Stochastic and deterministic processes together determine alpine meadow plant community composition on the Tibetan Plateau

To predict the consequences of environmental change on the structure and composition of communities, it is necessary to also understand the regional drivers underlying the structuring of these communities. Here, we have taken a hypothesis-based approach to test the relative importance of niche versus neutral processes using niche overlap, species traits and population asynchrony in two crossed treatments of fertilization and grazing in an alpine meadow community. Our results suggested that the observed species biomass overlap was not significantly different between treatments of grazing, grazing × fertilization and grazer exclusion. In contrast, the species biomass overlap was higher than expected in fertilization treatments when grazers were excluded. On the one hand, we found no relationship between species traits and relative abundance in grazing, grazing × fertilization and grazer-exclusion treatments; on the other hand, mechanistic trait-based theory could be used to predict species relative abundance patterns in fertilization treatments when grazers were excluded. From grazing to fertilization, when grazers were excluded, there was a slight increase in species synchrony, which indicated that the complementary dynamic of species gradually changed from complete independence into synchronously fluctuating with increasing fertilization. Based on the above results, we concluded that stochastic and deterministic processes formed ends of a continuum from grazing to fertilization when grazers were excluded in an alpine meadow plant community, and the importance of niche differences between species in structuring grassland communities increased with increasing fertilization and decreased with grazing.     

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Effects of disturbance intensity on seasonal dynamics of alpine meadow soil seed banks on the Tibetan Plateau

Background and Aims

Information on soil seed bank processes is crucial for understanding vegetation dynamics. Despite the documented importance of soil seed banks in many ecosystems, their role is not fully understood in some sensitive habitats, such as the alpine meadows of the Tibetan Plateau.

Methods

We studied the seasonal dynamics of the germinable soil seed bank under four disturbance intensities in an alpine meadow on the Tibetan Plateau as well as seed size distribution relative to disturbance intensity. Composition of the seed bank was compared with that of the standing vegetation.

Results

Density of buried seeds increased with disturbance intensity, but species richness and species diversity decreased. Seed density and species richness of the seed bank varied seasonally in all layers (0–2, 2–7, 7–12 cm) and the whole (0–12 cm). The species composition of seed bank was not significantly influenced by season. There was no trend in seed size distribution as disturbance increased. Seasonal seed bank turnover rates increased with increase in disturbance. The result of the NMDS showed that species composition of seed bank and vegetation exhibited a fairly uniform pattern in each season.

Conclusions

Although as a whole the species composition of the vegetation and seed bank showed a relatively low degree of similarity in each season, similarity was highest in the most disturbed habitat. There was no alteration in species composition of seed bank regardless of disturbance intensity, but seed density decreased as disturbance increased. Disturbances in alpine plant communities might increase persistence of regeneration niches. Regeneration from the seed bank together with vegetative reproduction contributed to aboveground vegetation in highly disturbed habitats. Clonal species played an important role in regeneration of vegetation in slightly disturbed areas, where there was little contribution of ruderals from soil seed banks.     

Indicator of flower status derived from in situ hyperspectral measurement in an alpine meadow on the Tibetan Plateau

Flowering status including flowering date and flower amount could reflect ecological process in assessing plant phenological response to global warming. However, little information is available so far for monitoring flowering status through remote sensing. To provide an ecological indicator for monitoring plant phenology from remotely sensed data, we conducted a field survey in an alpine meadow on the Tibetan Plateau where flower color in July is dominantly yellow due to flowering of Halerpestes tricuspis (Ranunculaceae). We used flower coverage to indicate the flowering status of this species and proposed a flower index derived from in situ hyperspectral data (HFI) to estimate the flower coverage. Results demonstrate that the flower coverage of H. tricuspis can be estimated with high accuracy from the hyperspectral measurements. The indicating ability was further improved when the flower coverage was higher than 0.10 or the fractional coverage of soil was low or known in advance. A simulation also shows that a quadrat or pixel with flower coverage higher than 0.066 can be detected with existence of flower by HFI if soil fraction is less than 50%. These results indicate that HFI is applicable for estimating flower coverage of this species from hyperspectral measurement. The study suggests that the hyperspectral remote sensing technique can be applied for monitoring flowering status, and therefore the technique can provide an important ecological indicator for monitoring plant phenology.     

光辐射对青藏高原高寒草甸凋落物分解的影响

光照对干旱半干旱生态系统凋落物分解有显著作用。然而,在太阳辐射强度大、高寒湿润的青藏高原东部高寒草甸,光辐射在凋落物分解中的作用尚不明确。本试验采用凋落物袋法,研究自然光照和遮光2种处理下10种高寒草甸常见物种(披针叶黄华Thermopsis lanceolata、珠芽蓼Polygonum viviparum、线叶嵩草Kobresia capillifolia、小花草玉梅Anemone rivularis、黄帚橐吾Ligularia virgaurea、麻花艽Gentiana straminea、大籽蒿Artemisia sieversiana、瑞苓草Saussurea nigrescens、垂穗披碱草Elymus nutans和矮藨草Scirpus pumilus)叶片凋落物分解特点。结果表明:(1)不同物种凋落物的木质素含量存在显著差异,其中珠芽蓼木质素含量最高(达29.44%),黄帚橐吾木质素含量最低(为4.91%);(2)整体上,遮光显著降低了凋落物质量损失率,而光照的影响程度也因物种而异;(3)无论在自然光照还是在遮光处理下,凋落物质量损失率与初始木质素含量均呈显著负相关关系;遮光明显降低木质素质量损失率,其降低程度也因物种而异。     

青藏高原高寒草甸非生长季温室气体排放特征及其年度贡献

高寒草甸是青藏高原地区的主要植被类型,目前对其温室气体研究多集中于生长季.本文利用静态箱-气相色谱法,对非生长季高寒草甸温室气体排放特征及其与主要环境因子的关系进行了研究.结果表明:非生长季高寒草甸表现为CO2和N2O的源、CH4的汇.其中非生长季CO2通量平均值为89.33 mg·m-2·h-1,累积排放通量为280.01g· m-2;CH4通量平均值为-11.35 μg·m-2·h-1,累积吸收通量为124.74 mg·m-2;N2O通量平均值为8.02 μg·m-2·h-1,累积排放通量为39.51 mg·m-2.非生长季CO2、CH4和N2O累积排放通量分别占全年的13.33％、53.47％和62.67％.冻融期(2012年4月)CH4累积吸收通量较小,只占非生长季的4.5％;而CO2和N2O累积排放通量较大,分别占非生长季的25.8％和20.8％.非生长季CO2通量与温度(气温、5和10 cm土壤温度)和5 cm土壤湿度均存在显著正相关关系,而CH4和N2O通量仅与5 cm土壤湿度存在显著正相关.研究表明,虽然冻融期CH4累积吸收通量在非生长季累积量中比重较小,但非生长季CH4和N2O累积排放量却占全年累积排放量的1/2以上,在温室气体累积通量评估中不容忽视.     

Methanotrophic community structure and activity under warming and grazing of alpine meadow on the Tibetan Plateau

Knowledge about methanotrophs and their activities is important to understand the microbial mediation of the greenhouse gas CH₄ under climate change and human activities in terrestrial ecosystems. The effects of simulated warming and sheep grazing on methanotrophic abundance, community composition, and activity were studied in an alpine meadow soil on the Tibetan Plateau. There was high abundance of methanotrophs (1.2–3.4 × 10⁸ pmoA gene copies per gram of dry weight soil) assessed by real-time PCR, and warming significantly increased the abundance regardless of grazing. A total of 64 methanotrophic operational taxonomic units (OTUs) were obtained from 1,439 clone sequences, of these OTUs; 63 OTUs (98.4%) belonged to type I methanotrophs, and only one OTU was Methylocystis of type II methanotrophs. The methanotroph community composition and diversity were not apparently affected by the treatments. Warming and grazing significantly enhanced the potential CH₄ oxidation activity. There were significantly negative correlations between methanotrophic abundance and soil moisture and between methanotrophic abundance and NH₄–N content. The study suggests that type I methanotrophs, as the dominance, may play a key role in CH₄ oxidation, and the alpine meadow has great potential to consume more CH₄ under future warmer and grazing conditions on the Tibetan Plateau.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

Temperature controls ecosystem CO2 exchange of an alpine meadow on the northeastern Tibetan Plateau

Alpine ecosystems are extremely vulnerable to climate change. To address the potential variability of the responses of alpine ecosystems to climate change, we examined daily CO₂ exchange in relation to major environmental variables. A dataset was obtained from an alpine meadow on the Qinghai‐Tibetan Plateau from eddy covariance measurements taken over 3 years (2002–2004). Path analysis showed that soil temperature at 5 cm depth (T_s5) had the greatest effect on daily variation in ecosystem CO₂ exchange all year around, whereas photosynthetic photon flux density (PPFD) had a high direct effect on daily variation in CO₂ flux during the growing season. The combined effects of temperature and light regimes on net ecosystem CO₂ exchange (NEE) could be clearly categorized into three areas depending on the change in T_s5: (1) almost no NEE change irrespective of variations in light and temperature when T_s5 was below 0 °C; (2) an NEE increase (i.e. CO₂ released from the ecosystem) with increasing T_s5, but little response to variation in light regime when 0 °C≤T_s5≤8 °C; and (3) an NEE decrease with increase in T_s5 and PPFD when T_s5 was approximately >8 °C. The highest daily net ecosystem CO₂ uptake was observed under the conditions of daily mean T_s5 of about 15 °C and daily mean PPFD of about 50 mol m⁻² day⁻¹. The results suggested that temperature is the most critical determinant of CO₂ exchange in this alpine meadow ecosystem and may play an important role in the ecosystem carbon budget under future global warming conditions.     

Weakened priming effect along soil profile in alpine grasslands on the Tibetan Plateau

Subsoils hold a substantial reservoir of organic carbon（C）, and its dynamics can be greatly influenced by fresh C inputs through priming effect, potentially altering the magnitude of soil C-climate feedback. Despite the importance of soil C dynamics in regulating this feedback, our understanding of how soil C release and the priming effect vary along the soil profile remains limited, especially in alpine grasslands on the Tibetan Plateau. In particular, the relative importance of abiotic and biotic factors, such as soil physicochemical properties, aggregate and mineral protection, substrate quantity and quality, and plant and microbial properties（e.g., microbial biomass and diversity）, in mediating vertical variations in soil C release and the priming effect is still unclear. Using 1-meter-deep soil profiles from five sites on the plateau, our ¹³C isotope labeling incubation experiments revealed a significant decline in both C release and the priming effect with increasing soil depth. We found that variations in soil C release along the profile were primarily influenced by soil properties（soil moisture and p H）, mineral protection(the molar ratios of amorphous Fe/Al oxides to soil organic C（SOC） and soil mineral specific surface area), and hydrolase activity. In addition, vertical variations in the priming effect were dominantly affected by soil properties（soil moisture and p H）, mineral and aggregate protection（the molar ratio of exchangeable Ca to SOC and the proportion of C occluded in clay+silt fractions）, and microbial properties（oxidase activity and the copy number of bacterial ribosomal RNA gene operons）. These findings provide valuable insights into the complex soil C cycling across profiles and its feedback to climate change.     

The maintenance of offspring diversity in response to land use: sexual and asexual recruitment in an alpine meadow on the Tibetan Plateau

Human activities have caused dramatic land use changes, impacting plant community composition, diversity and function. Fertilization and grazing are the two most common land use modes in grasslands. To understand the effects of grazing and fertilization on sexual and asexual recruitment in alpine grasslands, we conducted a demographic field investigation of species recruitment in an alpine meadow on the Tibetan Plateau. Grazing and fertilization had different effects on the quantity and diversity of sexual and asexual recruitment. Sexual recruitment increased significantly in grazed plots, but decreased significantly in fertilized plots. Asexual recruitment increased significantly in fertilized plots, but decreased significantly in grazed plots. For functional groups, grazing significantly reduced offspring recruitment of graminoids, but significantly increased offspring recruitment of forbs and legumes; fertilization significantly reduced offspring recruitment of forbs and legumes, but significantly increased offspring recruitment of graminoids. Furthermore, offspring diversity from sexual recruitment was significantly higher than from asexual recruitment in grazed plots, and as compared to non‐grazed and fertilized grasslands. Our studies indicate that moderate grazing disturbance has positive effects on seedling recruitment and offspring diversity, and fertilization has negative effects on offspring diversity, but may significantly increase asexual recruitment.     

Effects of grazing and fertilization on the relationship between species abundance and functional traits in an alpine meadow community on the Tibetan Plateau

Trait‐based approaches can provide a useful tool for linking plant attributes to community structure and ecosystem function. Seed mass and plant height play important roles in the dynamics of plant communities, but few empirical community level studies have tested this, especially in stressful environments. The aim of the present study was to determine if there is a relationship between functional traits (seed mass and plant height) and changes in species relative abundance (SRA) in response to grazing and fertilization. We measured SRA and plant functional traits for 40 common species in a Tibetan Plateau alpine meadow. In the fertilized meadow, seed mass and plant height was significantly positively correlated with the relative abundance of the species. In the grazed meadow, these variables were significantly negatively correlated. Our results demonstrates that plant functional traits can be used to predict the change of SRA in plant community. Grazing promotes the dominance of small‐seeded and short‐stature species, and fertilization facilitates the occurrence of large‐seeded and tall‐stature species.     

Calibration of Terra/MODIS gross primary production over an irrigated cropland on the North China Plain and an alpine meadow on the Tibetan Plateau

This paper evaluated the MODerate resolution Imaging Spectroradiometer (MODIS) gross primary production (GPP) product (MOD17) by using estimated GPP from eddy‐covariance flux measurements over an irrigated winter wheat and maize double‐cropping field on the North China Plain in 2003–2004, and an alpine meadow on the Tibetan Plateau in 2002–2003. The mean annual GPP from MOD17 accounted for 1/2–2/3 of the surface estimated mean annual GPP for the alpine meadow, but only about 1/5–1/3 for the cropland. This underestimation was partly attributed to low estimates of leaf area index by a MODIS product (MOD15) because it is used to calculate absorbed photosynthetically active radiation in the MOD17 algorithm. The main reason is that the parameter maximum light use efficiency (ε_max) in the MOD17 algorithm was underestimated for the two biomes, especially for the cropland. Contrasted to the default, ε_max was optimized using surface measurements. The optimized ε_max for winter wheat, maize and meadow was 1.18, 1.81 and 0.73 g C/MJ, respectively. By using the surface measurements and optimized ε_max , the MOD17 algorithm significantly improved the accuracy of GPP estimates. The optimum MOD17 algorithm explained about 82%, 68%, and 79% of GPP variance for winter wheat, maize, and meadow, respectively. These results suggest that it is necessary to adjust the MOD17 parameters for the estimation of cropland and meadow GPP, particularly over cropland.     

Litter species traits, but not richness, contribute to carbon and nitrogen dynamics in an alpine meadow on the Tibetan Plateau

Aims

Litter, as afterlife of plants, plays an important role in driving belowground decomposition processes. Here we tested effects of litter species identity and diversity on carbon (C) and nitrogen (N) dynamics during litter decomposition in N-limited alpine meadow soil from the Qinghai–Tibet Plateau.

Methods

We incubated litters of four meadow species, a sedge (“S”, Kobresia humilis), a grass (“G”, Elymus nutans), a herb (“H”, Saussurea superba), and a legume (“L”, Oxytropis falcata), in monoculture and in mixture with meadow soil. CO₂ release was measured 21 times during the incubation, and soil available N and microbial biomass C and N were measured before and after the experiment.

Results

The organic C decay rate did not differ much among soils amended with monocultures or mixtures of litter, except in the H, S, L, and S+H treatments, which had much higher decay rates. Potential decomposable C pools were lowest in the control, highest in the L treatment, and intermediate in the S treatment. Mineralized N was completely immobilized by soil microbes in all treatments except the control, S+L, and S+G+L treatments. Litter mixtures had both additive and non-additive effects on CO₂-C emission (mainly antagonistic effects), net N mineralization (mainly synergistic), and microbial biomass C and N (both). Overall, these parameters were not significantly correlated with litter species richness. Similarly, microbial C or N was not significantly correlated with litter N content or C/N. However, cumulative CO₂-C emission and net N mineralization were positively correlated with litter N content and negatively correlated with litter C/N.

Conclusions

Litter N content and C/N rather than litter species richness drove the release of CO₂-C and net available N in this ecosystem. The antagonistic effects of litter mixtures contributed to a modest release of CO₂-C, but their synergistic effects enhanced net available N. We suggest that in alpine meadow communities, balancing species with high and low N contents will benefit soil carbon sequestration and plant competition for available N with soil microbes.     

Small-scale species richness and its spatial variation in an alpine meadow on the Qinghai-Tibet Plateau

We investigated how the high small-scale species richness of an alpine meadow on the Qinghai-Tibet Plateau, China, is maintained. This area is characterized by strong wind and severe cold during long winters. In winter, most livestock is grazed on dead leaves in small pastures near farmers’ residences, whereas in the short summer, livestock is grazed in mountainous areas far from farmers’ residences. The number of plant species and the aboveground biomass were surveyed for three adjacent pastures differing in grazing management: a late-winter grazing pasture grazed moderately from 1 February to 30 April, an early-winter grazing pasture grazed lightly from 20 September to late October, and a whole-year grazing pasture grazed intensively throughout the entire year. In each pasture, we harvested the aboveground biomass from 80 or 100 quadrats of 0.01 m² along a transect and classified the contents by species. We observed 15.5–19.7 species per 0.01 m², which is high richness per 0.01 m² on a worldwide scale. The species richness in the two winter grazing pastures was higher than that in the whole-year grazing pasture. The spatial variation in species richness and species composition in the two winter grazing pastures in which species richness was high was greater than that in the whole-year grazing pasture in which species richness was lower. Most of the leaves that are preserved on the winter grazing pastures during summer are blown away by strong winds during winter, and the remaining leaves are completely exhausted in winter by livestock grazing. A pasture with a high richess is accompanied by a high spatial variation in species richness and species composition. There is a high possibility that the characteristic of spatial variation is also caused by traditional grazing practices in this area.     

Warming and grazing interact to affect root dynamics in an alpine meadow

Plant and Soil - Arbuscular Mycorrhizal Fungi (AMF) are ubiquitous soil microorganisms playing a vital role in the functioning of agricultural ecosystems. Although AMF are generally considered to...     

No temperature acclimation of soil extracellular enzymes to experimental warming in an alpine grassland ecosystem on the Tibetan Plateau

Alpine grassland soils store large amounts of soil organic carbon (SOC) and are susceptible to rising air temperature. Soil extracellular enzymes catalyze the rate-limiting step in SOC decomposition and their catalysis, production and degradation rates are regulated by temperature. Therefore, the responses of these enzymes to warming could have a profound impact on carbon cycling in the alpine grassland ecosystems. This study was conducted to measure the responses of soil extracellular enzyme activity and temperature sensitivity (Q₁₀) to experimental warming in samples from an alpine grassland ecosystem on the Tibetan Plateau. A free air-temperature enhancement system was set up in May 2006. We measured soil microbial biomass, nutrient availability and the activity of five extracellular enzymes in 2009 and 2010. The Q₁₀ of each enzyme was calculated using a simple first-order exponential equation. We found that warming had no significant effects on soil microbial biomass C, the labile C or N content, or nutrient availability. Significant differences in the activity of most extracellular enzymes among sampling dates were found, with typically higher enzyme activity during the warm period of the year. The effects of warming on the activity of the five extracellular enzymes at 20 °C were not significant. Enzyme activity in vitro strongly increased with temperature up to 27 °C or over 30 °C (optimum temperature; T_opt). Seasonal variations in the Q₁₀ were found, but the effects of warming on Q₁₀ were not significant. We conclude that soil extracellular enzymes adapted to seasonal temperature variations, but did not acclimate to the field experimental warming.