Embolism induced by the freeze-thaw commonly occurs in the woody plant in mid- and high-latitude regions. In cold environments, the resistance capacity to freeze-thaw embolism of woody plants is critical for their growth and successful survival throughout winter, which directly determines their distribution in the earth. During freeze-thaw cycles, the freeze-thaw embolism of vessels is induced by the bubbles that are from dissolved frozen water because of the low solubility of ice. The resistance capacity to freeze-thaw embolism varies from plants with different anatomical structures. Plants can also reduce potential damages of freeze-thaw embolism through adjusting the xylem positive pressure by refilling embolism and metabolism, and/or increasing the sap solute contents to avoid the stress. Compared to embolism induced by drought, however, few studies have been conducted on embolism by freeze-thaw, and the underlying mechanisms of plant responses and regulation are poorly understood. In this paper, we first reviewed the process of formation and repair of freeze-thaw embolism of xylem, including the strategies of escaping freezing, tolerance, and repairing freeze-thaw cavitation. Then we summarized physiological characteristics of plant resistance to low-temperature stress, influencing factors and evaluating criteria, based on the multiple trade-offs between low-temperature resistance, drought resistance, and hydraulic efficiency. Finally, we proposed five priorities for future study in this field: (1) the threshold of freezing temperature for different plants; (2) the vulnerability segmentation of frost-induced embolism; (3) the relationships between freeze-thaw cavitation repair and metabolism exhaustion; (4) the potential trade-offs between low-temperature resistance, drought resistance and hydraulic efficiency; and (5) integration of the traits of freeze-thaw embolism resistance into the economic spectrum of plants.

In order to understand the research progress of litter decomposition and its underlying mechanisms, this paper presented a bibliometric analysis of litter decomposition in China from 1986 to 2018 based on the four common literature databases, including CNKI, ISI Web of Science, ScienceDirect and Springer Link. Litter decomposition researches are mainly from forest ecosystems (65%), and focus on above-ground litter. This suggests that the studies on below-ground litter decomposition should be strengthened in the future. About 68% studies focused on the litters from dominant species, which couldn’t represent the natural decomposition characteristics due to the mixed effects among litters from multiple species. Besides carbon, nitrogen and phosphorus, we should pay more attention to other key chemical components related with decomposition (e.g. K, Fe, Mn, lignin, tannin, etc.) and the heavy metal elements related with environmental pollution. Meanwhile, ecological stoichiometry is an effective method to interlink the biogeochemical cycle in the plant-litter-soil system. Nitrogen deposition and climate change are hot topics in the field of litter decomposition, especially the interactions of multiple factors (e.g. nitrogen, phosphorus, etc.), temperature sensitivity of litter decomposition and underlying mechanisms in permafrost under climate warming context.

Aims The aims of this study were to explore how vegetation restoration affects leaf, litter and soil C, N, P stoichiometry dynamics and nutrients cycling, and to characterize the homeostasis and nutrient use strategy of plants at different vegetation restoration stages in the mid-subtropical area of China.
Methods Four vegetation types representing the successional sequence in the secondary forests were selected using the “space for time substitution” approach in central hilly area of Hunan Province, China, which consists of Loropetalum chinense + Vaccinium bracteatum + Rhododendron simsii scrub-grass-land (LVR), Loropetalum chinense + Cunninghamia lanceolata + Quercus fabri shrubbery (LCQ), Pinus massoniana + Lithocarpus glaber + Loropetalum chinense coniferous-broad leaved mixed forest (PLL), and Lithocarpus glaber + Cleyera japonica + Cyclobalanopsis glauca evergreen broad-leaved forest (LCC). Permanent plots were established in each community. The organic carbon (C), total nitrogen (N) and total phosphorus (P) contents in leaf, undecomposed litter layer and 0-30 cm soil layer were quantified at each stage. The response and nutrient use strategy of plant to environmental changes were estimated by allometric growth, nutrient use efficiency and nutrient reabsorption efficiency.
Important findings 1) Along vegetation restoration, the leaf C:N, C:P ratios decreased significantly and the highest values were in LVR. Leaf C, N, P contents, soil C, N contents and soil C:N, C:P, N:P ratios increased significantly, in which leaf C, N contents and soil C, N contents, N:P in LCC were higher than those in LVR, LCQ and PLL, and leaf P content and soil C:N, C:P in PLL were higher than those in LVR, LCQ and LCC. Leaf N:P (>20) indicated that all restoration stages were P limited. C, N, P contents and their stoichiometry of litter fluctuated greatly. 2) The relationships between litter and leaf or soil nutrients and their stoichiometry were weak, and the significant correlations were found in the relationships between leaf and soil nutrients and their stoichiometry. Leaf C, N and P were positively correlated with soil C, N, C:N (except leaf C, N contents), C:P and N:P, while leaf C:N was negatively correlated with soil C, N, C:P and N:P, leaf C:P was negatively correlated with soil C content, C:N and C:P, and leaf N:P were negatively correlated with soil C:N. 3) During vegetation restoration, leaf N and P had significantly allometric growth relationship (p < 0.01) with the allometric index being 1.45. The use efficiency of N and P nutrients in leaf showed decreasing trends and reabsorption efficiency showed increasing trends, and the lowest N use efficiency was observed in LCC and the lowest P use efficiency was in PLL, but the highest N, P reabsorption efficiency were both in PLL. 4) The leaf N content had weak homeostasis, and leaf P content had strong homeostasis to maintain P balance in plant under P limited in soil. Vegetation restoration had significant effects on leaf, litter and soil C, N, P contents and their stoichiometry. The C, N, P contents and their stoichiometry had significant correlations between leaf and soil. Plants could adapt to the shortage of soil nutrient supply mainly by reducing nutrient use efficiency and improving nutrient reabsorption capacity. The N and P cycles of the leaf-litter-soil system gradually reached the “stoichiometric equilibrium” during vegetation restoration.

Aims Enclosure is one of the important measures for vegetation restoration of degraded sandy land. Understanding the plant community change of different initial types in long-term state of enclosing is vital for us to understand the vegetation restoration process or re-vegetation in sandy land. This paper aims to analyze the changes of plant communities and its comparative responses to long-term enclosure (2005-2017) of mobile dunes (enclosed in 2005), fixed dunes (enclosed in 1985) and sandy grassland (enclosed in 1997), in relation to soil seed bank, soil nutrient and precipitation and air temperature.
Methods The species composition, height, coverage and above-ground biomass were measured by quadrats in every year during 2005-2017. The soil organic carbon, soil nutrient and soil seed bank were measured by soil cores from the above quadrats in 2008 and 2017. Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to explore the relationship between plant community species composition and soil seed bank soil nutrients and soil moisture.
Important findings The results showed that the vegetation coverage and community species richness of mobile dune was significantly increased, but the community biomass had no obvious trend. The biomass of fixed dune community, shrubs, semi-shrubs and the perennial legume functional group dominances decreased significantly, but the dominance of the annual and perennial forbs increased significantly. The annual forbs were the dominant functional group, and community species richness and perennial grass dominances decreased significantly. The soil seed bank had no remarkable change in the three enclosed sandy land communities during 2008-2017, while the seed richness significantly increased in mobile dunes, and that of the fixed dunes and grassland presented downward trends. The soil available nitrogen and available phosphorus were increased significantly. Regression analysis showed the annual variation of plant community biomass was significantly affected by air temperature and precipitation, but there was a little effect on the inter-annual variation of plant community biomass and species richness. Detrended correspondence analysis (DCA) displayed that there was a high similarity between soil seed bank and plant community, and typical correlation analysis (CCA) results showed that grassland community species composition was closely related to soil nutrients. However, fixed dune community species composition was mainly related to soil moisture. In summary, the fixed dune plant community enclosed for 33 years and the sandy grassland plant community enclosed for 21 years present degraded trends, while the mobile dune plant community enclosed for 11 years is slowly recovering, thus, long-term enclosure is not always conducive to the restoration of degraded sandy land vegetation. We suggested that the influence of degradation degree of vegetation, soil nutrients and moisture, soil seed bank and precipitation should be synthetically considered when we set the duration of enclosure for restoration.

Aims The response of plant leaf functional traits to flooding in salt marshes is not only helpful in exploring the internal correlation between leaf plasticity mechanism and photosynthetic characteristics, but also of vital significance for gaining a better understanding on the stress resistance strategies of plants in salt marsh wetlands. The aim of this study is to investigate the responses of leaf functional traits of Saussurea salsa to flooding with the changes of water-logging durations.
Methods The research site was located in provincial migratory bird nature reserve in Xiao Sugan Lake, Gansu Province, China (39.22°-39.35° N, 94.45°-94.59° E). A sample belt was selected from the edge of the lakeshore to the end of the perennial tidewater line in the low-lying area along the north side of Xiao Sugan River. The duration of water-logging in the salt marsh wetland was measured base on the water level marked by the flood rise and retreat marks in Xiao Sugan Lake over the years. The sample belt was divided into 3 plots according to the water-logging duration: low flooding area (water-logging duration: 30-90 days); medium flooding area (water- logging duration: 90-150 days); and deep flooding area (water-logging duration: 150-210 days). Six subplots (2 m × 2 m) of S. salsa were selected from each of the three plots for a total of 18 subplots. We investigated community characteristics and population traits of S. salsa, soil moisture and soil electrical conductivity (EC). Six plants per subplot were selected for photosynthesis and chlorophyll fluorescence measurements. Foliar samples collected from each of the six S. salsa were taken to the laboratory for measurements of leaf traits (leaf area, thickness, dry mass and chlorophyll content).
Important findings The results showed that S. salsa changed the covariation strategy of S. salsa foliar morphology and photosynthesis with the extension of water-logging duration. In the low flooding area, S. salsa adopted fleshy lobular pattern with small specific leaf area (SLA), high effective quantum yield of photosynthetic system II (PSII) photochemistry in light (Y(II)) and low quantum yield of regulated energy dissipation (Y(NPQ)). However, S. salsa grew in the deep flooding area adopted completely opposite covariation strategy in foliar morphology and photosynthesis compared with those grew in the low flooding area. We observed a significant correlation between SLA and Y(II), photochemical quenching (QP), and Y(NPQ), as well as a significant positive correlation between the quantum yield of non-regulated energy dissipation (Y(NO)) and chlorophyll a content (C_a), chlorophyll b content (C_b) in all of the three plots. Under the influence of the spatio-temporal evolution pattern of still water in the flooded area of Xiao Sugan Lake, S. salsa population achieved the balance of the photosynthetic carbon budget by changing the morphological characteristics of leaves, such as leaf area, leaf thickness and SLA, timely adjusting the photosynthetic characteristics, such as Y(NPQ) and Y(II). Saussurea salsa showed strong tolerance to water and salt heterogeneity, reflecting the leaf plasticity and resistance mechanism of salt marsh wetland plant under extreme environments.

Aims As an important link between plants and atmospheric environment, foliar organs have strong responses to stress. Understanding the adaptive mechanisms of plants to environments based on leaf traits is of great significance for establishment of plant communities in saline-alkali land.
Methods Eleven tree species used for afforestation were studied under three soil conditions in the coastal saline-‌alkali land of Shandong Province. The foliar anatomical traits were measured, and the responses of these traits to saline-alkali soil environment were determined to reveal the relationships between foliar functional traits and soil conditions.
Important findings (1) The leaves of the 11 tree species studied were thicker on the saline-alkali sites than on other sites, with 3-5 layers of well-developed palisade tissue closely arranged on the paraxial surface of the leaf mesophyll. The thickness ratio of palisade tissue to spongy tissue (PT/ST) was generally high but with large variations among the tree species. (2) The foliar anatomical traits differed among the three sites in different tree species. (3) Both correlation analysis and redundancy analysis (RDA) showed that the foliar anatomical traits were closely related to soil conditions; PT/ST was highly significantly correlated with soil indexes, positively with soil pH and soil conductivity at 25 ℃, and negatively with soil nitrate nitrogen content. Leaf characteristics and vein characteristics could explain 84% of the variations in leaf functional characteristics with environments. Overall, the foliar anatomical traits were closely related to soil conditions in saline-alkali land. The analysis of foliar anatomical traits could be used to study the adaptation of tree species to saline-alkali land, and as basis for tree species selection for vegetation restoration and community establishment.

Aims The microstructure of root in cross section is composed of connected skeleton and interconnected or closed holes, which is closely related to the tensile mechanical properties of root system.
Methods The relationship of the microstructure of root in cross section and the tensile performances of root, in this study, was discussed by means of single root drawing test and scanning electron microscope (SEM).
Important findings The main results showed that: 1) the ability of root in cross section to bear tension and tensile strain decreased with the increase of root diameter in per unit area; 2) the tensile strength and ductile behaviors of root decreased with the increase of average pore size, and the uniformity of pore size also had a certain influence on them; with the increase of root diameter, the arrangement mode of vessels presents: single vessel → multiple vessel → chain of vessel → cluster of vessel. The arrangement mode of vessel and the uniformity of distribution have influences on the tensile strength characteristics of root, the influence of the arrangement mode has a greater influence than the uniformity distribution; 4) the influence of area ratio of root in cross section on root tensile properties, is also influenced by xylem, root bark and vessel characteristics. In conclusion, this study revealed that the increase of root diameter influenced the microstructure in cross section and affected the tensile properties of roots from the perspective of root microstructure, which provided a certain theoretical basis to further analysis of the soil fixation mechanism of shrub root systems.

Aims Long-time continuing cropping of peanut (Arachis hypogaea) would result in soil deterioration, which would seriously affect the productivity and the quality of peanut. Arbuscular mycorrhizal fungi (AMF) have been used as a fertilizer that may improve root microenvironment, increase nutrient uptake and stress resistance of the plants. This study investigated the effects of Funneliformis mosseae on peanut rhizosphere microenvironment under continuing peanut cropping.
Methods We conducted an experiment to examine soil properties, peanut productivity and quality between the treatments of: (1) peanut seedlings inoculated with F. mosseae in continuous cropping soil, and (2) peanut seedlings without the inoculation.
Important findings We observed that F. mosseae significantly enhanced the activity of sucrase, urease, alkaline phosphatase and nitrate reductase in soil, significantly increased the soil contents of total nitrogen, total phosphorus, total potassium, available phosphorus and available potassium. Meanwhile, the abundances of Aspergillus, Fusarium and Gibberella in the rhizosphere soil of continuous cropping were decreased, while the abundances of Gaiella was significantly increased comparing to the treatment without F. mosseae inoculation. In addition, F. mosseae significantly increased the peanut yield and quality, including protein, oleic acid and linoleic acid content. Our results suggested that F. mosseae improve the peanut rhizosphere environment, alleviate the obstacles of continuous cropping.