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• Elevated CO 2 significantly advanced spring phenology in kobresia pygmaea. • Alpine plants on the tibetan plateau are more sensitive to elevated CO 2. • Elevated CO 2 reduced overlapping flowering between species. • Advancement in K. pygmaea's phenology was linked to higher water-use efficiency. Phenological changes in global vegetation are often attributed to climate warming. However, climate warming and elevated atmospheric CO 2 concentration (e CO 2) are two co-occurring global change factors, and how e CO 2 would affect vegetation phenology has received less attention. The partial pressure of atmospheric CO 2 on the Tibetan Plateau (TP) is lower than that in regions of lower altitudes. Consequently, the growth and phenology of alpine plants in this region could be more sensitive to e CO 2 , but this hypothesis is not yet supported by empirical evidence. Here we explored the effect of e CO 2 on plant phenology (including phenophases of green-up, budding, and flowering) through a 5-year field manipulation experiment in a high-altitude (4600 m above sea level) alpine grassland on the TP. Our results showed that e CO 2 significantly advanced the spring phenology of an early-flowering species (Kobresia pygmaea), while it had no impact on the phenology of two mid-flowering species (Potentilla saundersiana and Potentilla cuneata). Compared to other low-altitude regions, plant phenology on the TP underwent greater alterations under e CO 2 , which supports our hypothesis that the growth of high-altitude plants is more sensitive to e CO 2. Furthermore, we found that e CO 2 significantly reduced the overlapping of flowering between contrasting plant species, mainly due to the phenological advancement of the K. pygmaea induced by e CO 2. The observed advancement of the spring phenology in K. pygmaea under e CO 2 was associated with increasing ecosystem water-use efficiency (WUE), thereby advancing its subsequent phenological development, such as budding and flowering. Our findings provide experimental evidence that atmospheric CO 2 enrichment can accelerate plant growth processes in high-altitude regions, and suggest that large-scale model simulations should consider the effects of elevated atmospheric CO 2 concentration on plant growth and phenology. [Display omitted] [ABSTRACT FROM AUTHOR]

Climate change is receiving mounting attentions from various fields and phenology is a commonly used indicator signaling vegetation responses to climate change. Previous phenology studies have mostly focused on vegetation greening-up and its climatic driving factors, while autumn phenology has been barely touched upon. In this study, vegetation phenological metrics were extracted from MODIS NDVI data and their temporal and spatial patterns were explored on the Tibetan Plateau (TP). The results showed that the start of season (SOS) has significantly earlier trend in the first decade, while the end of season (EOS) has slightly (not significant) earlier trend. In the spatial dimension, similar patterns were also identified. The SOS plays a more significant role in regulating vegetation growing season length than EOS does. The EOS and driving effects from each factor exhibited spatially heterogeneous patterns. Biological factor is the dominant factor regulating the spatial pattern of EOS, while climate factors control its inter-annual variation. [ABSTRACT FROM AUTHOR]

Soil moisture is a key limiting factor in grass growth and restoration in alpine meadow ecosystems on the northern Tibetan Plateau. In the deeper layers, soil moisture influences the processes of freeze-thaw, erosion and water cycle. Cosmic-ray neutron probe (CRNP) is a new method for continuously monitoring mean soil water content (SWC) at hectometer scale, which has been applied in an alpine meadow at a high accuracy. However, with CRNP measuring depth of only 30 cm, depth-scaling is needed for sufficient insight into deep-layer soil moisture. This study evaluated the accuracy of CRNP measurement of SWC in the 2015 and 2016 growing seasons and the performance of temporal stability (TS) analysis in depth-scaling CRNP-estimated SWC. During the study period, 11 field samplings were done for calibration of CRNP-estimated SWC. Using 22 occasions of neutron probe measurements for each of 113 investigated locations, the TS of SWC was analyzed and its performance in depth-scaling CRNP-estimated SWC at five soil depths (10, 20, 30, 40 and 50 cm) was evaluated. The results showed that the mean SWCs to the depth of 50 cm were 12.9 and 17.0%, respectively in 2015 and 2016 growing seasons and were temporally influenced by precipitation and spatially by soil depth. The accuracy of the CRNP-measured SWC was high, with root mean square error and Nash-Sutcliffe efficiency coefficient (NSE) of 2.1% and 0.832, respectively. Representative locations for TS existed in all the soil layers, which increased with increasing soil depth. For the various soil layers, TS-estimated SWC was close to field-measured value. Only a relatively small error and high NSE were noted, suggesting that TS was reliable in application in CRNP depth-scaling. The study provided further scientific basis for the application of CRNP and an effective way of depth-scaling CRNP-estimated mean SWC in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

• Cold environment suppressed the accumulation of microbial-derived C in the soil. • Warming increased microbial residues in the soil, but N addition decreased it. • Warming weakened the N-driven negative effect on soil C stability. Microbial-derived soil organic carbon has attracted increased attention as an important source of soil organic matter that may affect soil carbon persistence. Ecosystems at high elevation and latitude contain massive amounts of soil organic carbon that is potentially vulnerable to rapid climate change. However, little is known about the mechanisms that govern the production and accumulation of carbon derived from microbes in the context of global changes for these regions. Here, we investigate soil biomarkers from membrane lipids and microbial residues in an experiment with six-year warming and nitrogen (N) addition treatments in an alpine meadow of the Tibetan Plateau. The results showed low accumulation of amino sugars in soil (167.51 to 297.36 μg·g−1) for this alpine meadow. After the six-year manipulative experiment, warming promoted accumulation of microbial-derived carbon in soil with both increased bacterial (increased by 53.56%) and fungal carbon (increased by 25.60%). Nitrogen addition inhibited microbial-derived carbon accumulation (decreased by 25.37%) and significantly decreased the fungal carbon content relative to bacterial carbon content. Climate warming tended to weaken this N-driven negative effect. As microbial residues relate closely with soil organic matter stability, these results highlight the potential of global change factors (warming and N addition) to alter the structure and persistence of soil organic matter, with important implications in refining carbon cycle-climate models. [ABSTRACT FROM AUTHOR]

[Display omitted] Structural information of grassland changes on the Tibetan Plateau is essential for understanding alterations in critical ecosystem functioning and their underlying drivers that may reflect environmental changes. However, such information at the regional scale is still lacking due to methodological limitations. Beyond remote sensing indicators only recognizing vegetation productivity, we utilized multivariate data fusion and deep learning to characterize formation-based plant community structure in alpine grasslands at the regional scale of the Tibetan Plateau for the first time and compared it with the earlier version of Vegetation Map of China for historical changes. Over the past 40 years, we revealed that (1) the proportion of alpine meadows in alpine grasslands increased from 50% to 69%, well-reflecting the warming and wetting trend; (2) dominances of Kobresia pygmaea and Stipa purpurea formations in alpine meadows and steppes were strengthened to 76% and 92%, respectively; (3) the climate factor mainly drove the distribution of Stipa purpurea formation, but not the recent distribution of Kobresia pygmaea formation that was likely shaped by human activities. Therefore, the underlying mechanisms of grassland changes over the past 40 years were considered to be formation dependent. Overall, the first exploration for structural information of plant community changes in this study not only provides a new perspective to understand drivers of grassland changes and their spatial heterogeneity at the regional scale of the Tibetan Plateau, but also innovates large-scale vegetation study paradigm. [ABSTRACT FROM AUTHOR]

• Precipitation mainly drives biomass allocation (R/S) in temperate grasslands. • Both precipitation and temperature drive R/S in alpine grasslands. • Plant functional group compositions affect R/S more than soil nutrition. • Effects of forbs on R/S is larger than those other dominant plant groups. Elucidating biomass partitioning between above- and belowground parts in plant communities is vital to better assessing the ecological services of terrestrial ecosystems. However, the knowledge of biomass allocation in grasslands is scant, especially for the divergent mechanisms under both contrasting conditions. On two large geographic transects, Tibetan Plateau and Inner Mongolian Plateau, we analyzed the general and differential mechanisms of biomass allocation (plant biomass ratio of root to shoot, R/S) in temperate vs. alpine grasslands. We found that the R/S exhibited large variations, ranging from 3.89 to 45.10 in temperate grasslands and from 4.74 to 80.72 in alpine grasslands. The R/S decreased with increasing precipitation in both grasslands, whereas it weakly related to temperature. Plant functional group had highest importance on R/S than mean annual precipitation, mean annual temperature, soil nitrogen and phosphorus availabilities, and plant richness and diversity index. Climate factors indirectly but strongly drove R/S by mainly changing plant functional group compositions. Particularly, forbs affected R/S more than other dominant plant functional types. This information can provide a profound insight into the biomass allocation dynamics in grasslands at regional level under climate change scenarios. [ABSTRACT FROM AUTHOR]

• The soil water content and temperature determined the maximum photosynthesis. • Sufficient soil water elevated the optimal temperature and maximum photosynthesis. • The optimal environmental configuration for the alpine meadow was detected. • If soil water is high, higher temperature and VPD are beneficial for carbon uptake. The changes in the carbon sink function of sensitive and fragile alpine ecosystems under climate change are widely concerned. Radiation, temperature, and water resources play important roles in regulating carbon assimilation. Therefore, it is important to detect the optimal configuration of environmental factors for the photosynthesis of alpine meadow ecosystems. In this study, based on 10-year flux and corresponding environmental observations in a typical alpine meadow ecosystem, the optimal environmental configuration that could realistically occur in the natural world for photosynthesis was detected. The results indicated that the temperature and water conditions were the primary limiting factors, as radiation resources are abundant on the Tibetan Plateau. The optimal temperature for photosynthesis (TA opt) was determined to be 11.92 ℃ based on the entire observation dataset, with a maximum photosynthetic capacity (GPP max) of 0.20 mg CO 2 m−2 s−1. However, the TA opt and the corresponding GPP max could be elevated when the soil water content (SWC) was abundant. When SWC exceeded 0.26 m3 m−3, TA opt increased to 13.41 ℃, and the corresponding GPP max reached as high as 0.30 mg CO 2 m−2 s−1, which is the maximum the ecosystem could reach. Thus, the optimal environmental configuration for photosynthesis that could realistically occur in the natural world in this alpine meadow was air temperature (TA)=13.41 ℃, SWC=0.28 m3 m−3, vapor pressure deficit (VPD)=0.61 kPa, net radiation (RN)=411.53 J m−2 s−1. According to this study, it could be inferred that within a reasonable range, higher temperatures and VPD would benefit photosynthesis in this alpine meadow as long as the soil water supply is sufficient. The climate on the Tibetan Plateau is arid with high VPD and is experiencing warming due to climate change. Consequently, alpine ecosystems situated in relatively humid regions with sufficient soil water resources may exhibit greater potential for carbon sequestration than previously estimated. [ABSTRACT FROM AUTHOR]

Greening of the Earth is among the world's most important ecological changes in recent decades, but its effects and triggers are multifaceted and varied across the globe. Climate change is the primary driver for greening of the Tibetan Plateau (TP), where vegetation is of great significance for resident survival, carbon sequestration and climate feedbacks. However, the spatial variability of greening and its drivers are not well-understood on the TP. Here, we compiled multi-source vegetation indices, climate and atmospheric data to investigate the greenness spatiotemporal variability and its linkage with large-scale climate oscillations. We found that greening dominated the TP during 1980s–2010s and positive trends in satellite-based vegetation indices account for >70% of the TP. Specifically, the arid and semi-arid TP experienced a significant and higher greening rate due to the remarkable increase in precipitation. We further clarified the domains of the South Asian monsoon (SAMI1 and SAMI2), the westerly wind (WI) and the plateau monsoon (TPMI) on the TP, and realized that the wetness increase in the arid and semi-arid was correlated with a weakening WI. Detecting the linkage between WI with large-scale climate oscillations, we suggested that the decadal weakening WI was induced by a warmer North Atlantic SST (positive AMO phase), which triggered wave train anomalies and regulated zonal and meridional vapor transport across Eurasia. Generally, during the weak/negative WI phase, anticyclone anomalies centered over Mongolia may strongly reduce vapor output across the east boundaries, while cyclone anomalies centered over the northwest outside the TP likely enhance meridional vapor transport into the western TP from the south ocean. Eventually, precipitation over the arid and semi-arid TP increased significantly in the past four decades, that stimulated plant growth and greening pattern on the TP. Our results established the linkage among climate oscillations, regional climate alteration and vegetation greening, providing a more thorough understanding in terrestrial greening. • Approximately 70% areas of Tibetan Plateau (TP) were greening at the past four decades. • The greening of TP was dominated by the wetting in arid and semi-arid areas. • The decadal weakening of westerly wind dominated wetting over the TP, thus actuated the greening of TP. [ABSTRACT FROM AUTHOR]

• Water conditions dominated plant phenology and photosynthetic capacity. • Plant phenology and photosynthetic capacity co-determined to annual GPP. • Plant photosynthetic capacity dominated annual GPP. Under global climate change, climate warming and changing precipitation patterns would greatly affect plant phenology and photosynthetic capacity and hence affect the gross primary productivity (GPP) of the sensitive alpine meadow. To deeply understand the variations in GPP, it is important to clarify the joint effects of plant phenology and photosynthetic capacity. In this study, 10-year continuous flux and microclimatic data and in situ phenology period observation data were used to explore the roles of growing season length (GSL) and the seasonal maximal capacity of carbon uptake (GPP max) on annual cumulative gross primary productivity (GPP ann). The results indicated that the temperature and water conditions, especially water conditions, dominated GSL and GPP max and hence determined GPP ann. Both GSL and GPP max were dominated by annual precipitation, and the soil water content in August also showed a strong influence over them. GSL and GPP max together could explain 87% of the variation in GPP ann. Compared with GSL, GPP max had stronger effects on the interannual variability (IAV) in GPP ann. This study provides a new perspective on clarifying the proximate causes of IAV in GPP ann. The IAV in GPP ann of alpine ecosystems could be determined by these two key underlying processes, the length of the carbon uptake period and amplitude. These results are of critical importance for improving our ability to predict future changes in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

Summary Cosmic-ray neutron sensing (CRNS) is a new method for continuously monitoring mean soil water content (SWC) on a hectometer scale. To evaluate the application and accuracy of the method for SWC observation in an alpine meadow ecosystem (AME), we installed the CRNS in a flat meadow near the Naqu prefecture on the northern Tibetan Plateau. We collecting soil samples and applying the system by the oven-drying method. A weather station was also installed near the CRNS for monitoring basic meteorological variables and the soil temperature and water content at various depths. Three Em-50 instruments for monitoring SWC and soil temperature were buried in three sub-quadrats northwest, northeast and southeast of the CRNS at distances of 460, 370 and 373 m, respectively, to observe the variation of SWC at the various depths. The footprint of the CRNS for SWC observation in the meadow was about 580 m, and the mean measuring depth was about 31 cm according to the general calculation equations. The reference neutron flux for dry soil ( N 0 ) had a mean and coefficient of variation of 8686 and 3%, respectively, and remained substantially invariant throughout the measuring period. The five SWCs from the independent field samples almost passed through the SWC trend of the CRNS, the root mean square error (RMSE) was 0.011 m 3 m −3 for the CRNS and oven-drying method. The time series of SWC measured by the CRNS agreed well with the mean SWC series to a depth of 20 cm measured by the weather station. The trend of SWC measured by the Em-50s generally agreed with the trend of SWC measured by the CRNS, but some values and variations of SWC differed between the Em-50s and CRNS data. Because of the good agreement between the CRNS and independent field samples, we suspect that this disagreement is due to an insufficient representativeness of point observations and the distances of the points from the CRNS. The diurnal variation of hourly SWC from the CRNS was sinusoidal during a dry period, peaking at 11:00 and was minimum at 18:00 (Beijing time), with a range of 1%. Overall, the CRNS measured SWC in the AME with an acceptable accuracy, providing a scientific basis for the promotion and application of the CRNS in high, cold ecosystems. [ABSTRACT FROM AUTHOR]

Highlights: [•] We simulated the human-induced alpine grassland NPP over the Qinghai–Tibet plateau. [•] We separated the influences caused by climate change and anthropogenic activities. [•] We found the different driving forces for the consistently enhanced actual NPP. [•] National grassland protection policy has achieved positive ecological effects. [Copyright &y& Elsevier]

Abstract: Although grazing is a common land use type, few studies are available about the response of soil microbial biomass to grazing especially above 4300 m on the Tibetan Plateau. Therefore, three fenced enclosures were made at three alpine meadow sites along an elevation gradient (4313 m, winter pasture; 4513 m and 4693 m, summer pasture) in July 2008. Soil samples inside and outside the fenced enclosures were gathered in July, August and September 2011. Microbial biomass C (MBC) and N (MBN) were determined using the chloroform fumigation–extraction method. Grazing marginally declined MBC by 21.60%, 4.83% and 5.36% across sampling dates at elevation 4313 m, 4513 m and 4693 m, respectively. Grazing significantly declined MBN by 39.58% and 18.88% across sampling dates at elevation 4313 m and 4693 m, respectively, whereas it slightly declined MBN by 1.50% at elevation 4513 m. Microbial biomass at elevation 4693 m was significantly higher in comparison with elevation 4513 m and 4313 m, whereas soil temperature at elevation 4693 m was 2.3 °C and 2.8 °C lower than that at elevation 4513 m and 4313 m, respectively. Our findings suggest that MBN may respond more rapidly to grazing than MBC and that climate warming and grazing may decline microbial biomass for the alpine meadow. [Copyright &y& Elsevier]