Aims The nitrogen and phosphorus characteristics of plants represent plant features and responses to environmental factors. Our objectives are to distinguish leaf and litter C : N : P stoichiometric characteristics, nitrogen and phosphorus resorption of trees, and the relationship between stoichiometric ratio and temperature and precipitation for four typical regions in China.

Methods We studied temperate coniferous, subtropical evergreen broad-leaved, tropical monsoon and tropical plantation forest in the Changbaishan, Dinghushan, Xishuangbanna and Qianyanzhou Ecological Stations, respectively. We analyzed leaf and litter C : N : P, N, P and the relation of N, P nutrition limitation at each station.

Important findings Leaf C : N : P in temperate needle and broad-leaved mixed, subtropical evergreen broad-leaved, tropical rain and subtropical plantation forests were 321 : 13 : 1, 561 : 22 : 1, 442 : 19 : 1 and 728 : 18 : 1, respectively. Litter C : N : P of the four forest types were 552 : 14 : 1, 1 305 : 35 : 1, 723 : 24 : 1, 1 950 : 27 : 1, respectively. The C : N of evergreen coniferous forest is higher than in evergreen broad-leaved and deciduous broad-leaved forests, but C : P has no relationship with forest type. Leaf N : P was highest in evergreen broad-leaved forest and lowest in deciduous broad-leaved forest. Plant N : P has a linear relation with latitude and mean monthly temperature, but neither N or P concentration has such a relationship. Plant at high latitude are easily limited by N, those in low latitude are easily limited by P, but results show that plants limited by N or P don’t have higher N or P resorption. Stoichiometric ratios of leaf and litter are consistent, but environmental factors have different effects on different kinds of plant.

Aims Our objective was to analyze relationships between foliar stable carbon isotope composition (δ¹³C) and environmental factors and leaf element contents in Pinus tabulaeformis, which is widely distributed in northwestern China.

Methods Leaf samples were taken in ten natural populations of P. tabulaeformis from Gansu, Ningxia and Inner Mongolia. The δ¹³C was determined using an isotope mass spectrometer. Environmental factors were recorded by Magellan GPS320 and from nearby weather stations. Leaf element contents were measured with low vacuum scanning electron microscope and X-ray energy dispersive spectroscopy. Carbon content was estimated with the potassium dichromate method, and nitrogen content was measured with the micro-Kjeldahl method. Data were evaluated with Excel 2003 and SPSS 13.0 analysis of correlation and regression.

Important findings Foliar δ¹³C ranged from -28.68‰ to -25.02‰, with an average value of -26.82‰. Foliar δ¹³C did not closely correlate with altitude and longitude, but was significantly positively correlated to latitude and negatively correlated with annual mean precipitation and temperature. Therefore, δ¹³C in different natural populations of P. tabulaeformis is significantly influenced by environmental factors, and annual mean temperature and precipitation are essential factors in δ¹³C’s capability of fractionation and growth. Leaf N, P and K content were significantly negatively correlated with δ¹³C; however, Si, Ca and Fe content were significantly positively correlated with δ¹³C. Different δ¹³C in different natural populations reflects changes in plant nutrient element content. Response mode of climatic factors and element content under different habitat conditions reflected and affected properties of δ¹³C change in natural populations of P. tabulaeformis.

Aims Study of the spatial distributions of species is important to biogeography. Studies can provide the foundation for ecological conservation and restoration. Therefore, our objective is to estimate Qinghai spruce’s (Picea crassifolia) potential distribution and assess its growth status in the upper reaches of the Heihe River in the Qilian Mountains of China.

Methods We built a biogeographical model of Qinghai spruce in environmental resource space at three dimensions based on concepts of Hutchinson’s n-dimensional hypervolume and relationships between species and resource utilization. The model requires data on species distribution and environmental variables important in the growth of the species. Data on the distribution of Qinghai spruce were obtained using Landsat TM data with supervised classification and decision-trees technology. The three most important environmental variables, i.e., mean growing season air temperature, mean annual precipitation and solar radiation, were spatialized with GIS technology.

Important findings The optimal combination of the three environmental variables (mean growing season air temperature, mean annual precipitation and solar radiation) in the area of Qinghai spruce distribution is 9 °C, 360 mm and 1.9 × 10³ kW·h·m^-2, respectively. The model is built to estimate Qinghai spruce’s potential distribution in a large area and to assess the species’ growth status at corresponding geographical positions. Our model provides a universal function that incorporates multiple environmental variables to estimate the species’ potential distribution and growth status in other regions.

Aims Our objective was to estimate grassland vegetation coverage (VC) in Inner Mongolia prairie, China, using a statistical model and a sub-pixel model and determine which model was more applicable in this area.

Methods Field experiments were conducted around three experimental stations in the Inner Mongolia prairie using a digital camera and a LAI-2000 plant canopy analyzer. A spectrum information model was used to extract the measured VC value from the photos. Statistical models were built between those VC values and six vegetation indexes. Then the VC map was made by using the model with the highest R². The measured leaf area index (LAI) values were used in the sub-pixel model to make another VC map.

Important findings The simple ratio vegetation index extracted from the Landsat-5 TM (thematic mapper) image had the higher correlation with VC values calculated from the photos taken in the field (R²= 0.761). The VC spatial distribution maps generated by the two models were generally similar, but the VC average value gained from the statistical model was 0.09 lower than that of VC values retrieved by the sub-pixel model. These two methods had similar results in the areas with higher and lower VC values, but the results from the sub-pixel model were higher than the statistical model with mid VC values.

Aims Climate change is generally accepted to be a critical problem. It affects crop growth stages through changes in sunlight, heat and moisture. Our objective is to investigate the development of winter wheat growth stages under climate changes in northern China to determine possible causes of changes.

Methods Based on data of winter wheat growth stages and meteorology, we used multiple stepwise regression + residual interpolation to determine changes in winter wheat growth stages in northern China since 2000. Changes were investigated for two periods: 1971-1980 and after 2000.

Important findings The north part of northern China, including Beijing, Tianjin, Hebei and Shanxi Provinces, showed a significant warming and drying trend. In Henan and Shandong Provinces, temperature and precipitation had increased and sunlight had decreased. Jiangsu and Anhui Provinces also showed a trend of decreased sunlight and increased annual average temperature and accumulated temperature over 10 °C; however, the changes were small. Variations in climate cause changes in the growth stages of winter wheat. Compared to the 1970s, the sowing period had been delayed about 7-10 days after 2000 in most parts of northern China except Jiangsu and Anhui Provinces. The greening stage had advanced in the southeast, but was delayed in the northwest part of northern China. The jointing stage had advanced in northern China, especially in Beijing, Tianjin, Hebei, Shanxi and Shandong Provinces. It postponed the heading stage about 2-15 days. The harvesting stage had been postponed in most parts of northern China by 5-10 days. Variations in climate factors, mainly sunlight, temperature and precipitation, are the main influences on winter wheat growth stages. Greening and jointing stages showed significant correlations to annual average sunlight hours. An increase of annual average temperature more strongly affected the heading stage. An increase of accumulated temperature over 10 °C can postpone maturity of the stage. Precipitation can promote the stages of jointing and heading.

Aims Phosphorus (P) is an essential macronutrient for plant growth and development. Phosphorus deficiency is one of the major constraints limiting plant growth in many types of soils. To cope with P deficiency, plants have evolved numerous strategies including changes in root architecture and exudation of organic acids in general and citrate in particular to maximize P uptake and mobilize soil P. To unravel the adaptive strategies of Medicago falcata, a native legume species in temperate grassland in northern China, and M. truncatula, a legume model plant, to P deficiency, we compared biomass, root traits and exudation of citrate of the two species under two levels of P supply.

Methods Pot experiments were conducted to compare biomass, root traits and exudation of citrate in M. falcata and M. truncatula under two levels of P supply (500 and 5 µmol·L^-1).

Important findings Shoot biomass was reduced while root biomass was not affected in both M. falcata and M. truncatula when exposed to P-deficient medium, leading to an increase in root/shoot ratio. The lateral root length and total root length of M. falcata seedlings grown in P-deficient medium were markedly reduced compared to those grown in P-sufficient medium, while no significant effect of P deficiency on these parameters in M. truncatula seedlings was observed. A greater amount of citrate was exuded from M. falcata roots than M. truncatula roots under P-sufficient conditions. Citrate exudation from both species was stimulated in response to P deficiency with the stimulatory effect being much more evident in M. falcata than in M. truncatula. Taken together, these findings reveal that M. falcata and M. truncatula have developed different strategies to cope with P deficiency. Medicago falcata had a small root system and exuded large amount of citrate to mobilize soil P bound to soil particles. In contrast, M. truncatula had limited capacity to mobilize soil P by releasing citrate, but it maintained a large root system to more effectively explore sparse soil P and maximize P uptake by its large root surface area.

Aims Kobresia humilis meadow is a disturbance-dependent (disclimax) community resulting from long-term and heavy grazing. Our objectives were to (a) examine how the compensatory growth of K. humilis (including ramet density, compensatory height and compensatory aboveground biomass per ramet) varies with different levels of clipping, fertilizing and watering and (b) examine changes in specific leaf area (SLA), lamina net photosynthetic rate (P_n) and relative growth rate (RGR) in order to reveal the compensatory mechanisms of the species.

Methods Our field experiment was carried out at the Haibei Research Station of the Chinese Academy of Sciences from 2007 to 2009. The experiment used a split-plot design with clipping treatment on the whole plot (stubbled 1 cm, 3 cm and unclipped) and both fertilizer (fertilized and unfertilized) and water (watered and unwatered) treatments in subplots. We analyzed effects of clipping, fertilizing and watering on the response variables by multivariate ANOVA. We used stepwise regression analysis to determine the contribution of SLA, P_n and RGR to compensatory growth and Pearson’s correlation to determine the correlation among ramet density, compensatory height and compensatory aboveground biomass.

Important findings Compensatory height and SLA were reduced after clipping; RGR of ramet height and aboveground biomass, ramet density and compensatory biomass were increased. Fertilizing can increase ramet density, compensatory height, compensatory biomass, RGR of ramet height and aboveground biomass, SLA and P_n. The effect of watering on compensatory height and RGR of ramet height under stubbled 1 cm clipping was not obvious, but reduced them under stubbled 3 cm clipping. The interactions of clipping, fertilizing and watering showed that the relation between clipping and fertilizing is “antagonistic” and the relation between clipping and watering is “cooperative”. Although K. humilis appeared under compensation in plant height, fertilizing could enhance the compensation ability and promote tolerance against defoliation at a certain level of clipping.

Aims Our objective was to clarify the mechanism of drought tolerance for Agropyron cristatum on the Loess Plateau of China through investigation of drought stress on the biosynthetic and recycling metabolism of glutathione and ascorbate in leaves.

Methods We investigated the levels and redox status of glutathione and ascorbate and the activities of galactonolactone dehydrogenase (GalLDH), gamma glutamyl cysteine synthetase (γ-ECS), ascorbate peroxidase glutathione reductase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) in the leaves of A. cristatum under different levels of artificial drought stress using pot experiments.

Important findings Levels of glutathione and ascorbate and their redox status were maintained under drought stress by increasing activities of GalLDH, γ-ECS, APX, GR, DHAR and MDHAR involved in the metabolism of glutathione and ascorbate. This protected A. cristatum from oxidative damage induced by drought stress. However, there were differences in the responses of the metabolism of glutathione and ascorbate related to different drought stress levels with prolonged stress. With 24 or fewer days of stress treatment, the metabolism of glutathione and ascorbate was enhanced under severe drought stress. After 24 days of stress treatment, the activities of GalLDH, γ-ECS, APX, GR, DHAR and MDHAR decreased significantly under severe drought stress, because A. cristatum suffered more severe oxidative damage. However, the activities of these enzymes were maintained throughout the whole period of stress under moderate drought stress. This suggested that A. cristatum could effectively protect itself from oxidative damage induced by moderate drought stress for long time periods. However, A. cristatum could only effectively protect itself from oxidative damage induced by severe drought stress for limited time, and its antioxidant capacity decreased significantly with prolonged stress. Our results also suggested that ascorbate and glutathione had important roles in protecting against oxidative damage under drought stress.

Aims Senecio subdentatus (Asteraceae) is an annual ephemeral in the Gurbantunggut Desert, northern Xinjiang, China. It produces two morphs of achenes in the same infructescence. Our aim was to determine the morphological, dispersal and germination characteristics of the heteromorphic achenes and the relationship between size and yield of plants produced by them.

Methods Shape, size, surface ornamentation (via SEM micrographs), dispersal, germination of the two morphs of achenes, number of peripheral and central achenes per infructescence and per plant and plant biomass were determined in the laboratory. The mode and duration of achene dispersal were measured in the field.

Important findings The peripheral and central achenes are columnar, but they are significantly different in size, color, length of pappus and pericarp micro-morphology. Peripheral achenes are pale yellow and central ones brown. There are no differences in rate of descent in still air or dispersal distance at wind speeds of either 1 or 2 m·s^-1. Therefore, the dispersal ability of the two achene morphs was not significantly affected by achene size or length of pappus. The germination percentages were higher for 1-month old peripheral achenes at 5/2, 15/2, 20/10, 25/15 and 30/15 °C, and there were significant differences among temperatures in both achene morphs. Light did not affect germination significantly. There are more peripheral than central achenes in a capitulum. The proportion of peripheral achenes in an individual was negatively correlated with plant size, while the proportion of central achenes was positively correlated with plant size. This suggests that small plants produce more peripheral achenes with higher germination percentages, while large plants produce more central achenes with lower germination percentages. In the unpredictable environment of the Gurbantunggut Desert, plant size of S. subdentatus is closely related to micro-habitat. This species can reduce competition among siblings and increase adaptive responses to different micro-environmental conditions by making full use of germination differences between peripheral and central achenes and regulating yield of both achenes in different plant sizes.

Aims Qianshan Pasture in the Kunlun Mountains and Cele Oasis of China are different habitats with different elevations. We compared six pasture species in terms of chlorophyll (Chl) content and fluorescence parameters to investigate the effect of habitat on photosystem II (PSII).

Methods Agropyron cristatum, Bromus inermis, Festuca elata, Elymus dahuricus, Onobrychis pulchella and Medicago sativa var. luxurians, were grown in both sites. Leaf pigment content was measured by 95% ethanol method, and PSII chlorophyll fluorescence parameters were monitored by using a Plant Efficiency Analyzer (PEA).

Important findings The measured physiological parameters differed greatly in all six species growing in the two habitats. In Qianshan Pasture, all species showed obviously higher Chl a, Chl b and total Chl content, while those growing in Cele Oasis had higher Chl a/Chl b. Plants in Kunlun Mountains also had markedly higher maximum fluorescence yield (F_m), maximum photochemical efficiency of PSII (F_v/F_m), potential activity of PSII (F_v/F_o) and active reaction centers per cross-section (RC/CS_o), as well as lower minimum fluorescence yield (F_o), absorption flux per reaction center (ABC/RC), maximal trapping flux per reaction center (TR_o/RC), flux of dissipated excitation energy per reaction center (DI_o/RC) and initial slope of fluorescence intensity (M_o) than those in the Cele Oasis. These parameters also showed large differences in variation amplitude between habitats. The results indicated that environmental conditions in the Cele Oasis were stressful to pasture species. It decreased chlorophyll content and inactivated PSII. The habitat in Kunlun Mountains was relatively favorable for these species.

Aims Our aim was to study effects of salt stress and high irradiance on photoinhibition of photosystem II (PSII) and I (PSI) in leaves of sweet pepper (Capsicum annuum).

Methods We used the chlorophyll a fluorescence technique and the JIP-test to study the responses of sweet pepper leaves to salt stress with different NaCl concentrations and under high irradiance (1 000 μmol·m^-2·s^-1).

Important findings The chlorophyll a fluorescence induction curves changed greatly as induced by NaCl stress under high irradiance, and photoinhibition of PSII was aggravated with apparent effects on PSII reaction centers and the acceptor side. Also, the donor side of PSII was damaged by high NaCl concentration under high irradiance. Additionally, the activity of PSI reaction centers (P700⁺) was inhibited by NaCl stress under high irradiance. Therefore, NaCl stress could aggravate photoinhibition of sweet pepper photosystem under high irradiance, and photoinhibition was greater with higher NaCl concentration. However, PSI photoinhibition was less than PSII photoinhibition. Additionally, high NaCl concentration could induce both damage to PSII donor sides and PSI.