Alpine Meadow Steppe Research Articles

Different responses of community temporal stability to nitrogen and phosphorus addition in a non-degraded alpine grassland

Fertilization has been widely used in grassland management, but different types of fertilizers may have inconsistent impacts on grassland productivity and stability. However, less is known about the responses of plant community stability and the shifts of driving mechanisms under different fertilizers. We conducted a long-term nitrogen (N) and/or phosphorus (P) addition experiment in an alpine steppe meadow, using two N levels (5 (N5), 10 (N10) g N/m−2(−|-) year−1), one P level (5 g N m−2 year−1, P5) and the combined treatments (N5P5 and N10P5), to examine their effects on community temporal stability and the underlying mechanisms. Both N addition alone and the combined addition significantly increased community aboveground biomass, while P5 did not affect community aboveground biomass. However, only N10, N5P5, and N10P5 decreased annual average species richness, while other treatments showed no pronounced effect. Except for N5, community stability decreased under both N and P additions. The most remarkable finding is that the driving mechanisms underlying community stability shifted with nutrient types. The decrease in species richness induced by N addition decreased community stability. However, the decrease in dominant species asynchrony induced by P addition affected community asynchrony, which contributed to the decrease in community stability. N and P combined addition could additively affect community stability both by species richness and dominant species asynchrony and further community asynchrony. Our results indicate that fertilizer types can alter the regulatory effect of plant diversity on community stability. This study reveals that attention should be paid to the different mechanisms driving community stability under different nutrient types, which provides a scientific basis for alpine grassland management.

Vulnerability of grassland ecosystems to climate change in the Qilian Mountains, northwest China

With climate warming and frequent extreme weather events, the structure and function of grassland ecosystems have been significantly altered. The Qilian Mountains are a wet island, serving as an important ecological barrier in China and central Asia. It is highly sensitive and prone to climate change, yet the vulnerability of grasslands to climate variability in this region remains unclear. In this study, based a NDVI dataset and meteorological data, we evaluated the relative vulnerability of grassland ecosystems to short-term climate variability by combining three indices: exposure, sensitivity, and resilience, and explored its relationship with water use efficiency (WUE). The results show that the spatial distribution patterns of grassland vulnerability in the Qilian Mountains from 2000 to 2018 were largely determined by exposure, and sensitivity is positively correlated with exposure and negatively correlated with resilience. The vulnerability of alpine meadow and alpine steppe is higher than that of temperate typical steppe and temperate desert steppe. Climate and topography gradients together affect the degree of grassland vulnerability. Grasslands with higher vulnerability are mainly found at medium elevation (3200–4000 m) area, where the topography is flat. The northern grasslands are primarily influenced by temperature, while the western and southern parts are mainly affected by precipitation. Alpine meadow and alpine steppe are more vulnerable to temperature and temperate desert steppe is more vulnerable to changes in precipitation. The higher the vulnerability, the lower the WUE of the grasslands. This study provides a reference for assessing the impact of climate change on grassland ecosystems and developing grassland vegetation management strategies.

Contrasting Trends in Water Use Efficiency of the Alpine Grassland in Tibetan Plateau

AbstractAlpine grassland is the dominant land cover type of the Tibetan Plateau (TP). Previous studies suggested that TP's climate and vegetation condition have changed substantially in past few decades. However, the impacts of such changes on ecosystem water use efficiency (WUE), a vital index for depicting the tradeoff between water loss and carbon gain, of alpine grassland remain poorly understood. This study used a water‐carbon coupled model, Penman‐Monteith‐Leuning Version 2 (PML_V2), to investigate the variation in WUE of alpine grassland in the TP during 1982–2016 and its drivers. PML_V2 performs well in estimating WUE at nine eddy‐covariance flux sites, noticeably better than other main‐stream products. The long‐term mean WUE decreases from the southeastern to the northwestern TP, generally following the spatial pattens of surface greenness and climatic condition. During 1982–2016, WUE increased in most parts (71%) of alpine grassland but declined in certain regions of western TP. At the ecosystem scale, contrasting trends in WUE emerged between alpine meadow and alpine steppe over the last 35 years. The former's significant increasing WUE was primarily driven by increased leaf area index (LAI) and CO2 concentration (eCO2). While greening also occurred in the latter, its WUE remained stable because (a) the large negative effect of precipitation offset the positive effect of LAI on WUE, and; (b) the enhancement of photosynthesis resulted from elevated eCO2 was weaker because of low LAI. Our results are of importance for understanding the dynamics and mechanisms of water‐carbon coupling in high‐elevation ecosystems under a changing climate.

Climate-soil interactions improve the stability of grassland ecosystem by driving alpine plant diversity

The grassland ecosystem plays an important role in maintaining ecological security. This study intends to better understand the effects of climate-soil interaction on the alpine grassland ecosystem and to further deepen the understanding of plant diversity and its influencing factors in the alpine meadow, alpine steppe, and desert steppe of the Qilian Mountains of China. Therefore, the soil nutrient contents such as soil organic carbon, nitrogen, available phosphorus, ammonium nitrogen, and nitrate-nitrogen were measured from the uppermost 0–40 cm soil layer. In addition, the regional vegetation characteristics were also investigated and plant diversity index values were determined. The three grassland types showed obvious variations in soil nutrient content and plant diversity index values. Alpine meadow and alpine steppe showed the highest plant diversity and soil nutrients, evident by greater soil organic carbon and nitrogen contents compared to the desert steppe. With the increase in precipitation amounts, the plant height and number of typical plants increased, but no significant impact of elevation was found. Plant diversity was positively correlated with annual precipitation and soil organic carbon, which were the primary factors determining plant diversity in the Qilian Mountains. Compared to other grassland types, alpine meadows showed the most complex network, indicating the greatest ecological stability and strong resistance to environmental changes. These findings revealed a close link between the species diversity, soil nutrients, and major climatic factors in the Qilian Mountains, which are critical for predicting plant diversity in the context of climate change.

Estimation of alpine grassland above-ground biomass and its response to climate on the Qinghai-Tibet Plateau during 2001 to 2019

The Qinghai-Tibet Plateau (QTP) grassland is a critical part of the carbon pool of terrestrial ecosystems and provides important animal husbandry resources. To embody the stability of grassland ecology and the response of grassland above-ground biomass (AGB) to climate change on the QTP, four estimations models (partial least squares regression (PLSR), random forest (RF), Back-Propagation neural network (BPNN) and deep belief network (DBN)), which are based on statistics, machine learning and deep learning regression methods, respectively, were established to estimate the alpine meadow and alpine steppe AGB from 2001–2019. The results showed that: (1) the RF model performs well on the AGB estimation with the highest accuracy (R2 =0.84, RMSE=8.51gC/m2, MAE=6.46gC/m2) and stability (R2 =0.76, RMSE=9.24gC/m2, MAE =8.30gC/m2); (2) in spatial pattern, the AGB decreased from southeast to northwest on the QTP, and present a significant increasing with a rate at 0.19gC/m2a and 0.06gC/m2a on alpine meadow and alpine steppe during 2001–2019, respectively; (3) in the past 19 years, the AGB variations on the QTP showed a relatively stability with the average CV of 0.1; (4) the sustainability of the AGB show a weak anti-persistence (Hurst = 0.43), and it indicates that the future trend of the AGB is opposite to the current trend and the AGB is likely to be decreased in the future; (5) temperature and precipitation have a positive promotion on the most alpine meadow and alpine steppe vegetation growth on the QTP, while warming and wetting have negative promotion effects on the northwest of the plateau. These research results can provide useful reference materials for the study of the impact of climate change on alpine meadow and alpine steppe on the QTP.

Divergent Climate Sensitivities of the Alpine Grasslands to Early Growing Season Precipitation on the Tibetan Plateau

Warming is expected to intensify hydrological processes and reshape precipitation regimes, which is closely related to water availability for terrestrial ecosystems. Effects of the inter-annual precipitation changes on plant growth are widely concerned. However, it is not well-known how plant growth responds to intra-annual precipitation regime changes. Here, we compiled reanalysis climate data (ERA5) and four satellite-based vegetation indices, including the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), the Solar-induced Chlorophyll Fluorescence (SIF), and the Modified Triangular Vegetation Index (MTVI2), to evaluate the response of alpine grasslands (including alpine meadow and alpine steppe) to the change of precipitation regimes, especially to the intra-annual precipitation regimes on the Tibetan Plateau. We found monthly precipitation over the alpine steppe significantly increased in the growing season (May–September), but precipitation over the alpine meadow significantly increased only in the early growing season (May–June) (MJP) during the past four decades (1979–2019). The inter-annual plant growth (vegetation indices changes) on the alpine meadow was dominated by temperature, but it was driven by precipitation for the alpine steppe. On the intra-annual scale, the temperature sensitivity of the vegetation indices generally decreased but precipitation sensitivity increased during the growing season for both the alpine meadow and steppe. In response to the increase in MJP, we found the temperature sensitivity of the vegetation indices during the mid-growing season (July–August) (MGNDVI, MGEVI, MGSIF, and MGMTVI2) in the alpine meadow significantly increased (p < 0.01) while its precipitation sensitivity significantly decreased (p < 0.01). We infer that more MJP over the meadow may be the result of enhanced evapotranspiration, which is at the expense of soil moisture and even induces soil “drought” in the early growing season. This may be to elevate community water acquisition capacity through altering root mass allocation and community composition, consequently regulating the divergent climate sensitivities of vegetation growth in the mid-growing season. Our findings highlight that it is inadequate to regard precipitation as an indicator of water availability conditions for plant growth, which may limit our understanding of the response and acclimatization of plants to climate change.

Fungi and Archaea Control Soil N2O Production Potential in Chinese Grasslands Rather Than Bacteria.

Nitrous oxide (N2O) is a powerful greenhouse gas and the predominant stratospheric ozone-depleting substance. Soil is a major source of N2O but remains largely uncertain due to the complicated processes of nitrification and denitrification performed by various groups of microbes such as bacteria, fungi, and archaea. We used incubation experiments to measure the total fungal, archaeal, and bacterial N2O production potential and the microbial functional genes in soils along 3,000 km Chinese grassland transect, including meadow steppe, typical steppe, desert steppe, alpine meadow, and alpine steppe. The results indicated that fungi, archaea, and bacteria contributed 25, 34, and 19% to nitrification and 46, 29, and 15% to denitrification, respectively. The AOA and AOB genes were notably correlated with the total nitrification enzyme activity (TNEA), whereas both narG and nirK genes were significantly correlated with total denitrification enzyme activity (TDEA) at p < 0.01. The correlations between AOA and ANEA (archaeal nitrification enzyme activity), AOB and BNEA (bacterial nitrification enzyme activity), and narG, nirK, and BDEA (bacterial denitrification enzyme activity) showed higher coefficients than those between the functional genes and TNEA/TDEA. The structural equation modeling (SEM) results showed that fungi are dominant in N2O production processes, followed by archaea in the northern Chinese grasslands. Our findings indicate that the microbial functional genes are powerful predictors of the N2O production potential, after distinguishing bacterial, fungal, and archaeal processes. The key variables of N2O production and the nitrogen (N) cycle depend on the dominant microbial functional groups in the N-cycle in soils.

Development of place-based catenal models for grassland ecosystems of the Upper Yellow River, Western China

Careful development of place-based catenal models and their application as transferable archetypes provides an integrative and generalisable framework for scientifically-informed approaches to environmental management. A workshop and field excursion to the Source Zone of the Yellow River in western China in July 2019 brought together local experts and a team of international researchers to co-develop cross-disciplinary, process-based catenal models that summarise controls on the character and behaviour of grassland environments in alpine meadow (3800 m asl) and alpine steppe (4200 m asl) landscapes. Water, sediment and nutrient fluxes, soil material properties, ground cover (especially percentage bare ground) and the role of small mammals (especially pika) are critical drivers of process linkages in the catenal models. Geologic, climatic and anthropogenic controls on these drivers vary in alpine steppe and alpine meadow settings. While these process interactions have supported grazing adapted ecosystems and sustained biodiversity values in this area for millennia, anthropogenic and climate change disturbances threaten the integrity of these landscapes and their capacity to provide ecosystem services and sustain livelihoods into the future. The place-based catenal models presented in this paper can be used to support applications that appropriately target key attributes, linkages and fluxes that may require conservation, maintenance or treatment in the management of alpine steppe and alpine meadow landscapes.

Diurnal Evapotranspiration and Its Controlling Factors of Alpine Ecosystems during the Growing Season in Northeast Qinghai-Tibet Plateau

It is generally believed that evapotranspiration at night is too miniscule to be considered. Thus, few studies focus on the nocturnal evapotranspiration (ETN) in alpine region. In this study, based on the half-hour eddy and meteorological data of the growing season (from May to September) in 2019, we quantified the ETN of alpine desert (AD), alpine meadow (AM), alpine meadow steppe (AMS), and alpine steppe (AS) in the Qinghai Lake Basin and clarified the different response of evapotranspiration to climate variables in daytime and nighttime with the variation of elevation. The results show that: (1) ETN accounts for 9.88~15.08% of total daily evapotranspiration and is relatively higher in AMS (15.08%) and AD (12.13%); (2) in the daytime, net radiation (Rn), temperature difference (TD), vapor pressure difference (VPD), and soil moisture have remarkable influence on evapotranspiration, and Rn and VPD are more important at high altitudes, while TD is the main factor at low altitudes; (3) in the nighttime, VPD and wind speed (WS) control ETN at high altitudes, and TD and WS drive ETN at low altitudes. Our results are of great significance in understanding ETN in the alpine regions and provide reference for further improving in the evapotranspiration estimation model.

青藏高原两种高寒草地植被变化及其水温驱动因素分析

高寒草甸和高寒草原作为青藏高原两种重要植被类型，研究其植被变化与气候变化相关性，有助于为青藏高原两种高寒草地生态系统应对全球气候变化管理提供参考。以位于同纬度的三江源高寒草甸和阿里高寒草原为研究对象，基于植被净初级生产力（Net Primary Productivity，NPP）变化表征植被变化，利用NPP数据和气象数据，分别分析两地2000-2017年植被NPP、降水和气温时空变化差异；利用Sen+Mann-Kendall趋势检验，研究两种高寒草地气候与植被净初级生产力变化趋势；以县域统计年鉴牛羊肉产量表征放牧强度，研究放牧活动对高寒草地植被变化的影响；通过Pearson相关和偏相关分析方法，分别研究降水和气温对两种高寒草地植被NPP变化影响差异。研究结果表明：（1）2000-2017年三江源高寒草甸和阿里高寒草原区年平均气温以0.085℃/a和0.084℃/a的趋势上升，降水以平均每年3.87 mm和2.23 mm的趋势增加，高寒草甸区变暖变湿速率较高寒草原区快。（2）三江源高寒草甸和阿里高寒草原植被NPP均呈现由东南向西北逐渐降低空间格局；2000-2017年高寒草甸区57.7%植被NPP呈现上升趋势，而高寒草原区50.96%植被NPP呈现上升趋势，高寒草甸植被NPP增长趋势较高寒草原明显。（3）三江源高寒草甸和阿里高寒草原两种草地植被变化受放牧影响不大，受气候变化影响较大。（4）三江源高寒草甸大部分植被NPP增长与气温呈显著正相关而与降水呈显著负相关，高寒草甸植被NPP主要受温度调控，而阿里高寒草原植被NPP受降水和温度的双重调控。;Alpine meadow and alpine steppe are two important vegetation types on the Qinghai-Tibet Plateau. Studying the correlation between vegetation change and climate change in alpine meadow and alpine steppe is helpful to provide reference for the management of the two alpine grassland ecosystems on the Qinghai-Tibet Plateau in response to global climate change. In this study, the alpine meadow of the Three-River Headwaters Region and the alpine steppe of Ali located in the same latitude were taken as the research objects. Vegetation changes were characterized based on the changes of Net Primary Productivity (NPP) of vegetation. Based on NPP data and meteorological data, the spatial and temporal variations of NPP, precipitation and air temperature between 2000 and 2017 were analyzed. Sen+ Mann-Kendall trend test was used to study the change trend of climate and vegetation net primary productivity in two alpine grasslands. The effects of grazing activities on the vegetation change of alpine grassland were studied by using the yield of beef and mutton in county statistical yearbooks. Through Pearson correlation and partial correlation analysis, the variation trend of precipitation and air temperature on vegetation NPP and the difference of precipitation and air temperature on NPP of two alpine grasslands were studied respectively. The results show that:(1) from 2000 to 2017, the annual average temperature in Three-River Headwaters Region alpine meadow and Ali alpine steppe increased at 0.085℃/a and 0.084℃/a, and the annual average precipitation increased at 3.87 mm and 2.23 mm, respectively. The warming and wetness rate in the alpine meadow was faster than that in the alpine steppe. (2) The NPP of the alpine meadow in the Three-River Headwaters Region and the alpine steppe in Ali showed a decreasing spatial pattern from southeast to northwest. From 2000 to 2017, the NPP of 57.7% of alpine meadow vegetation presented an upward trend, while that of 50.96% of alpine steppe vegetation showed an upward trend. The NPP of alpine meadow vegetation increased more obviously than that of alpine steppe. (3) The alpine meadow in the Three-River Headwaters Region and the alpine steppe in Ali region were less affected by grazing and more affected by climate change. (4) The NPP growth of most alpine meadow vegetation in the Three-River Headwaters Region was significantly positively correlated with air temperature and negatively correlated with precipitation. The NPP of alpine meadow vegetation was mainly regulated by temperature, while the NPP of alpine steppe vegetation in the Ali region was controlled by both precipitation and temperature.

Divergences of soil carbon turnover and regulation in alpine steppes and meadows on the Tibetan Plateau

The grasslands of the Tibetan Plateau store approximately 2.5% of global soil organic carbon (SOC) and considerable soil inorganic carbon (SIC) and have the potential to become a vast carbon source or sink as climate change progresses. However, the soil carbon (C) sequestration mechanisms that occur across large-scale natural gradients remain unclear. Here, humic substances (HS) were utilized to trace soil C turnover at 0–20 cm, and we compared divergences among three main grassland types (alpine meadow, alpine steppe, and artificial plantation) using structural equation modeling (SEM). The results showed that the alpine meadows sequestered the most soil C (63.99 ± 4.41 g kg−1 SOC and 4.11 ± 0.63 g kg−1 SIC), sequestering 2–3 times more than the alpine steppe ecosystems (19.78 ± 1.98 g kg−1 SOC and 9.21 ± 0.66 g kg−1 SIC). The alpine steppe and artificial plantation regions have strong C sink potential due to their low C/N ratios (P < 0.05). Importantly, SIC played an important role in the alpine steppes, accounting for nearly 26–37% of soil C. The ratios of recalcitrant HS to SOC were estimated as 46.50%, 65.09%, and 78.17% in the alpine meadow, alpine steppe, and artificial plantation ecosystems, respectively, indicating that SOC in the alpine meadow was the most sensitive to climate change. Fulvic acid (FA) accounted for 50.86% of SOC in the 0–20-cm interval, contributing most to the formation of SOC in all vegetation types. In addition, in contrast to climatic controls on soil C turnover in the alpine meadow, climate conditions rarely controlled C turnover in the alpine steppe. Moreover, sand and silt were the main soil minerals involved in C turnover in alpine meadow and alpine steppe ecosystems, respectively. Our study improves understanding of the mechanism by which soil C sinks form on the Tibetan Plateau under warming and wetting conditions.

Phosphorus addition promotes Nitrogen retention in alpine grassland plants while increasing N deposition

Nitrogen (N) and phosphorus (P) are common limiting nutrients affecting plant growth in alpine ecosystems. Anthropogenic activities significantly increase N input in ecosystems, thereby increasing P limitation when in excess. The increase of N input and P limitation may affect N retention, which is a critical ecosystem function. To understand how ecosystem N retention and N loss respond to N and P addition, this study investigated the effects of N and P addition on NH3 volatilization, N2O emissions, and N pools of plants and soil in two alpine grasslands (alpine meadow and alpine steppe). The results showed that P addition (with or without N) increased N retention, mainly by promoting plant N uptake, underlining the importance of P availability in N retention in the QTP alpine grassland ecosystems while increasing N deposition. N addition without P showed neutral effects of N retention of plant, soil and microbes, and N addition (with or without P) showed positive effects on gaseous losses of N. N loss through NH3 volatilization and N2O emissions was quite low (less than 1%), suggesting that N gaseous loss was not a significant pathway of N loss from the alpine grassland ecosystems. These results were meaningful for estimating the impacts of N retention and N loss under future N deposition scenarios in alpine grassland ecosystems in the QTP.

Analysis of the Radiation Fluxes over Complex Surfaces on the Tibetan Plateau

Analysis of long-term, ground-based observation data on the Tibetan Plateau help to enhance our understanding of land-atmosphere interactions and their influence on weather and climate in this region. In this paper, the daily, monthly, and annual averages of radiative fluxes, surface albedo, surface temperature, and air temperature were calculated for the period of 2006 to 2019 at six research stations on the Tibetan Plateau. The surface energy balance characteristics of these six stations, which include alpine meadow, alpine desert, and alpine steppe, were then compared. The downward shortwave radiation at stations BJ, QOMS, and NAMORS was found to decrease during the study period, due to increasing cloudiness. Meanwhile, the upward shortwave radiation and surface albedo at all stations were found to have decreased overall. Downward longwave radiation, upward longwave radiation, net radiation, surface temperature, and air temperature showed increasing trends on inter-annual time scales at most stations. Downward shortwave radiation was maximum in spring at BJ, QOMS, NADORS, and NAMORS, due to the influence of the summer monsoon. Upward shortwave radiation peaked in October and November due to the greater snow cover. BJ, QOMS, NADORS, and NAMORS showed strong sensible heat fluxes in the spring while MAWORS showed strong sensible heat fluxes in the summer. The monthly and diurnal variations of surface albedo at each station were “U” shaped. The diurnal variability of downward longwave radiation at each station was small, ranging from 220 to 295 W·m−2.The diurnal variation in surface temperature at each station slightly lagged behind changes in downward shortwave radiation, and the air temperature, in turn, slightly lagged behind the surface temperature.

Soil Macropores Affect the Plant Biomass of Alpine Grassland on the Northeastern Tibetan Plateau

Macropores are an important part of soil structure. However, in alpine regions, the effects of soil macropores on soil properties and vegetation growth are not clear. We used the X-ray computed tomography (CT) method to obtain 3D images and visualize the distribution and morphology of soil macropores. By combining principal component analysis (PCA) and stepwise regression methods, we studied the relationships between soil macropores and both soil properties and vegetation growth in three types of grassland [alpine degraded steppe (ADS), alpine typical steppe (ATS), and alpine meadow steppe (AMS)] on the Tibetan Plateau. More tubular and continuous macropores occurred in the soil profiles of the AMS and ATS than in that of the ADS. In addition, the AMS soil had the highest macropore number (925 ± 189), while the ADS soil had the lowest macropore number (537 ± 137). PCA and correlation analysis suggested that macroporosity (MP) has significant positive correlations with the contents of soil organic matter, total nitrogen (TN), available phosphorus (AP) and total phosphorus (TP) (p &amp;lt; 0.05). The two parameters with the greatest influence on aboveground and belowground biomass were the shape factor (p &amp;lt; 0.05) and MP (p &amp;lt; 0.05), respectively. However, there was no significant correlation between plant diversity and soil macropores. We conclude that the irregularity of soil macropores restricts the growth space of roots and causes plants to sacrifice the accumulation of aboveground biomass for that of roots to find suitable sites for nutrient and water absorption.

The Relative Contributions of Climate and Grazing on the Dynamics of Grassland NPP and PUE on the Qinghai-Tibet Plateau

Net primary productivity (NPP) and precipitation-use efficiency (PUE) are crucial indicators in understanding the responses of vegetation to global change. However, the relative contributions of climate change and human interference to the dynamics of NPP and PUE remain unclear. During the past few decades, the impacts of climate change and human activities on alpine grasslands on the Qinghai-Tibet Plateau (QTP) have been intensifying. The aims of the study were to investigate the spatiotemporal patterns of grassland NPP and PUE on the QTP during 2000–2017 and quantify how much of the variance in NPP and PUE can be attributed to the climatic factors (precipitation and temperature) and grazing intensity. The results showed that: (1) grassland NPP significantly increased with a rate of 0.6 g C m−2 year−1 over the past 18 years, mainly induced by the increased temperature and the enhanced precipitation. The temperature was the dominant factor for NPP interannual variation in mid-eastern QTP, and precipitation restrained vegetation growth most in the southwest and northeast. (2) The PUE was higher on the eastern and western parts of the plateau, but lower at the center. Regarding grassland types, the PUE of alpine steppe (0.19 g C m−2 mm−1) was significantly lower than those of alpine meadow (0.31 g C m−2 mm−1) and desert steppe (0.32 g C m−2 mm−1). (3) Precipitation was significantly and negatively correlated with PUE and contributed the most to the temporal variation of grassland PUE on the QTP (52.7%). (4) Furthermore, we found that the grazing activities had the lowest contributions to both NPP and PUE interannual variation, compared to temperature and precipitation. Thus, it is suggested that climate variability rather than grazing activities dominated vegetation changes on the QTP.

Effects of grazing on plant species and phylogenetic diversity in alpine grasslands, Northern Tibet

Plant diversity plays an important role in maintaining and upgrading ecosystem structure and functions, and there remain uncertainties on plant species and especially phylogenetic diversity to grazing in alpine grasslands. Therefore, this study compared the differences of plant species and phylogenetic α- and β-diversity between grazing and fencing conditions at three alpine grasslands (ASMWG: alpine steppe meadow for winter grazing; ASMSG: alpine steppe meadow for summer grazing; AMSG: alpine meadow for summer grazing), Northern Tibet. At the ASMSG site, grazing significantly decreased mean nearest taxon distance (MNTD) by 19.33%, and increased Pielou by 7.06%. Plant community compositions between fencing and grazing conditions were different (p = 0.021) at the ASMSG site. However, the Pielou, MNTD and plant community composition were not significantly different between the fencing and grazing conditions at the ASMWG and AMSG site. Grazing significantly increased the niche overlap at the ASMWG (levins: 51.36%, schooner: 28.97%, pianka: 28.11%, Czech: 28.97%; morisita: 28.75%) and ASMSG site (schooner: 35.19%, petraitis: 4.63%, pianka: 26.45%, czech: 35.19%; morisita: 30.86%), indicating that grazing may increase plant species competition and competitive exclusion at the ASMWG and ASMSG site. In contrast, grazing did not significantly affect niche overlap at the AMSG site. Therefore, the response of plant diversity to grazing can be different between species and phylogenetic diversity, and among alpine grasslands.

Atmospheric water vapor and soil moisture jointly determine the spatiotemporal variations of CO2 fluxes and evapotranspiration across the Qinghai-Tibetan Plateau grasslands

Alpine grasslands play important functions in mitigating climate change and regulating water resources. However, the spatiotemporal variability of their carbon and water budgets remains unquantified. Here, 47 site-year observations of CO2 and water vapor fluxes (ET) are analyzed at sites situated along a hydrothermal gradient across the Qinghai-Tibetan Plateau, including an alpine wetland (wettest), an alpine shrub (coldest), an alpine meadow, an alpine meadow-steppe, and an alpine steppe (driest and warmest). The results show that the benchmarks for annual net ecosystem exchange (NEE) are −79.3, −77.8, −66.7, 20.2, and 100.9 g C m−2 year−1 at the meadow, shrub, meadow-steppe, steppe, and wetland, respectively. The peak daily NEE normalized by peak leaf area index converges to 0.93 g C m−2 d−1 at the 5 sites. Except in the wetland (722.8 mm), the benchmarks of annual ET fluctuate from 511.0 mm in the steppe to 589.2 mm in the meadow. Boosted regression trees-based analysis suggests that the enhanced vegetation index (EVI) and net radiation (Rn) determine the variations of growing season monthly CO2 fluxes and ET, respectively, although the effect is to some extent site-specific. Inter-annual variability in NEE, ecosystem respiration (RES), and ET are tightly (R2 > 0.60) related to the inter-growing season NEE, RES, and ET, respectively. Both annual RES and annual NEE are significantly constrained by annual gross primary productivity (GPP), with 85% of the per-unit GPP contributing to RES (R2 = 0.84) and 15% to NEE (R2 = 0.12). Annual GPP significantly correlates with annual ET alone at the drier sites of the meadow-steppe and the steppe, suggesting the coupling of carbon and water is moisture-dependent in alpine grasslands. Over half of the inter-annual spatial variability in GPP, RES, NEE, and ET is explained by EVI, atmospheric water vapor, topsoil water content, and bulk surface resistance (rs), respectively. Because the spatial variations of EVI and rs are strongly regulated by atmospheric water vapor (R2 = 0.48) and topsoil water content (R2 = 0.54), respectively, we conclude that atmospheric water vapor and topsoil water content, rather than the expected air/soil temperatures, drive the spatiotemporal variations in CO2 fluxes and ET across temperature-limited grasslands. These findings are critical for improving predictions of the carbon sequestration and water holding capacity of alpine grasslands.

Different response of alpine meadow and alpine steppe to climatic and anthropogenic disturbance on the Qinghai-Tibetan Plateau

Climate change and anthropogenic disturbance are two main drivers for vegetation dynamics on the Qinghai-Tibetan Plateau (QTP). Alpine meadow and alpine steppe are the primary rangeland ecosystem types on the QTP. However, the vegetation trends of the two land cover types and the underlying mechanisms behind their variation remain under debate. In this study, we used Global Inventory Modeling and Mapping Studies (GIMMS) 3g Normalized Difference Vegetation Index (NDVI) (i.e., GIMMS NDVI3g) by coupling the Breaks for Additive Season and Trend (BFAST) model and the Boosted Regression Tree (BRT) model to analyze alpine meadow and alpine steppe vegetation trends on the QTP between 1982 and 2015. We also assessed vegetation variation response to climatic and anthropogenic indicators in conjunction with climatic and human footprint datasets. Results show that growing season NDVI (GSNDVI) values increased overall for both alpine meadow (0.0001 year−1, p = 0.33) and alpine steppe (0.0002 year−1, p < 0.05) throughout 1982–2015. Significant greening trends in both alpine meadow (0.0007 year−1; p < 0.05) and alpine steppe (0.0005 year−1; p < 0.05) ecosystems were obtained before 1998 and 2001, respectively. However, browning trends ascertained by GSNDVI (−0.0006 year−1; p = 0.12) in alpine meadows were observed throughout 1998–2015, while greening trends ascertained by GSNDVI (0.0002 year−1; p = 0.12) in alpine steppes were observed throughout during 2001–2015. Opposing trends in precipitation, solar radiation, and the Standardized Precipitation Evapotranspiration Index (SPEI) occurred before and after breakpoints in both ecosystems. For the alpine meadow ecosystem, adverse precipitation trends caused browning before 1998 followed by greening after 1998 in the Three-River-Source National Park (TNP). Conversely, opposing changes in precipitation, solar radiation, and SPEI resulted in greening before 1998 followed by browning after 1998 in southern Tibet and the southeastern QTP. Alpine meadow vegetation trends were generally dominated by solar radiation before 1998 and jointly by precipitation and solar radiation after 1998. Prior to 2001 variation in alpine steppe greenness was controlled by precipitation, while after 2001 solar radiation dominated. Along with an increase in human footprint pressure (HFP) gradients, greenness trends gradually increased before 1998 but reversed after 1998 in the alpine meadow ecosystem. Additionally, greenness trends gradually decreased before 2001 but remained unchanged after 2001 for the alpine steppe ecosystem. These results highlight the different effects that climate change and anthropogenic disturbances have had on alpine meadow and alpine steppe ecosystems on the QTP over different time frames.

Fungal spore indicators of vegetation and highland pastoralism in modern topsoil and dung, eastern Tibetan Plateau

Recent research on the Qinghai-Tibetan Plateau (QTP) has focused on the human-land relationship and its evolution through time; fossil fungal spores are promising proxies for recovering information on paleoecological changes and ancient anthropogenic activity. Modern analog fungal spore datasets are crucial for accurate paleoecological interpretation of fossil fungal spores, however, these are lacking for the QTP. In this study, we analyzed 130 modern samples of topsoil and herbivore dung from eastern QTP to assess fungal assemblages in different habitats and vegetational communities. The results indicate that fungal spores in different types of domestic herbivore dung samples are quite similar and dominated by Sporormiella spp., Urocystis sp., Sordaria spp., Pleospora spp., QTPF-01 (Qinghai-Tibetan Plateau Fungi). Topsoil samples, collected from different vegetation zones, show more variation. Savoryella spp. is the dominant taxon in cropland samples, while Meliola spp. and Coniochaeta spp. are well associated with alpine forest environments. Fungal spore assemblages in alpine shrub, alpine meadow, and alpine steppe are mostly dominated by Glomus spp., together with Savoryella spp., Valsaria sp., Entorrhiza spp., and QTPF-03. Comparison of fungal assemblages in herbivore dung and topsoil samples suggests that Sporormiella spp., Urocystis sp., Sordaria spp., Pleospora spp., QTPF-01, Podospora spp., Delitschia A., and QTPF-07 are the most reliable dung indicators. The total concentration of coprophilous fungal spores is sensitive to grazing pressure and can be used to diagnose the intensity of pastoralism on the QTP. Our work provides vital modern analog data for fossil fungal spore studies on the QTP.

Soil Bacterial Communities and Diversity in Alpine Grasslands on the Tibetan Plateau Based on 16S rRNA Gene Sequencing

The Tibetan Plateau, widely known as the world’s “Third Pole,” has gained extensive attention due to its susceptibility to climate change. Alpine grasslands are the dominant ecosystem on the Tibetan Plateau, albeit little is known about the microbial community and diversity among different alpine grassland types. Here, soil bacterial composition and diversity in the upper soils of five alpine grassland ecosystems, alpine meadow (AM), alpine steppe (AS), alpine meadow steppe (AMS), alpine desert (AD), and alpine desert steppe (ADS), were investigated based on the 16S rRNA gene sequencing technology. Actinobacteria (46.12%) and Proteobacteria (29.67%) were the two dominant soil bacteria at the phylum level in alpine grasslands. There were significant differences in the relative abundance at the genus level among the five different grassland types, especially for the Rubrobacter, Solirubrobacter, Pseudonocardia, Gaiella, Haliangium, and Geodermatophilus. Six alpha diversity indices were calculated based on the operational taxonomic units (OTUs), including Good’s coverage index, phylogenetic diversity (PD) whole tree index, Chao1 index, observed species index, Shannon index, and Simpson index. The Good’s coverage index value was around 0.97 for all the grassland types in the study area, meaning the soil bacteria samplings sequenced sufficiently. No statistically significant difference was shown in other diversity indices’ value, indicating the similar richness and evenness of soil bacteria in these alpine grasslands. The beta diversity, represented by Bray–Curtis dissimilarity and the non-metric multidimensional scaling (NMDS), showed that OTUs were clustered within alpine grasslands, indicating a clear separation of soil bacterial communities. In addition, soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), pH, and soil water content (SWC) were closely related to the variations in soil bacterial compositions. These results indicated that soil bacterial taxonomic compositions were similar, while soil bacterial community structures were different among the five alpine grassland types. The environmental conditions, including SOM, TN, TP, pH, and SWC, might influence the soil bacterial communities on the Tibetan Plateau.