Meadow Steppe Research Articles

Mowing enhances the positive effects of nitrogen addition on ecosystem carbon fluxes and water use efficiency in a semi-arid meadow steppe

Grasslands are now facing a continuously increasing supply of nitrogen (N) fertilizers, resulting in alterations in ecosystem functioning, including changes in carbon (C) and water cycling. Mowing, one of the most widely used grassland management techniques, has been shown to mitigate the negative impacts of increased N availability on species richness. However, knowledge of how N addition and mowing, alone and/or in combination, affect ecosystem-level C fluxes and water use efficiency (WN) is still limited. We experimentally manipulated N fertilization (0 and 10 g N m−2 yr−1) and mowing (once per year at the end of the growing season) following a randomized block design in a meadow steppe characterized by salinization and alkalinization in northeastern China. We found that, compared to the control plots, N addition, mowing, and their interaction increased net ecosystem CO2 exchange by 65.1%, 14.7%, and 133%, and WN by 40.7%, 18.5%, and 96.1%, respectively. Nitrogen enrichment also decreased soil pH, which resulted in greater aboveground biomass (AGB). Moreover, N addition indirectly increased AGB by inducing changes in species richness. Our results indicate that mowing enhances the positive effects of N addition on ecosystem C fluxes and WN. Therefore, appropriate grassland management practices are essential to improve ecosystem C sequestration, WN, and mitigate future species diversity declines due to ecosystem eutrophication.

Hot Moments of N2O Emission Under Water and Nitrogen Management in Three Types of Steppe

AbstractHot moments of soil nitrous oxide (N2O) emission contribute to large fractions of total annual emissions. Magnitudes of N2O emission during hot moments to increasing water and nitrogen (N) availabilities in different steppe types were not well understood. Based on soil monoliths collected from three steppes (meadow, typical, and desert steppes) in northern China, we found that the typical steppe held the lowest N2O emission among the three steppes. Water and N co‐addition could increase soil N2O emission by up to 245‐fold. Water addition significantly enhanced soil N2O emission in all the three steppes, but the sensitivity of soil N2O emission to water addition decreased from desert steppe (r2 = 0.83) to typical (r2 = 0.73) and meadow (r2 = 0.66) steppes. Nitrogen addition significantly promoted soil N2O emission only in the meadow steppe where soil C:N was relatively higher than the other two steppes. Water and N addition displayed strong positive interaction on soil N2O emission in the meadow steppe while negative interaction in the typical steppe. Different responses of plants, microorganisms and soil NO3− concentration drove the variation in magnitude of soil N2O emission between the two steppes. Our results illustrate different interactions of water and N addition on hot moments of soil N2O emission in the three steppes. We suggest that the pulsed feature of N2O emission should be taken into account in prediction of greenhouse gas emission in the global arid and semiarid grasslands.

Interannual variations in natural abundance of nitrogen stable isotopes in soil and plant and their implications in a meadow steppe

Nitrogen is the most limiting nutrient for ecosystems. The natural abundance of δ15N (15N/14N) can efficiently indicate ecosystem nitrogen cycling processes. We investigated the interannual variations in natural abundance of δ15N in soil-plant system and soil net nitrogen mineralization in a meadow steppe of Inner Mongolia. Results across the four sampling years (2017-2020) showed that the content of soil NO3--N (9.83-14.79 mg·kg-1) was significantly higher than that of NH4+-N (3.92-5.00 mg·kg-1) and that δ15N value of soil NH4+ (13.3‰-18.3‰) was significantly higher than that of NO3-(3.76‰-6.14‰). The δ15N value of soil NO3- was negatively correlated with soil NO3- content. The δ15N value of soil NH4+ was relatively higher in the dry years, while the δ15N value of soil NO3- significantly decreased in the wetter and drier years. Soil net mineralization and ammonification rates were significantly higher in the dry years than that of the wet years, while soil nitrification rates showed no correlation with annual precipitation. The δ15N values of plants were not related to that of soils, but nega-tively correlated with plant nitrogen content. Both δ15N values and nitrogen contents were significantly and positively correlated between the leguminous and non-leguminous plants, suggesting that legume could facilitate nitrogen uptake of non-leguminous plants. These results could provide supporting data for nitrogen cycling and their responses to changes in precipitation in grassland soil-plant systems.

Climate warming enhances precipitation sensitivity of flowering phenology in temperate steppes on the Mongolian Plateau

The phenological sensitivity of terrestrial plants to precipitation change (precipitation sensitivity, Sp) and whether Sp is affected by climatic warming remain largely unknown. A four-year (2015–2018) field experiment with warming and increased/decreased precipitation was conducted to investigate the impacts of climate change on Sp in three dominant temperate grasslands (i.e., desert, typical, and meadow steppes) on the Mongolian Plateau of Northern China. Results showed that Sp of flowering phenology to reduced and increased precipitation was symmetric in each of the three steppes. Experimental warming stimulated Sp by 0.30 day/(10 mm· °C) over the three steppes, however, by 1.20 day/(10 mm· °C) in the desert steppe, which could be primarily attributed to enhanced dependences of Sp on soil moisture and normalized difference vegetation index (NDVI) during the flowering stage. These findings suggest that warming has the greater potential to stimulate Sp of phenological events in arid rather than semiarid and mesic grasslands and that Sp is jointly controlled by soil water availability and community development indicated by NDVI in different flowering stages in temperate grasslands. Considering the above two factors in phenology models will promote robust prediction of grassland reproductive phenology in temperate regions under concurrent climate warming and changing precipitation.

Soil inoculum identity and rate jointly steer microbiomes and plant communities in the field

Inoculation with soil from different ecosystems can induce changes in plant and soil communities and promote the restoration of degraded ecosystems. However, it is unknown how such inoculations influence the plant and soil communities, how much inoculum is needed, and whether inocula collected from similar ecosystems will steer soil and plant communities in different directions. We conducted a three-year soil inoculation experiment at a degraded grassland and used two different soil inocula both from grasslands with three inoculation rates. We measured the development of the soil and plant communities over a period of three years. Our results show that soil inoculation steers the soil microbiome and plant communities at the inoculated site into different directions and these effects were stronger with higher amount of soil used to inoculate. Network analyses showed that inoculation with upland meadow soil introduced more genera occupying the central position in the biotic network and resulted in more complex networks in the soil than inoculation with meadow steppe soil. Our findings emphasize that there are specific effects of donor soil on soil microbiomes as well as plant communities and that the direction and speed of development depend on the origin and the amount of soil inoculum used. Our findings have important implications for the restoration of biodiversity and ecosystem functioning in degraded grassland ecosystems.

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.

Response of soil microbial biomass C, N, and P and microbial quotient to agriculture and agricultural abandonment in a meadow steppe of northeast China

The carbon and nutrients in the soil microbial biomass (SMB) and their proportion in soil elements, namely soil microbial quotient (SMQ), are the key indicators of the soil quality and quickly respond to the shift in the land-use pattern. However, the degree of the land-use shift influences on the soil microbial biomass, and the soil microbial quotient in the meadow steppe is not well-understood. The soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorous (MBP), and SMQ were determined separately in grassland, cropland, and abandoned cropland in four sites within 50 km at a meadow steppe in northeast China. The results showed that the soil MBC, MBN, and MBP declined significantly as soil depth increased among the three land-use patterns. Agriculture has resulted in a significant decrease in the soil MBC, MBN, MBP, and SMQ compared to their corresponding values in grassland. The soil MBC content significantly was increased at the 0–10 cm soil layer after agriculture abandonment, but the SMQ had no significant improvement in the entire profile when compared to cropland. Besides, for the three types of land-use patterns, the C, N, and P stoichiometry of the soil microbial biomass was relatively stable, except for the stoichiometry in the relatively deep soil layer. Overall, the present agricultural abandonment (~ 19 years) got a certain of restoration in soil microbial biomass. Their corresponding values, however, were significantly lower than those found in native grassland. This highlighted that the restoration of the poor soil quality caused by the long term agriculture cultivation is a slow and hard process; it is important to maintain natural grasslands for protecting and maintaining soil microbial biomass and SMQ in the meadow steppe of northeast China.

Effects of Rust on Plant Growth and Stoichiometry of Leymuschinensis under Different Grazing Intensities in Hulunber Grassland

Grazing is the main utilization of native grassland, and forage fungal disease is one of the limiting factors of grassland productivity. The present research in the Hulunber meadow steppe grassland was conducted to investigate the responses of the dominant plant Leymus chinensis (Trin.) to beef cattle grazing, rust, and their interaction influence. Six grazing intensity treatments with three replicates were established. The response of L. chinensis to grazing and rust was systematically studied for two consecutive years. The main findings were that grazing and rust had significant effects (p < 0.05) on the growth and nutrient elements content of L. chinensis. Compared with the 0 cattle ha−1 treatment, the dry matter of L. chinensis in the 0.42, 0.63, and 1.67 cattle ha−1 treatments decreased by 42.2%, 90.5%, and 339.5%, respectively. Compared with non-infected plants, dry matter of rust-infected L. chinensis plants decreased by 45.6%. The N:C and P:C ratios of rust-infected plants were lower than in non-infected plants, and positively correlated with their relative growth rates. Therefore, we concluded that the growth rate hypothesis still applied in L. chinensis under the interactive effects of grazing and disease. Additionally, grazing can alleviate the loss of dry matter caused by disease.

Soil water retention capacity surpasses climate humidity in determining soil organic carbon content but not plant production in the steppe zone of Northern China

Soil hydraulic properties determine water transport, distribution and storage in soils, thus affect ecosystem functioning. Soil water retention parameters, such as saturation water content (SAT), water drained upper limit (DUL) and lower limit (DLL), characterize soil water-holding capacity, thus are important for plant growth and carbon and nutrient cycling in grassland ecosystem. However, we are still unclear to what a content these soil water retention parameters may regulate plant production and soil organic matter content in natural ecosystems in semi-arid grassland environment. We investigated plant species richness (SR) and aboveground biomass (AGB), soil organic carbon (SOC) and nitrogen (SON) contents, and soil water retention and other physical parameters in the steppe ecosystems along a climate transect, which spans the desert steppe, typical steppe and meadow steppe zones in Northern China. We analyzed the relative importance of climate factors, soil water retention parameters and other soil physical parameters in regulating plant AGB and SOC and SON contents using partial correlation analysis, and quantified the contribution of each separate and combined climate and soil physical parameters in predicting plant AGB and SOC and SON contents using geographic detectors method. We found that (i) plant SR and AGB, soil SAT and DUL, and SOC and SON are all significantly inter-correlated, and increased with the increase of climate humidity. (ii) Climate, but not soil physical parameters, was the most preponderant factor in predicting plant SR and AGB. (iii) Soil physical parameters were more important than climate parameters in regulating soil organic matter content; soil DUL was the best predictor to SOC content. Our results suggest that soil water retention parameters especially soil DUL, may play a greater role than climate factors in modulating SOC content, though climate is more predominant in regulating plant SR and production. Our results imply that we need to be cautious in estimating SOC content based on climate factors and vegetation biomass or remotely-sensed vegetation indices, and that soil physical properties need also to be incorporated along with vegetation parameters for improving SOC estimation at a regional scale.

Soil Microbial Community Varied with Vegetation Types on a Small Regional Scale of the Qilian Mountains

Clarifying the response of soil microbial communities to the change of different vegetation types on a small regional scale is of great significance for understanding the sustainability of grassland development. However, the distribution patterns and driving factors of the microbial community are not well understood in the Qilian Mountains. Therefore, we characterized and compared the soil microbial communities underlying the four vegetation types in a national natural reserve (reseeded grassland, swamp meadow, steppe meadow, and cultivated grassland) using high-throughput sequencing of the 16S rRNA and ITS. Meanwhile, the plant community and soil physicochemical characteristics were also determined. The results showed that bacterial and fungal communities in all vegetation types had the same dominant species, but the relative abundance differed substantially, which caused significant spatial heterogeneities on the small regional scale. Specifically, bacteria showed higher variability among different vegetation types than fungi, among which the bacterial and fungal communities were more sensitive to the changes in soil than to plant characteristics. Furthermore, soil organic carbon affected the widest portion of the microbial community, nitrate-nitrogen was the main factor affecting bacteria, and aboveground plant biomass was the main factor affecting fungi. Collectively, these results demonstrate the value of considering multiple small regional spatial scales when studying the relationship between the soil microbial community and environmental characteristics. Our study may have important implications for grassland management following natural disturbances or human alterations.

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.

Estimating the impact of climate change on the carbon exchange of a temperate meadow steppe in China

Temperate meadow steppe is a representative ecosystem in northeast China. A process-oriented biogeochemistry model, denitrification–decomposition (DNDC) biogeochemistry model was employed and adapted to interpret and integrate field observations to determine whether temperate meadow steppe ecosystem was a carbon dioxide (CO2) sink during the growing seasons from 2010 to 2011. Then, we applied the model to predict the long-term impacts of climate change on the carbon dynamics in the ecosystem. Daily weather data for 2010–2011 in conjunction with the soil properties data and management practices data for this site were utilized as inputs to simulate the grass growth and soil carbon dynamics. The modeled carbon fluxes were compared with eddy tower data. The observed and modeled CO2 fluxes data were consistent, with both showing that the temperate meadow steppe is a stable carbon sink. Prediction tests were conducted with a baseline and eight alternative climate scenarios for the period 2017–2050. Simulations for the years 2017–2050 found that (1) the temperate meadow steppe will be a carbon sink under the baseline climate conditions; (2) the decelerated warming and drier climate scenario produced the lowest grass production and lowest carbon sequestration; and (3) the accelerated warming and wetter climate scenario produced the highest biomass and highest carbon sequestration. By using DNDC model we can understand carbon dynamics in temperate meadow steppe. The ecosystem production is limited by precipitation, a wetter future climate would substantially elevate the carbon sequestration capacity of the ecosystem. However, the carbon sequestration potential could significantly decrease if the climate is more arid from 2017 to 2050.

Variation of plant CSR strategies across a precipitation gradient in the alpine grasslands on the northern Tibet Plateau

Identifying ecological strategies based on functional traits can help us better understand plants' adaptations and changes in ecological processes, and thus predict the impact of climate change on ecosystems, especially in the vulnerable alpine grasslands. Herein, we investigated the plant CSR strategies of four grassland types (alpine meadows, AM; alpine meadow steppes, AMS; alpine steppes, AS; and alpine desert steppes, ADS) and its functional groups (grasses, sedges, legumes, and forbs) along the east-to-west gradient of decreasing precipitation on the northern Tibetan grasslands by using Grime's CSR (C: competitor, S: stress tolerator, and R: ruderal) analysis. Although alpine grasslands were dominated by S-strategy, our results also indicated that AM with higher water, nitrogen (N) and phosphorus (P) availability had significantly lower S-strategy values and relatively higher C- and R-strategy values (C: S: R = 6: 63: 31 %) than those in AMS (C: S: R = 3: 94: 3 %,), AS (C: S: R = 3: 87: 10 %), and ADS (C: S: R = 1: 94: 5 %). The CSR strategy values of forbs and legumes showed greater variability compared with grasses and sedges in the environmental gradient. Furthermore, water variability on the precipitation gradient eventually affected plant traits and CSR strategies through soil N and P availability and pH. Our findings highlighted that plant CSR strategies were regulated by the availability of soil resources, and plants adopted more flexible adaptation strategies in relatively resource-rich environments. This study sheds light on the mechanisms of plant adaptation to the changing environment in the alpine grasslands.

Effect of nitrogen addition on leaf photosynthesis and water use efficiency of the dominant species Leymus chinensis (Trin.) Tzvelev in a semi-arid meadow steppe

Effect of nitrogen addition on leaf photosynthesis and water use efficiency of the dominant species Leymus chinensis (Trin.) Tzvelev in a semi-arid meadow steppe

Dynamics of aboveground phytomass in the communities of the Ob River forest-steppe and Northern Kulunda during protective regime

The formation of the herbage of plant communities depends on the natural and climatic features of the area and the modes of their economic use. In 2013-2021, the comparison of dynamics of aboveground phytomass in steppe and meadow communities of Western Siberia at an absolute protective regime was carried out. The state of vegetation was studied on the territory of the Novosibirsk region on the example of two communities of the steppe meadow (the right-bank part of the Ob River forest-steppe) and two communities of the solonetzic steppe (the northern part of the Kulunda steppe). The vegetation cover was described according to the standard geobotanical techniques. It is shown that at an absolute protective regime of meadow communities, along with weather conditions, the considerable impact on the condition of the herbage is significantly affected by the excessive accumulation of plant litter, which reaches up to 57-77 % of the total aboveground phytomass. This leads to decrease in the density and mass of green shoots, while the overgrowth of weed plant species is often observed. The meteorological conditions of separate years have the main impact on formation of the aboveground phytomass of solonetzic steppes. Both in dry and highly humid years, the participation of grasses in the herbage decreases and the role of salt-tolerant forbs, as well as sagebrushs and especially Halimione verrucifera, increases.

Contrasting Responses of Vegetation Production to Rainfall Anomalies Across the Northeast China Transect

AbstractIncreases in rainfall variability due to climate change influence gross primary production (GPP) in drylands, with important consequences for terrestrial carbon budget. However, it is not well understood if GPP responds in heterogeneous or even contrasting ways along the temperate bioclimate gradients. Here, based on satellite observations from 2001 to 2020, we identified rainfall anomalies as the magnitude of rainfall beyond its mean of one standard deviation, and examined differential responses of GPP to rainfall anomalies across Northeast China Transect (NECT). We found wet anomalies caused major GPP increase in western arid part of NECT, and the magnitude of increased GPP declined gradually toward the eastern humid region. As a result, GPP increased in desert steppe while decreased in mesic forest in response to wet anomalies. On the contrary, dry anomalies reduced GPP across the ecosystems except for mesic forest. The meadow steppe in the transition region between semi‐arid and sub‐humid regions, which lacks of drought‐tolerant species, showed greater GPP decline in response to dry anomalies than desert and typical steppe in the arid region. The heterogeneous responses of GPP to wet and dry anomalies resulted in contrasting asymmetries in diverse biomes, showing positive asymmetry in drier region, that is, greater magnitude of GPP increase in response to wet anomalies than GPP decrease in response to dry anomalies, and negative asymmetry in wetter region, with a transition between arid and semi‐arid regions. Our findings highlight that increased rainfall variability will enhance heterogeneous and even contrasting GPP changes along temperate bioclimate gradients.

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.

Diet selection of sheep shifted from quality to quantity characteristics of forages as sward availability decreased

Understanding the dynamic inter-relationship between grazing animals and the pasture sward is critical for sustainable grazing management. A field study was conducted to investigate the relationships between the quantity and quality characteristics of forages and diet selection of 30-kg Mongolian ewes in different seasons in Bromus inermis improved meadow steppe. Using a residual herbage mass method (1 000 kg/ha in spring, 800 kg/ha in summer and 600 kg/ha in autumn) to adjust stocking rate, three seasonal rest grazing strategies (spring rest, summer rest and autumn rest) combined with continuous grazing were studied. In each season, diet selection of sheep, quantified by diet composition and selectivity index, was estimated using a plant wax marker technique. Quantity (dry weight proportion, coverage, frequency, density and height) and quality (CP, NDF, ADF and DM digestibility) characteristics of forages consumed by sheep were determined simultaneously. Our results showed that in spring with the highest sward availability (herbage mass), diet selection of sheep could be predicted by the positive linear relationship between quality characteristics of forages and their selectivity index. In summer, the diet selection could be predicted by both positive linear relationship between quality characteristics and selectivity index, and the relationship between quantity characteristics and diet composition. While in autumn with the lowest sward availability, it could be predicted by the positive linear relationship between quantity characteristics and diet composition. During the whole grazing season, the diet composition of B. inermis (40.4%) and Leymus chinensis (35.1%) were larger than that of Potentilla bifurca (9.5%) and Carex (15.1%) and the selectivity index of P. bifurca was the highest (0.62), followed by Carex (0.17) and B. inermis (0.05), and L. chinensis (−0.29) was the lowest. The two parameters of diet selection above were positively related to their quantity and quality characteristics, respectively. These results suggested that the influence of quality characteristics on diet selection of sheep decreased gradually as a result of the decline in sward availability, while the influence of quantity characteristics increased. This study provides a comprehensive understanding of diet selection of sheep examining the trade-offs between quantity and quality characteristics of forages. The knowledge of diet selection of sheep and the corresponding prediction regressions acquired in this study could give the basis for designing appropriate grazing management strategies.

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.

Pollen–vegetation relationship based on lake surface sediments of arid and semi-arid northern China; considerations and limitations

In this paper, we investigated pollen assemblages of 92 lake center surface sediments of arid and semi-arid northern China to statistically disclose the pollen–vegetation relationships at the regional scale and explore considerations and limitations for lacustrine pollen-based reconstructions. As a result, the Principal component analysis (PCA), the Indicator species analysis, the Discriminate analysis (DA), the Random forest (RF), and the Supportive vector machine (SVM) generally reveal positive correlations between pollen assemblages and vegetation types or ecotones and suggests capabilities of pollen to represent regional vegetation and climate. However, the DA, RF, and SVM reveal that the samples of desert steppe, typical steppe, and meadow steppe cannot be differentiated by pollen assemblages, suggesting weaknesses to represent regional vegetation and climate. Mental test analyses of pollen assemblages and azonal vegetation of the dune desert and the Gobi desert yield better correlations than that of regional vegetation, suggesting that the pollen of azonal vegetation distort the regional pollen–vegetation relationships and thus weaken the capabilities to represent regional vegetation and climate. Our results present considerations and limitations for pollen-based past vegetation and climate reconstructions and provide references for interpretations of lacustrine fossil pollen records in arid and semi-arid northern China.