Don't judge toxic weeds on whether they are native but on their ecological effects

Toxic nonpreferred species accelerate the natural restoration of plant productivity and diversity in degraded grasslands

Long-term overgrazing may lead to the degradation of grasslands which are often characterized by an increase in nonpreferred species, especially toxic plants. However, the impact of these toxic nonpreferred species on the restoration processes of degraded grasslands is not well understood, particularly their interactions with soil properties and other plant functional groups. To address this knowledge gap, we conducted an in situ grazing exclusion experiment in a temperate degraded grassland of Inner Mongolia, China. The objective of this study was to investigate how toxic nonpreferred plants influence the recovery of plant diversity and productivity in degraded grasslands and whether these effects can be explained by changes in soil properties. Our findings revealed that Stellera chamaejasme, a toxic nonpreferred species widely distributed in North China, directly altered plant community composition and improved species diversity in degraded grasslands dominated by Asteraceae plants. The presence of S. chamaejasme could inhibit Asteraceae abundance and increase soil copper content in this study area, because Asteraceae plants have a high copper accumulation capacity. Within the communities with S. chamaejasme, the alleviation of soil copper limitation to plants may subsequently enhance the abundance and aboveground productivity of Poaceae and Forbs. Our study demonstrated that the strong direct and indirect interactions of toxic nonpreferred species with other ecosystem components promoted competitive release in terms of biomass accumulation and species diversity. The change of soil limiting microelements content caused by toxic species exerts an important mediation function during the recovery process of degraded grasslands. Thus, these toxic nonpreferred species can act primarily as accelerators for the restoration of community structure and ecosystem function in degraded grasslands.

Short-term robust plant overcompensatory growth was observed in a degraded alpine meadow on the southeastern Qinghai-Tibetan Plateau

Plant overcompensatory growth (OCG) is an important mechanism by which plant communities adapt to environmental disturbance. However, it is not clear whether plant OCG can occur in degraded alpine meadows. Here, we conducted a mowing experiment in an alpine meadow at three degradation levels (i.e., severe degradation, SD; moderate degradation, MD; and light degradation, LD) on the southeastern Qinghai-Tibetan Plateau from 2018 to 2020 to investigate plant OCG and its relationships with soil available nutrients, plant nutrient use efficiency (i.e., nitrogen use efficiency, NUE; and phosphorus use efficiency, PUE), and precipitation. The results showed that 1) the OCG of the plant community generally occurred across all degradation levels, and the OCG strength of the plant community decreased with mowing duration. Moreover, the OCG strength of the plant community in the SD treatment was significantly greater than that in the MD and LD treatments after two years of mowing (p < 0.05). 2) In LD and MD, the soil nitrate nitrogen (NO3-) and available phosphorus (AP) concentrations exhibited a decreasing trend (p < 0.05), while the soil ammonium nitrogen (NH4+) concentration did not change from 2018 to 2020 (p > 0.05). In the SD treatment, the soil NO3- concentration tended to decrease (p < 0.05), the NH4+ concentration tended to increase (p < 0.05), and the AP concentration exhibited an inverse parabolic trend (p < 0.05) from 2018 to 2020. 3) From 2018 to 2020, plant NUE and PUE exhibited decreasing trends at all degradation levels. 4) Plant nutrient use efficiency, which is regulated by complex plant-soil interactions, strongly controlled the OCG of the plant community along each degradation gradient. Moreover, precipitation not only directly promoted the OCG of the plant community but also indirectly affected it by regulating the structure of the plant community and plant nutrient use efficiency. These results suggest that the OCG of the plant community in degraded alpine meadows may benefit not only from the strong self-regulating capacity of the plant-soil system but also from humid climatic conditions.

Impacts of climate change and human activity on the potential distribution of Aconitum leucostomum in China

Aconitum leucostomum is a poisonous grass that disturbs grassland populations and livestock development, and its spread is influenced by climate change and human activities. Therefore, exploring its potential distribution area under such conditions is crucial to maintain grassland ecological security and livestock development. The present study initially selected 39 variables that may influence the spatial distribution of A. leucostomum, including bioclimate, soil, topography, solar radiation, and human footprint data; the variables were screened by Spearman's correlation coefficient and the jackknife method. Twenty variables were finally identified, and three types of models based on the maximum entropy (MaxEnt) model were constructed to predict the distribution of A. leucostomum within China under three shared economy pathways (SSP126, SSP245, and SSP585): A: prediction of environmental variables under the current climate model; B: prediction of environmental variables + human footprint under the current climate model; and C: prediction of environmental variables under the future climate model (including the 2030s, 2050s, and 2070s). The effects of human activities and climate change on the potential geographic distribution of A. leucostomum were explored separately. The results show that precipitation seasonality, human footprint, solar radiation and mean diurnal range are the main factors affecting the distribution of A. leucostomum. Human activities inhibit the spread of A. leucostomum, and climate change promotes its growth, with areas of high suitability and area variation mainly in northern Xinjiang and northern Yunnan. With climate change, in the future, the distribution center of A. leucostomum shows a tendency to migrate to the southeast on the horizontal gradient and to move to higher altitudes on the vertical gradient. This study provides a positive reference value for the control of A. leucostomum and the maintenance of grassland ecological security.

Endophytic Fungal Community of Stellera chamaejasme L. and Its Possible Role in Improving Host Plants' Ecological Flexibility in Degraded Grasslands

Stellera chamaejasme L. is a widely distributed poisonous plant in Chinese degraded grasslands. To investigate the role of endophytic fungi (EF) in S. chamaejasme's quick spread in grasslands, the endophytic fungal community of S. chamaejasme was studied through culture-dependent and culture-independent methods, and the plant-growth-promoting (PGP) traits of some culturable isolates were tested. Further, the growth-promoting effects of 8 isolates which showed better PGP traits were evaluated by pot experiments. The results showed that a total of 546 culturable EF were isolated from 1114 plant tissue segments, and the colonization rate (CR) of EF in roots (33.27%) was significantly higher than that in shoots (22.39%). Consistent with this, the number of specific types of EF was greater in roots (8 genera) than in shoots (1 genus). The same phenomenon was found in culture-independent study. There were 95 specific genera found in roots, while only 18 specific genera were found in shoots. In addition, the dominant EF were different between the two study methods. Cladosporium (18.13%) and Penicillium (15.93%) were the dominant EF in culture-dependent study, while Apiotrichum (13.21%) and Athelopsis (5.62%) were the dominant EF in culture-independent study. PGP trait tests indicated that 91.30% of the tested isolates (69) showed phosphorus solubilization, IAA production, or siderophores production activity. The benefit of 8 isolates on host plants' growth was further studied by pot experiments, and the results indicated that all of the isolates can improve host plants' growth. Among them, STL3G74 (Aspergillus niger) showed the best growth-promotion effect; it can increase the plant's shoot and root dry biomass by 68.44% and 74.50%, respectively, when compared with the controls. Our findings revealed that S. chamaejasme has a wide range of fungal endophytic assemblages, and most of them possess PGP activities, which may play a key role in its quick spread in degraded grasslands.

Trends in grassland science: Based on the shift analysis of research themes since the early 1900s

Grasslands, one of the major terrestrial ecosystems, are essential for the maintenance of ecological and production functions; however, they are undergoing extensive degradation. The development and cutting-edge explorations in grassland science are critical to addressing challenges such as climate change and the increasing influence of human activities. To identify research trends in grassland science, latent Dirichlet allocation (LDA) topic modelling was used to conduct an automated content analysis on 123,829 papers available on Web of Science Core Collection from 1900 to 2020. Results from this analysis showed that grassland research has become increasing multidisciplinary, accompanied by a pronounced reduction in the relative frequency of traditional production-oriented research and an increase in the themes focusing on ecological functions and modern technologies. Changes in research activities have been uneven globally, with a significant increase in the number of publications in China and Brazil, which probably reflects an increased support from various governmental agencies in these countries. Additionally, in 2019, China surpassed the United States in terms of the total number of publications. Further, this study identified important topics and emerging challenges in grassland research, such as biodiversity conservation, climate changes, and genetic considerations. Comprehensive improvement of education, research, global cooperation, and funding strategies will be necessary to promote grassland science research on frontier themes and to effectively address the social and environmental challenges in the new era.

Research Progress of Grassland Ecosystem Structure and Stability and Inspiration for Improving Its Service Capacity in the Karst Desertification Control

The structure and stability of grassland ecosystems have a significant impact on biodiversity, material cycling and productivity for ecosystem services. However, the issue of the structure and stability of grassland ecosystems has not been systematically reviewed. Based on the Web of Science (WOS) and China National Knowledge Infrastructure (CNKI) databases, we used the systematic-review method and screened 133 papers to describe and analyze the frontiers of research into the structure and stability of grassland ecosystems. The research results showed that: (1) The number of articles about the structure and stability of grassland ecosystems is gradually increasing, and the research themes are becoming increasingly diverse. (2) There is a high degree of consistency between the study area and the spatial distribution of grassland. (3) Based on the changes in ecosystem patterns and their interrelationships with ecosystem processes, we reviewed the research progress and landmark results on the structure, stability, structure-stability relationship and their influencing factors of grassland ecosystems; among them, the study of structure is the main research focus (51.12%), followed by the study of the influencing factors of structure and stability (37.57%). (4) Key scientific questions on structural optimization, stability enhancement and harmonizing the relationship between structure and stability are explored. (5) Based on the background of karst desertification control (KDC) and its geographical characteristics, three insights are proposed to optimize the spatial allocation, enhance the stability of grassland for rocky desertification control and coordinate the regulation mechanism of grassland structure and stability. This study provided some references for grassland managers and relevant policy makers to optimize the structure and enhance the stability of grassland ecosystems. It also provided important insights to enhance the service capacity of grassland ecosystems in KDC.

Ecological thresholds of toxic plants for sheep production and ecosystem multifunctionality and their trade-off in an alpine meadow

Toxic plants are a natural component of alpine meadow which co-evolved with Tibetan sheep for thousands of years. One challenge for indigenous herders is to know the ecological thresholds of toxic plants and maintain their vital functions in ways that are compatible with economic income and ecological conservation. To achieve this, field trials with Tibetan sheep grazing in alpine meadow were conducted to examine the ecological thresholds of toxic plants for sheep production and ecosystem functions and their trade-offs. Our results demonstrated that the changing point values of biomass proportion of toxic plants for dry matter intake and liveweight gain of sheep were 17% and 22%, respectively. The changing point value of biomass (richness) proportion of toxic plants for soil carbon accumulation index was 31% (59%), for soil nutrient cycling index was 38% (42%), and for ecosystem multifunctionality index was 28% (50%). The trade-off between liveweight gain of sheep and ecosystem multifunctionality first decreased and then increased along the gradient of biomass proportion of toxic plants (the value of changing point was 37%), and had a significant negative correlation with richness of toxic plants. In addition, structural equation modeling indicated that toxic plants can affect the trade-off between liveweight gain of sheep and ecosystem multifunctionality though increasing acid detergent fiber of plant and decreasing plant species richness, belowground biomass and soil total phosphorus. Consequently, opinions towards toxic plants should shift from the conventional view that they are serious threat to grassland ecosystem health to an inclusive understanding that they are beneficial to livestock and ecosystem functions under certain ecological thresholds.

Precipitation and soil nutrients determine the spatial variability of grassland productivity at large scales in China

Changes in net primary productivity (NPP) to global change have been studied, yet the relative impacts of global change on grassland productivity at large scales remain poorly understood. Using 182 grassland samples established in 17 alpine meadows (AM) and 21 desert steppes (DS) in China, we show that NPP of AM was significantly higher than that of DS. NPP increased significantly with increasing leaf nitrogen content (LN) and leaf phosphorus content (LP) but decreased significantly with increasing leaf dry matter content (LDMC). Among all abiotic factors, soil nutrient factor was the dominant factor affecting the variation of NPP of AM, while the NPP of DS was mainly influenced by the changing of precipitation. All abiotic factors accounted for 62.4% of the spatial variation in the NPP of AM, which was higher than the ability to explain the spatial variation in the NPP of DS (43.5%). Leaf traits together with soil nutrients and climatic factors determined the changes of the grassland productivity, but the relative contributions varied somewhat among different grassland types. We quantified the effects of biotic and abiotic factors on grassland NPP, and provided theoretical guidance for predicting the impacts of global change on the NPP of grasslands.

Biogeographic Patterns of Leaf Element Stoichiometry of Stellera chamaejasme L. in Degraded Grasslands on Inner Mongolia Plateau and Qinghai-Tibetan Plateau

Plant leaf stoichiometry reflects its adaptation to the environment. Leaf stoichiometry variations across different environments have been extensively studied in grassland plants, but little is known about intraspecific leaf stoichiometry, especially for widely distributed species, such as Stellera chamaejasme L. We present the first study on the leaf stoichiometry of S. chamaejasme and evaluate its relationships with environmental variables. S. chamaejasme leaf and soil samples from 29 invaded sites in the two plateaus of distinct environments [the Inner Mongolian Plateau (IM) and Qinghai-Tibet Plateau (QT)] in Northern China were collected. Leaf C, N, P, and K and their stoichiometric ratios, and soil physicochemical properties were determined and compared with climate information from each sampling site. The results showed that mean leaf C, N, P, and K concentrations were 498.60, 19.95, 2.15, and 6.57 g kg⁻¹; the average C:N, C:P, N:P, N:K and K:P ratios were 25.20, 245.57, 9.81, 3.13, and 3.21, respectively. The N:P:K-ratios in S. chamaejasme leaf might imply that its growth is restricted by K- or K+N. Moreover, the soil physicochemical properties in the S. chamaejasme-infested areas varied remarkably, and few significant correlations between S. chamaejasme leaf ecological stoichiometry and soil physicochemical properties were observed. These indicate the nutrient concentrations and stoichiometry of S. chamaejasme tend to be insensitive to variations in the soil nutrient availability, resulting in their broad distributions in China’s grasslands. Besides, different homeostasis strength of the C, N, K, and their ratios in S. chamaejasme leaves across all sites were observed, which means S. chamaejasme could be more conservative in their use of nutrients improving their adaptation to diverse conditions. Moreover, the leaf C and N contents of S. chamaejasm were unaffected by any climate factors. However, the correlation between leaf P content and climate factors was significant only in IM, while the leaf K happened to be significant in QT. Besides, MAP or MAT contribution was stronger in the leaf elements than soil by using mixed effects models, which illustrated once more the relatively weak effect of the soil physicochemical properties on the leaf elements. Finally, partial least squares path modeling suggested that leaf P or K contents were affected by different mechanisms in QT and IM regions, suggesting that S. chamaejasme can adapt to changing environments by adjusting its relationships with the climate or soil factors to improve its survival opportunities in degraded grasslands.

Ecosystem Coupling and Ecosystem Multifunctionality May Evaluate the Plant Succession Induced by Grazing in Alpine Meadow

Most alpine meadow on the Tibetan Plateau are at different stages of community succession induced by grazing practices. Quantifying the succession sequence and assessing the dynamics of plant composition, ecosystem coupling, and multifunctionality across successional stages are essential for reasonable restoration of degraded alpine meadow. Here, we selected areas with different grazing disturbance histories and used them as a space series (i.e., space-for-time substitution) to study the community succession. Our work quantified the plant succession sequence of alpine meadow induced by grazing with plant functional group approach. The plant succession sequence is from the tall sedge community with erect growth to the short undesirable toxic forbs community with prostrate growth. Ecosystem coupling, ecosystem multifunctionality and their relationships were all the lowest in Stage 4. Compared to Stage 4, the ecosystem multifunctionality index increased in Stages 1, 2, and 3 by 102.6, 89.8, and 207.6%, respectively; the extent of ecosystem coupling increased by 20.0, 16.8, and 21.2%, respectively. Our results indicated that the driving factors of ecosystem coupling and ecosystem multifunctionality were soil factor individual in early successional stage to plant-soil simultaneously in late successional stage. Our results also highlighted the importance of toxic weeds during the late stage of degraded succession and suggest that the expansion of toxic plants is a consequence of their greater suitability from a successional perspective. The findings of this study would provide valuable guidance for optimizing the management and restoration practice of alpine meadow.

Stability response of alpine meadow communities to temperature and precipitation changes on the Northern Tibetan Plateau

Biomass temporal stability plays a key role in maintaining sustainable ecosystem functions and services of grasslands, and climate change has exerted a profound impact on plant biomass. However, it remains unclear how the community biomass stability in alpine meadows responds to changes in some climate factors (e.g., temperature and precipitation). Long‐term field aboveground biomass monitoring was conducted in four alpine meadows (Haiyan [HY], Henan [HN], Gande [GD], and Qumalai [QML]) on the Qinghai‐Tibet Plateau. We found that climate factors and ecological factors together affected the community biomass stability and only the stability of HY had a significant decrease over the study period. The community biomass stability at each site was positively correlated with both the stability of the dominant functional group and functional groups asynchrony. The effect of dominant functional groups on community stability decreased with the increase of the effect of functional groups asynchrony on community stability and there may be a ‘trade‐off’ relationship between the effects of these two factors on community stability. Climatic factors directly or indirectly affect community biomass stability by influencing the stability of the dominant functional group or functional groups asynchrony. Air temperature and precipitation indirectly affected the community stability of HY and HN, but air temperature in the growing season and nongrowing season had direct negative and direct positive effects on the community stability of GD and QML, respectively. The underlying mechanisms varied between community composition and local climate conditions. Our findings highlighted the role of dominant functional group and functional groups asynchrony in maintaining community biomass stability in alpine meadows and we highlighted the importance of the environmental context when exploring the stability influence mechanism. Studies of community stability in alpine meadows along with different precipitation and temperature gradients are needed to improve our comprehensive understanding of the mechanisms controlling alpine meadow stability.

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.

Quantitative Analysis of the Research Trends and Areas in Grassland Remote Sensing: A Scientometrics Analysis of Web of Science from 1980 to 2020

Grassland remote sensing (GRS) is an important research topic that applies remote sensing technology to grassland ecosystems, reflects the number of grassland resources and grassland health promptly, and provides inversion information used in sustainable development management. A scientometrics analysis based on Science Citation Index-Expanded (SCI-E) was performed to understand the research trends and areas of focus in GRS research studies. A total of 2692 papers related to GRS research studies and 82,208 references published from 1980 to 2020 were selected as the research objects. A comprehensive overview of the field based on the annual documents, research areas, institutions, influential journals, core authors, and temporal trends in keywords were presented in this study. The results showed that the annual number of documents increased exponentially, and more than 100 papers were published each year since 2010. Remote sensing, environmental sciences, and ecology were the most popular Web of Science research areas. The journal Remote Sensing was one of the most popular for researchers to publish documents and shows high development and publishing potential in GRS research studies. The institution with the greatest research documents and most citations was the Chinese Academy of Sciences. Guo X.L., Hill M.J., and Zhang L. were the most productive authors across the 40-year study period in terms of the number of articles published. Seven clusters of research areas were identified that generated contributions to this topic by keyword co-occurrence analysis. We also detected 17 main future directions of GRS research studies by document co-citation analysis. Emerging or underutilized methodologies and technologies, such as unmanned aerial systems (UASs), cloud computing, and deep learning, will continue to further enhance GRS research in the process of achieving sustainable development goals. These results can help related researchers better understand the past and future of GRS research studies.

The role of the locoweed (Astragalus variabilis Bunge) in improving the soil properties of desert grasslands

Astragalus variabilis Bunge is a widespread locoweed that threatens livestock production in desert grassland. No research has reported its possible ecological functions due to focus being on its negative effect on livestock production. This study aimed to assess the effects of A. variabilis on soil properties and its possible role in improving soil quality in desert grassland. Soil samples were collected in Astragalus patches and the adjacent bare patches over two successive growing seasons in Alxa desert grassland where A. variabilis was favoured to spread. Soil properties including texture, water content, dry bulk density, porosity, available nutrients, organic matter, and soil microbial biomass were determined at 15 study sites. There was no significant difference in soil texture between Astragalus-dominant and bare patches; but organic matter (OM), available N and P, and microbial biomass in surface soil (0–30cm) were significantly higher in Astragalus patches. Furthermore, microbial biomass showed a significantly positive correlation with available nutrients and OM. Levels of water soluble salt were significantly lower in A. variabilis surface soils under drought conditions. Results suggested that A. variabilis was associated with some positive changes in soil properties, and was potentially important in improving soil chemical and microbial properties in desert grassland ecosystems. Consequently, total elimination of locoweed should not necessarily be considered the best solution to locoweed poisoning in livestock.