Aims Precipitation and its spatiotemporal changes are crucial for determining the effects of climate on plant community assembly and functioning of ecosystem (CAFE) in arid and semi-arid regions. Plant functional groups (PFGs) - an effective representation of CAFE—have been widely reported for their identity-dependent response to the changing climate. Here, we examine the responses of different PFGs to the temporal changes in precipitation by using aboveground biomass (AGB) as the dependent variable.
Methods We conducted a long-term ecological research of AGB since 1982 in a typical steppe grassland of Nei Mongol, China. We used the monthly-observation dataset from 1982 through 2015 to quantify the empirical relationships between AGB of different PDFs and precipitation.
Important findings We found that: 1) the decline in precipitation-concentration degree (PCD) was coupled with an increase in small rainfall events (≤5 mm) during the 35-year study period; 2) temporal changes in precipitation resulted in AGB decreases of annuals and biennials (AB), perennial forbs (PF) and perennial rhizome grass (PR), whereas AGB increased for perennial bunchgrasses (PB); 3) AGB, PF and semi-shrubs (SS) were positively correlated with growing season rainfall with AGB positively correlated with PCD while AGB of the shrubs (S) was negatively correlated with PCD; 4) AGB showed no significant correlation with precipitation frequency, but a significant negative correlation for type I to II precipitation 0.1-10 mm; 5) AGB had negative correlations with frequency and amount of type I (0.1-5.0 mm) and type VI (20-25 mm) precipitation. We concluded that the increase in small precipitation events will significantly reduce the AGB. These small precipitation events should be further explored for their ecological significances in the arid and semi-arid regions.

Aims Nitrogen use efficiency (NUE) is a key functional trait in plants, which closely relates to ecosystem functions. However, it is still unclear about the regional patterns and affecting factors of plant NUE.
Methods This study quantified leaf and root NUE in 139 grassland plant species and explored their relationships with environmental factors and plant functional groups across 82 sampling sites in Nei Mongol and Qinghai-‌Xizang Plateau.
Important findings 1) We found that leaf NUE (53 g·g ^-1) in meadow steppe was significantly greater than those in alpine meadow (46 g·g ^-1), desert steppe (41 g·g ^-1) and typical steppe (39 g·g ^-1). Root NUE (108 g·g ^-1) in alpine meadow was higher than those in other ecosystems. 2) Leaf NUE was more sensitive to temperature than root NUE, but with increasing drought index they all showed a significant decrease. 3) Leaf and root NUE in forbs were significantly lower than sedges and grasses. In addition, leaf and root NUE of legume were 48% and 60% lower than those of non-legume, respectively. 4) Plant NUE did not show any significant relationship with soil nitrogen content. Overall, there was significant difference between leaf and root NUE in their spatial patterns in the Nei Mongol and Qinghai-Xizang Plateau grasslands. The main impacting factors were plant functional group and drought index. The findings are helpful for better understanding the mechanisms underlying the variation of grassland productivity in China, and also provide more scientific basis for grassland management.

Aims In this study, we aim to understand how grazing would influence the partitioning of assimilated carbon in an alpine meadow on the Qinghai-Xizang Plateau.
Methods Measurements on carbon partitioning were made in a long-term grazing experiment consisting of light winter grazing and enclosure treatments. The ¹³C tracer method was used to determine the partitioning and transportation of assimilated carbon into different carbon pools.
Important findings On the 30th day following the labeling, shoots retained 32% of the initial ¹³C, and roots and soil together retained 22%; about 30% of the initial ¹³C were lost through shoot respiration. There were significant differences in the retention in soil, and the respiratory emission from soil, of assimilated carbon between the light grazing and enclosure treatments. Under light grazing, plants invested more assimilated carbon into the root and soil carbon pools. The rate of ¹³C transportation from shoots to soil and the rate of respiratory ¹³C release from soil were both greater, and the retention of ¹³C in and respiratory release from shoots were lower, under light grazing than under enclosure. Our results suggest that grazing is an important mechanism for maintenance of grassland. Grazing may cause changes in the structure and functioning of ecosystems, and induce large variations in soil carbon storage. Alpine meadow in the Qinghai-Xizang Plateau is amongst the grasslands with highest elevation in the world, and has large soil carbon storage due to low temperatures. We found no difference in soil C stocks between light grazing and enclosure treatments, indicating that light grazing would have no significant impact on soil carbon stocks.

Aims Armeniaca mandshurica is an important species which serves the need of ornamental, wood and other economical uses. This species has been in wild or semi-wild state for a long time and few studies about this species have been conducted. This paper aimed to provide an important reference for the collection, evaluation and protection of germplasm resources of A. mandshurica.
Methods The status of the germplasm resources in the main distribution area of A. mandshurica was investigated. In total, 47 typical sample trees from Liaoning, Jilin and Heilongjiang Provinces were selected. For each tree, 22 quantitative traits and 7 qualitative traits were measured. The diversity of quantitative traits was represented by indicators such as coefficient of variation, and the diversity of qualitative traits was represented by indicators such as frequency distribution. Trend surface analysis was used to explore the geographical variation in the quantitative traits. A total of 115 pairs of SSR primers were used for PCR amplification for 47 A. mandshurica germplasms, and the genetic diversity of A. mandshurica germplasms was analyzed using genetic similarity coefficient. Germplasms were classified by cluster analysis based on phenotypic traits and SSR markers separately.
Important findings High phenotypic diversity was found among different germplasms in A. mandshurica. The coefficients of variation (CV) of the 19 quantitative traits ranged from 9.40% to 55.98%. Among the 19 traits, twig length had the highest CV and kernel width had the lowest CV. The Shannon-Wiener index of 7 qualitative traits ranged from 0.58 to 1.22. Due to the significant correlation between geographical locations and main climatic factors within the study area, the quantitative traits of A. mandshurica germplasms were closely related to their geographical locations. Among them, twig length increased from east to west, twig thickness and seed mass increased from north to south, and fruit handle length increased from northeast to southwest. Twig length and fruit handle length were positively correlated with altitude, twig thickness was negatively correlated with altitude, and seed mass was not correlated with altitude. Clustering analysis based on 26 phenotypic traits showed that the 47 A. mandshurica germplasms could be divided into 4 categories, which mainly reflected the difference of the germplasm characteristics in A. mandshurica and to some extent also reflected the difference of germplasm productions. Clustering analysis based on genetic similarity coefficients showed that the 47 A. mandshurica germplasms were also divided into 4 categories, which reflected the difference of germplasm productions. Chi-square test showed that the correlation between the two clustering results was not significant, and that the external environment was the key factor affecting phenotypic variations in A. mandshurica.

Aims Invasive plants are a subset of naturalized plants, and naturalization is a fundamental precondition for plant invasion and so compiling a complete inventory of naturalized alien species is necessary for predicting and hence preventing such invasion.
Methods Based on field investigations and various literature reports, we compiled a nationwide list of the naturalized plant species of China.
Important findings The results showed that there are 1 099 naturalized plants belonging to 112 families and 578 genera in China, 70% of them were introduced intentionally and were of American origin, followed by those with European origins, herbs were prevalent among naturalized species. Compositae, Gramineae and Leguminosae were the three most dominant families. Spatially, species diversity and density decreased from the southeast coast to inland northwest. Latitude and gross domestic product is the primary driver of this pattern, however, colonial history, distribution of botanical gardens and the number of surveys may also affect formation. In temporal terms, species diversity is currently in a period of rapid growth and will continue for 20 to 30 years. Social and economic development, human disturbance, land use and global change are the main drivers. This study enriches the background information about naturalized plants and improves our knowledge on threats associated with plant invasions in China. The gained information can be used to provide arguments for efforts on control and management of alien plants or conservation of native biodiversity in the country.

Aims Null model is an important basis for nesting judgment. Highly asymmetric structures often appear in plant symbolic fungal networks. This study aims to explore the influence of matrix asymmetric changes on network nesting judgment.
Methods The study was conducted based on various null model construction methods.
Important findings Constraints vary with changing null models, with reducing null space when additional qualifications were added during null model establishment. Highly constrained nulls are prone to causing type II errors. Highly asymmetric networks increase matrix temperature (NT) deviation based on random (Equiprobable- equiprobable, r00) null model while reducing overlap and decreasing fill (NODF) deviation. Values of z-score show that highly asymmetric networks contribute to the significant determination level of NT and NODF. The impacts on the judgment of nestedness of asymmetric networks differ between row and column fixed null models. The effects of network asymmetry change on nesting detection based on column constrained (c0) nulls are similar to that of random null model, but with smaller nesting deviation and standard deviations. No significant differences in both NT and NT deviations were observed among different asymmetry networks based on the row fixed (r0) nulls, with a lower NODF deviation in highly asymmetric network based on c0 nulls. To more accurately determine whether the asymmetric networks would have nested structures, we recommend using a combination of random and constrained null models. Our results also demonstrate that the r0 null model performs better than either the r00 null model or the c0 null model when comparing nesting level of different asymmetric networks.

Aims Soil aggregate is the main habitat for decomposition and transformation of soil organic carbon (SOC) and is important to regulate SOC sequestration. The mechanisms of the stability of SOC fractions may vary among different aggregate sizes. The aims of this study were to explore the characteristics of SOC “fractionation” in soil aggregates, and to reveal the mechanisms of carbon (C) sequestration in soil aggregates of mollisols after 31-year changes in vegetation cover.
Methods A long-term field experiment with different vegetation cover (grassland, farmland and bareland) was established in National Observation Station of Hailun Agro-ecosystem System. Soil aggregate fractionation, the density and humus fractionation within different aggregate sizes were further carried out.
Important findings The results showed that after 31 years of land cover change, the surface SOC and total nitrogen (TN) contents in grassland with higher C inputs increased significantly with time, while the SOC and TN contents decreased significantly in bareland, but with no statistical significance in farmland. The 2-0.25 mm (include 2 mm, the same below) aggregates was the excellent fraction for SOC sequestration under all three land cover. The stability of soil aggregate was in the order of: grassland > farmland > bareland. The mass proportion of soil aggregate and its associated content were highest in grassland, while the proportion of microaggregate and its carbon allocation rate were lowest in grassland. However, due to the lower C inputs in farmland and bareland, the distribution of aggregates was in the order of microaggregate > macroaggregate > silt-clay fraction under these two types of land cover, and organic carbon (OC) content was highest in microaggregates. Different vegetation cover changed the C “fractionation” of density and humus fractions in aggregates. Compared with farmland and bareland soils, OC contents in light fractions in >2 mm and 2-0.25 mm aggregates were higher in grassland, and the OC contents in furic acid, humic acid and humin were highest in 2-0.25 mm aggregates in grassland, while the humus OC accumulated in microaggregates in farmland and bareland. Our results indicated that the plant-derived C entered macroaggregates first, and long-term grass cover enhanced free and light C fractions in macroaggregate, which consequently improved the stability of soil aggregates and enhanced the “fractionation” effects of large aggregates on the humus fractions. Our results revealed the characteristics of carbon sequestration in soil aggregates under different vegetation cover.