The alpine meadow ecosystem in Tibet is fragile and sensitive, and its carbon sink function with respect to climate change has become a matter of widespread concern. Therefore, this study aims to clarify the inter-annual variations (IAVs) in the carbon fluxes in an alpine meadow and to further quantify the contributions of the driving factors to the IAVs. Based on 7 years of flux data (2012–2018) and the corresponding climatic and biotic data, a set of look-up tables was used to separate and quantify the IAV sources. Furthermore, linear perturbation analyses were employed to quantify the contributions of each key factor. During 2012–2018, the net ecosystem productivity (NEP), gross primary productivity (GPP) and ecosystem respiration (Re) of this alpine meadow were 3.31 ± 26.90, 210.18 ± 48.35 and 206.88 ± 28.45 g C m⁻² y⁻¹, respectively, which indicated relatively large IAVs. When the contributions of climatic and biotic effects were distinguished and quantified, the dominant effects of biotic factors emerged. Additionally, negative interactions between climatic and biotic effects were detected. Among the climatic factors, only soil water content contributed relatively more to the IAVs and played a role in regulating the interactions between climatic and biotic effects. These results suggest that biotic effects must be carefully considered to reduce the uncertainties associated with future carbon flux estimates.

The increase in atmospheric nitrogen (N) deposition has profound effects on soil respiration (SR). However, the responses of SR to the addition of different N compounds, particularly in saline–alkaline grasslands remain unclear. A 3-year controlled field experiment was conducted to investigate the responses of SR to different N compounds (NH₄NO₃, (NH₄)₂SO₄ and NH₄HCO₃) during the growing seasons in a saline–alkaline grassland located in the agro-pastoral ecotone of northern China. Our results demonstrated that SR showed a bimodal pattern and a significant interannual difference that was regulated by air or soil temperature and precipitation. Nitrogen addition had a significant effect on SR, and the effect of N addition on SR varied yearly, which was related to seasonal precipitation. The mean SR across 3 years (2017–2019) was increased by 19.9%, 13.0% and 16.6% with the addition of NH₄NO₃, (NH₄)₂SO₄ and NH₄HCO₃, respectively. The highest effect of NH₄NO₃ addition on SR across 3 years was ascribed to the highest aboveground net primary production, belowground net primary production (BNPP) and soil NO₃⁻ concentrations. SR (C loss) was significantly increased while plant productivity (C input) did not significantly change under NH₄HCO₃ addition, indicating a decrease in C sequestration. In addition, BNPP was the main direct factor influencing SR in this saline–alkaline grassland, and soil salinization (e.g. soil base cations and pH) indirectly affected SR through soil microorganisms. Notably, NH₄NO₃ addition overestimated the response of SR to N addition, and different N compounds should be considered, especially in saline–alkaline grassland.

Fisher discriminant analysis can comprehensively take multiple factors into consideration and effectively conduct separations between two classes. If it can be used to detect the occurrences of drought, drought can be detected more effectively and accurately. Based on 9-year carbon flux and corresponding meteorological data, soil water content (SWC) and vapor pressure deficit (VPD) were selected as the discriminant factors. Drought occurrences were detected by applying the Fisher discriminant analysis method in an alpine ecosystem in Tibet. Fisher discriminant analysis was successfully applied to detect drought occurrence in an alpine meadow ecosystem. The soil water deficit and atmospheric water deficit were comprehensively taken into consideration. Consequently, this method could detect the onset and end date of droughts more accurately and reasonably. Based on the characteristics of drought and non-drought samples, the discriminant equation was constructed as y = 24.46SWC − 4.60VPD. When y > 1, the days were distributed above the critical line. In addition, when y was greater than one for more than 10 days, it was labeled as one drought event. If the interval between two drought processes was less than 2 days, it was considered one drought event. With increasing the study period and continued accumulation of observation data, the discriminant equation could be further optimized in the future, resulting in more accurate drought detection.

The Tibetan Plateau is generally referred to as the Chinese water tower, and evapotranspiration (ET) affects the water budget and stability of alpine meadows on the Tibetan Plateau. However, its variability and controlling mechanisms have not been well documented under the drier conditions induced by global warming. Therefore, this study aimed to clarify whether meteorological or biological factors primarily affected the variability in ET under contrasting water conditions in the alpine meadow ecosystem on the Tibetan Plateau. Based on 6-year (2013–2018) eddy covariance observations and the corresponding meteorological and biological data, linear perturbation analyses were employed to isolate the contributions of meteorological and biological factors to the variability in evapotranspiration (δET). The results showed that δET was mainly driven by meteorological factors in wet peak seasons (July and August), and was dominated by net radiation (Rn) and air temperature (Ta), indicating that the inadequate available energy is the factor limiting ET. However, the dominant factors affecting δET shifted from meteorological to biological in dry peak seasons when the canopy stomatal conductance (gs) and leaf area index were dominant. At this point, the ecosystem was limited by the water conditions. These results provide empirical insights into how meteorological and biological factors regulate variability in ET under contrasting water conditions. These findings can further improve our understanding of water cycle processes and can help effectively manage water resources in alpine meadow ecosystems under future climate change conditions.

Wetlands store large amounts of carbon stocks and are essential in both global carbon cycling and regional ecosystem services. Understanding the dynamics of wetland carbon exchange is crucial for assessing carbon budgets and predicting their future evolution. Although many studies have been conducted on the effects of climate change on the ecosystem carbon cycle, little is known regarding carbon emissions from the alpine wetlands in arid northwest China. In this study, we used an automatic chamber system (LI-8100A) to measure ecosystem respiration (ER) in the Bayinbuluk alpine wetland in northwest China. The ER showed a significant bimodal diurnal variation, with peak values appearing at 16:30 and 23:30 (Beijing time, UTC + 8). A clear seasonal pattern in ER was observed, with the highest value (19.38 µmol m⁻² s⁻¹) occurring in August and the lowest value (0.11 µmol m⁻² s⁻¹) occurring in late December. The annual ER in 2018 was 678 g C m⁻² and respiration during the non-growing season accounted for 13% of the annual sum. Nonlinear regression revealed that soil temperature at 5 cm depth and soil water content (SWC) were the main factors controlling the seasonal variation in ER. The diurnal variation in ER was mainly controlled by air temperature and solar radiation. Higher temperature sensitivity (Q₁₀) occurred under conditions of lower soil temperatures and medium SWC (25% ≤ SWC ≤ 40%). The present study deepens our understanding of CO₂ emissions in alpine wetland ecosystems and helps evaluate the carbon budget in alpine wetlands in arid regions.

The carbon and water cycle, an important biophysical process of terrestrial ecosystems, is changed by anthropogenic revegetation in arid and semiarid areas. However, there is still a lack of understanding of the mechanisms of carbon and water coupling in intrinsic ecosystems in the context of human activities. Based on the CO₂ and H₂O flux measurements of the desert steppe with the planted shrub Caragana liouana, this study explored the carbon and water flux coupling of the ecosystem by analyzing the variations in gross primary productivity (GPP), evapotranspiration (ET) and water use efficiency (WUE) and discussing the driving mechanisms of biological factors. The seasonal variation in climate factors induced a periodic variation pattern of biophysical traits and carbon and water fluxes. The GPP and ET fluctuated in seasons, but the WUE was relatively stable in the growing season. The GPP, ET and WUE were significantly driven by global radiation (R_g), temperature (T_a and T_s), water vapor pressure deficit, leaf area index and plant water stress index (PWSI). However, R_g, temperature and PWSI were the most important factors regulating WUE. R_g and temperature directly affected WUE with a positive effect but indirectly inhibited WUE by rising PWSI. Plant water stress inhibited photosynthesis and transpiration of the planted shrub community in the desert steppe. When the plant water stress exceeded a threshold (PWSI >0.54), the WUE would decrease since the GPP responded more quickly to the plant water stress than ET. Our findings suggest that policies related to large-scale carbon sequestration initiatives under afforestation must first fully consider the status of water consumption and WUE.

The ecosystem apparent quantum yield (α), maximum rate of gross CO₂ assimilation (P_max) and daytime ecosystem respiration rate (R_d), reflecting the physiological functioning of ecosystem, are vital photosynthetic parameters for the estimation of ecosystem carbon budget. Climatic drivers may affect photosynthetic parameters both directly and indirectly by altering the response of vegetation. However, the relative contribution and regulation pathway of environmental and physiological controls remain unclear, especially in semi-arid grasslands. We analyzed seasonal and interannual variations of photosynthetic parameters derived from eddy-covariance observation in a typical semi-arid grassland in Inner Mongolia, Northern China, over 12 years from 2006 to 2017. Regression analyses and a structural equation model (SEM) were adopted to separate the contributions of environmental and physiological effects. The photosynthetic parameters showed unimodal seasonal patterns and significantly interannual variations. Variations of air temperature (T_a) and soil water content (SWC) drove the seasonal patterns of photosynthetic parameters, while SWC predominated their interannual variations. Moreover, contrasting with the predominant roles of T_a on α and R_d, SWC explained more variance of P_max than T_a. Results of SEM revealed that environmental factors impacted photosynthetic parameters both directly and indirectly through regulating physiological responses reflected by stomatal conductance at the canopy level. Moreover, leaf area index (LAI) directly affected α, P_max and R_d and dominated the variation of P_max. On the other hand, SWC influenced photosynthetic parameters indirectly through LAI and canopy surface conductance (gc). Our findings highlight the importance of physiological regulation on the photosynthetic parameters and carbon assimilation capacity, especially in water-limited grassland ecosystems.

Warming and grazing, and litter quality jointly determine litter decomposition and nutrient releases in grazing ecosystems. However, their effects have previously been studied in isolation. We conducted a two factorial experiment with asymmetric warming using infrared heaters and moderate grazing in an alpine meadow. Litter samples were collected from all plots in each treatment, among which some subsamples were placed in their original plots and other samples were translocated to other treatment plots to test the relative effects of each treatment on litter decomposition and nutrient releases. We found that warming rather than grazing alone significantly increased total losses of litter mass, total organic carbon, total nitrogen (TN) and total phosphorus (TP) per unit area due to increases in both mass loss rates and litter biomass. However, grazing with warming did not affect their total mass losses because increased mass loss was offset by decreased litter biomass compared with the control. Seasonal mean soil temperature better predicted litter decomposition than litter lignin content or carbon to nitrogen ratio. There were interactions between warming and grazing, but there were no interactions between them and litter quality on litter decomposition. The temperature sensitivity of TN loss was higher than that of TP loss per unit area. Our results suggest that increased temperature has a greater effect on litter decomposition and nutrient release than change in litter quality, and that more N release from litter could result in greater P deficiency in the alpine meadow.

All organisms need elements in fixed proportions for carrying out normal metabolic processes and how flexible they are depends on how effective they are utilizing these resources from external sources. It is important to understand the interactions among plant, soil and microbial biomass carbon (C), nitrogen (N) and phosphorus (P) stoichiometry under different conditions of resource supply. We conducted a pot experiment on 1-year-old Robinia pseudoacacia seedlings for nearly 5 months under different water, nitrogen and phosphorus supplies, and we determined plant, soil and microbial biomass C, N and P stoichiometry. We found that plant, soil and microbial nutrients and stoichiometry exhibited a certain degree of plasticity in response to the changes in water and nutrient conditions in their environments. Variation partitioning analysis showed that root stoichiometry accounted for a large part of the variance in microbial stoichiometry. Structural equation modeling further revealed that root stoichiometry and leaf stoichiometry were two direct factors affecting microbial biomass C:N and C:P, and that root stoichiometry had the greatest direct effect. In addition, the degree of homeostasis for microbial biomass C and C:P was more sensitive to changes in soil nutrients than changes in other factors, and other elements and elemental ratios displayed strict homeostasis. These results highlight the importance of studying microbial stoichiometry in improving our understanding of nutrient cycling of the plant–soil system under different water and nutrient supply.

Rapid spread and growth of plants that are poisonous to animals produce large amounts of plant litter in degraded grasslands. Nitrogen (N) input may promote the growth of these poisonous plants and alter the rhizosphere microbes and arbuscular mycorrhizal fungi (AMF) in particular. However, it is unclear how poisonous plant litter affects the growth of palatable plants and their associated AMF in the rhizosphere and whether and how N deposition may mediate these effects. A greenhouse experiment was performed to test the combined effects of litter addition of a poisonous plant, Stellera chamaejasme, and N addition on the growth of a dominant grass, Leymus chinensis, AMF characteristics and soil properties. Litter addition significantly increased the ramet number and aboveground biomass of L. chinensis and soil available phosphorus (AP) concentration and decreased the spore density of AMF. However, the interaction of both treatments had no significant effects on traits of L. chinensis and AMF properties. Stellera chamaejasme litter positively affected L. chinensis by increasing AP and negatively affected AMF by combining balanced changes in soil nutrients and litter-induced allelopathic compositions. High N addition may alleviate soil N limitation and inhibit litter decomposition, thus overriding the litter’s effects on L. chinensis and AMF. These findings imply that it is necessary to objectively and comprehensively evaluate the ecological functions of poisonous plants beyond their harmful effects on livestock. Simultaneously, N deposition should be an indispensable factor in predicting the relationships between poisonous plants and edible plants in degraded grasslands.

Forest productivity and carbon (C) sequestration largely depend on soil N and P availability. To date, however, the temporal variation of nutrient limitation along forest succession is still under debate. Leaf stoichiometry and nutrient resorption are important indicators for predicting nutrient limitation of plant growth. Here, we measured nitrogen (N) and phosphorus (P) concentrations in green leaves and leaf litter for all woody species at four stages of temperate forest succession, and analyzed how abiotic and biotic factors affect leaf stoichiometry and nutrient resorption along forest succession. At the individual scale, leaf N and P concentrations had a significant increase at the end of the succession, while no change in leaf N:P ratio was detected. Nitrogen resorption efficiency (NRE) increased significantly with succession, but P resorption efficiency (PRE) first increased and then decreased. Significant increases in NRE:PRE ratios only occurred at the end of the succession. Moreover, plant N cycling was less responsive to soil nutrient than P cycling. At the community scale, we found that leaf N and P concentrations first decreased and then increased along forest succession, which were mainly affected by Shannon–Wiener index and species richness. Leaf N:P ratio significantly varied with succession and was mainly determined by community-weighted mean diameter at breast height (DBH). NRE increased and was significantly influenced by species richness and DBH, while PRE was relatively stable along forest succession. Thus, the NRE:PRE ratios significantly increased, indicating that N limitation is exacerbated with the temperate forest succession. These results might reflect the intense interspecific competition for limiting resource in a higher biodiversity community. In conclusion, our findings highlight the importance of biotic factors in driving forest ecosystem nutrient cycling and provide valuable information for sustainable fertilizer management practices in China’s temperate and boreal forests.

Acid rain (AR), which occurs frequently in southern China, negatively affects the growth of subtropical tree species. Arbuscular mycorrhizal fungi (AMF) mitigate the detrimental effects induced by AR. However, the mechanisms by which AMF protect Zelkova serrata, an economically important tree species in southern China, from AR stress remain unclear. We conducted a greenhouse experiment in which Z. serrata plants were inoculated with AMF species Rhizophagus intraradices and Diversispora versiformis, either alone or as a mixed culture, or with a sterilized inoculum (negative control). The plants were subjected to three levels of simulated sulfuric AR and nitric AR (pH 2.5, 4.0 and 5.6) to examine any interactive effects on growth, photosynthetic capabilities, antioxidant enzymes, osmotic adjustment and soil enzymes. AR significantly decreased dry weight, chlorophyll content, net photosynthetic rate and soluble protein (SP) of non-mycorrhizal plants. Mycorrhizal inoculation, especially a combination of R. intraradices and D. versiformis, notably improved dry weight, photosynthetic capabilities, catalase, peroxidase, superoxide dismutase, SP and root acid phosphatase activity of Z. serrata under harsh AR stress. Moreover, the benefits from AMF symbionts depended on the identity of AM fungal species and the gradient of AR stress. Our results indicate that AM fungi protect Z. serrata against AR stress by synchronously activating photosynthetic ability, antioxidant enzymes and osmolyte accumulation. These findings suggest that a combination of R. intraradices and D. versiformis may be a preferable choice for culturing Z. serrata in southern China.

热带草原棕榈树的空间格局及其时间变异性
物种的空间分布和关联可以提供林分的基本信息及动态。然而，由于树木的空间分布和关联依赖于不同尺度下的环境因子，要弄清环境异质性和植物间相互作用的影响，需要选择合适的空间模型进行分析。本研究分析了同一热带草原棕榈树的种群空间分布，并假设这些分布在20年内发生了变化。为了消除大规模环境异质性的影响，采用异质性泊松零模型下的非均质L-函数。研究结果发现：(i)与20年前不同，稀树草原地区的成年树种呈规则分布而非聚集分布；(ii)虽然幼苗和幼树的空间分布总是呈聚集状，但聚集区域面积(强度)在减少；(iii)除幼树期，其它阶段均与富养斑块无关联，这也与20年前不同；(iv)没有两个研究地点在结构上有特别的差异，只是雌性棕榈树在空间上与保护区的整叶幼苗有关联，而在乡村，它们在空间上是相互独立的。我们的研究证明，空间分布发生了部分变化，同时对空间异质性的管理也更加完善，模拟结果也更精确。

The pollution of freshwater ecosystems is threatening freshwater plant species diversity worldwide. Freshwater plants, such as the common duckweed (Lemna minor), are potentially sensitive to novel stressful environments. To test if ecotype diversity could increase resistance to stressful environments, I used seven L. minor populations and measured their growth rates with and without moderate salt stress across an ecotype diversity gradient. The L. minor populations were grown over 5 months in 92 experimental mesocosms, either in ecotype monocultures or in polyculture with either one or three conspecific ecotypes (23 unique compositions). After growing the duckweed in unperturbed conditions (phase 1), the cultures were subjected to moderate salt stress (50 mmol/L NaCl) for several weeks (phase 2). The experiment was conducted in the presence of the natural epimicrobial community associated with the different ecotypes. In phase 2, a subset of these algae added an unintentional second stressor to the experiment. The ecotypes differed in their growth rates, the fastest growing at twice the rate of others. The diversity context further shaped the ecotype growth rates. Ecotype polycultures showed higher abundances towards the end of the experiment, thus over time, as the environment deteriorated, ecotype diversity gained in importance. These findings show that within-species variation in growth rates can translate to a positive effect of ecotype diversity on population abundance. Exposure of L. minor to moderate salt levels did not significantly impact growth rates, although the effect may have been masked by reduced algal stress in the saline environments.

Clonal integration benefits clonal plants by buffering environmental stress and increasing resource extraction efficiency. However, the number of connected ramet generations that benefit from clonal integration in a clonal system has received relatively little attention. A pot experiment was conducted to evaluate the extent of physiological integration within the clonal system of Vallisneria natans consisting of a mother ramet and three sequentially connected offspring ramets. Mother ramets were grown in full sunlight, and offspring ramets were heavily shaded with limited light availability. Stolons between mother ramets and offspring ramets were severed or connected, but connection among the three offspring ramets remained. The photosynthetic ability of unshaded mother ramets of V. natans was significantly enhanced, but their biomass accumulation was greatly reduced when connected to shaded offspring ramets. Clonal integration significantly increased biomass accumulation, C and N availabilities, extracellular enzyme activities and microbial biomass of the first ramet generation (adjacent ramet), but not later ramet generations. Our results indicate that support from the mother ramet of V. natans may be limited to the adjacent offspring ramet in a clonal system under severe light stress, implying an effect of ramet generation. Our results contribute to a better understanding of the hierarchy and segmentation of clonal plants. These findings suggest that the extent of clonal integration plays a vital role in ecological interactions of the ramet population.

The response of plant leaf and root phenology and biomass in the Arctic to global change remains unclear due to the lack of synchronous measurements of above- and belowground parts. Our objective was to determine the phenological dynamics of the above- and belowground parts of Eriophorum vaginatum in the Arctic and its response to warming. We established a common garden located at Toolik Lake Field Station; tussocks of E. vaginatum from three locations, Coldfoot, Toolik Lake and Sagwon, were transplanted into the common garden. Control and warming treatments for E. vaginatum were set up at the Toolik Lake during the growing seasons of 2016 and 2017. Digital cameras, a handheld sensor and minirhizotrons were used to simultaneously observe leaf greenness, normalized difference vegetation index and root length dynamics, respectively. Leaf and root growth rates of E. vaginatum were asynchronous such that the timing of maximal leaf growth (mid-July) was about 28 days earlier than that of root growth. Warming of air temperature by 1 °C delayed the timing of leaf senescence and thus prolonged the growing season, but the temperature increase had no significant effect on root phenology. The seasonal dynamics of leaf biomass were affected by air temperature, whereas root biomass was correlated with soil thaw depth. Therefore, we suggest that leaf and root components should be considered comprehensively when using carbon and nutrient cycle models, as above- and belowground productivity and functional traits may have a different response to climate warming.