Based on niche theory, recruitment limitation is to discuss population regeneration through the population demographic characteristics in different life stages, such as seed germination, seedling survival and growth, propagule dispersal, and varied environmental factors (soil moisture, nutrients, litter, etc.). The core of recruitment limitation consists of seed limitation and microsite limitation; however, there is no universal conclusion on which of them is more dominant. Seed limitation is related to insufficient propagules of seed production, soil seed bank and underground bud bank. Specifically speaking, inter-annual climate fluctuation, soil seed bank life time and animal predation can lead to seed limitation. Soil seed bank is often regarded as the vault of population recruitment, which promotes population recruitment together with the above ground seed rain. In contrast, if soil seeds have high death rate and dormancy rate, the role of seed bank will decline. The contribution of underground bud bank and its clonal ramets for population regeneration are more reflected on stronger resilience to disturbance. Dispersal limitation is a common phenomenon in population regeneration, which is related to many factors including seed production, dispersal ability, spreading media and seedling density. In terms of microsite limitation, embodying in how abiotic factors such as water, nutrients, litters, and biotic factors such as competition and predation effect on seed vigor, germination, seedling viability, and biomass allocation, its importance often varies with the stages of plant life history. In the future, it is necessary to conduct comprehensive and long-term experiments to focus on the underlying mechanisms involving evolution and ecology on seed limitation and its associated ecological processes, so as to obtain a more profound understanding of population recruitment, and establish a more comprehensive and systematic framework of population recruitment.

Aims Nitrogen (N) supply and planting density are two important factors influencing plant growth. Legumes are important to ecosystem N input because of their capacity of biological N₂ fixation (BNF). Increasing atmospheric N deposition may promote the growth of leguminous plants, but it may also inhibit strongly their BNF capacity. Planting density can also influence the growth and BNF capacity of legumes due to intraspecific competition. However, few studies up to date have focused on the effects of N fertilization and planting density on the growth and BNF capacity of legumes. In this study, we aimed to explore the potentially interactive effects of N fertilization and planting density on the growth and BNF capacity of Lespedeza davurica, a leguminous plant species which is widely distributed throughout the northern China grasslands.
Methods A pot experiment was conducted in a greenhouse. The experiment contained four levels of N addition (0, 5, 10, 20 g·m^-2·a^-1) by NH₄NO₃fertilizer and three levels of planting densities (1, 3, 6 Ind.·pot^-1, i.e. 32, 96, 192 Ind.·m^-2).
Important findings 1) Our results showed that both N addition and planting density could impact the biomass production ofL. davurica. N addition increased plant leaf carbon (C) and N contents and leaf-level net photosynthetic rate. Besides, N addition also stimulated the plant growth at both pot and individual levels, and the yield reached maximum at N addition of 10 g·m^-2·a^-1. Increasing planting density decreased leaf C and N contents, leaf-level net photosynthetic rate, and individual growth, but increased total biomass in each pot. 2) Nitrogen addition reduced the capacity of BNF of L. davurica, while increasing planting density could weaken this suppression effect to some extent. The combination of N addition of 10 g·m^-2·a^-1 and planting density of 3 Ind.·pot^-1 or N addition of 10 g·m^-2·a^-1 and planting density of 6 Ind.·pot^-1 could maximize the effects of N application on individual yield and the effects of increasing planting density on the alleviation of BNF suppression. Nitrogen addition suppressed the BNF of L. davurica through reducing plant investment to nodulation and nodule biomass production. The intraspecific competition and resource limitation caused by increasing planting density led to improvement in the investment to nodulation and nodule growth. 3) Structural equation model analyses showed that N addition and planting density combined explained variations in the plant biomass and nodule production either directly or indirectly by 64% and 42%, respectively. The results indicate that it is important to optimize the amount of fertilizer application and appropriate planting density when considering plantation and management of artificial and degraded grasslands.

Aims Our objective was to explore mechanisms underlying the effects of transgressive overyielding and plant diversity effects associated with legume-grass mixtures in Zhaosu Basin, Yili Prefecture, Xinjiang Uygur Autonomous Region, China.
Methods Replicate plots (n = 3) were sown in 2013 as legume and grass monocultures (8 in total, including Vicia sativa, Avena sativa, Onobrychis viciaefolia, Bromus inermis, Trifolium pratense, Phleum pratense, Medicage sativa, Dactylis glomerata), a two-species mixture (AM2, annual legume and grass; PM2, perennial legume and grass), a four-species mixture (PM4, 2 perennial legumes and 2 grasses), a six-species mixture (PM6, 3 perennial legumes and 3 grasses), and three mixed-ratio combinations (legume : grass = 6:4, 5:5 and 4:6). Total plot productivity was measured as forage yield (dry matter) each summer between 2013 to 2015, which were used to calculate transgressive overyielding effect (OV), transgressive overyielding effect 1 (OV₁), and transgressive overyielding effect 2 (OV₂). Plant diversity effects were determined by calculating the complementary effect (CE), selection effect (SEF), and net biodiversity effects (∆Y).
Important findings The OV amplitudes of AM2 in 2013, 2014 and 3 years average were less than those of PM2 and PM6. The OV amplitude of AM2 in 2015 was greater than those of PM2, PM4 and PM6. The difference between the productivity of the mixed community and the yield of the most productive product species in the community components and the average yield of each species showed similar laws. CE of AM2 in 2013, 2014, and 3 years average was greater than PM2, PM4 and PM6 in the respective years. While SEF were much smaller than CE in AM2, changes in CE were relatively stable in PM2, PM4 and PM6. Fitted curves of species richness, species evenness, and forage yield (community productivity) predominantly showed a single maximum in PM4, yet productivity was highest in the 5:5 ratio plot. CE, SEF,and ∆ Y in perennial legume-grass mixtures decreased over time, which also led to declines in OV amplitude, OV₁ and OV₂, and their stability. Thus, in the initial study year, CE and SEF jointly dominated OV, OV₁ and OV₂ in perennial legume-grass mixtures. Over the next two growing years, SEF became the main factor that influenced OV, OV₁ and OV₂ in perennial legume-grass mixtures. OV, OV₁ and OV₂ of annual legume-grass mixtures were all influenced by CE from 2013 to 2015.

Aims The mixed broadleaved-Korean pine (Pinus koraiensis) forest is a zonal vegetation type in Northeast China. Clarifying the influencing factors of productivity is of great significance for understanding the maintenance mechanism of productivity in temperate forests.
Methods The study was conducted based on survey data of a 9 hm² permanent plot in a typical mixed broadleaved- Korean pine forest in the Xiaoxing’an Mountains. Census data of 2005 and 2015 for all individuals with diameter at breast height >6.5 cm within each 30 m × 30 m sub-plot were used to calculate canopy structural complexity, species diversity and stand basal area. We analyzed the direct and indirect factors affecting forest productivity using structural equation model and data on topography and soil physicochemical properties in each sub-plot.
Important findings Both the canopy structural complexity and the species diversity were positively related to productivity, with canopy structural complexity having a significantly greater effect than species diversity. The effects of canopy structural complexity on productivity consisted of two attributes: the vertical stratification and the plasticity. The vertical stratification played a significant role in driving the changes in forest productivity, while plasticity had no significant effect. The stand basal area was positively related to productivity, but with less effect than the canopy structural complexity. Both the canopy structural complexity and the species diversity had indirect effects on productivity via stand basal area. However, the environmental factor representative of slope and soil total phosphorus played differential roles on productivity for varying canopy structral complexity, and it was significantly and negatively correlated with productivity after removing the effects of canopy vertical stratification. In short, in a typical mixed broadleaved-Korean pine forest, the canopy structural complexity is more crucial for explaining the diversity-productivity relationship than species diversity, and the roles of other biotic and abiotic factors in productivity should not be ignored.

Aims Heterostyly is a heritable style polymorphism controlled by genes, which was emphasized by Darwin as an adaptation to promote accurate pollen transfer between floral morphs by depositing two types of pollen on different parts of insect bodies. This hypothesis has received much attention, but little is known about the effects of different pollinator groups on the reproductive success of different morphs in some populations with large variation in floral morphs composition and unstable pollination system.
Methods We investigated the floral morph composition, morph ratio, pollinator types and pollination efficiencies inLimonium kaschgaricum which distributed in the south slope of Tianshan in Xinjiang, China, a heterostylous population with coexistence of homostylyous morph, in order to understand the function of different pollinator groups and their influence on the fruit set of different morphs and floral morph ratios.
Important findings Results showed that: 1) The population mainly composed of L-morph, S-morph flowers with reciprocal placement of anthers and stigmas and H-morph flowers with equal pistils and stamens height. There were no differences in corolla diameter, corolla tube length and pollen production among morphs, but the morphology of pollen ornamentation and stigma mastoid cells were dimorphic. H-morph flowers were identical with L- (or S-) morph flowers in morphology of pollen ornamentation and stigma mastoid cells. 2) The results of heteromorphic incompatibility test showed that intramorph and self-pollination were incompatible in all floral morphs. The intermorph pollination was compatible when pollen ornamentation and stigma mastoid cells were hetermorphic, otherwise it was incompatible. 3) There were two types of pollinator functional groups, long and short-tongued. But they did not appear synchronously, short-tongued insects appeared in early and medium stages of flowering, and long-tongued insects appeared in the later flowering period. When the short-tongued insects were main pollinators at the early and medium stages of flowering, the number of intermorph pollen on stigma of long- and homo-styled morphs was significantly higher than that of short-styled morphs. The intermorph pollen transfer efficiency between high-sexual organs were higher than between low sexual organs. In the later stage of flowering, when long-tongued insects were main pollinators, there were no differences in the intermorph pollen loads on stigma of long- and short-styled morphs, and the intermorph pollen transfer proficiency between high-sexual organs were symmetrical with low sexual organs. At the same time, the fruit sets in different flowering stages were obviously different. 4) Long- and short-tongued insects have significantly different pollen transfer efficiencies, the short-tongued insects only transfer pollen between high sex organs, were low efficient pollinators with low visiting frequency and intermorph pollen transfer proficiency. However, long-tongued insects were efficient pollinators for all floral morphs, and were high efficient pollinators with high visiting frequency and intermorph pollen transfer proficiency. The unstable pollination system caused by periodical appearance of long-tongued insects may make the short-tongued insects become the main driving force causing floral morphs variation and S-morph flowers may suffer greater driving force. H-morph flowers may exist as an alternative floral morph in this unstable pollination system due to overcoming the drawback of sunken stigma.

Aims The aim of the present study was to investigate the responses of understory vegetation to soil nutrients and bacterial communities.
Methods This study investigated the understory vegetation biomasses and species composition as well as soil physical and chemical properties in 17-year-old Chinese fir plantations with three densities (high-density (KH), medium-density (KM), and low-density (KL)) in Kaihua, Zhejiang. The changes of bacterial community structures were analyzed via 16S rDNA high-throughput sequencing techniques.
Important findings The result showed that the total above-ground biomass of the understory vegetation ranged from 0.10 to 2.10 t·hm^-2 and the dominant plant species varied in three Chinese fir plantations. The soil pH and available phosphorus content were significantly different between high-density and low-density forest stands. Correlation analysis showed that soil pH was positively correlated with the biomass of herbs, shrubs and the total of understory vegetation, while the content of soil organic matter was just positively related with the last two factors, and the available potassium content was only affected by the biomass of shrub. Based on the analysis of the soil microbial community, the Acidobacteria, Proteobacteria, Actinobacteria, and Chloroflexi were the dominant phyla in the three Chinese fir plantations. Redundancy analysis showed that soil pH, available nitrogen, available phosphorus and available potassium contents played a crucial role in regulating the soil bacterial community structures. Gp2, Gp1, Gp3 and Gp6 were the dominant subgroups of Acidobacteria, accounting for 51.32%- 57.38% of the Acidobacteria. With the decline of the Chinese fir density, the biomass of understory vegetation and the proportion of Gp1 increased, while the proportion of Gp2 and Gp6 decreased and the relative abundance of Gp6 was negatively correlated with soil pH. Obviously, the moderate reduction in stand density of pure Chinese fir forests was beneficial in the growth of understory vegetation and in maintaining a reasonable bacterial community structure, which helps to maintain the soil fertility of the Chinese fir forests and to achieve sustainable management in the long run.

Aims Human disturbance is one of the main obstacles to the forward succession of karst grassland, exploring the response of grassland to disturbance in terms of soil microorganism can provide the basis for the restoration and rational utilization of karst land. Our objective was to study the effects of different human disturbances on soil microorganisms and the underlying mechanisms in a karst grassland ecosystem of northwestern Guangxi, China.
Methods Three patterns of disturbances (burning, mowing, and mowing plus root removal) and one control treatment (enclosure) were conducted at the long-term monitoring plots in the Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences. We analyzed the changes of soil microbial diversity and community structure by high-throughput sequencing, and determined their relationships with environmental factors (slope position, soil physicochemical properties).
Important findings 1) For α diversity, at both middle and lower slope positions, the burning treatment significantly reduced the fungal Chao1 index, while the mowing treatment significantly reduced the bacterial Shannon index and Pedigree diversity index. However, the mowing plus root removal treatment significantly reduced the fungal Chao1 index and the bacterial Shannon index, respectively, at middle and lower slope positions. 2) For microbial community structure, burning, mowing and mowing plus root removal treatments significantly reduced the relative abundance of Acidobacteria at both middle and lower slope positions, while the fire treatment significantly reduced the relative abundance of Ascomycota from 74.49% to 34.72% at the lower slope position. 3) Redundancy analysis showed that soil microbial biomass carbon explained 29.8% and 26.8% of the changes of bacterial and fungal α diversity, respectively, and 31.7% of the changes of bacterial community structure. Root biomass explained 13.9% and 10.3% of the changes of bacterial α diversity and fungal community structure, respectively. In conclusion, the three studied human disturbances have significantly negative influence on soil microbial α diversity as well as having a significant change in and changed community structure, and the degree of influence varied among the pattern of disturbances and the type of microorganisms. Moreover, the effects were also regulated by slope position. Long-term human disturbances mainly affected the diversity and structure of soil microbial communities by changing soil microbial biomass carbon and root biomass. The decreases of α diversity and Ascomycota will not be conducive to the maintenance of soil ecosystem stability, and the decrease of Acidobacteria will not facilitate to soil organic matter degradation and iron cycling. Therefore, the long-term human disturbances such as burning and mowing will induce the functional degradation of grassland ecosystem.

Aims The salinized grassland ecosystem has low level of nutrients, and how increasing nitrogen (N) deposition affects N cycling has not been solved yet.
Methods An experimental platform was set up to simulate increasing N deposition in the saline alkaline grassland ecosystem in Youyu County, Shanxi Province, eight levels of N addition were designed, expressed by 0, 1, 2, 4, 8, 16, 24, 32 g·m^-2·a^-1 (N0, N1, N2, N4, N8, N16, N24, N32). The same amount of ammonium nitrate (NH₄NO₃) was sprayed at the beginning of each month during the growing seasons from May to September every year. From May of 2017, net N mineralization rate was measured once a month using top-cover PVC cylinders. The experiment has been conducted for three years from 2017 to 2019.
Important findings The results showed that, (1) High N addition levels (N16, N24, N32) significantly increased soil inorganic N pool. (2) Soil N mineralization in the salinized grassland ecosystem was dominated by nitrification. After three years of N addition, high levels of N addition (N24, N32) significantly promoted the rate of soil net nitrification, and different N levels showed different effects in different months and years. (3) The sensitivity of soil net N mineralization to different levels of N addition showed significant difference among three years with different precipitation. Lower N addition improved the sensitivity of soil net N mineralization, while higher N addition reduced the sensitivity of soil net N mineralization. (4) There was a positive correlation between soil net N mineralization rate and soil temperature, water content, and a negative correlation between soil net N mineralization rate and soil pH value. The sensitivity of soil N cycle to N addition was higher at low level of N addition than at high level of N addition in the saline-alkaline grassland of north China. The influence of increasing N deposition on soil N cycle should be considered in process-based models in the future.

Dashiwei Tiankeng group is rich in plant diversity resources. In order to further understand the plant community characteristics of the Dashiwei Tiankeng group, a total of 25 sample plots were set up in different positions (mouth pit, waist pit, bottom pit) and outside the pit in the Dashiwei Tiankeng group. 269 species of vascular plants belonging to 176 genera and 89 families were investigated by the sampling method, and Lauraceae, Rosaceae, Urticaceae and so on are the most abundant families in the community. The stratification is obvious and the distribution of dominant species was more uniform in each layer of the community. Among them,Manglietia aromatica, Choerospondias axillaris, Handeliodendron bodinieri were dominant species in the tree layer; Metapanax davidii, Itea yunnanensi were dominant species in the shrub layer, and the dominant herb layer is Elatostema. The seasonal phase is obvious of the community. It is mainly evergreen and deciduous broad-leaved mixed forest, with the evergreen broad-leaved forest at the bottom pit and the deciduous broad-leaved forest at the edge of mouth pit. The life form spectrum of the community is 55.76% for phanerophytes, 29.74% for aboveground plants, 6.69% for aboveground plants, 5.21% for cryptophytes and 2.60% for annual plants. The characteristics of the community types, composition and structure of Tiankeng were analyzed in this paper, which provided basic information for studies of plant communities in Dashiwei.