Aims Our objective was to examine seeds of Echinochloa crusgalli, Xanthium sibiricum, Aeschynomene indica and Polygonum hydropiper, four common annual species growing in the water-level-fluctuation zone of Three Gorges Reservoir in China, in regard to tolerance of submergence and germination in the changing environment of the zone.
Methods We divided the water-level-fluctuation zone into nine elevational zones according to the Wusong Elevation criterion. We then observed seed submergence tolerance and the effects of submergence on seed germination in a “water impoundment-water recession” cycle after seeds of the four species were sown in the nine elevational zones. Seed germination was divided into seeds with and without sand sediment on surface.
Important findings Ripe seeds (or fruits) of the four species kept their vitality after submergence. The starting time for seed germination after the end of submergence and seed germination duration both gradually decreased with decreasing elevation (p < 0.01), and the starting time for seed germination was significantly reduced below 169 m (p < 0.01). Overall, within the same elevational zone, the starting time for seed germination after the end of submergence was longest for A. indica, followed by X. sibiricum, and shortest for P. hydropiper. For E. crusgalli, X. sibiricum and P. hydropiper, seed germination percentage first increased and then decreased with decreasing elevation, while it decreased for A. indica. Sand sedimentation had little effect on seed germination; the effect was positive for X. sibiricum and negative for A. indica. We concluded that seeds (or fruits) of the four annuals can tolerate submergence and germinate successfully in the changing environment of water-level-fluctuation zone. Therefore, these species are useful in the revegetation and restoration of this zone of the Three Gorges Reservoir.

Aims Our objectives were to compare the established vegetation and soil seed bank of tidal flat and tributary habitats of the Danjiangkou Reservoir of China’s Han River and to identify the contribution of hydrochory (dispersal of seeds by water) from the tributaries to the reservoir tidal flats.
Methods We sampled the vegetation and soil seed banks of four tributaries and the reservoir tidal flats of their estuaries. Using data from 91 sample plots, we compared the species composition of the established vegetation and soil seed banks of the tributaries and their tidal flats using Sørensen’s coefficient of similarity, detrended correspondence analysis (DCA) and two way indicator species analysis (TWINSPAN).
Important findings Species richness of the established vegetation and seed bank along the tributaries was considerably higher than in the reservoir tidal flats, but the density of seedlings germinated from the soil seed bank was not significantly different between the two habitats. TWINSPAN and DCA showed a separation of sites from the tributary and tidal flat zones. In the DCA ordination diagram, however, most species were grouped between the tributaries and tidal flats, indicating a close relationship of species composition between the two habitats. Sørensen’s coefficient of similarity also revealed a high similarity of the established vegetation and seed bank between the tributaries and the tidal flats. Seeds were germinated from all three layers of the top soil (0-15 cm) in the Han reservoir tidal flats, while only one species, Cynodon dactylon, was found in the top soil of 0-5 cm in the Dan Reservoir tidal flats. Our results indicate a potential contribution of hydrochory from the tributaries to the vegetation of the reservoir tidal flats.

Aims Precipitation change and nitrogen deposition are two important aspects of global climate change affecting forest ecosystems. Meteorological data from Gutian Mountain, which has typical subtropical evergreen broad-leaved forest, shows a slow downward trend in precipitation, particularly in the last ten years. Our objective was to investigate the effects of precipitation change and nitrogen deposition on physiological and ecological processes of subtropical tree species to better understand change patterns and mechanisms.
Methods We set up a controlled-experiment with two factors: nitrogen (control and addition of 10 g·m^-2·a^-1 NH₄NO₃) and precipitation (natural precipitation as control and reduction of 30%). We collected seeds of five tree species of Gutian Mountain in the fall of 2006, sowed them in the spring of 2007 and grew the tree seedlings under the treatments. We periodically investigated traits of growth, biomass and allocation for three years.
Important findings Nitrogen significantly promoted growth in height, stem basal diameter and biomass of Elaeocarpus glabripetalus, Liquidambar formosana and Schima superba, but only increased branch and leaf biomass of Pinus massoniana and branch biomass of Cyclobalanopsis glauca. After two years of treatment, nitrogen increased leaf weight ratio of E. glabripetalus and S. superba and increased branch weight ratio of L. formosana and C. glauca. With three years, nitrogen increased stem weight ratio of E. glabripetalus, L. formosana and S. superba and branch weight ratio of C. glauca and P. massoniana. The increase of the aboveground weight ratio is at the cost of decreased underground weight ratio. Nitrogen reduced the root weight ratio of E. glabripetalus, L. formosana, S. superba and C. glauca. Reducing precipitation by 30% had no significant effects on growth.

Aims Increasing N deposition and frequent abnormal weather are two characteristics of global climate change. Our objective was to study the physiological responses of bryophytes to simulated nitrogen treatments and low temperature stress to provide insights to the relationship between N deposition and compensatory effect after low temperature stress.
Methods Physiological responses of three bryophyte species, Pogonatum cirratum subsp. fuscatum, Hypnum plumaeforme and Reboulia hemisphaerica, which were subjected to a 2-year-simulated N deposition and suffered a low temperature stress in early 2008, were studied and compared with the results of the bryophytes that experienced a 1-year-simulated N deposition and normal weather conditions. N treatments (Control, 20, 40 and 60 kg N·hm^-2·a^-1) in three replicates were established for each species. The N additions were divided into four applications per year.
Important findings The results of 2008 showed that net photosynthetic rate and concentration of starch decreased with increasing N addition doses within the range of 0-60 kg N·hm^-2·a^-1. The concentration of total N increased with increasing N treatment doses within the range of 0-40 kg N·hm^-2·a^-1, but decreased at N addition doses of 60 kg N·hm^-2·a^-1. At control and low N addition conditions (20 kg N·hm^-2·a^-1), most of the indices of carbon and nitrogen metabolism of the three bryophytes were higher than the results of 2007 of the same species at the same N treatments, but N addition depressed the increase. At high N treatment conditions (≥ 40 kg N·hm^-2·a^-1), the results of 2007 and 2008 were usually similar. Results indicate that the bryophytes exhibit overcompensatory growth after experiencing a low temperature stress, but at stimulated N deposition conditions, their compensatory ability decreases.

Aims Methane (CH₄) is an important atmospheric trace gas contributing to global warming and atmospheric chemistry. Aerated soils are a biological sink for atmospheric CH₄. Our objectives were to quantify CH₄ uptake by soils in typical grasslands of Inner Mongolia and examine the effects of plant functional type on the uptake.
Methods We used static chamber sampling and gas chromatography measurement to examine the effects of four plant functional types (PFTs) ― perennial rhizome forbs (PR), perennial bunchgrass (PB), perennial forb (PF) and annuals and biennials (AB) ― on CH₄ uptake by aerated soils.
Important findings CH₄ uptake by soils showed seasonal change related to soil water content and temperature. Over most of observed periods in 2008 and 2009, there were no significant differences in soil CH₄ uptake rates among the various PFTs. During rapid plant growth in August, however, there were significant differences in the soil CH₄ uptake rates. The soil CH₄ uptake rates were lower with PBs. AB and PF increased the uptake of CH₄ by soils, while PR and PB had little influence. Soil physico-chemical factors such as temperature, water content and gas diffusion affect CH₄ uptake. Differences in CH₄ uptake rates by soils may be explained using these environmental factors affected by PFTs. With regard to trace gas, PFT has prominent effects on this ecosystem. Sub-dominant species and companion species (AB and PF), by regulating soil microbe and environmental factors, have important and irreplaceable roles on the uptake of CH₄ by soils.

Aims Although the relationship between size and density is central to plant ecology, research has focused on the above-ground parts of plants and few studies have examined size-density relationships of below-ground modules. Density-dependent regulation in plants can be viewed not only at population and individual levels, but also at plant part level, since growth in plants is a modular process. Our aims were to (1) investigate size-density relationships at plant part level and at individual plant level for both below- and above-ground parts of plants and (2) compare size-density relationships at the levels of plant parts and individuals.
Methods Allium cepa var. proliferum was planted in a completely random design at five plant densities (36, 49, 64, 121 and 225 plants·m^-2) and three replicates. Plot size was 5 m × 5 m. At harvest time (July 11), ten plants from each plot were destructively sampled to measure plant height, number of leaves, length of leaf blade, bulb diameter and mean weight per tiller. Each plant was separated into root, bulb, sheath and leaf blade, oven-dried for 120 h at 80 ℃, and then weighed. Regression analysis was conducted to explore the relationship between the characters measured and plant density and the allometric relationships of mean masses of below- and above-ground modules with density at plant part level and individual level.
Important findings Relationships between plant density and dry weight of root, bulb, leaf blade and sheath showed negative power functions with different allometric exponents for different plant modules: bulb (-1.14) < leaf blade (-1.03) < root (-0.78) < sheath (-0.49). Masses of below- and above-ground dry matter, as well as the whole plant, were decreased significantly with increased plant density. The allometric exponents were -1.13, -0.95 and -0.98 for below-ground, above-ground and individual plant dry matter, respectively. We concluded that the effect of density-dependent regulation on below-ground modules may be greater than that on above-ground modules. Blow-ground parts may be more closely constrained by plant density in contrast to above-ground parts and the whole plant. Consequently, competition for below-ground resources is predominant in the A. cepa var. proliferum populations.

Aims Daxing’an Mountains, the northernmost and largest forest area in China, showed significant warming in the 20th century. Scots pine (Pinus sylvestris var. mongolica) is one of the two major conifer tree species in the boreal forests of these mountains. Our objective was to detect possible influence of climate change on boreal forests in this area through dendroclimatology.
Methods We investigated climate-growth relationships of Scots pine using tree ring-width chronologies (total of 139 cores) developed from Mohe, Mengkeshan and Mangui in the northern Daxing’an Mountains. Four residual chronologies were developed from the cross-dated ring width series using the program ARSTAN. The relationships between climate and tree-ring index were analyzed using bootstrapped response functions analysis of the software DENDROCLIM2002. Redundancy analysis (RDA), a multivariate “direct” gradient analysis, and its ordination axes are constrained to represent linear combinations of supplied environmental variables. It was carried out on the four Scots pine chronologies and 24 climate variables for 1973-2006 using the program CANOCO. STATISTICA for Windows, release 6.0 (StatSoft Inc., Tulsa, Oklahoma, USA) was used for other statistical analysis.
Important findings Tree growth from the four sites showed different responses to local climates. Response analysis showed that the chronologies of Mohe I and Mohe II were negatively correlated with June temperature of the current year, while the chronologies of Mangui and Mengkeshan were positively correlated with October temperature of the previous year. The analysis indicated that temperature is the major limiting factor for the growth of Scots pine in the northern Daxing’an Mountains. Redundancy analysis between four Scots pine chronologies and regional climate data verified this conclusion.

Aims Our objective was to determine the characteristics of sulfur (S) accumulation and allocation by Suaeda salsa in two different habitats of the intertidal zone in the Yellow River Estuary of China.
Methods We established two plots (30 m × 30 m) in two different tidal flats and collected different organs of S. salsa every 20 days from May to November 2008 from four or five sites selected at random from the two plots.
Important findings The biomass of different organs of S. salsa in middle tidal flat (JP1) and low tidal flat (JP2) showed significant seasonal dynamics, and the values of JP1 tended to be higher than those of JP2. For JP1 and JP2, the ratios of aboveground biomass to belowground biomass changed similarly, but the values of JP2 tended to be larger than those of JP1. The litter biomass of JP1 and JP2 increased exponentially during the growing season. Although the total sulfur (TS) contents in leaves, stems or litter of JP1 and JP2 fluctuated greatly during the growing season, they all initially increased and then decreased. In comparison, values in roots decreased at all times following an exponential decay model. The TS accumulation amount and rate (V_S) of different organs and litter also had seasonal changes, and the values of above-ground organs of JP1 were much higher than those of JP2. Moreover, the TS accumulation amount and V_S of above-ground parts were higher than those of below-ground parts. The S allocation ratios of different parts of JP1 and JP2 also had significant differences. Leaves had the highest allocation ratio, and values of JP1 and JP2 were (38.34 ± 16.19)% and (66.27 ± 12.09)%, respectively, indicating that leaves were the most important site of S storage.

Aims Stipa purpurea is a dominant species of alpine grassland in northern Tibet, a region sensitive to climate change. Environmental variations in temperature, water and atmospheric carbon dioxide (CO₂) concentration caused by regional climate change should affect the photosynthetic physiology of plants. Our objective was to investigate the response of S. purpurea to environmental variation.
Methods We irrigated natural alpine grassland to imitate rainfall increasing soil moisture and then used a portable gas exchange system (LI-6400) to measure photosynthesis parameters at the irrigated field (H) and a natural field (CK). During measurement, we set three levels of leaf temperature (15, 20 and 25 °C) and gradually elevated CO₂ concentration from 50 to 1 500 μmol·mol^-1.
Important findings CO₂, temperature and soil moisture affected photosynthesis parameters of S. purpurea significantly, and interactions were apparent among these three factors. Rising CO₂ gradually increased net photosynthetic rate (P_n) until it decreased at extremely high CO₂. Higher temperature caused P_n to decrease, but sufficient water supply partially alleviated the negative effect. Sufficient water supply increased P_n. With rising CO₂, intercellular CO₂ concentration (C_i) increased, transpiration rate (T_r) decreased, water use efficiency (WUE) increased and stomatal conductance (G_s) decreased; higher temperature caused G_s to decrease faster. The photosynthesis parameters above were also impacted by the combined actions of humidity and temperature. G_s was maximized at 20 °C, and sufficient water supply increased G_s further. C_i increased with increased temperature, and this response was more pronounced with sufficient water. T_r was positively correlated with temperature and soil water. Vapor pressure deficit (VPD) was directly proportional to temperature and decreased with sufficient water. Increased temperature and increased water supply made WUE decline. Therefore, increased soil water had compensatory effects on the relationship between high temperature and photosynthetic parameters.

Aims Our objective was to investigate the threshold effects of photosynthetically active radiation (PAR) and soil mass water content (W_m) on photosynthetic and physiological parameters of Prunus sibirica, and understand the adaptability of P. sibirica to light and soil moisture conditions. We determined optimal W_m and PAR for P. sibirica to maintain higher net photosynthetic rate (P_n) and water use efficiency (WUE).
Methods Using CIRAS-2 portable photosynthesis system, we measured P_n, transpiration rate (T_r), WUE and other photosynthetic and physiological parameters of three-year-old P. sibirica under different soil moisture conditions.
Important findings P_n, T_r and WUE of P. sibirica had the critical response to soil moisture content and PAR. With increases in W_m (6.5%-18.6%), the light compensation point decreased and light saturation point, apparent quantum yield and maximum P_n increased. When W_m was about 18.6%, the low and high light use efficiency of P. sibirica was maximal. The index of P_n, T_r obviously increased with increasing W_m (9.2% to 18.6%), but P_n, T_r decreased when W_m was too high or low. When PAR ranged from 800 to 1 200 µmol∙m^-2∙s^-1, P_n and WUE were higher and the light saturation points of P_n and WUE were from 983 to 1 365 µmol∙m ^-2∙s^-1. These indicated that P. sibirica possessed higher adaptability to light conditions. Based on photosynthetic and physiological parameters, the soil water availability and productivity of P. sibirica were classified and evaluated. For P. sibirica woodland, < 9.2% or >22.3% were low productivity and middle WUE, 20.5%-22.3% was middle productivity and low WUE, 9.2%-12.9% of W_m was middle productivity and middle WUE and 12.9%-20.5% of W_m was high productivity and high WUE. The optimum high productivity and high WUE of W_m were about 18.6%, and the corresponding optimum PAR was about 1 365 µmol∙m ^-2∙s^-1.

Aims Our objectives was to investigate the genetic structure and diversity of ten natural populations of Jatropha curcas.
Methods Seven loci encodings were detected by vertical polyacrylamide gel electrophoresis.
Important findings The genetic diversity of J. curcas was high at the species level with a mean number of alleles per locus of 2.428 6, percentage of polymorphic loci of 97.14% and a mean expected heterozygosity of 0.396 4. A low level of genetic differentiation among populations (0.041 3) and a high estimate of gene flow (5.808 9) were detected together with a high level of genetic identity among populations (0.921 7-0.995 3). Unweighted pair-group mean arithmetic cluster analysis suggested that the genetic distances among populations were weakly correlated with their geographic distances. The results provided a good genetic base to select varieties.

Natural systems are essentially complex. In most cases, fully understanding natural systems requires the capacity to examine simultaneous influences and responses among multiple interacting factors. Compared with traditional multivariate methods, structural equation model (SEM) could specify the causal or dependent relationships among variables using the prior knowledge of researchers before conducting relevant experiments, i.e. initial models. SEM could not only identify the individual path coefficient for each relationship, but also estimate the whole model fit to determine whether to revise the initial models. We attempt to introduce SEM from the following aspects: definition and types of variables in SEM, detailed procedures for how to analyze data through SEM, some applications of SEM in ecology and recommended software. We encourage more researchers to apply SEM in ecological data analyses in order to improve understanding of natural systems and advance the field of ecology.

Desertification is widely recognized as one of the most serious global environmental threats occurring in ecological fragile areas. To assess desertification at a regional scale is of practical significance in understanding environmental change and strengthening ecosystem management. We critically reviewed the development of desertification concepts, as well as common assessment methodologies. We emphasized desertification assessment objects and indicators, as well as scales. We showed that, compared to other indicators, indicators derived from vegetation information are more reliable and effective in desertification assessment. We proposed future research prospects on desertification assessment.