Degraded Alpine Meadow Research Articles

Degradation shifts plant communities from S- to R-strategy in an alpine meadow, Tibetan Plateau

The replacement of dominant sedges/grasses with secondary forbs is common in alpine rangelands, but the underlying plant ecological strategies and their relevance to leaf traits and their variabilities of different plant functional groups remain largely unknown. Here, we measured key leaf traits and analyzed the competitor, stress-tolerator and ruderal (CSR) strategies of major species with different functional groups (sedges, grasses and forbs) in an alpine meadow along a degradation gradient on the Tibetan Plateau. Our results indicated that S-selected species were dominant in both non-degraded (C:S:R = 1:95:4%) and severely degraded (C:S:R = 2:87:11%) meadows. However, there was a shift from S- to R-strategy in the communities after rangeland degradation. More specifically, sedges and grasses with a “conservative” strategy maintained stronger S-strategy to tolerate degraded and stressful conditions. In contrast, forbs with an “opportunistic” strategy (increase 9.5% in R-score) tended to adapt to degraded stages. Moreover, 51.1% and 23.9% of the increased R-scores in forbs were accounted by leaf mass per area and specific leaf area, respectively. Generally, higher leaf water and nitrogen contents coupled with larger variations in leaf traits and flexible SR strategies in forbs enabled them to capitalize on lower soil water and nutrient availability. Our findings highlighted that the contrasting strategies of plant species in response to the decrease in available resources might lead to niche expansion of secondary forbs and loss of diversity in the degraded alpine meadow. The emerging alternative stable states in the degraded rangelands might bring about a predicament for rangeland restoration.

Meadow degradation increases spatial turnover rates of the fungal community through both niche selection and dispersal limitation

The alpine meadow ecosystem, as the main ecosystem of the Qinghai-Tibet Plateau, has been heavily degraded over the past several decades due to overgrazing and climate change. Although soil microorganisms play key roles in the stability and succession of grassland ecosystems, their response to grassland degradation has not been investigated at spatial scale. Here, we systematically analyzed the spatial turnover rates of soil prokaryotic and fungal communities in degraded and undegraded meadows through distance-decay relationship (DDR) and species area relationship (SAR), as well as the community assembly mechanisms behind them. Although the composition and structure of both fungal and prokaryotic communities showed significant changes between undegraded and degraded meadows, steeper spatial turnover rates were only observed in fungi (Degraded Alpine Meadow β = 0.0142, Undegraded Alpine Meadow β = 0.0077, P < 0.05). Mantel tests indicated that edaphic variables and vegetation factors showed significant correlations to the β diversity of fungal community only in degraded meadow, suggesting soil and vegetation heterogeneity both contributed to the variation of fungal community in that system. Correspondingly, a novel phylogenetic null model analysis demonstrated that environmental selection was enhanced in the fungal community assembly process during meadow degradation. Interestingly, dispersal limitation was also enhanced for the fungal community in the degraded meadow, and its relative contribution to other assembly process (i.e. selection and drift) showed a significant linear increase with spatial distance, suggesting that dispersal limitation played a greater role as distance increased. Our findings indicated the spatial scaling of the fungal community is altered during meadow degradation by both niche selection and dispersal limitation. This study provides a new perspective for the assessment of soil microbial responses to vegetation changes in alpine areas.

Evapotranspiration and Its Partitioning in Alpine Meadow of Three-River Source Region on the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau (QTP) is generally considered to be the water source region for its surrounding lowlands. However, there have only been a few studies that have focused on quantifying alpine meadow evapotranspiration (ET) and its partitioning, which are important components of water balance. This paper used the Shuttleworth–Wallace (S–W) model to quantify soil evaporation (E) and plant transpiration (T) in a degraded alpine meadow (34°24′ N, 100°24′ E, 3963 m a.s.l) located at the QTP from September 2006 to December 2008. The results showed that the annual ET estimated by the S–W model (ETSW) was 511.5 mm (2007) and 499.8 mm (2008), while E estimated by the model (ESW) was 306.0 mm and 281.7 mm for 2007 and 2008, respectively, which was 49% and 29% higher than plant transpiration (TSW). Model analysis showed that ET, E, and T were mainly dominated by net radiation (Rn), while leaf area index (LAI) and soil water content at a 5 cm depth (SWC5cm) were the most important factors influencing ET partitioning. The study results suggest that meadow degradation may increase water loss through increasing E, and reduce the water conservation capability of the alpine meadow ecosystem.

Root dynamics along a restoration chronosequence of revegetated grasslands in degraded alpine meadows of the Qinghai‐Tibetan Plateau, China

AbstractRevegetation using perennial grass, such as Elymus nutans is an effective approach in abating grassland degradation on the Qinghai‐Tibetan Plateau. However, knowledge of root dynamics along restoration chronosequences of revegetated grasslands are limited. To gain better insight into root dynamics, we used minirhizotrons to examine changes in root traits, including root standing crop, production, mortality, and turnover along a restoration chronosequence of three revegetated grasslands (cultivated in 2002, 2006, and 2010 corresponding to late, middle, and early recovery stage) in four consecutive growing seasons (2014, 2015, 2016, and 2017). We found that the aboveground plant biomass was higher in the middle recovery stage than that in the early and late recovery stages, while plant species richness showed an increasing trend from the early to late recovery stage. Similar with the plant biomass response, the belowground root standing crop and production were significantly higher in the middle recovery stage than in the early and late recovery stages. Root mortality was decreased and root turnover tend to be stable from the early to late recovery stage. Root turnover was positively correlated with soil NO3−‐N content and negatively correlated with aboveground plant biomass. Moreover, root standing crop and mortality increased from May to September across all growing seasons; however, root production decreased within this period. Our study highlighted significant links between aboveground and belowground plant parts that may help effectively manage revegetated grassland on the Qinghai‐Tibetan Plateau.

Suitable duration of grazing exclusion for restoration of a degraded alpine meadow on the eastern Qinghai-Tibetan Plateau

Grazing exclusion (GE) is a key national ecological restoration project widely applied to rehabilitate degraded grasslands. To date, there have been many debates on the effectiveness of GE for grassland recovery, and it is still poorly understood how degraded alpine meadows in the semi-humid area with relative rich rainfall in the Qinghai-Tibetan Plateau respond to GE with different durations. We selected a chronosequence of grazing-excluded alpine meadows to examine the dynamics of grassland functions. The results showed that the dominant functional group shifted from forbs to graminoids after 8 to 10 years GE and then returned to forbs. The plant population density increased significantly with GE time (P < 0.01). The total biomass, species richness, Shannon diversity index, and relative importance value of graminoids first increased and then decreased with increasing plant population density, whereas the relative importance value of forbs presented the opposite trend. All turning points were between 1350 and 1650 plants per m2 which occurred from 8 to 10 years of GE. As the duration of GE increased, the ecosystem multifunctionality first dramatically increased and then became steady after 8 to 10 years of GE. These findings imply that GE is of great practical significance to prevent degradation in semi-humid regions of the Qinghai-Tibetan Plateau, where we recommend that GE should cease after approximately 8 to 10 years to restore degraded alpine meadows from the perspective of plant productivity, diversity, community structure, and ecosystem multifunctionality.

Grazing Exclusion Changed the Complexity and Keystone Species of Alpine Meadows on the Qinghai-Tibetan Plateau

Grazing exclusion is an effective approach to restore degraded grasslands. However, the effects of grazing exclusion on keystone species and the complexity of plant community were poorly investigated. Here, we conducted a field survey among different grazing exclusion durations, i.e., Grazing, grazing exclusion below 5 years, grazing exclusion with 5 years, grazing exclusion with 7 years, and grazing exclusion over 7 years, in alpine meadows on the central Qinghai-Tibetan Plateau (QTP). The complexity and keystone species of alpine meadows were analyzed by a network analysis. The results showed the following: (1) The species richness did not change, but aboveground biomass and the coverage of the plant community tended to increase with the extension of the grazing exclusion duration. (2) The soil nutrients, i.e., total nitrogen, total organic carbon, available nitrogen, and available potassium, remained stable, while the soil bulk density decreased under grazing exclusion conditions. (3) There was a hump-shaped change of the complexity (i.e., average connectivity and average clustering coefficient) of the plant community along with the extension of the grazing exclusion duration. Moreover, the keystone species were different among the grazing exclusion treatments. Based on the complexity of the plant community and the changes of keystone species, the optimum duration of grazing exclusion for alpine meadows should be between 5 and 7 years. Our results suggest that besides the productivity, the change of the complexity and keystone species of plant community should be considered when grazing exclusion is adopted to restore the degraded alpine meadows.

Mapping of Kobresia pygmaea Community Based on Umanned Aerial Vehicle Technology and Gaofen Remote Sensing Data in Alpine Meadow Grassland: A Case Study in Eastern of Qinghai–Tibetan Plateau

The Kobresia pygmaea (KP) community is a key succession stage of alpine meadow degradation on the Qinghai–Tibet Plateau (QTP). However, most of the grassland classification and mapping studies have been performed at the grassland type level. The spatial distribution and impact factors of KP on the QTP are still unclear. In this study, field measurements of the grassland vegetation community in the eastern part of the QTP (Counties of Zeku, Henan and Maqu) from 2015 to 2019 were acquired using unmanned aerial vehicle (UAV) technology. The machine learning algorithms for grassland vegetation community classification were constructed by combining Gaofen satellite images and topographic indices. Then, the spatial distribution of KP community was mapped. The results showed that: (1) For all field observed sites, the alpine meadow vegetation communities demonstrated a considerable spatial heterogeneity. The traditional classification methods can hardly distinguish those communities due to the high similarity of their spectral characteristics. (2) The random forest method based on the combination of satellite vegetation indices, texture feature and topographic indices exhibited the best performance in three counties, with overall accuracy and Kappa coefficient ranged from 74.06% to 83.92% and 0.65 to 0.80, respectively. (3) As a whole, the area of KP community reached 1434.07 km2, and accounted for 7.20% of the study area. We concluded that the combination of satellite remote sensing, UAV surveying and machine learning can be used for KP classification and mapping at community level.

Plant nitrogen concentration is more sensitive in response to degradation than phosphorus concentration in alpine meadow

Exploring the early warning signals of plant and soil characteristics in response to degradation is of great theoretical significance to illuminate the underlying driving mechanism of alpine meadow degradation. Here, in alpine meadow of Damxung County on the Tibetan Plateau, a field survey along degradation gradients was conducted to measure plant and soil properties: from non-degradation to severely degradation, simultaneous impairments in plant total biomass, plant coverage, plant N concentration, soil moisture, soil total C and N were detected, which significantly (P<0.05) decreased by 85.36%, 54.30%, 25.23%, 26.95%, 28.94% and 12.85% in sequence, while no significant change in plant P concentration was observed. As grassland degradation increased, a consistent decrease of plant N: P ratios (<14) not only demonstrated that N was a strong limiting factor for plants growth, but also supported our hypothesis that plant N concentration was more sensitive in response to degradation than plant P concentration. Path analyses further indicated that soil moisture and soil total nitrogen were the most critical factors that mediated the plant N: P in response to degradation. Although our findings suggested that N resorption efficiency was improved after grassland degradation, the increase in grassland degradation would enhance the N concentration decoupling from P concentration, which in turn might further exacerbate the N limitation and the degradation in alpine ecosystems. The early-warning signals of grassland degradation obtained here not only can improve the understanding of grassland degradation, but also can provide guidance for the restoration and management of degraded alpine meadow on the Tibetan Plateau.

Nutrients available in the soil regulate the changes of soil microbial community alongside degradation of alpine meadows in the northeast of the Qinghai-Tibet Plateau

The alpine meadow in the Qinghai-Tibet Plateau has been seriously degraded due to human activities and climate change in recent decades. Understanding the changes of the soil microbial community in response to the degradation process helps reveal the mechanism underlying the degradation process of alpine meadows. We surveyed and analyzed changes of the vegetation, soil physicochemical properties, and soil microbial community in three degradation levels, namely, non-degradation (ND), moderate degradation (MD), and severe degradation (SD), of the alpine meadows in the northeastern Qinghai-Tibet Plateau. We found that as the level of degradation increased, plant cover, plant density (PD), above-ground biomass (AGB), plant Shannon-Wiener index (PS), soil water content (SWC), soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP), and available potassium (AK) decreased significantly, while the soil pH increased from 7.20 to 8.57. Alpine meadow degradation significantly changed the composition of soil bacterial and fungal communities but had no significant impact on the diversity of the microbial communities. Functional predictions indicated that meadow degradation increased the relative abundances of aerobic_chemoheterotrophy, undefined_saprotroph, and plant_pathogen, likely increasing the risk of plant diseases. Redundancy analysis revealed that in ND, the soil microbial community was mainly regulated by PS, PH, PD, SWC, and soil pH. In MD, the soil microbial community was regulated by the soil's available nutrients and SOC. In SD, the soil microbial community was not only regulated by the soil's available nutrients but also influenced by plant characteristics. These results indicate that during alpine meadow degradation, while the changes in the plants and soil environmental factors both affect the composition of the soil microbial community, the influence of soil factors is greater. The soil's available nutrients are the main driving factors regulating the change in the soil microbial community's composition alongside degradation levels.

Soil degradation influences soil bacterial and fungal community diversity in overgrazed alpine meadows of the Qinghai-Tibet Plateau

Over half of the alpine meadows in the Qinghai-Tibet Plateau (QTP) are degraded due to human activities. Soil degradation from overgrazing is the most direct cause of grassland degradation. It is thus important to synthesize the effects of multiple soil degradation indicators on the belowground biomass of plants and soil microorganisms in the degraded QTP. We studied the diversities and structures of soil bacterial and fungal communities using soil bacterial 16S rRNA and the fungal ITS gene under four degradation gradients, D1: lightly degraded, D2: moderately degraded, D3: highly degraded, and a non-degraded control site (CK). The bacterial Shannon diversity in D3 was significantly lower than that in D1 (p < 0.001), and the bacterial richness index in D3 was significantly lower than that in D1 (p < 0.001). There was no difference in soil fungal diversity among the different degradation levels; however, soil fungal richness decreased significantly from CK to D3. The phyla Actinobacteria, Acidobacteria and the genus Mortierella were differed significantly under the four degradation gradients. Plant litter mass and root C/N ratio were important factors associated with bacterial and fungal diversity and richness. These results indicated that alpine meadow degradation can lead to variations in both microbial diversity and the potential functioning of micro-organisms in the QTP.

Changes in plant communities, soil characteristics, and microbial communities in alpine meadows degraded to different degrees by pika on the Qinghai–Tibetan Plateau

Alpine meadows, the main natural grassland on the Qinghai–Tibet Plateau, are the material basis of plateau meadow husbandry and ecological barrier. However, interference by pika (Ochotona curzoniae) has caused the degradation of alpine meadows to varying degrees. Knowledge about the vegetation, soil characteristics, and microbial community structure and function following degradation is limited. Thus, we studied the changes in vegetation, soil characteristics, and microbial communities in alpine meadows in Qinghai under different degrees of pika-induced degradation (non-degraded “CK”; lightly degraded “LD”; moderately degraded “MD”; and heavily degraded “HD”). The results showed coverage and biomass of the vegetation and species diversity decreased as the degree of degradation increased, and the dominant vegetation changed from members of the family Gramineae to poisonous plants. As the degree of degradation increased, total nitrogen (TN), available nitrogen (AN), and organic matter (OM) in the soil increased first and then decreased, while total potassium (TK), available potassium (AK), and available phosphorus (AP) increased. Among alpine meadows with different degrees of degradation, the alpha diversity of bacteria and fungi in the LD plot were significantly lower than that of CK, MD, and HD. Hierarchical clustering results showed that the composition of bacteria in the MD and HD plots was similar, and the classification of fungi among samples was more obvious. Results from a LEfSe showed that the majority of biomarker bacteria were found in the LD plot, but the majority of biomarker fungi were found in the MD plot. In addition, Tax4Fun and FUNGuild prediction analyses showed that compared with other plots, bacteria causing human infectious diseases were significantly higher in MD, there were more saprotrophic fungi in CK, and there was a significant increase in pathogenic fungi in MD and HD. Thus, pika-induced degradation affected vegetation, soil, and microbial community structure and function in the Qinghai–Tibetan Plateau, with fungal communities responding more strongly to the degradation than bacterial communities.

Fractal and multifractal analysis of soil particle-size distribution and correlation with soil hydrological properties in active layer of Qinghai–Tibet Plateau, China

The soil hydrological properties (HPs) in active layers regulate soil water migration and affect the evolution of permafrost and biogeochemical cycles. There is relatively little understanding of the changes in soil HPs as a result of soil particle-size distribution (PSD) in the permafrost regions of the Qinghai–Tibet Plateau. In this study, different land-cover types, namely alpine wet meadow (AWM), extremely degraded wet meadow (EDWM), alpine meadow (AM), degraded alpine meadow (DAM), alpine steppe (AS), and alpine desert (AD), were used to determine the relationships between soil PSD and HPs. Soil HPs were determined using borehole nuclear magnetic resonance, and soil PSD (0–2 mm) was analyzed using fractal and multifractal theory. The results showed that vegetation degradation caused coarse soil particle accumulation in the root zone, reflected by the soil single-fractal dimension (Dv: 2.12–2.50). The Dv values showed significant positive correlations with soil clay and silt content as well as a negative correlation with sand content. Results from the generalized fractal and singularity spectra suggested an even distribution of soil particles in the root zone of the AWM, AM, and AD sites. Vegetation degradation increased the soil PSD capacity dimension (D0), information dimension (D1), correlation dimension, D1/D0 ratio, and Hölder exponent of order zero. This was indicative of a narrow soil PSD range and non-uniform soil texture in the root zone. Although capillary water dominated the total soil water content in the alpine soils, soil water composition was also dependent on the vegetation type and soil stratigraphic distribution. Soil particle composition and multifractal dimensions were found to be effective parameters for forecasting soil HPs in the non-root zone of alpine soils. However, the presence of vegetation weakens the relationship between soil PSD and HPs. This study contributes to the accurate evaluation of hydrological and biogeochemical cycles in permafrost regions.

Restoration of Degraded Alpine Meadows Improves Pollination Network Robustness and Function in the Tibetan Plateau

Ecological restoration is widely used to mitigate the negative impacts of anthropogenic activities. There is an increasing demand to identify suitable restoration management strategies for specific habitat and disturbance types to restore interactions between organisms of degraded habitats, such as pollination. In the Tibetan Plateau, alpine meadows have suffered severe degradation due to overgrazing and climate change. Protecting vegetation by fencing during the growing season is a widely applied management regime for restoration of degraded grasslands in this region. Here, we investigated the effect of this restoration strategy on plant–pollinator communities and plant reproduction in the eastern Tibetan Plateau. We collected interaction and seed set data monthly across three grazed (grazed all year) and three ungrazed (fenced during growing season) alpine meadows in growing seasons of two consecutive years. We found ungrazed meadows produced more flowers and attracted more pollinator visits. Many common network metrics, such as nestedness, connectance, network specialization, and modularity, did not differ between grazing treatments. However, plants in ungrazed meadows were more robust to secondary species extinction than those in grazed meadows. The observed changes in the networks corresponded with higher seed set of plants that rely on pollinators for reproduction. Our results indicate that protection from grazing in growing seasons improves pollination network stability and function and thus is a viable restoration approach for degraded meadows.

Grazing-to-fencing conversion affects soil microbial composition, functional profiles by altering plant functional groups in a Tibetan alpine meadow

Information on the effect of anthropogenic activities on soil microbial communities can improve our ability to formulate restoration strategies in the Tibetan Plateau, where meadow degradation has been an increasing concern. In this study, we evaluated plant functional groups, soil properties, microbial communities, and their functional groups in four types of meadows (grazed, fenced, fenced + reseeded, and intact) in the Tibetan Plateau. Based on amplicon sequencing data, both the bacterial and fungal composition, their potential functions and their possible drivers have been investigated. Fencing was the most effective approach to restore degraded alpine meadows, characterized by higher aboveground biomass, soil nutrient levels, and microbial diversity. The soil carbon, ammonium nitrogen, and the above- and below-ground biomass (mainly sedge and grass groups) were closely related to the changes in bacterial and fungal composition, while the diversity of sedge, grass, legumes, and forbs was largely associated with N cycling and fungal functional guilds. Grazing-to-fencing conversion also caused changes in the microbial community composition, specifically resulting in a shift from oligotrophs to copiotrophs, an increased abundance of diazotrophs and plant pathogens, and a decreased abundance of nitrifiers. Our results revealed the effects of grazing-to-fencing conversion on an alpine meadow and can provide significant guidance for the sustainable management of degraded grassland ecosystems.

Artificial Measures Are Not Necessarily Better Than Natural Recovery for the Extremely Degraded Alpine Meadow: The Results of Simulated Degradation Restoration After Three Years

Natural and artificial restoration measures are widely used to restore degraded ecosystems, such as degraded alpine meadow. The objective of this research was to evaluate the advantages and disadvantages of natural and artificial measures for extremely degraded alpine meadows. We removed the surface soil (0–10 cm) of the alpine meadow to simulate the extremely degraded “black soil beach,” and set artificial measures (planting Festuca sinensis (E) and Elymus sibircus L. cv. chuan-cao No. 1 (F)) and natural recovery (N) (without any artificial auxiliary measures) in the northeastern part of the Qinghai-Tibet Plateau (QTP), China. After 3 years, we determined the characteristics of community and soil in the artificial and natural treatment. The results show that the species number, above-and below-ground biomass (AB, BB), root-shoot ratio (R/S) in N is significantly higher than that in artificial restoration (E and F); while the community coverage and concentration of soil total carbon, total nitrogen, microbial biomass carbon, microbial biomass nitrogen and microbial biomass phosphorus (TC, TN, MBC, MBN and MBP) in artificial restoration is significantly higher than that in N. In conclusion, compared with N, artificial measures (E and F) are not completely beneficial to the development of plant community diversity and the restoration of soil nutrients in the extremely degraded meadow. Thus, the establishment of artificial grassland is not necessarily better than natural recovery for the extremely degraded alpine meadow.

The characteristics of soil fungal community in degraded alpine meadow in the Three Rivers Source Region, China.

To clarify the variation of species composition, diversity, and functional structure of soil fungi community along alpine meadow degradation,we examined the characteristics and controlling factors of soil fungal communities in non-degraded, lightly degraded, moderately degraded, severely degraded and extremely degraded (black soil beach) alpine meadows at the Three Rivers Source Region, based on the high-throughput gene sequencing and FUNGuild functional prediction. The results showed that the dominant phyla in alpine meadow soil were Ascomycota, Basidiomycetes, and Mortierellomycota. Species composition of soil fungal community varied greatly in alpine meadow under different levels of degradation. The abundance of Cladosporium flabelliforme, Entoloma sodale, Hygrocybe conica, Inocybe sp. and Trichocladium opacum increased, while that of Gibberella tricincta and Dactylonectria macrodidyma decreased following grassland degradation. The meadow under severe degradation had higher soil fungal Chao1 index, while that under light degradation had lower Shannon index and Simpson index. The abundance of pathologic, symbiotic, and saprophytic types of fungi varied among different alpine meadows. Along with the grassland degradation, the abundance of soil symbiotic fungi decreased, while that of pathological fungi increased. The soil fungal community and functional compositions changed obviously with degradation in the alpine meadow. Plant aboveground biomass, soil water content, pH, total organic carbon, total nitrogen, ammonium nitrogen, available phosphate, total potassium and AN/AP (ratio of available nitrogen and available phosphorus) were the main driving factors for the variations in soil fungal community structure.

Changes in soil organic carbon content and their causes during the degradation of alpine meadows in Zoig&amp;#x000ea; Wetland

Changes in soil organic carbon content and their causes during the degradation of alpine meadows in Zoig&amp;#x000ea; Wetland

土壤蒸发和植被蒸腾对三江源退化高寒草甸蒸散的影响

蒸散（ET）主要由土壤蒸发（E）和植被蒸腾（T）组成，然而难以把E与T从陆地生态系统ET中区分开。为阐明位于青海省境内的三江源区（89°24'—102°23'E，31°39'—36°16'N）高寒草甸E和T对生态系统ET的影响，利用小型蒸渗仪和微气象系统定量研究了三江源退化高寒草甸ET、E和T的变化，以及植被和环境因子对其的影响。结果表明：2017和2018年的ET分别为467.7 mm和479.2 mm，其中生长季（5—9月）约占72%，且E对生态系统ET的贡献（56%）大于T（44%），年降水量（P）的90%以上通过ET返回大气（ET/P > 90%）。根据生长季中不同植被覆盖度的蒸渗仪观测结果发现，ET随植被覆盖度的降低而增加。逐步回归分析表明，净辐射（R_n）是驱动生态系统ET、E、T最主要的因子；另外，E对饱和水汽压差（VPD）的响应更敏感，而T受空气温度（T_a）的影响更大；土壤含水量（SWC₅）对蒸散的影响相对较小，可能由于研究区降水相对较多的原因。结果说明，草甸退化将加剧土壤蒸发，进而导致生态系统散失更多的水分。;Evapotranspiration (ET) is mainly composed of evaporation (E) and transpiration (T), however it is often difficult to separate E and T from ET in a terrestrial ecosystem. To better understand the effects of E and T on ET of alpine meadow in the Three-River Source Region (TRSR) (89°24'—102°23'E, 31°39'—36°16'N) in Qinghai Province, lysimeter and micrometeorological measurements were used to quantitatively study the variations of ET, E and T, and effects of vegetation and environmental factors on evapotranspiration processes. The results showed that the ET in 2017 and 2018 was 467.7 mm and 479.2 mm, respectively, and ET during the growing season (May-September) accounted for about 72% of annual total ET, while the contribution of E to ET (56%) was greater than that of T (44%). Furthermore, more than 90% of annual precipitation was returned to the atmosphere through the ET processes (ET/P > 90%). Based on the results from lysimeters with different vegetation coverages during the growing season, it was found that ET increased with the decreasing of vegetation coverage. The results of stepwise regression analysis indicated that net radiation (R_n) was the most important factor driving ecosystem ET, E and T; in addition, the response of E to vapor pressure deficit (VPD) was more sensitive than T, while the air temperature (T_a) was more significantly associated with T in comparison to the E. The effect of soil water content (SWC₅) on ET was relatively small maybe due to the precipitation was relatively abundant in this alpine meadow. The results suggested that the degradation of alpine meadow would lead to the increase of E, as a result more water is lost from the ecosystem through evapotranspiration.

生长季放牧对高寒草甸传粉网络的影响

植物-传粉者相互作用面临人类活动的威胁。在青藏高原地区，放牧是一项主要的人类活动干扰，过度放牧导致高寒草甸植被严重退化。然而在该区域放牧如何影响植物-传粉者相互作用网络还不得而知。在青藏高原东部的高寒草甸选取了两个研究样点，每个样点包括一块禁牧样地（生长季禁牧）和放牧样地（全年放牧）。在2016年至2018年每年的生长季（7月和8月）进行了连续3年观测，共构建16个传粉网络。结果发现，在研究区域的高寒草甸生态系统中，传粉者群落的物种组成以双翅目昆虫为主。放牧后双翅目、鳞翅目以及鞘翅目传粉者的种类数减少，但膜翅目传粉者的种类数未受到放牧的影响。放牧干扰显著降低了群落中植物、传粉者以及它们之间相互作用的多样性，但对传粉网络的嵌套性和特化程度（H₂'）没有显著影响，说明网络的稳定性和恢复力没有受到放牧的影响。探讨了放牧对传粉网络的影响，发现区域放牧强度过大，降低了传粉昆虫和传粉网络的多样性。未来需要进一步深入研究高寒草甸生态系统中放牧强度对传粉网络的影响模式，以期为合理的放牧制度模式提供理论依据。;Plant-pollinator interactions are threatened by human activities. In the Tibetan Plateau, grazing is one of the main anthropic disturbances, and overgrazing has led to degradation of alpine meadow. However, it is still unclear how grazing affects plant-pollinator interaction network of alpine meadows. Here we collected data from two locations in the eastern Tibetan Plateau to evaluate the effects of grazing on the pollination networks in alpine meadow. Each location comprised one ungrazed plot (fenced during growing season) and one grazed plot (grazed all year). We conducted surveys during growing seasons (July and August) in three consecutive years (2016-2018), and constructed 16 pollination networks in total. The plants-pollinators interactions were surveyed using timed observation method by cameras. We found that the pollinator communities of alpine meadow were dominated by Diptera species. The richness of Diptera, Lepidoptera and Coleoptera pollinators were decreased after grazing, while the richness of Hymenoptera pollinators was unaffected by grazing. Grazing reduced the diversity of plants, pollinators and their interactions. The generalism of plants was also decreased by grazing, but the nestedness and specialization (H₂') of networks were similar between treatments, indicated that the stability and resilience of pollination networks are unaffected by grazing. Furthermore, the network metrics were similar between different months, and grazing had similar impacts on pollination networks in July and August. Our study firstly investigated the effect of the grazing on the pollination networks of alpine meadows in the Tibetan Plateau. The results highlighted that the grazing intensity of study area is exceeded the optimum intensity as it decreased the diversity of pollinators and pollination networks. Further studies should focus on how pollination networks response to grazing intensity in alpine meadow to find a reasonable grazing regime.

Effects of alpine meadow degradation on nitrifying and denitrifying microbial communities, and N

Meadow degradation is often accompanied by significant changes in nitrogen (N)-cycling and nitrous oxide (N2O) emission potential, and leads to challenges in meadow management. However, the mechanisms of soil N-cycling and N2O emissions remain poorly understood, especially in alpine ecosystems. In this study, we investigated the soil N-cycling process in four alpine meadows on the Tibetan Plateau along a degradation gradient using real-time quantitative polymerase chain reaction and amplicon sequencing to elucidate the mechanisms. Compared to non-degraded meadows, meadow degradation reduced N2O emissions by 38.5–140.2%. Meadow degradation reduced the abundance of amoA of ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB), whereas light and moderate degradation increased the abundance of genes nirS, nirK, and nosZ. Lightly degraded meadow exhibited the highest α-diversity of both nitrifiers and denitrifiers, but this higher diversity was not accompanied by higher N2O emissions, with only 32.3% of the microbial taxa identified as predictors of N2O emissions, suggesting that functional redundancy exists in the N-cycling process in meadow ecosystems. Nitrosospira and Mesorhizobium from the AOB and nirK communities, respectively, were identified as the key taxa that may contribute to N2O emissions. Soil properties, especially N reaction substrates, including ammonium-N, nitrate-N, dissolved organic N, and total N, were the primary drivers for N2O emissions via mediation of the N-cycling community, especially nitrifiers. Our results emphasised the importance of environmental factors in shaping nitrifying, denitrifying, and N2O emissions, providing insights for the restoration of degraded meadow ecosystems.