Alpine Steppe Research Articles

Retrieving Soil Moisture in the Permafrost Environment by Sentinel-1/2 Temporal Data on the Qinghai–Tibet Plateau

Soil moisture (SM) products presently available in permafrost regions, especially on the Qinghai–Tibet Plateau (QTP), hardly meet the demands of evaluating and modeling climatic, hydrological, and ecological processes, due to their significant bias and low spatial resolution. This study developed an algorithm to generate high-spatial-resolution SM during the thawing season using Sentinel-1 (S1) and Sentinel-2 (S2) temporal data in the permafrost environment. This algorithm utilizes the seasonal backscatter differences to reduce the effect of surface roughness and uses the normalized difference vegetation index (NDVI) and the normalized difference moisture index (NDMI) to characterize vegetation contribution. Then, the SM map with a grid spacing of 50 m × 50 m in the hinterland of the QTP with an area of 505 km × 246 km was generated. The results were independently validated based on in situ data from active layer monitoring sites. It shows that this algorithm can retrieve SM well in the study area. The coefficient of determination (R2) and root-mean-square error (RMSE) are 0.82 and 0.06 m3/m3, respectively. This study analyzed the SM distribution of different vegetation types: the alpine swamp meadow had the largest SM of 0.26 m3/m3, followed by the alpine meadow (0.23), alpine steppe (0.2), and alpine desert (0.16), taking the Tuotuo River basin as an example. We also found a significantly negative correlation between the coefficient of variation (CV) and SM in the permafrost area, and the variability of SM is higher in drier environments and lower in wetter environments. The comparison with ERA5-Land, GLDAS, and ESA CCI showed that the proposed method can provide more spatial details and achieve better performance in permafrost areas on QTP. The results also indicated that the developed algorithm has the potential to be applied in the entire permafrost regions on the QTP.

Different responses of soil bacterial species diversity and phylogenetic diversity to short-term nitrogen input in an alpine steppe at the source of Brahmaputra

The Qinghai-Tibet Plateau has experienced an increase in N deposition/input due to global change. However, it remains unclear how the responses and whether the responses of soil bacterial diversity to short-term N input are consistent at different levels. Here, we investigated soil bacterial species and phylogenetic α-diversity and community composition based on a short-term nitrogen input experiment (five levels: 0, 2.5, 5, 10, and 20 g N m−2 y−1) in an alpine steppe at the source of Brahmaputra, using high-throughput sequencing technology. Short-term nitrogen input did not affect the species α-diversity and β-diversity of soil bacteria. However, soil bacterial phylogenetic α-diversity and dissimilarity increased with increasing nitrogen input. Different relative contributions and correlations of primary factors to species and phylogenetic diversity under short-term nitrogen input may result in different responses, in which ecological processes also play a role. Therefore, studying the response of soil bacteria to short-term nitrogen input should take into account not only the species level but also the phylogenetic level. We should pay close attention to the potential influence of short-term nitrogen deposition/fertilization on the soil bacterial community in the alpine steppe on the Tibetan Plateau.

Responses of carbon dynamics to grazing exclusion in natural alpine grassland ecosystems on the QingZang Plateau.

In the context of "Carbon Emissions Peak" and "Carbon Neutrality", grazing exclusion (GE) has been applied widely to rehabilitate degraded grasslands and increase carbon sequestration. However, on the QingZang Plateau (QZP), the impacts of GE on the carbon dynamics of alpine grasslands are poorly understood, particularly at a regional scale. To fill this knowledge gap, we evaluated the responses of carbon sequestration to GE in different alpine grasslands across QZP by using meta-analysis. The effects of GE on ecosystem carbon fractions were dependent on GE duration, grassland types and climate factors. Specifically, our results indicated that GE had more obviously positive effects on carbon stock across the alpine meadow than the alpine steppe. However, when considering different GE duration, the longer duration of GE was more effective for increasing ecosystem carbon sequestration (R 2 = 0.52, P<0.0001) in the alpine steppe. Our results further demonstrated that annual mean precipitation (AMP) and temperature (AMT) began to dominate ecosystem carbon sequestration after three years of GE duration across the alpine meadow; and AMP was an important climate factor limiting ecosystem carbon sequestration (R 2 = 0.34, P<0.01) in the alpine steppe. In terms of plant carbon fraction, GE generated continuous positive effect (P<0.05) on aboveground biomass with the increased GE duration in the alpine meadow, while this positive effect disappeared after the 8th year of GE duration. And no positive effects were found on belowground biomass in the 11th year in alpine steppe. For soil organic carbon (SOC), there existed periodic fluctuations (increased and then decreased) on SOC in response to GE. For microbial biomass carbon, there were no obvious trends in response to GE duration. In general, we highlighted that the responses of different carbon fractions (plant-soil-microbe) to GE were non-uniform at spatial and temporal scales, thereby we should adopt different carbon management practices for sustainable development of different grasslands.

Comparison of Root Ecological Stoichiometry between Non-Growing Season and Growing Season of Grassland on the Chang Tang Plateau.

Root C: N: P stoichiometry affect the geochemical cycles of ecosystems, while a few studies were conducted on it and its relationship with soil nutrients, especially in the non-growing season. In this study, we investigated the root C:N:P stoichiometry of alpine steppe(AS), alpine meadow steppe(AMS), and alpine meadow(AM) in April (non-growing season) and August(growing season) in 2013. The results showed that: (1) There were no differences in root C, N, P, C: N, C:P, and N:P with a depth of AS in April. However, root C and C: N increased with depth, while N and N:P decreased with a depth of AS in August. In both months, the variation of root C, N, P, C: N with depth in AM was consistent with that of AS in August, and root C at deep layer decreased in August, which indicated roots of AM began to grow in April No significant difference of root C, N, C: N and N:P with depth was found, while P and C:P varied between the two months of AMS. Root P content at 20-30 cm was higher than that of other soil layers in April, which was significantly higher than that of AS, while no significant difference was found in August. (2) Grassland types had significant effects on soil nutrients (except TP) at 0-10 cm and 20-30 cm soil layers. (3) No significant correlation between soil nutrients and root C, N, P, C: N, C: P, and N: P was found in April. Soil TN and AN content had a significant positive correlation with root N: P, indicating that soil nitrogen was the primary N source of the root. Soil TP and AP were significantly negatively correlated with root C and C: N in August. This study can provide basic data and provide theoretical support for further understanding the role of grassland roots in nutrient cycling.

Determining the role of richness and evenness in alpine grassland productivity across climatic and edaphic gradients.

Spatial heterogeneity of climatic and edaphic gradients can substantially affect the grassland productivity function. However, few studies have tested the importance of species richness and evenness on regulating grassland productivity across spatial-scale climatic and edaphic changes. This study examines the complex mechanisms by which species richness and evenness regulate productivity in alpine meadow and steppe. We used field survey data to explore above-ground productivity formation and sensitivity to spatial-scale climatic and edaphic response of alpine grassland based on species richness and evenness. Results showed that the growing season solar radiation was the main driving factor of above-ground biomass and was strongly negatively correlated with above-ground biomass. Furthermore, compared with alpine steppe, above-ground biomass in alpine meadow was more responsive to climatic variables, but less responsive to soil variables. Unexpectedly, we found that the regulation patterns of species richness and evenness on above-ground biomass were different in both habitats by a structural equation model analysis. Our study demonstrated that species evenness and richness were both important in co-regulating above-ground biomass in alpine meadow, whereas species richness mattered more than species evenness in regulating above-ground biomass in alpine steppe. Our results offer further support for species richness and evenness co-regulating grassland productivity across spatial-scale climatic and edaphic gradients, which helps maintain the benefits of plant diversity and alpine grassland ecosystems.

Response of forage nutrient storages to grazing in alpine grasslands

Forage nutrient storages can determine livestock size and husbandry development. There is insufficient research on the response of forage nutrient storages to grazing and related driving mechanisms in alpine grasslands, especially on the Tibetan Plateau. This study conducted a grazing experiment in three alpine grassland sites along an elevation gradient (two warm-season pastures and one cold-season pasture; two alpine steppe meadow sites and one alpine meadow) of Northern Tibet. Different types of alpine grassland ecosystems, at least for forage nutrient storages, may have different responses to grazing. Warm-season grazing significantly reduced crude protein (CP) storage, acid detergent fiber (ADF) storage, and neutral detergent fiber (NDF) storage of high-quality forage by 53.29, 63.82, and 63.26%, respectively, but cold-season grazing did not significantly alter the CP, ADF and NDF storages of high-quality forage. Warm-season grazing significantly reduced CP, ADF, NDF, crude ash (Ash), ether extract (EE) and water-soluble carbohydrate (WSC) storages of the plant community by 46.61, 62.47, 55.96, 64.94, 60.34, and 52.68%, and forbs by 62.33, 77.50, 73.69, 65.05, 57.75, and 62.44% in the alpine meadow site but not the alpine steppe meadow site, respectively. Plant species and phylogenetic diversity had different relationships with forage nutrient storages. The elevation distribution of forage nutrient storages under fencing conditions were different from those under grazing conditions. Therefore, cold-season grazing can have lower negative effects on forage nutrient storages than warm-season grazing. Combined plant species with phylogenetic diversity and composition can be better in predicting forage nutrient storages. Grazing can restructure the elevation distribution of forage nutrient storages in alpine grasslands.

A Meta-Analysis of Soil Organic Carbon Response to Livestock Grazing in Grassland of the Tibetan Plateau

Known as the “roof of the world”, the Tibetan Plateau hosts the largest pastoral alpine ecosystem in the world. Nevertheless, there is currently no consensus on how soil organic carbon (SOC) stock changes after livestock grazing on the grassland of this region. Here, a meta-analysis was performed based on 55 published studies to quantify the livestock grazing-induced changes in SOC stock (0–30 cm) in grassland on the Tibetan Plateau. The results showed that livestock grazing significantly increased bulk density by an average of 11.5%, indicating that significant soil compaction was caused by livestock grazing. In contrast, SOC content and stock significantly decreased by 14.4% and 11.9% after livestock grazing, respectively. The decline rate of SOC stock was higher in alpine meadow (−12.4%) than that in alpine steppe (−8.8%), but there was no significant difference between the two rates. The SOC stocks decreased by 10.1%, 6.2% and 20.1% under light grazing, moderate grazing and heavy grazing, respectively. The decline rate of SOC stock under moderate grazing was significantly lower than that under heavy grazing. For different livestock types, it was observed that yak grazing significantly decreased SOC stock by 15.3%. Although the decline rate induced by yak grazing was higher than those induced by Tibetan sheep grazing and mixed grazing, no significant difference was detected among them. Similarly, the grazing-induced SOC declines also did not differ significantly among subgroups of grazing season. The positive relationships between SOC stock and plant biomass indicated that the decreased plant biomass was a likely reason for the declined SOC stock under grazing condition. The findings suggested that moderate grazing with Tibetan sheep in the warm season may minimize SOC losses from grazing activities in alpine grassland on the Tibetan Plateau.

Effects of Dynamic Changes of Soil Moisture and Salinity on Plant Community in the Bosten Lake Basin

To estimate the potential risks of plant diversity reduction and soil salinization in the Bosten Lake Basin, the dynamic changes in the plant community and species diversity affected by soil moisture and salinity were analyzed from 2000 to 2020 based on remote sensing technology and field experiments. A model for simulating soil moisture, salinity, and the productivity of the plant communities was proposed. The results demonstrated that: (1) The soil moisture index (SMI) increased but the soil salinity index (SSI) decreased from 2000 to 2020 in the study areas. Accordingly, the plant community productivity indices, including the vegetation index (NDVI), enhanced vegetation index (EVI), and ratio vegetation index (RVI), exhibited an increasing trend. It was found that the Alpine meadow, Alpine steppe, and temperate steppe desert were the main types of plant communities in the study areas, accounting for 69% of its total area. (2) With increasing SMI or decreasing SSI, the vegetation productivity such as NDVI, RVI, and EVI all exhibited an increasing trend. With the increment of SMI, the species diversity indices of the Simpson, Shannon–Wiener, and Margalef exhibited a distinctly increasing trend. However, the indices of the Simpson, Shannon–Wiener, and Alatalo increased with the decreasing SSI. (3) The study discovered from the SVM model that the species diversity index was optimal when the soil salinity was 0–15 g/kg and the soil moisture was 12–30% in the study areas. It was found that soil moisture, not soil salinity, controls the plant species diversity change in the study areas. (4) A multiple linear regression model was established for simulating the effect of soil water-salinity on the vegetation productivity index at the watershed scale. The model indicated that higher salinity would reduce vegetation productivity and higher soil moisture would promote vegetation growth (except for RVI). The SSI had a higher impact on NDVI and EVI than the SMI in the study areas. This study would support decision-making on grassland ecosystem restoration and management in the other arid areas.

Palynological evidence reveals an arid early Holocene for the northeast Tibetan Plateau

Abstract. Situated within the triangle of the East Asian monsoon, the Indian monsoon, and the westerlies, the Holocene patterns of climate and vegetation changes on the northeast Tibetan Plateau are still unclear or even contradictory. By investigating the distribution of modern pollen taxa on the east Tibetan Plateau, we infer the past vegetation and climate since 14.2 ka BP (1000 years before present) from a fossil pollen record extracted from Gahai Lake (102.3133∘ E, 34.2398∘ N; 3444 m a.s.l.) together with multiple proxies (grain size, contents of total organic carbon and total nitrogen) on the northeast Tibetan Plateau. Results indicate that the Gahai Basin was covered by arid alpine steppe or even desert between 14.2 and 7.4 ka BP with dry climatic conditions, and high percentages of arboreal pollen are thought to be long-distance wind- transported grains. Montane forest (dominated by Abies, Picea, and Pinus) migrated into the Gahai Basin between 7.4 and 3.8 ka BP driven by wet and warm climatic conditions (the climate optimum within the Holocene) but reverted to alpine steppe between 3.8 and 2.3 ka BP, indicating a drying climate trend. After 2.3 ka BP, vegetation shifted to alpine meadow represented by increasing abundances of Cyperaceae, which may reflect a cooling climate. The strange pollen spectra with high abundances of Cyperaceae and high total pollen concentrations after ca. 0.24 ka BP (1710 CE) could be an indication of disturbance by human activities to some extent, but needs more direct evidence to be confirmed. Our study confirms the occurrence of a climate optimum in the mid-Holocene on the northeast Tibetan Plateau, which is consistent with climate records from the fringe areas of the East Asian summer monsoon, and provides new insights into the fluctuations in the intensity and extent of the Asian monsoon system.

Interannual Variations of Water and Carbon Dioxide Fluxes over a Semiarid Alpine Steppe on the Tibetan Plateau

Water and carbon exchanges between grassland and the atmosphere are important processes for water balance and carbon balance. Based on eddy covariance observations over a semiarid alpine steppe ecosystem in Bange on the Tibetan Plateau during the growing season from 2014 to 2017, the variations in evapotranspiration (ET), net ecosystem exchange (NEE), and their components and the associated driving factors were analyzed. Linear and nonlinear models were applied to investigate the relationships between fluxes and their controlling factors over different timescales. The results show that the average ET for the growing season ranged from 1.1 to 2.4 mm/d with an average of 2.0 mm/d for the four consecutive years. Drought conditions reduced the surface conductance and hence the Priestley–Taylor coefficient. Mean T/ET was low (0.34) due to low vegetation cover. Plant growth increased the T/ET ratio during the growing season, whereas soil water content (SWC) explained most of the variation of ET and E on daily and monthly scales. The Enhanced Vegetation Index (EVI) was the most important controlling factor for temperature. Transpiration increased with SWC in dry conditions. For the growing season in 2014, 2016, and 2017, Bange was a carbon sink, while it was a carbon source in 2015. The largest CO2 flux was higher and the temperature sensitivity coefficient (Q10) was lower for 2015 than for the other three years. SWC affected these photosynthesis and respiration parameters. The ratio of respiration (Re) to gross primary production (GPP) was the highest during the 2015 growing season. Both on daily and monthly scales, Re was positively and linearly correlated with GPP. The most important controlling factor for the CO2 flux was EVI on daily and monthly scales.

Patterns, Dynamics, and Drivers of Soil Available Nitrogen and Phosphorus in Alpine Grasslands across the QingZang Plateau

Soil available nutrient contents are critical for regulating ecosystem structure and function; therefore, exploring patterns, dynamics, and drivers of soil available nutrient contents is helpful for understanding the geochemical cycle at the regional scale. However, learning the patterns and dynamics of soil available nutrients across a regional scale is quite limited, especially the soil available nitrogen (SAN) and soil available phosphorus (SAP) in alpine grasslands. In this study, we used machine learning (Random Forest) to map the SAN and SAP at a soil depth of 0–30 cm in alpine grasslands across the QingZang Plateau (QZP) in 2015. Our results showed that the current (2015) contents of the SAN and SAP in alpine grasslands on the QZP were 139.96 mg kg−1 and 2.63 mg kg−1, respectively. Compared to the 1980s, the SAN significantly increased by 18.12 mg kg−1 (14.83%, p &lt; 0.05) and the SAP decreased by 1.71 mg kg−1 (39.40%, p &lt; 0.05). The SAN and SAP contents of alpine meadows were higher than those of alpine steppes. The increases in SAN were not significantly (p &gt; 0.05) different between those two grassland types, while the decrease in SAP was significantly (p &lt; 0.05) higher in alpine meadows than in alpine grasslands. Combined with redundancy analysis, we quantified the impact of environmental drivers, and 80% of the spatial variation in SAN and SAP could be explained by environmental factors. Our findings also highlighted that in the context of global change, the increase in SAP and decrease in SAP might lead to weakening of nitrogen limitation and intensification of phosphorus limitation, especially in alpine meadows. In general, this study expanded the knowledge about the patterns and dynamics of SAN and SAP, and deepened the understanding of the driving mechanisms, which provided a basis for sustainable management of grasslands and optimization of ecological security barrier functions on the QZP.

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.

Short-term warming and N deposition alter the photosynthetic pigments trade-off in leaves of Leymus secalinus growing in different alpine grassland habitats on Qinghai-Tibetan plateau.

Warming and N (nitrogen) deposition are the two main driving factors of global change. We examined the effects of increased N deposition (8 kg ha-1 year-1) and warming, as well as their combined effect on the leaf photosynthetic pigments of Leymus secalinus, which is one of the key alpine plants growing in different grassland habitats on Qinghai-Tibetan plateau. In 2014, the experiments were established in 12 plots (2×5m) of three types of habitats including alpinemeadow (AM), alpine steppe (AS), and cultivated grassland (CG) with the following treatments: CK (control treatment), N (only N deposition), W (only warming), and W&N (warming combined with N deposition). Results showed that the effects of warming and N deposition on photosynthetic pigments of Leymus secalinus varied with different grassland habitat types. In three grassland types, warming led to no significant effects on the total chlorophyllcontent of L.secalinus, while N deposition alone only significantly enhanced total chlorophyllcontent in alpine meadow and cultivated grassland. N deposition combined with warming only significantly enhanced total chlorophyllcontent of L.secalinus in alpine steppe and cultivated grassland. Chla content plays an important role in determining the variation of total chlorophyll content. Chla/Chlb ratioof L.secalinus was more stable in alpine meadow compared with that of L. secalinus in the other two grassland types. Car/Chl ratioof L.secalinus was not prone to be affected by warming and N deposition in all grassland types. Leaf N content was obviously positively correlated with photosynthetic pigments, especially Chla content. Warming and N deposition all affected photosynthetic pigment dynamics and tended to increase Chla by enhancing its weight. Our results highlighted that both warming and N deposition as well as their combination can alter the trade-off of photosynthetic pigments through enhancing the Chla ratio in L. secalinus. In addition, growing habitats should be within consideration when studying alpine plants adaptation mechanism to global change in the future.

The Three Rivers Source Region Alpine Grassland Ecosystem Was a Weak Carbon Sink Based on BEPS Model Analysis

The Three Rivers Source Region (TRSR) is a natural habitat for rare animals and a genetic treasure trove of plateau organisms. It is an important eco-safety barrier in China and even Asia, and a priority of China’s to promote ecological advancement. Precisely assessing the dynamics and mechanisms of alpine grassland ecosystem carbon budgets is beneficial for quantifying the response to climate change on a regional scale. The spatial distribution and dynamic changes in carbon fluxes in the TRSR from 1985 to 2018 were analyzed by the Theil–Sen + Mann–Kendall and ensemble empirical mode decomposition (EEMD) methods, and multiple linear regression was used to quantify the contribution of meteorological elements to the carbon flux trends. The results indicated that (1) the alpine grassland ecosystem was a weak carbon sink. The multiyear mean gross primary production (GPP) and net ecosystem production (NEP) in the TRSR were 147.86 and 11.27 g C/m2/yr, respectively. The distribution of carbon fluxes progressively decreased from east to west. (2) The carbon fluxes of the alpine grassland ecosystem were dominated by a monotonically increasing trend, with increasing rates of GPP and NEP of 1.31 and 0.40 g C/m2/yr, respectively. A total of 48.60% of the alpine grassland showed a significant increase in NEP, whereas only 0.21% showed a significant decrease during the research term. (3) The alpine meadow sequestered carbon better than the alpine steppe did and accounted for more than 60% of the regional carbon sink. (4) In a correlation analysis between NEP and temperature, precipitation and solar radiation, the positive correlation accounted for 89.67%, 90.51%, and 21.16% of the TRSR, respectively. Rising temperatures and increased precipitation were the main drivers contributing to the increase in NEP. Research on carbon budget variability and mechanisms can help guide preservation zoning initiatives in national parks.

Six Years of Grassland Cultivation Promotes CO2, N2O Emissions and CH4 Uptake with Increasing N Deposition on Qinghai-Tibetan Plateau

Nitrogen (N) deposition has become an important factor of vital changes in the Qinghai-Tibetan Plateau (QTP), one of the key eco-regions in the world. To investigate how N deposition affects the fluxes of GHGs (CH4, CO2, N2O) in the alpine grassland ecosystem, the dominant ecosystems on QTP, we conducted control experiments in three types of alpine grasslands, including the alpine meadow (AM), alpine steppe (AS), and cultivated grassland (CG) on the QTP. In this study, four N addition gradients (0 kg Nha−1yr−1, 8 kg Nha−1yr−1, 24 kg Nha−1yr−1, and 40 kg Nha−1yr−1) were set up using ammonium nitrate from 2015 to 2020 in order to simulate N deposition at different levels, and after 6 years of continuous N application, greenhouse gases were collected from sampling plots. The results showed that simulated N deposition had no significant effect on soil GHG fluxes, while the grassland type had an extremely significant effect on soil GHG fluxes. Under the same N deposition conditions, the CH4 absorption in the cultivated grassland was higher than that in the other two types of grasslands. At low N deposition levels (CK, N1), the CO2 emission in the cultivated grassland was higher than that in the other two types of grasslands. At high N deposition levels (N2 and N3), the N2O emission in the cultivated grassland increased more significantly than it did in the other two types of grasslands. Control of grassland cultivation should be proposed as a reliable form of land-use management to reduce GHG emissions on the QTP in the era of increasing N deposition.

Pollen-based biome reconstruction on the Qinghai-Tibetan Plateau during the past 15,000 years

Reconstruction of past vegetation change is critical for better understanding the potential impact of future global change on the fragile alpine ecosystems of the Qinghai-Tibetan Plateau (QTP). In this paper, pollen assemblages comprising 58 records from the QTP, spanning the past 15 kyrs, were collected to reconstruct biome compositions using a standard approach. Six forest biomes were identified mainly on the southeastern plateau, exhibiting a pattern of gradual expansion along the eastern margin during early to mid-Holocene times. The alpine meadow biome was separately identified based on an updated scheme, and showed notable westward expansions towards lower latitudes and higher altitudes during early Holocene times. Consistent patterns of migration could also be identified for the alpine steppe biome, which moved eastward during the late Holocene after 4 ka. As the dominant biome type, temperate steppe was distributed widely over the QTP with minor migration patterns, except for a progressive expansion to lower altitudes in the late Holocene times. The desert biome was inferred mainly as covering the northwestern plateau and the Qaidam Basin, in relatively restricted areas. The spatial distribution of the reconstructed biomes represent the large-scale vegetation gradient on the QTP. Monsoonal precipitation expressed predominant controls on the development of alpine ecosystems, while the variations in desert vegetation responded to regional moisture brought by the mid-latitude Westerlies. Temperature changes played relatively minor roles in the variations of alpine vegetation, but exerted more significant impacts on the forest biomes.

Effects of N and P enrichment on plant photosynthetic traits in alpine steppe of the Qinghai-Tibetan Plateau

BackgroundN (nitrogen) and P (phosphorus) play important roles in plant growth and fitness, and both are the most important limiting factors that affect grassland structure and function. However, we still know little about plant physiological responses to N and P enrichment in alpine grassland of the Qinghai-Tibetan Plateau. In our experiment, five dominant common herbaceous species were selected and their photosynthetic parameters, leaf N content, and aboveground biomass were measured.ResultsWe found that species-specific responses to N and P enrichment were obvious at individual level. N addition (72 kg Nha−1 yr−1), P addition (36 kg Pha−1 yr−1) and NP addition (72 kg Nha−1 yr−1and 36 kg P ha−1 yr−1, simultaneously) significantly promoted net photosynthetic rate of Leymus secalinus. Differential responses also existed in the same functional groups. Responses of forb species to the nutrients addition varied, Aconitum carmichaeli was more sensitive to nutrients addition including N addition (72 kg Nha−1 yr−1), P addition (36 kg Pha−1 yr−1) and NP addition (72 kg Nha−1 yr−1and 36 kg P ha−1 yr−1). Responses of plant community photosynthetic traits were not so sensitive as those of plant individuals under N and P enrichment.ConclusionsOur findings highlighted that photosynthetic responses of alpine plants to N and P enrichment were species-specific. Grass species Leymus secalinus had a higher competitive advantage compared with other species under nutrient enrichment. Additionally, soil pH variation and nutrients imbalance induced by N and P enrichment is the main cause that affect photosynthetic traits of plant in alpine steppe of the Qinghai-Tibetan Plateau.

Impact of Snow Cover Phenology on the Vegetation Green-Up Date on the Tibetan Plateau

Variations in snow cover resulting from global warming inevitably affect alpine vegetation growth on the Tibetan Plateau (TP), but our knowledge of such influences is still limited. Here, we investigated the relationship between snow cover and alpine vegetation during 2003–2020 on the TP using the satellite-derived vegetation green-up date (GUD) and metrics of snow cover phenology, namely the snow cover onset date (SCOD), snow cover end date (SCED), snow cover duration (SCD), and snowmelt onset date (SMOD). In this study, we first analyzed the spatiotemporal changes in the GUD and the snow cover phenology metrics on the TP. Pearson’s correlation, gray relation analysis, and linear regression were then employed to determine the impact of snow cover phenology on the GUD. Overall, with the SCOD, SCED, and SMOD delayed by one day, the GUD was advanced by 0.07 and 0.03 days and was postponed by 0.32 days, respectively, and a one-day extension of the SCD resulted in a 0.04-day advance in the GUD. In addition, the roles of vegetation type, topography, and climate factors (temperature and precipitation) in modulating the relationships between snow cover phenology and the GUD were evaluated. The GUD of alpine steppes was negatively correlated with the SCOD and SCED, contrary to that of the other vegetation types. The GUD of alpine steppes was also more sensitive to snow cover phenology than that of other vegetation types. The increase in elevation generally enhanced the sensitivity of the GUD to snow cover phenology. The GUD showed a stronger negative sensitivity to the SCD in warmer areas and a stronger positive sensitivity to the SMOD in wetter areas. Our findings revealed the essential impact of variation in snow cover phenology on the GUD and indicated the complex interference of environmental factors in the relationship between snow cover and vegetation growth.

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.

Classification of Alpine Grasslands in Cold and High Altitudes Based on Multispectral Landsat-8 Images: A Case Study in Sanjiangyuan National Park, China

Land-use–cover change (LUCC)/vegetation cover plays a critical role in Earth system science and is a reflection of human activities and environmental changes. LUCC will affect the structure and function of ecosystems and a series of other terrestrial surface processes, such as energy exchange, water circulation, biogeochemical circulation, and vegetation productivity. Therefore, accurate LUCC mapping and vegetation cover monitoring are the bases for simulating the global carbon and hydrological cycles, studying the interactions of the land surface and climate, and assessing land degradation. Based on field GPS surveys and UAV data, with cloud-free and snow/glacier algorithms and the SVM classifier to train and model alpine grassland, the alpine grassland and LUCC were extracted by using Landsat-8 OLI satellite images in Sanjiangyuan National Park in this paper. The latest datasets of vegetation types with 30 m × 30 m spatial resolution in the three parks were prepared and formed. The classification results show that the SVM classifier could better distinguish the major land-use types, and the overall classification accuracy was very high. However, in the alpine grassland subcategories, the classification accuracies of the four typical grasslands were relatively low, especially between desert steppes and alpine meadows, and desert steppes and alpine steppes. It manifests the limitations of Landsat-8 multispectral remote sensing imageries in finer-resolution grassland classifications of high-altitude alpine mountains. The method can be utilized for other multispectral satellite imageries with the same band matching, such as Landsat 7, Landsat 9, Sentinel-2, etc. The method described in this paper can rapidly and efficiently process annual alpine grassland maps of the source areas of the Yellow River, the Yangtze River, and the Lancang River. It can provide timely and high-spatial-resolution datasets for supporting scientific decisions for the sustainable management of Sanjiangyuan National Park.