Competition and facilitation among plants are an important part of the mechanisms regulating the responses of species, communities, and ecosystems to environmental changes. Basically, facilitation plays a more important role than competition in stress environments. The present review summarized the research progress of shrubland facilitation in arid and semi-arid regions in the past 30 years, on the aspects of trends of facilitation along a water availability gradient, mechanisms of shrubland facilitation, and the application of shrubland facilitation in vegetation restoration. As the most important empirical model that predicts the trend of plant-plant interactions along abiotic stress gradient, stress gradient hypothesis (SGH) posits that facilitation increases with increasing abiotic stress. However, SGH has been largely debated in water limiting system. Shrubland facilitation does not always increase linearly with the decreasing water availability, which is not only closely related to the complex impact of shrub on soil water, but also associates with the species identities, plant life history stages, study methods, scales and indicators. Arid shrubs could affect the survival, growth, and reproduction of its understory plants via ameliorating soil moisture and nutrient, mediating soil microbial community compositions, buffering extreme temperature and radiation, and resisting predators. Removal studies have shown that the effects of shrub on belowground play a stronger role than aboveground on shrubland facilitation. Shrubland facilitations are commonly considered a potential effective approach of vegetation restoration in degraded ecosystems. Whereas, the success of vegetation restoration is comprehensively affected by the abiotic stress, species identities, plant life history stages and land use. Under the dual influence of climate change and human activities, the areas of drylands and shrublands are assumed to be expanding. Thus, the studies on mechanisms of shrubland facilitation and trends of shrub-herbs interactions along drought gradient, as well as the effects of shrub on herbs, are critical for predicting the responses of species and grassland to climate change in arid and semi-arid regions. Based on the current research progresses and environmental changes in arid and semi-arid regions, the present review proposes the following issues in the future studies: 1) exploring the biological indicators to the abiotic stress and plant-plant interactions; 2) clarifying the relative contribution of shrub belowground versus aboveground on facilitation, in order to disclose the mechanisms of shrubland facilitation; 3) exploring the feedback effect of beneficiaries on facilitators; 4) assessing the comprehensive role of nurse plants on vegetation restoration; 5) popularizing the study methods of manipulative water experiments and long-term positioning observations on shrubland facilitation; and 6) addressing the study of plant facilitation on related research issues, such as the relationship between facilitation and species invasion, shrub encroachment, biodiversity and ecosystem function, and the response of facilitation to climate change.

Alpine shrublands, as a zonal vegetation, are widely distributed on the Qingzang Plateau, and play ecological functions such as soil and water conservation, conservation of water resources, and biodiversity protection in the regional ecosystem. This study used a unified and standardized method to set up 58 quadrats to investigate the species composition, quantitative characteristics and habitat information of the main alpine shrublands of the Haizi Mountain in Batang, China. Based on “Vegegraphy of China” compilation and research standards, the alpine shrublands in this area can be divided into three vegetation types, 11 alliances, and 21 associations. This paper presents descriptions on the characteristics for each association and provides the original data from all survey plots.

Ostryopsis davidiana shrubland is one of the most important types of deciduous broad-leaved shrublands that widely occur on mountain sites in northern and northwestern China. Determination of the distribution and characteristics of the O. davidiana communities helps facilitate the biodiversity conservation and establishment of eco-safety buffer on the Mongolian Plateau. In this study, we investigated the O. davidiana shrubland communities across 37 sites in Nei Mongol, and identified the community characteristics, including species composition, ecological traits and classification of O. davidiana alliance based on site survey data collected during 2016-2020. A total of 300 vascular species, belonging to 158 genera from 52 families, were recorded, consisting 298 angiosperm species belonging to 156 genera from 50 families and 2 fern species belonging to 2 genera from 2 families. Perennial herbs are the dominant life form, accounting for 73% of the total species, and mesophytes are the dominant water ecological type, accounting for 65% of the total species. Temperate plants contributed largely to the geographical floristic composition, with majority of the species being East Palaearctic, followed by East Asia element. Most of plant species in the O. davidiana communities were rare species. Based on the survey data, the O. davidiana alliance in Nei Mongol could be divided into 3 association groups and 17 associations.

Aims The karst ecosystem in southwest China is fragile and easily degraded by disturbance. However, the process and mechanism of karst shrub tussock responding to grazing disturbance in the early stage of restoration are still poorly understood.
Methods In this study, four grazing intensity of heavy, moderate, light and none grazing were selected in shrub tussock communities in karst mountains of Mashan County, Guangxi. Species composition and community structure of the vegetation were investigated, and their responses to grazing were further analyzed.
Important findings The results show that: (1) There are 272 species of vascular plants in the sample plots, belonging to 77 families and 200 genera. Poaceae, Asteraceae, Fabaceae and Verbenaceae are the dominant families. With the increase of grazing intensity, the number of families, genera and species in the shrub layer decreased monotonously. However, in the herb layer a single peak was shown in the light grazing area. (2) The increase in grazing intensity did not significantly change the dominant species, but the important values of species changed. For example, the important values of shrub Vitex negundo, herbaceous invasive species Chromolaena odorata and Bidens pilosa, increased with the increase of grazing intensity. (3) Non-metric multi-dimensional scaling (NMDS) analysis showed that grazing led to the vegetation homogenization of shrub layer, and the species composition of most plots tended to be similar, but the species composition of the same grazing gradient also had great differences. Species in the shrub layer were mainly affected by grazing intensity and rock exposure rate, while species in the herb layer were affected by terrain and soil factors such as altitude, soil organic matter content, soil pH, in addition to grazing intensity. (4) Community height, coverage of shrub layer and aboveground biomass decreased significantly with the increase of grazing intensity. The response of community structure in shrub layer to grazing intensity was more obvious than that in herb layer, which may be related to the preference of goats to eat shrubs. Overall, the increase in grazing disturbance intensity will drive the simplification, sparseness and dwarfing of the species composition and community structure of shrub tussock, thereby significantly reducing aboveground biomass and enhancing the potential risk of vegetation degradation in karst mountains, and further hindering natural recovery of karst ecosystem.

Aims Trait-trait relationships and trait-environment relationships are critical for understanding species distribution, community assembly and plant strategy to environmental change. Caragana species distribute widely in arid Mongolia Plateau, and shape the alternative distribution along the environmental gradients. To understand the plant strategies to climate and soil, the trait-trait relationships and trait-environmental relationships were documented for Caragana species in Nei Mongol region.
Methods We measured eight morphological and chemical traits from nine Caragana species distributed across 41 sites in Nei Mongol, including plant height (h), leaf dry matter content (LDMC), specific leaf area (SLA), stem tissue density (STD), leaf area (LA), leaf nitrogen content (LNC), leaf phosphorus content (LPC), and leaf nitrogen to phosphorus ratio (N:P). We tested the trait-trait relationships between species and within species, and explored their relationships with aridity, soil nitrogen content and pH at the genus levels.
Important findings We found that the aridity index explained more than 29% of the variation in most functional traits (except N:P), with h, SLA, LA, LNC, and LPC decreased, while LDMC and STD increased with increasing drought. h, SLA, LDMC, STD and LNC were also affected by soil total nitrogen content. However, soil pH explained less for Caragana traits. The synchronous correlations or trade-offs among different functional traits were stronger at the genus level, while the trait-trait relationships within species were weak. There was a consistent increase in h, SLA, LA, LNC and LPC, but a decrease in LDMC and STD to declined aridity. We only observed significant correlations at the within-species level for C. korshinskii and C. microphylla, which were the most widely distributed. Plants with smaller distribution range have weak intraspecific covariation relationships among functional traits, which indicated that the plant economic spectrum theory is not necessarily suitable to explain the utilization of resources and the adaptation strategies of plants to environmental changes at the local scale at the individual level.

Aims The dynamic changes of plant leaf traits during sand dune fixation in semi-arid areas is of significance to reveal the ecologically adaptative strategies of sand plants to environmental changes in the region.
Methods Eleven leaf traits of a typical species, Artemisia ordosica, in the Mau Us Sandy Land, was measured at its four different stages of sand dune fixation (semi-fixed dune, fixed dune, fixed dune with biological soil crusts, fixed dune with abundant herbaceous plants). The variations in the leaf traits and their relationships over different sand-fixation stages were tested using correlation, principal component and similarity analysis.
Important findings (1) The leaf area (LA) and maximum net photosynthetic rate (A_max) in the semi-fixed dune stage were significantly higher than the other three stages of sand dune fixation, leaf carbon content increased significantly and then decreased slightly as the dunes gradually fixed. Leaf phosphorus content at the semi-fixed dune and the fixed dune with biological soil crusts was significantly higher than that at other two stages, and being highest at the semi-fixed dune stage. These variations in leaf traits indicated that A. ordosica tended to enhance photosynthesis for survival in the semi-fixed dune stage. (2) Photosynthesis-related traits of LA and specific leaf area (SLA) were significantly and negatively correlated with physically defense-related structure traits of leaf dry matter content (LDMC) and leaf tissue density, respectively, regardless of the stage of sand dune fixation. The results demonstrated that in order to adapt to the dry environment, the investment in leaf photosynthetic capacity would decrease with the increase of investment in physical defense structure. It also showed that there was a trade-off between the plant growth and defense capability enhancement at different stages of sand dune fixation. (3) The leaf chemical traits were significantly correlated with the leaf morphological and physiological traits. And the leaf traits tended to develop from “rapid investment - return” to “slow investment - return” as sand dune fixed. The results demonstrated that sandy plants could adjust their resource use trade-off strategy to adapt to different environmental changes through leaf traits and interrelationship optimization.

Aims The subject of plant functional traits, which reflect plant growth and survival strategies, is an important trending topic in plant ecology. In this study, we selected three native warm temperate shrub species in China: Vitex negundo var. heterophylla, Ziziphus jujuba var. spinosa, Grewia biloba var. parviflora and two alien species: Amorpha fruticosa and Rhus typhina, to compare their growth and survival strategies.
Methods The differences in growth strategies were analyzed by measuring plant functional traits. This experiment was carried out at Fanggan ecological experimental station of Shandong University in Jinan. The seeds of five shrubs were germinated and 20 seedlings of each species were planted in pots containing a mixture of sand and humus (2:1, by volume). The leaf structural traits, leaf nitrogen, phosphorus and potassium, chlorophyll content, leaf gas exchange parameters, chlorophyll fluorescence, biomass, and seedling growth parameters were measured.
Important findings Physiological and ecological characteristics of five shrub seedlings were different under the same condition. 1) The alien species, i.e. R. typhina and A. fruticosa, showed high photosynthetic rates and had a strong ability to adjust to the instantaneous change of light intensity, which may be beneficial for plants to make full use of sunflecks under the canopy. The high supporting structure ratio of A. fruticosa is also more conducive to leaf movements, thus adjusting to the changeable light environment. The supporting structure ratio of A. fruticosa was as high as 0.434, which is conducive to its leaf movement and adjustment to the changeable light environment. The high photosynthetic capacity of R. typhina and A. fruticosa was the vital reason for the establishment of the two alien species. 2) The local species V. negundo var. heterophylla and Z. jujuba var. spinosa demonstrated relatively conservative utilization of resources, and in both of them low gas exchange values were measured. However, V. negundo var. heterophylla could make full use of the midday light. 3) G. biloba var. parviflora showed strong gas exchange capacity, and the actual quantum yield of diurnal variation of fluorescence was slow to recover in the afternoon as demonstrated by the chlorophyll fluorescence analysis. In addition, Grewia biloba var. parviflora had the lowest specific leaf mass (44.539 g·m^-2) and chlorophyll a/b (2.215), which confirmed that it is a shade-tolerant plant. Traits such as good seed dispersal ability, leaf heat dissipation ability, conservative light energy utilization strategy, and strong ability to recover after disturbance, help V. negundo var. heterophylla become the dominant species in the warm temperate zone of China. In summary, our study is helpful to understand the growth strategy and niche differentiation mechanisms in several common shrubs of warm temperate zone of China and provides theoretical basis for vegetation restoration, vegetation reconstruction and sustainable development of regional ecological environment.

Aims Niche studies play an important role in understanding the interspecific associations of communities. The study of interspecific relationship is important for understanding community structure, function and classification. Our objective was to quantitatively analyze the niche and interspecific association of main woody plants in Myrica nana shrubland in central Yunnan.
Methods Methods including niche measurement, variance ratio, chi-square test, Pearson correlation test, and Spearman rank correlation test, were used.
Important findings Results showed that: (1) Levins and Shannon’s niche width measurement results showed that M. nana had the widest niche width in the community, while Rubus aurantiacus var. obtusifolius had the narrowest one. There was no consistent relationship between niche width and important value, and some species had larger niche width and smaller important value, indicating that these species had greater development potential. (2) The value of the niche overlap index was generally small, which showed that the demand similarity for environmental resources among the main species was small, and the competition between main species was not strong. (3) Species with larger niche width had a higher probability of niche overlap with other species, but this was not absolute, some species with small niche width value also had a higher degree of niche overlap. (4) The overall connectivity of the main species showed a significant positive correlation, indicating that the community species could coexist harmoniously and the community structure tended to be stable. (5) Chi-square test, Pearson correlation test and Spearman rank correlation test revealed no significant relationship between most of the species pairs, indicating that the connection between species pairs in the community was weak, and each species showed an independent distribution pattern. (6) Pearson correlation coefficient and Spearman rank correlation coefficient among main species both had a very significant positive correlation with the niche overlap index, indicating that when the species pairs showed extremely significant positive associations, the niche overlap value was large, and when the species pairs showed extremely significant negative associations, the niche overlap value was small.

Aims Environmental factors are among the key factors governing the community species spatial turnover. As one of the widely distributed vegetation types in subtropical region of China, it is important to understand the species composition differences among shrubland communities, which will provide important information for protecting biodiversity and eco-security shield construction in the northern mid-subtropical region, China. However, little is known about which factors drive the species spatial turnover in subtropical region of China. The objectives of this study were to investigate the shrubland community spatial turnover pattern and to determine the key factors that shape the present distribution of species based on the field-based data in northern mid-subtropical China.
Methods Based on the field-based data in mid-subtropical shrublands, we used the generalized dissimilarity modelling (GDM) by the Bray-Curtis dissimilarity index to explore the driving effects of climate, soil, topography, and human disturbance on species turnover of the shrubland communities in northern mid-subtropical China.
Important findings The results showed that climatic factors had significant effects on plant species turnover of the shrubland communities in northern mid-subtropical China. With the increase of altitude, slope, soil total nitrogen content and gross domestic product (GDP), species turnover rate increased significantly. Mean annual air temperature and the distance to road had no significant effects on species turnover. The species turnover rate decreased when soil pH became too acidic or too alkaline, and the rate reached the maximum value when pH was 5. Climate, soil, topography, and human disturbance explained 33.55% of the species turnover deviances, of which, soil accounted for 26.54% of the GDM deviances, while climate and topography, human disturbance accounted for 13.39% and 3.17% of the GDM deviances, respectively. Soil pH contributed 37.28% of the deviances to the species turnover of shrubland community. In conclusion, environmental factors (especially soil pH) were the major drivers of the species spatial turnover in northern mid-subtropical shrubland communities.

Aims The study of the spatial distribution pattern of soil nutrient stoichiometry along the altitude gradient is helpful to clarify the status of nutrient limitation in the mountainous ecosystems, to reveal the potential influencing factors to nutrient limitation, and to provide a scientific basis for regional ecological protection and vegetation restoration.
Methods The sampling plots were established along an altitude gradient from 3 100 m to 3 700 m in Pailugou watershed of the Qilian Mountains. Soil samples from 0-10 and 10-20 cm layers were collected separately to analyze the distribution of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) contents, and stoichiometric characteristics among different altitude and soil layers. Correlations between these nutrient characteristics and climatic factors, aboveground biomass, and soil pH were analyzed.
Important findings Results showed that: (1) Soil nutrients were concentrated on the soil surface and decreased with soil depth. The SOC and TN contents increased initially and then decreased as altitude increased. Soil TP content increased with increasing altitude. (2) Soil N:P increased from 3 100 m to 3 400 m, reached the maximum value at 3 400 m, and then decreased with increasing altitude and soil N:P was less than 14, suggesting an increase in N limitation along the altitude gradient. The C:P in soil was lower at high altitude than that at medium or low altitude, while C:N decreased with increasing altitude. (3) SOC content was positively correlated with TN content and negatively correlated with TP content. TN and TP content had no significant correlation. (4) Mean air temperature and total precipitation in the growing season showed positive correlation with SOC content and C:N:P, negative correlation with TP content, and no correlation with TN content. SOC and TN content had positive correlations with the aboveground biomass of shrublands. Soil pH was negatively correlated with TP content, but did not affect SOC and TN content significantly. Our results indicate that the productivity of alpine shrublands in the watershed was mainly limited by N and an appropriate amount of N application could alleviate the limitation.