leaved forest reached 19.09 Mg·hm^-2 in 2011, with the highest value found in Quercus forest and the lowest in Betula forest. The carbon storage increased gradually during three inventory periods, indicating that the broad-leaved forest was well protected to maintain a healthy growth by the forest protection project of Qinghai Province and Xizang Autonomous Region.

Aims Our objectives were to disclose why evergreen shrubs, but not deciduous shrubs, dominate above timberline in humid southeastern Qinghai-Xizang Plateau, and to test if different functional types converge in response to the warming climate from aspect of nitrogen limitation.

Methods Based on investigations of nitrogen concentration in senesced leaves of seven shrubs across timberline ecotones in the Sergymla Mountains, Southeast Xizang, we analyzed differences in leaf mass- and area-based nitrogen resorption proficiency among different functional types (evergreen vs. deciduous), altitudes and aspects at 4200-4400 m a.s.l.

Important findings Leaf mass-based nitrogen resorption proficiency was higher in the evergreen shrub Rhododendron aganniphum var. schizopeplum than in deciduous shrubs. However, the leaf area-based N resorption proficiency was relatively higher in deciduous shrubs due to their lower leaf mass per unit area. Although no significant difference in the resorption proficiency was found between altitudes or aspects for the deciduous shrubs of Salix oritrepha and Berberis hemsleyana, leaf mass-based N resorption proficiency was higher at higher altitude for the evergreen shrub Rhododendron aganniphum var. schizopeplum. Decreasing N concentration in senesced leaves, i.e., increasing resorption proficiency, which can improve N use efficiency, is an important strategy for the evergreen shrub to cope with the stressful alpine environment across timberline ecotones. Compared with the deciduous shrubs, N resorption proficiency in the evergreen shrub Rhododendron aganniphum var. schizopeplum is assumed to be more sensitive to future climate warming.

Aims Our purpose was to characterize the effects of nitrogen (N) addition on plant carbon (C), N, phosphorus (P), and C:N:P ecological stoichiometric characteristics in six dominant plant species, including Kobresia myosuroides, Elymus nutans, Anemone rivularis, Pedicularis kansuensis, Potentilla fragarioides and Oxytropis ochrocephala, of alpine meadow on the Qinghai-Xizang Plateau, China.
Methods N was added at four levels. Concentrations of C, N and P were measured, and C:N:P was estimated in the six plant species following the N addition treatments.
Important findings Significant differences in leaf N and P concentrations existed among the six species under natural conditions. The N and P concentrations were highest in O. ochrocephala, at 24.5 and 2.51 g·kg^-1, respectively. The leaf N concentration was significantly lower and leaf P concentration was significantly higher in O. ochrocephala than in legume plants of other grasslands in China. Leaf N and P concentrations in the other five species were in the ranges of 11.5-18.1 and 1.49-1.72 g·kg ^-1, respectively. Kokresia myosuroides had the lowest N concentrations and E. nutans had the lowest P concentrations; they were significantly lower than the non-legume plants in other grasslands in China (p < 0.001). P and C concentrations did not respond to N addition in all the six plant species, but N concentration significantly increased with N addition in five species other than O. ochrocephala, which did not respond to N addition. Values of the N:P varied in the range of 7.3-11.2 in treatment without N addition, indicating that the plant growth was limited by N in the alpine meadow. Values of the N:P increased and were greater than 16 with N addition in five species other than O. ochrocephala, indicating that N addition induced P deficiency in these five species. Our results point to very low leaf N concentration and limitation of N on plant growth in alpine meadow on the Qinghai-Xizang Plateau, China, but different species had different responses to N addition. The legume plant O. ochrocephala was not susceptible to N addition, but leaf N concentration in other five plant species was increased by N addition. Findings in this study highlight the importance for fertilization and management of alpine meadow.

Aims Our objective was to study vegetation dynamics and changes in ecosystem functions based on plant traits and characteristics of plant functional types (PFTs).
Methods A field manipulation experiment with a split-plot design was conducted in alpine meadow at the Haibei Research Station of the Chinese Academy of Sciences from 2007 to 2011. Three clipping levels (stubbled 1 cm, 3 cm and unclipped) were used on the whole plot and subplots were treated with or without fertilizer and watering. The recursive algorithm and multivariate analysis were implemented to search for optimal trait subsets and plant response types (PRTs), which could response to experimental treatments, and to identify plant effect types (PETs) impacting the aboveground net primary productivity of community.
Important findings Under each of four resource conditions, i.e., unfertilized and unwatered (NFNW), fertilized only (F), watered only (W) and fertilized and watered (FW), the optimal response trait subsets were different, i.e., leaf margin-plant height-leaf weight-specific leaf area, life cycle-plant height-leaf weight-specific leaf area, life cycle-chlorophyll content-leaf surface-plant height-leaf weight-specific leaf area, and propagative organ-leaf margin-plant height, respectively. Of these responses, plant height, leaf weight and specific leaf area were more sensitive to treatments than others. Under the resource conditions, we found 14 optimal PRTs and four PETs in all of the PFTs. These PETs can explain 50.3%-86.4% of variation in productivity. The optimal PRTs and the optimal PETs account for 70% and 20% of all PFTs. PETs account for 28.5% of PRTs, therefore, there was partial overlap between PETs and PRTs. These results indicate that both vegetation response to grazing disturbance and ecosystem functioning changes could be accurately reflected by easily measurable plant functional traits. However, the optimal trait subsets and PFTs could be different depending on when the heterogeneity of resources is taken into account. PETs are the optimal PFTs reflecting vegetation response to grazing disturbance, but also changes in ecosystem functioning.

Aims Recent theoretical and empirical work suggests that species diversity enhances the primary productivity and stability of communities. However, the relationships between the diversity of different species types (i.e., total species, response species, effect species and common species), the special function of ecosystems, and the potential mechanism driving stability remain unclear. Our objective is to address the question by comparing the diversity effect of these different types on aboveground net primary productivity (ANPP) and community stability.
Methods Our experiment was conducted in alpine meadow at the Haibei Research Station of the Chinese Academy of Sciences from 2007 to 2011. We used a split-plot design with clipping treatment in the whole plot using three clipping levels (stubbled 1 cm, 3 cm and unclipped). Subplots were treated with fertilizer (urea 7.5 g·m^-2·a^-1+ ammonium phosphate 1.8 g·m^-2·a^-1 and unfertilized) and watering (20.1 kg·m^-2·a^-1 and unwatered).
Important findings We observed that the diversity of different species types affected ecosystem functioning differently. ANPP was mainly affected by the diversity of response species and effect species, whereas community stability was largely affected by that of common species. The maintenance of stability depended on increasing diversity in common species, and the potential mechanism was the portfolio effect. Both the over-yielding effect and asynchrony effect, however, had no influence on stabilizing the community. Clipping had enormous effects on the diversity of total species, whereas the changes in diversity of response species mainly connected with resource availability. Thus, clipping and fertilization had reverse effects on species diversity, ANPP and stability, i.e., the former increased both species diversity and stability and decreased ANPP, while the latter had opposite effects on them. Our results suggest that ANPP is driven by the diversity of a few effect species because they have a great influence on ANPP, while stability is driven by the diversity of a large number of common species because they can coexist stably in the community. The portfolio effect is the main mechanism of the diversity-stability relationship. The diversity effect of different species differs among each other; therefore, in terms of specific ecosystem functioning, we infer that “functional identity” of species in community is more important than diversity per se and it may be incorrect if we did not discriminate when defining the relationship between species diversity and ecosystem function in any situation.

Aims Meadow grassland is a dominant vegetation type on the Qinghai-Tibetan Plateau, but its mechanisms controlling the exchange of CO₂ across a spectrum of time scales and carbon budget remain unclear. Our objective was to investigate the main drivers of ecosystem carbon dynamics and understand the potential response to future climate warming.
Methods We used the eddy covariance method for continuously measuring net ecosystem CO₂ exchange (NEE) and environmental factors over meadow grassland on the northern shore of Qinghai Lake from July 1, 2010 to June 30, 2011.
Important findings Diurnal changes of NEE were controlled by photosynthetic photon flux density (PPFD) during the middle growing season (May to September), and air temperature (T_a) was the determining factor on diurnal NEE during other periods. The maximum daily CO₂ uptake and release rate were 11.37 g CO₂∙m^-2∙d^-1 on July 1 and 4.04 g CO₂∙m^-2∙d^-1 on October 21, respectively. T_a was the primary environmental factor related to daily NEE, and the correlation was described by an exponential-linear equation (R ²= 0.54, p < 0.01). A significant asymptotical response of daily NEE with increasing leaf area index (LAI) and enhanced vegetation index (EVI) was observed. The interaction effect of LAI and T_a was significant (p < 0.05), while main effect of EVI versus T_a was more important (p < 0.001). Respiration quotient (Q₁₀) was 2.42 and ecosystem total respiration (R_eco) consumed 74% of gross primary production (GPP). The proper magnitude of diurnal temperature range (<14.8 ℃) could be propitious to ecosystem carbon sequestration. The meadow grassland acted as carbon sink and absorbed 271.31 g CO₂∙m ^-2from the atmosphere during the study period.

Aims Although much attention has been paid for the relationship between the biodiversity and ecosystem functioning, little is known about the ecosystem consequences of changes. In addition, the question whether species diversity (SD) can co-vary with functional diversity (FD) is unanswered. We used the Rao index to quantify FD in order to explore this question through comparison of different components of biodiversity and their effects on primary productivity (PP).
Methods A field manipulation experiment was conducted in alpine meadow at the Haibei Research Station of the Chinese Academy of Sciences from 2007 to 2010. The experiment used a split-plot design with clipping treatment in the whole plot using three clipping levels (stubbled 1 cm, 3 cm and unclipped). Subplots were treated with fertilizer (urea 7.5 g·m^-2·a^-1+ ammonium phosphate 1.8 g·m^-2·a^-1and unfertilized) and watering (20.1 kg·m^-2·a^-1and unwatered). General linear model univariate ANOVA, correlation analysis and regression analysis were used to analyze the effects of the treatments on plant functional traits, biodiversity and PP, as well as the relationship among them.
Important findings The clipping and fertilizing treatments had important effects on plant traits and PP. There were no significant differences in SD among the clipping treatments, whereas the FD quantified by distinct traits showed different trends across clipping treatments. Fertilization increased PP but decreased both SD and FD(except FD_H). Watering affected weakly only the Richness index (R). Correlation analysis indicated that the FD_{6 traits}reflected functional divergence better than the FDcalculated by the single trait. Regression analysis showed that there was a significantly positive and no correlation between SD and FD, respectively, and the relationship among SD, FD and PP followed three patterns: significantly positive, negative or no correlation, i.e., the relation forms were not only trait-dependent but also were related to disturbance and soil resources. Plant functional traits have a more direct influence on PP.

Aims It is important to the study of the carbon cycle and ecological issues to understand seasonal variation in CO₂ flux and the influence of environmental factors on the artificial grassland in the source region of the three rivers on the Qinghai-Tibetan Plateau.

Methods We utilized the eddy covariance method to observe net ecosystem CO₂ exchange (NEE) and biological and environmental factors and their variation at the Elymus nutans artificial grassland from September 1, 2005 to August 31, 2006.

Important findings The daily maximum uptake of CO₂ was 2.38 g C·m^-2·d^-1 on July 30. The ratio of daily uptake and emission in August were observed, -6.82 and 2.95 μmol CO ₂·m^-2·s^-1, respectively. In the growing seasons, daily NEE was dominated by the variation of photosynthetically active radiation (PAR). At the same time, daily NEE combined with leaf area and community diversity to control photosynthetic rate and photosynthetic efficiency. Maximum photosynthetic rate was 2.46-10.39 μmol CO ₂·m^-2·s^-1, and the apparent quantum yield (denoting the maximum efficiency of light utilization in photosynthesis) was 0.013-0.070 μmol CO ₂·μmol^-1 PAR. The influence of temperature, Q₁₀ (1.8) in the growing season was less than in the non-growing season. The respiration of the ecosystem was mainly dominated by temperature and leaf area. Carbon absorption was not dominated by the larger temperature difference of the day and night in the growing season. Our study proved that the artificial grassland ecosystem was a carbon sink with a carbon absorption of -49.35 g C·m^-2·a^-1. Our study also proved that the source and sink function of carbon was influenced by the amount, intensity and seasonal allocation of annual precipitation, as well as by plant community diversity.

To advance our understanding of the effects of climate change on grassland ecosystems, we used a time series (1982-1999) data set of the Normalized Difference Vegetation Index (NDVI) together with historical climate data to analyze interannual variations in grassland vegetation cover and explore the relationships between NDVI and climatic factors on the grasslands of the Tibetan Plateau. The NDVI increased significantly by a ratio of 0.41% a^-1 and a magnitude of 0.001 0 a^-1 during the growing season (p=0.015). An increase in NDVI during the growing season resulted from both the advanced growing season and accelerated vegetation activity. The largest NDVI increase was in the spring with a ratio of 0.92% a^-1 and a magnitude of 0.001 4 a^-1. The NDVI increase in the summer was a secondary contributor to the NDVI increase during the growing season with a ratio of 0.37% a^-1 and a magnitude of 0.001 0 a^-1. In the spring, the NDVI increased significantly in the alpine grasslands (alpine meadow and alpine steppe) and temperate steppe (p<0.01;p=0.001; p=0.002). During the summer, a significant NDVI increase was found in alpine meadows (p=0.027). However, the NDVI increase in alpine and temperate steppe was not significant (p=0.106; p=0.087). In the autumn, no significant increase was found in the three grasslands (p=0.585; p=0.461; p=0.143). In the spring, the NDVI increase in three grasslands was corresponded to an increase in temperature. In the summer, the NDVI was related to temperature and sensitive to precipitation in the spring in the alpine grasslands (alpine meadow and alpine steppe). However, no significant statistical relationship was found between NDVI and climatic factors in temperate steppe. Significant lagged correlations between precipitation and NDVI were found for alpine grasslands (alpine meadow, alpine steppe).

Understanding the relationship between biodiversity and ecosystem function is central to the Global Change and Terrestrial Ecosystem (GCTE) project as well as to ecosystem ecology in general. Productivity and nutrient maintenance, both critical ecosystem functions, can be measured in relation to species diversity to test how ecosystem functions are affected by species diversity. We studied changes in species diversity and productivity along elevational gradients in alpine meadow grasslands to assess the influence of changing species diversity on productivity.The results show a significant unimodal pattern of species diversity across the elevation gradient with species diversity highest at mid-elevations. Maximum species diversity occurred at intermediate levels of productivity and species richness. Above-ground biomass decreased with increasing elevation and below-ground biomass was greatest at low and high elevations and lowest at mid-elevations. Below-ground biomass varied during the growing season and vertically within the soil profile: maximum underground biomass occurred at the beginning and end of plant growing and was distributed primarily in the top 0-10 cm of soil.

With increased study of global change, the function of vegetation has become more widely appreciated. As one of the characters of the vegetation, the net primary productivity (NPP) becomes an important approach to study the structure and function of the v

The effectiveness and accordance of four popular climate vegetation classification models (Penman model, Thornthwaite model, Holdridge Life Zone System and Kira model) for the geographic distribution of Chinese vegetation is compared using the KAPPA agreement statistic method. The results indicate that those four classification models work well in the first level of Chinese Vegetation Division. The KAPPA value of the Holdridge Life Zone System (0.57) is the largest among the four models. It showed that it is the best one among four models in simulating the geographical distribution of Chinese vegetation. However, regarding some specific regions, for example, the Tibetan plateau, all of the models need to be refined or to take new affecting factors into account in order to obtain a better simulation of the geographic distribution of vegetation. 1) The Penman model could simulate the geographic distribution of vegetation in temperate steppe and Tibetan plateau with a KAPPA value greater than 0.50. It is the best one for Tibetan plateau among the four models. 2) The Thornthwaite model could give the best simulation for tropical rain forests and seasonal rain forests with KAPPA value of 0.40. It might compensate for the ineffectiveness of the Holdridge Life Zone System in this area. 3) The Holdridge Life Zone System is the best one among the four models for simulating the geographical distribution of vegetation in China based on the first level of vegetation division in China, but it still fails to simulate west seasonal rain forests and rain forests (52), west temperate steppe (63), Tibetan plateau temperate desert (86) and Tibetan plateau temperate steppe (84). 4) The Kira model simulates the subtropical evergreen forest zone very well, and its KAPPA value in this zone is close to that of the Holdridge Life Zone System. Also it could simulate moderately well the geographical distribution of vegetation in low elevation and humid/semihumid area, but it was far from the ground truth when applied to the regions of temperate desert and Tibetan Plateau. This research also implies that it is urgent to develop better climate vegetati on classification model in order to provide better input to the general circulation models (GCMs) with the geographic distribution of vegetation and to accurately evaluate the possible effects of climate change on vegetation.

The effect of wetland carbon element change on the global climate change has been a focus in the world for a long time. Some works have been done in studying carbon cycles of marsh wetland at Sanjiang Plain in China, but little was reported on the same work in Ruoergai Plateau situated in north-east part of Qinghai-Tibetan Plateau, which is one of the most sensitive areas with respect to global climate change in the world. The cold climate and abundant water in this area, vast marsh area and rich reserves of the peat are all very specific in China and in the world. To evaluate the effect of wetland carbon cycles on global climate change, the author studied organic carbon distribution and flow from three kinds of plant (Carex muliensis, Carex meyeriana and Kobresia tibetica) communities to three kinds of soil (peat soil, peat bog soil and meadow bog soil) using a field decomposition approach. The purposes are as follows: 1) to investigate the amount and distribution of soil organic carbon and 2) to verify the amount of C lost and C retained in the wetlands in Ruoergai Plateau. The results showed that the content of soil organic carbon was high and decreased with the increase of the depth of the soil layer. The disappearing rate of the organic carbon was different in different chemical constituents at different stages of living plants, standing dead and litter. Among several chemical constituents, the disappearing rate of the easy-decomposing C was the highest and reached 61.37%, 69.59% and 66.34% respectively in the three marsh plant communities, while the disappearing rate of the lignin C (44.53%-52.98%) was slightly higher than that of the cellulose C (38.23%-43.86%). The total disappearing rates of the plant carbon were 53.8%, 60.03% and 55.18% respectively in the three communities. The amount of C retained in soil from litter after 1 and 2 years of decomposition was 30 g·m-2 and 25.5 g·m-2 respectively, while the amount retained in the residual roots was 179-223 g·m-2 and 161-208 g·m-2 respectively. These results indicated that residual plant roots was the main source of soil organic carbon and the amount of organic carbon flow was large because of the higher biomass in wetland ecosystem in Ruoergai Plateau.

The net primary production (NPP) of Chinese terrestrial vegetation in 1997 was estimated based on the CASA model. The geographic distribution of NPP was explored using GIS and remote sensing imagery (NOAA/AVHRR), together with spatial data on vegetation, climate, soil type and solar radiation. The model estimates China’s terrestrial NPP in 1997 as 1.95 Pg C, or about 4.0% of the worlds terrestrial total. NPP decreased from southeast China toward the northwest. Southern Hainan Island, Southwestern Yunnan and Southeastern Tibet showed large NPP values, with the value exceeding 900 g C•m^–2•a^–1, whereas the Takelamagan desert located in western China had very small values—less than 10 g C•^m–2•a^–1.