Aims Ulmus pumila, a widespread tree in the semiarid ecosystems of northern China, forms sparse woodlands in Hunshandake Sandy Land. However, the population of U. pumila has been declining in recent years because of a lowered water table, mainly caused by increased human water consumption. Our objective was to clarify the relationships between tree ecophysiological activity and the water table in order to provide information for protection of the species.
Methods Saplings of U. pumila were planted in pots with simulated water tables at 1, 2, 3 and 4 m depth (U1, U2, U3 and U4, respectively). The impacts of water tables on the photosynthetic characteristics and stress tole- rence of U. pumila were determined by analyzing photosynthetic light and CO₂ curves, pigments, predawn water potential, soluble sugar and proline.
Important findings Both the utilization efficiency of strong and weak light by U. pumila and the maximum photosynthetic rate decreased with lowered water table. Compared to U1, the light saturation point, apparent quantum yield and dark respiration rate of U2, U3 and U4 were significantly decreased successively (p < 0.05). The maximum net photosynthetic rate of U4 ((8.98 ± 0.08) μmol·m^-2·s^-1) was reduced by 16.9% compared to U1 ((10.81 ± 0.28) μmol·m ^-2·s^-1). Light energy conversion efficiency of U. pumila also declined with lowered water table. Compared to U1, the photorespiratory rate, maximum RuBP saturated rate of carboxylation, maximum rate of electron transport and rate of triose-phosphate utilization decreased (p < 0.05) and CO₂ compensation point increased in the order of U2 < U3 < U4 (p < 0.05). The soluble sugar and proline contents significantly increased (p < 0.05) with lowered water table, i.e. U1 < U2 < U3 < U4, indicating that the lowered water table induced the greater stress. Therefore, groundwater decline resulted in drought stress and down-graded photosynthetic ability. This implies that the decrease in U. pumila population might be caused by the lowered water table and that maintaining proper underground water table in Hunshandake Sandy Land is helpful to the sustainability of the sparse woodland ecosystem.

Aims Meadow grassland is a dominant vegetation type on the Qinghai-Tibetan Plateau, but its mechanisms controlling the exchange of CO₂ across a spectrum of time scales and carbon budget remain unclear. Our objective was to investigate the main drivers of ecosystem carbon dynamics and understand the potential response to future climate warming.
Methods We used the eddy covariance method for continuously measuring net ecosystem CO₂ exchange (NEE) and environmental factors over meadow grassland on the northern shore of Qinghai Lake from July 1, 2010 to June 30, 2011.
Important findings Diurnal changes of NEE were controlled by photosynthetic photon flux density (PPFD) during the middle growing season (May to September), and air temperature (T_a) was the determining factor on diurnal NEE during other periods. The maximum daily CO₂ uptake and release rate were 11.37 g CO₂∙m^-2∙d^-1 on July 1 and 4.04 g CO₂∙m^-2∙d^-1 on October 21, respectively. T_a was the primary environmental factor related to daily NEE, and the correlation was described by an exponential-linear equation (R ²= 0.54, p < 0.01). A significant asymptotical response of daily NEE with increasing leaf area index (LAI) and enhanced vegetation index (EVI) was observed. The interaction effect of LAI and T_a was significant (p < 0.05), while main effect of EVI versus T_a was more important (p < 0.001). Respiration quotient (Q₁₀) was 2.42 and ecosystem total respiration (R_eco) consumed 74% of gross primary production (GPP). The proper magnitude of diurnal temperature range (<14.8 ℃) could be propitious to ecosystem carbon sequestration. The meadow grassland acted as carbon sink and absorbed 271.31 g CO₂∙m ^-2from the atmosphere during the study period.

Aims Our objective was to determine the distribution pattern and climate characteristics of Kobresia in China.
Methods Distribution information was collected from the literature and floras. Climate information was collected from China’s meteorological stations. ArcGIS software and cluster analysis methods in SPSS software were applied to analyze data.
Important findings Kobresia is mainly distributed on the Tibetan Plateau and in northern China. Fifteen species have wide distributions, and ten have disjunct distributions. The ranges of elevation, latitude and longitude of Kobresia’s distribution are 1400-5 000 m, 23-46° N and 81-112 °E, respectively. Mean values of climatic factors are Holdridge annual biotemperature of 4-19 ℃, annual mean air temperature of 0-20 ℃, annual mean maximum and minimum air temperatures of 7-28 and -6-16 ℃, respectively, extreme high and low air temperatures of 25-40 and -37-0 ℃, respectively, mean air temperature in January and July of -14-13 and 11-24 ℃, respec- tively, highest air temperature in January and July of -7-23 and 18-30 ℃, respectively, lowest air temperature in January and July of -22-7 and 5-20 ℃, respectively, mean air temperature in spring, summer, autumn and winter of -4-19, 9-23, 6-21 and -11-15 ℃, respectively, Kira warmth index and coldness index of 23-159 and -36-0 ℃, respectively, annual precipitation of 154-1500 mm, precipitation in spring, summer, autumn and winter of 19-135, 53-662, 48-545 and 5-92 mm, respectively, Holdridge annual potential evapotranspiration of 261-1100 mm, Thornthwaite annual potential evapotranspiration, aridity index and humidity index of 399-895 mm, 67-786 and 179-816, respectively, and sunshine duration from April to October of 990-2100 h. Generally, there is a high richness of Kobresia under lower or medium mean value of thermal factors, medium mean value of pre- cipitation, aridity or humidity or higher mean value of sunshine duration. As for extreme value of climatic factors, the range for minimum-maximum value of annual air temperature, the range for minimum value of extreme lowest and maximum value of extreme highest air temperature, air temperature range for minimum value in coldest and maximum value in hottest month, air temperature range for minimum value in winter and maximum value in summer are -6-21, -12-28, -48-42, -32-33 and -20-25 ℃, respectively, and the ranges for minimum-maximum values of annual precipitation, aridity index and sunshine duration from April to October are 15-1800 mm, 7-890 and 701-2 300 h, respectively. There is a high species richness of Kobresia under lower and medium extreme values of thermal factors, medium extreme value of precipitation, aridity, and higher extreme value of sunshine duration. The results suggest that Kobresia in China is primarily adapted to microthermal subhumid or mesothermal humid climatic types.

Aims Leaf nitrogen (N) and phosphorus (P) and N : P stoichiometry have been studied intensively in different regions in China. Songnen grassland is a natural region. Its dominant vegetation is meadow, which is determined by soil properties, and its flora is complex. Our objective was to find the stoichiometric patterns for this region.
Methods Leaf samples of 80 herbaceous species were collected in Songnen grassland in August 2008. We determined leaf N, P, and N : P on both a mass and an area basis and tested the differences according to plant life forms and functional groups.
Important findings Leaf N and P concentrations were (24.2 ± 0.96) and (2.0 ± 0.10) mg·g^-1 on a mass basis and (13.0 ± 0.54) and (1.0 ± 0.05) mg·cm ^-2 on an area basis, respectively. N : P was (13.0 ± 0.39). Plant growth was limited by N in Songnen grassland. The concentration of leaf N and P and coefficient of variation were higher in annual plants than in other life forms on a mass basis, and there were no significant differences of leaf N concentration on an area basis and N : P between different life forms. Leaf N concentrations both on mass and area bases and N : P of legumes were higher than in other functional groups. There was no significant difference in the leaf P concentrations on an area basis among different life forms or functional groups. Our findings indicate that appropriately increasing the proportion of legume plants would improve both the yield and quality of primary productivity in Songnen grassland.

Aims Leaf area index (LAI) is one of the most frequently used parameters for analysis of canopy structure and forest productivity. It can be determined by direct or indirect methods. Sources of errors in indirect LAI measurements with optical instruments include non-photosynthetic components, element clumping, and a topography effect. Our objective was to find an appropriate method for evaluating LAI and its seasonal dynamics in typical temperate montane forest.
Methods We examined the applicability of three indirect methods (LAI-2000 plant canopy analyzer, CI-110 plant canopy analyzer and digital hemispherical photograph (DHP)) and one direct method (litterfall) to determine the seasonal dynamics of LAI in three typical temperate forests in Dongling Mountain of Beijing. The forests included a secondary deciduous broad-leaved forest dominated by Quercus mongolica and two plantations (Larix gmelinii var. principis-rupprechtii and Pinus tabuliformis). These represent deciduous broad-leaved forest, deciduous needle-leaved forest and evergreen needle-leaved forest, respectively. The optically-based effective leaf area index (LAI_e) values were corrected to eliminate wood elements and clumping effects and then compared with the direct measurement of LAI based on litter collection.
Important findings For all the three forests, the LAI seasonal dynamic shows a uni-modal pattern with a peak in July. Optically-based LAI_e underestimated the amplitude of the seasonal dynamics. Wood element elimination can strengthen the seasonal sensitivity of LAI, especially in the seasonally distinct deciduous forest. The order of LAI_e measured by the three optically-based methods in three stands is LAI-2000 plant canopy analyzer > DHP > CI-110 plant canopy analyzer. There was good correspondence with the direct measurement LAI for the corrected LAI of LAI-2000 plant canopy analyzer and DHP. The correction method based on gap-size distribution was appropriate for the LAI-2000 plant canopy analyzer and the method based on both gap fraction and gap-size distribution for DHP. Considering economics and practical convenience, we recommend DHP for LAI evaluation in these temperate montane forests.

Aims Our objective was to investigate the influence of light intensity on growth of Datura stramonium and D. stramonium var. tatual.
Methods Plants of D. stramonium var. tatual and D. stramonium were grown under five levels of light intensity in light incubators. We determined plant growth and pigment concentrations in leaves and stems and studied leaf anatomy.
Important findings Both plants grew best under 13 000 and 18 000 lx and had increased leaf and stem biomass with increased light intensity. Under the same light intensity, stem length, basal stem diameter, blade number, bud length, leaf and stem biomass of D. stramonium var. tatual appeared higher than those of D. stramonium, but most differences were statistically insignificant. Leaf, palisade and spongy thickness and stomatal density and index all increased with increasing light intensity. Under the same light intensity, stomatal density and index of D. stramonium var. tatual were significantly higher than those of D. stramonium. For both species, chlorophyll a, b and carotenoid concentrations in leaves showed open downward parabolic trends with increasing light intensity. Chlorophyll a, b and carotenoid concentrations increased with increasing light intensity in stems of D. stramonium var. tatual, but showed open downward parabolic trends in D. stramonium. Concentrations of anthocyanin, flavonoids, and total phenols of the two species increased with increasing light intensity. Stems of D. stramonium var. tatual became purple, presumably due to high levels of anthocyanin, flavonoids and total phenols. Higher levels of anthocyanin, flavonoids and total phenols, together with the higher stomatal density and index, may account for the wider distribution of D. stramonium var. tatual.

Aims Understanding the impacts of invasive plants on the dynamics of native plants in invaded communities is important for the control of the invasives. Our objectives were to analyze the dynamics of native plants in communities invaded by Solidago canadensis.
Methods We selected native plant communities that had experienced different densities of S. canadensis invasion. Four different invasive stages (based on S. canadensis densities of 0, 5-7, 11-13 and >18 ind. plant∙m^-2) were determined, and 10 plots were randomly distributed in each invasive stage. The plant height, density and aboveground biomass of native species in each plot were measured. The niche breadths and niche overlap coefficients of seven common native species were determined for each plot. Nitrate nitrogen (N), ammonia N, available phosphorus (P), total P and total N of soil were measured, as was total N of plant tissue for the seven common native species.
Important findings Solidago canadensis had more N accumulation than all seven common native species. The invasion of S. canadensis significantly decreased native species richness. With greater intensification of S. canadensis invasion, niche breadth significantly increased for native species with higher plant tissue total N but significantly decreased for native species with lower plant tissue total N, and the average value of niche overlap coefficients of each native species decreased. The invasion of S. canadensis significantly increased soil nitrate N, but decreased soil ammonia N, available P, total P and total N. Results indicated that the ability to accumulate nitrogen determined the dynamics of native plant species when communities were invaded by S. canadensis.

Aims Our objectives were to determine (a) the effects of quinclorac-degrading bacterium on ultrastructure of tobacco stem and leaves in quinclorac-contaminated soil and (b) the degradation of quinclorac in soil after bacterium HN36 treatment. The research will provide a theoretical basis for soil remediation by degrading bacteria and for tobacco production safety.
Methods We treated quinclorac-contaminated soil with degrading bacterium HN36 and then observed the ultrastructure of tobacco leaves using an electron microscope and determined chemical composition.
Important findings Cell ultrastructures of stem and tip leaves were more affected by quinclorac than those of middle leaves. The degrading bacterium HN36 had a positive effect on ultrastructure of tobacco leaves. The degradation of quinclorac in soil treated with the degrading bacterium was significantly faster than in untreated soil. Applying quinclorac-degrading bacterium HN36 to the soil contaminated by quinclorac could mitigate phytotoxicity of the tobacco cell ultrastructure and accelerate the degradation of quinclorac in soil.