Aims Soil respiration is an important component of terrestrial carbon budgets, but the effects of land use on soil respiration are inadequately understood. The objective of this study was to examine soil respiration in natural mixed (Betula platyphylla and Populus davidiana) secondary forest and primary Korean pine (Pinus koraiensis) forest in the Changbai Mountain, China.
Methods An infrared gas analyzer (IGA) linked to a 50 cm×50 cm×15 cm chamber without a bottom was used to measure soil respiration. Three 50 cm×50 cm steel frames were inserted into the soil to 3-5 cm in plots randomly selected one day prior to measurement. When measuring, the chamber was put on the steel frame to make a closed container capturing CO₂ from the soil surface. Soil respiration was measured every 20-30 days during the 2003-2005 growing seasons.
Important findings Diurnal variations of soil respiration were mainly affected by soil temperatures, and maximum soil respiration appeared 1-2 hours earlier in secondary than in primary forests. Soil respiration was the greatest in August. There was a significant exponential relationship with soil temperature at 5 cm depth, but no significant relationship with soil water content in both plots. During the growing season (May-September), total CO₂ efflux from natural secondary forests was estimated as 3 449.4 g·m^-2, about 1.3 times that in primary forests (2 674.4 g·m^-2). The secondary forests had higher soil temperature and lower soil water content, which enhanced roots activity and soil microorganism metabolism.

Aims Soil respiration is the second largest flux in the global carbon cycle. Human activity, especially forest cutting, had brought a lot release of soil carbon. But the effect of forest clear-cutting on soil respiration was not inadequately understood. We chose Korean pine forest in Changbai Mountain as the case study area. Our aim is to compare soil respiration in clear-cut fields with that in natural Korean pine forests, and to reveal the effects of clear-cut on the density and pattern of soil respiration.
Methods During the growing seasons from 2003 to 2005, soil respiration was measured by a closed static chamber system in the clear-cut fields and Korean pine forests in Changbai Mountain. Soil temperature and moisture during the upper layers were observed by geothermometer and TDR system.
Important findings The results showed that diurnal variation of soil respiration at clear-cut plot could be expressed as single peak curve, and the times of the maximum and the minimum soil respiration values appeared 2-4 hours earlier than those in forest plot, which consisted with that of soil temperature at 5 cm depth. Soil respiration rate at forest plot was higher with the proportion of 75% than that in clear-cut plot during the growing season. Soil temperature had a significant exponential relationship with soil respiration. Soil water may have negative effect on soil respiration during the growing season with high frequency precipitations. The soil respiration model including the effects of soil water and temperature could explain soil respiration variation much better than temperature-based soil respiration model. The management of clear-cut forest had obvious effect on the dynamic characteristics and intensity of soil respiration due to the decrease in biomass and the change in microclimate in Korean pine forest.

Aims Based on the dynamic measurements of soil respiration and its environmental factors in a maize (Zea mays) field during the growing season in 2005, the diurnal and seasonal variations of soil respiration (SR) and their responses to the coordinated effects of soil temperature and biotic factors were studied. Our objective was to describe the effects of biotic factors on the response of soil respiration to soil temperature and to determine the seasonal variation of soil respiration during the growth season of maize.
Methods Soil respiration rates were measured twice monthly during the growing season (May-September) in 2005 using a soil respiration chamber (LI-6400-09, Li-Cor Inc., Lincoln, NE) connected to a portable infrared gas analyzer (IRGA, LI-6400, Li-Cor Inc., Lincoln, NE). We inserted 15 soil collars into the soil and soil respiration was pooled over all 15 collars per plot. To catch the diurnal pattern, soil respiration rates were measured every hour from 6∶00 to 18∶00 on May 4, June 5, June 28, July 28, August 28 and September 22.
Important findings The diurnal variation of soil respiration showed asymmetric pattern, with the minimum value occurring around 6∶00-7∶00 hours (local time) and the maximum value around 13∶00 hours. Soil respiration fluctuated greatly during the growing season. The mean soil respiration rate was 3.16 μmol CO₂·m^-2·s^-1, with a maximum value of 4.87 μmol CO₂·m^-2·s^-1 on July 28 and a minimum value of 1.32 μmol CO₂·m^-2·s^-1 on May 4. During the diurnal variation of soil respiration, there was a significant linear relationship between soil respiration and soil temperature (T) at 10 cm depth. During the growing season, the coefficients of α and β were fluctuated because the net primary productivity (NPP) of maize markedly increased the slope (α) and the biomass (B) markedly influenced the intercept (β) of the linear equation. Thus, the dynamic model of soil respiration was developed. Most of the temporal variability (97%) in soil respiration could be explained by the variations in soil temperature, biomass and NPP of maize defined in the model. However, just taking account into the influence of soil temperature on soil respiration, an exponential equation over- or underestimated the magnitude of soil respiration.

Aims Soil respiration is important in carbon flux and changes in carbon cycling in forest soil. Understanding belowground response to fertilization is critical in assessing soil carbon dynamics and atmospheric nitrogen deposition. The objectives of this study were to: 1) compare soil respiration rates between Larix gmelinii and Fraxinus mandshurica plantations in the same field site, 2) examine the effects of nitrogen fertilization on soil respiration rate in both plantations, and 3) analyze the relationships between soil respiration rate and soil temperature, fine root biomass and nitrogen contents.
Methods In May 2002, we established six plots in each plantation type at the Maoershan Forest Research Station, and from 2003 to 2005 we fertilized three plots and left three unfertilized as the control. We placed ten soil chambers randomly in each plot in July 2004 and measured soil respiration rates using a soil chamber system from August 2004 to October 2005. At the same time, we estimated fine root standing biomass in each plot by sampling eight soil cores monthly, calculated dry mass and analyzed nitrogen content for each fine root sample.
Important findings Fine root standing biomass of F. mandshurica (229 g·m^-2) was greater than that of L. gmelinii (158 g·m^-2), and the average difference in soil respiration rates during the growing season was 19.8%. Nitrogen fertilization significantly decreased fine root standing biomass 27.4% in F. mandshurica and 18.4% in L. gmelinii; soil respiration rate was decreased 25.8% and 34.9% respectively. However, nitrogen fertilization did not change fine root nitrogen contents. In both plantation types, soil respiration rate exhibited significant exponential relationships with soil temperature (R ²=0.93-0.98). The range of Q₁₀ values was 2.45-2.62 for F. mandshurica and 3.02-3.29 for L. gmelinii. Nonetheless, nitrogen fertilization did not impact Q₁₀ values in either plantation type; the difference of Q₁₀ between fertilized and unfertilized was less 1%. Nitrogen fertilization in L. gmelinii and F. mandshurica did not alter nitrogen contents in fine roots or Q₁₀ values, but decreased fine root standing biomass, which suggests that reduction of soil respiration rate in fertilized plantations was caused by the decreases of fine root standing biomass.

Aims Soil CO₂ flux is driven primarily by the CO₂ diffusion gradient across the soil surface. Ideally, the soil CO₂ flux measurement should be made without affecting the diffusion gradient across the soil surface. With the closed chamber system, the soil CO₂ diffusion rate (∂c/∂t) is required to estimate the soil CO₂ flux. To obtain the ∂c/∂t, the chamber CO₂ concentration must be allowed to rise. Consequently, the ∂c/∂t will be affected by the CO₂ diffusion gradient across the soil surface because of the decreased CO₂ diffusion gradient in the soil chamber. Additionally, the ∂c/∂t will also be affected by the diurnal variation of the CO₂ concentration across the soil surface. Our objective was to compare linear and exponential fitting methods to estimate ∂c/∂t.
Methods Currently, the ∂c/∂t is commonly estimated using linear fitting regression. Instead of using the linear fitting method, an exponential fitting method is used to fit the time series of chamber CO₂ concentration adopted in the LI-8100 automated soil CO₂ flux system.
Important findings The ∂c/∂t estimated from the linear slopes was consistently underestimated as compared to that from exponential initial slopes. Nighttime ∂c/∂t was significantly negatively correlated with soil surface CO₂ concentration, suggesting that the decreased CO₂ diffusion gradient across the soil surface strongly influences the ∂c/∂t. For the closed-chamber method, linear curve fitting significantly underestimated the ∂c/∂t rate during the nighttime. These results demonstrated the importance of estimating the ∂c/∂t at ambient soil surface CO₂ concentration. The response of the ∂c/∂t to air temperature exhibited significant asymmetry characteristic, showing that it is a better way for exponential fitting to make long-term and continuous soil CO₂ flux measurement to elucidate the magnitudes and processes of soil CO₂ flux in the typical terrestrial ecosystem.

Two main components of soil respiration, i.e., root/rhizosphere and microbial respirations, respond differently to elevated atmospheric CO₂ concentrations both in mechanism and sensitivity because they have different substrates, derived from plant and soil organic matter, respectively. To model the carbon cycle and predict carbon source/sink of forest ecosystems, we must first understand the relative contributions of root/rhizosphere and microbial respirations to total soil respiration under elevated CO₂ concentrations. Root/rhizosphere and soil microbial respirations have been shown to increase, decrease and remain unchanged under elevated CO₂ concentrations. A significantly positive relationship between root biomass and root/rhizosphere respiration has been found. Fine roots respond more strongly to elevated CO₂ concentrations than coarse roots. Evidence suggests that soil microbial respiration is highly variable and uncertain under elevated CO₂ concentrations. Microbial biomass and activity are related or unrelated to rates of microbial respiration. Because substrate availability drives microbial metabolism in soil, it is likely that much of the variability in microbial respiration resulted from differences in the response of root growth to elevated CO₂ and subsequent change in substrate production. Biotic and abiotic factors influencing soil respiration were found to affect both root/rhizosphere and microbial respirations.

Winter CO₂ efflux from soils is a significant component of annual carbon budgets and can greatly determine carbon balance of ecosystems. However, present estimates of annual soil respiration are mostly based on measurements taken during the growing season and assume that microbial respiration in frozen or snow-covered soils is negligible. We analyze methods used, magnitude of winter soil respiration, and influencing factors. There are very few measurements of winter soil respiration except in tundra and alpine ecosystems. Winter CO₂ efflux from soils ranged from 0.002 to 1.359 μmol C·m^-2·s^-1 and 0.22 to 0.67 μmol C·m^-2·s^-1 in tundra and forest ecosystems, respectively. No direct relationship between soil temperature and winter CO₂ efflux from soils was found, but there is a critical threshold for active respiration, typically between -7 and -5 ℃, below which lack of free water limits microbial contributions to winter soil respiration. The depth, timing and duration of snow cover greatly influence the magnitude of winter CO₂ efflux from soils, with water availability an important limiting factor. If insulating snowpack is present, carbon availability also controls heterotrophic activity. We discuss current problems and future research needs.

Stem respiration is an important part of the annual carbon balance of forest ecosystems and consumes ca. 11%-33% of total net daytime carbon assimilation. Because of difficulties in measurement, little attention was paid to stem respiration studies in the past. However, with increasing atmospheric CO₂ concentration, studies of stem respiration have become popular. Several methods were applied in earlier studies, including gas exchange measurements and closed method. An open flow system is employed in recent studies. Results from recent research show that the diurnal pattern of stem respiration is bimodal with a midday depression and that rates are the greatest in the growing season. Controlling factors include meteorological factors (e.g., stem temperature, CO₂ concentration and humidity) and biological factors (tree species, tree age, diameter at breast height, sapwood size and nitrogen content in stem). Latitude, altitude and topographic factors indirectly influence respiration rates through meteorological or biological factors, in particular stem temperature. Stem respiration rate is positively correlated with stem temperature. The mechanism of stem respiration and its controlling factors will continue to be subjects of future research. Integration of meteorological and biological factors into models of stem respiration will provide insight into contribution of stem respiration to the carbon balance of forest ecosystems, role of forest ecosystems in reducing CO₂ concentration elevation in the atmosphere, response of forest ecosystems to global changes, and development of carbon cycle models of forest ecosystems. These issues and measurement techniques remain challenging and fruitful areas for future research.

Aims Net primary productivity (NPP) is a key component of the terrestrial carbon cycle. Model simulation is commonly used to estimate regional and global NPP given difficulties to directly measure NPP at such spatial scales. A number of NPP models have been developed in recent years as research issues related to food security and biotic response to climatic warming have become more compelling. However, large uncertainties still exist because of the complexity of ecosystems and difficulties in determining some key model parameters.
Methods We developed an estimation model of NPP based on geographic information system (GIS) and remote sensing (RS) technology. The vegetation types and their classification accuracy are simultaneously introduced to the computation of some key vegetation parameters, such as the maximum value of normalized difference vegetation index (NDVI) for different vegetation types. This can remove some noise from the remote sensing data and the statistical errors of vegetation classification. It also provides a basis for the sensitivity analysis of NPP on the classification accuracy. The maximum light use efficiency (LUE) for some typical vegetation types in China is simulated using a modified least squares function based on NOAA/AVHRR remote sensing data and field-observed NPP data. The simulated values of LUE are greater than the value used in the CASA model and less than the values simulated with the BIOME-BGC model. The computation of the water restriction factor is driven with ground meteorological data and remote sensing data, and complex soil parameters are avoided. Results are compared with other studies and models.
Important findings The simulated mean NPP in Chinese terrestrial vegetation from 1989-1993 is 3.12 Pg C (1 Pg=10¹⁵ g). The simulated NPP is close to the observed NPP, and the total mean relative error is 4.5% for 690 NPP observation stations distributed in the whole country. This illustrates the utility of the model for the estimation of terrestrial primary production over regional scales.

Aims Wolong Nature Reserve is the largest reserve for the endangered wild panda. Owing to historical factors, much of the forest has been disturbed and poorly restored, even in many sections of the core protection area. The relative importance of determinants for species recruitment after disturbance is poorly known. We investigated factors affecting regeneration of conifer-broadleaved mixed forest in forest gaps near Wuyipeng, an observation station for pandas.
Methods We examined environmental factors, gap characteristics and recruitment of new individuals in every forest gap along three line transects and analyzed the data using principal component analysis and correlation analysis in SPSS.
Important findings A range of natural and human disturbances influenced the pattern and characteristics of forest gaps in this area. Richness was higher for seedlings than for gap makers, but the ranking of species dominance was different between seedlings and gap makers. The establishment success of dominant species was affected by a variety of factors. For instance, the establishment of Betula spp. was correlated significantly with topographic factors, while that of Abies faxoniana was affected heavily by soil characteristics and that of Rhododendron spp. was correlated significantly with topographic factors and gap-maker characteristics. Moreover, all biodiversity indexes of regenerating seedlings were significantly correlated to principal components mostly reflecting soil characteristics. Therefore, soil characteristics appear to be the most important factor influencing regeneration of dominant species and seedling richness following gap disturbance.

Aims Leaf area index (LAI) and canopy openness are two important biophysical parameters of canopy structure related to water and carbon fluxes and light interception. They are not only key variables in ecological and climate models, but also important indicators of forest ecosystem productivity. Accurate estimates of LAI and canopy openness are required in studies of ecophysiology, atmosphere-ecosystem interactions, and global change. Many researches have been done in Dongling Mountain in the warm temperate zone; however, fewer studies have focused on LAI and canopy openness. The objective of this study is to measure and analyze the dynamics of LAI and canopy openness of three typical forest communities in this area. Results will be useful in testing and validating LAI estimations based on remote sensing, providing reference for study of canopy heterogeneity and its effects and modeling carbon, water and energy fluxes from stand to landscape levels.
Methods We used hemispherical photographs taken from the ground through a fisheye lens for indirectly assessing LAI and canopy openness. Photos were taken in May, August, September and November 2005 in deciduous broad-leaved forest (DBF), Larix principis-rupprechtii forest (LF) and Pinus tabulaeformis forest (PF).
Important findings The LAI of DBF and LF increased gradually with plant growth and development to August. The lowest value of canopy openness appeared in November. DBF maintained a higher LAI in August and was more open in November compared to LF. For PF, we observed few changes of LAI and canopy openness, which we attribute to leaf retention of this evergreen species. In all forests, canopy openness varied inversely with LAI. The correlation was exponential and significant; therefore, canopy openness is a good indicator of LAI in these forests.

Aims Vegetation-environment relationships are key to understanding and predicting vegetation patterns in mountainous areas. Much research has examined vegetation patterns related to climate and topography; however, most studies have focused on forest and shrubland and habitats or examined relationships of vegetation and environmental factors along elevation gradients. Meadows have rarely been studied. We examined a subalpine meadow on the top of Mt. Xiaowutai (summit 2 882 m), the highest mountain of Hebei Province, North China, to relate communities to environmental factors.
Methods We examined vegetation data in relation to temperature, solar radiation and a soil wetness index simulated by GIS. Two-way indicator species analysis (TWINSPAN) was used to classify herbaceous communities, and the relationship between plant communities and environment was examined using canonical correspondence analysis (CCA).
Important findings The thermal gradient of temperature and direct solar radiation was the key factor correlated with community distribution in the study area, followed by soil nutrients, grazing and soil moisture as reflected by the soil wetness index, which is controlled by topography. This study indicates that habitat conditions, including temperature, solar radiation, soil wetness and nutrients, are the key factors which determine the distribution pattern of alpine meadow communities.

Aims The Yangtze River Estuary (YRE) faces eutrophication caused by discharge of industrial and agricultural sewage. This leads to frequent harmful algae blooms involving the rapid proliferation of specific toxic species. The aims of our study are to: 1) apply scientific methods and field experiments to clarify the distributional relationship between phytoplankton species and the environment, and 2) provide data on species and environment informative for further research on estuary ecosystem protection.
Methods An interdisciplinary survey of hydrological, chemical and biological resources was conducted in the YRE on August 28 - September 6, 2004. Phytoplankton cell counts and species identification were performed with an inverted microscope at ×100-×400 magnification after sedimentation for 24 h in 25 ml Utermøhl chambers. Temperature, salinity, pH and dissolved oxygen were measured by a pre-calibrated YSI 6000. Other abiotic environmental data were obtained based on protocols of marine investigations. Canonical correspondence analysis (CCA) was applied to explore the relationship between phytoplankton species and environmental parameters using CANOCO4.0.
Important findings Turbidity and nutrient gradient along the Yangtze River runoff were the most important factors influencing the distribution of phytoplankton species. Transparency, nitrate and silicate were major factors affecting the phytoplankton community structure. The dominant phytoplankton species Proboscia alata f. gracillima preferred low nitrate concentration and clear offshore areas, while Skeletonema costatum preferred high nitrate and more turbid waters. CCA can be a useful tool to understand the spatial distribution of phytoplankton species in marine ecosystems.

Aims Some urban river segments are reconstructed, resulting in benefits as well as ecological problems including reduced species diversity and altered species composition. Therefore, better understanding of the ecological impacts of urban river reconstruction projects is important for effectively protecting river ecosystems in future projects.
Methods We studied a 4.2 km river reconstruction project of Fenhe River in Taiyuan, using 30 random sample plots before (1997) and after (2003) reconstruction. We examined impacts of the project on species diversity, frequency and community similarity of native vascular plants using the Shannon-Weiner index, Sørensen index and one-factor analysis of variance.
Important findings Species richness and Shannon-Weiner index values decreased after reconstruction. In addition, the number of low frequency species increased, the number of high frequency species decreased and the similarity of species composition before and after reconstruction was low. Therefore, the impacts of this urban river reconstruction project on the species diversity, frequency and community similarity of native vascular plants were substantial.

Aims Our objective was to estimate the gross caloric values (GCVs) for two vegetation types and examine the differences.
Methods Samples of fresh leaves were collected from 10 dominant species in mid-montane moist evergreen broad-leaved forest in Ailao Mountain and 11 dominant species in tropical seasonal rain forest in Xishuangbanna. Leaf litter samples also were collected and divided into fresh litter layer (L layer) and fermentation layer (F layer). Samples were dried to constant weight at 60℃, and GCV of each sample was measured by a PARR1261 oxygenic bomb calorimeter.
Important findings There was a wide range of GCVs in fresh leaves and litter layers in both forests. The GCVs in fresh leaves were higher than in leaf litter for all but six species, with the differences between fresh leaves and leaf litter dependent on species characteristics. In both forests, species with higher GCVs in fresh leaves also had higher GCVs in leaf litter. The difference of GCV between L layer and F layer samples was significantly higher in Xishuangbanna than in Ailao Mountain, suggesting that litter layer decomposes more rapidly in Xishuangbanna. The ranking of mean GCVs from the same stand was fresh leaves > leaf litters > L layer litters > F layer litters. Mean GCVs in Ailao Mountain were consistently higher than those in Xishuangbanna, indicating that GCV was correlated with geographical factors. Forest GCVs are higher at high altitude, high latitude evergreen broad-leaved forest than in low altitude, low latitude tropical seasonal rain forest.

Aims Mosses constitute major ground cover of the subalpine forests in the eastern Tibetan Plateau. Moss leaves have one layer of cells and are highly sensitive to environment changes. This attribute of mosses can be used to monitor environment conditions and guide restoration. However, moss growth and especially variables affecting moss growth in subalpine forest ecosystems are poorly known. Even worldwide, the growth rate of mosses has been rarely studied because of time-consuming, inaccurate measuring methods.
Methods Two plots, one in old-growth spruce forest and the other in nearby clear-cut land in subalpine western Sichuan, were selected. We labeled and measured mosses on May 7, 2001 and remeasured on August 7, 2001 and simultaneously recorded microclimate measurements.
Important findings The growth rate of mosses varied among species and habitats. Hylocomium splendens had the highest growth rate, while Dicranum assamicum had the lowest. The growth of H. splendens, D. assamicum, and Thuidium lepidoziaceum was faster in forest than in clear-cut land, where it was inversely correlated with distance to forest. Actinothuidium hookeri and Rhytidiadelphus triquetrus had moderate growth rates nearly independent of habitat. From May to July, the forest almost always had lower radiation level and vapor pressure deficit (VPD) favorable for moss growth. For habitat-insensitive species, microtopographic factors might have offset the effect of habitat. Moss growth rate and habitat VPD were strongly negatively correlated. Moss growth rate can be used to suggest favorable habitat.

Aims The semiarid loess hill and gully region of northwestern China is a pasture and agriculture transitional zone lacking water resources for irrigation. The agroecosystem in this region is very vulnerable. An effective method to improve the ecosystem is to combine agriculture with animal husbandry, in which productive grassland plays a key role. However, water is the main limitation on grassland production in this region. We studied the effect of alternating ridges and furrows (for harvesting rainfall) on alfalfa (Medicago sativa) forage yield and soil water.
Methods Five treatments were compared: 1) conventional cultivation in a flat plot without mulch (CK), 2) plastic mulched ridges with 30 cm wide ridges and furrows (M30), 3) plastic mulched ridges with 60 cm wide ridges and furrows (M60), 4) bare ridges with 30 cm wide ridges and furrows (B30), and 5) bare ridges with 60 cm wide ridges and furrows (B60). We measured soil water consumption, forage yield of alfalfa and soil water profile in all treatments.
Important findings The water use efficiency of M30 and M60 treatments was greater than that of CK by 13% and 41%, respectively. The total forage yield of alfalfa of M30 in four years was not significantly different from that of CK, but the total forage yield of M60 was significantly higher than that of CK by 41%. In both bare treatments, the total forage yield of alfalfa was lower than in CK. In the 3rd alfalfa growing season, the 150 cm soil layer was the balance point of water supplement and soil water consumption. Below 150 cm, soil water can not be supplied by rainfall. Water at 200-500 cm soil depth was close to the permanent wilting point after four years. This study indicates that the M60 treatment is the optimum model for alfalfa establishment in this region. However, after the alfalfa phase, the shallow-root crop must be planted to restore soil water content for subsequent conventional crops.

Aims Potaninia mongolica is a xerophytic shrublet in the East Alashan-West Erdos Region of Inner Mongolia. It is an ancient species in its own genus and well-adapted to drought environments with fissurate growth and summer dormancy.
Methods We selected Dengkou as our research site and collected three stages of P. mongolica (before, during and after fissuration) during August 1-5, 2002 and 2003. We used PV technique and measured several water relations parameters (${φ_{s}}^{sat}$、${φ_{s}}^{tlp}$, ROWC ^tlp, RWC ^tlp,Δφ, ε^max etc.) in different stages of fissurate growth for comparison and analysis of drought tolerance and its mechanisms.
Important findings Different stages of P. mongolica had different abilities and pathways to maintain turgor pressure. Fissurate growth decreased the species' drought tolerance and involved multiple adaptations to drought. Before fissuration, osmotic adjustment occurred by the addition of solute to cells (such as, proline) and decreasing the loss of water in the body, so that it could maintain normal turgor pressure during drought. After fissurate growth, turgor pressure was maintained by higher cell and tissue elasticity (lower ε^max) and by osmotic adjustment for normal plant development in dry conditions and to avoid dehydration under water stress during the drought period, but it also became more sensitive to water stress. The ability to maintain turgor pressure varied from strong to weak during the period before, during and after fissuration. Fissurate growth decreased the ability of P. mongolica to maintain turgor pressure and led to changes in walls related to drought tolerance.

Aims Prunus mongolica is a xeric shrub, a landscape plant, a water and soil conservation plant, and an ancient species in the desert and desert steppe of the Mongolian Plateau, China. It tolerates extreme aridity and poor soil, growing strongly in desert where annual rainfall is <200 mm and annual evaporation is 3 400-4 000 mm. Study of P. mongolica is useful for understanding plant succession on the Mongolian Plateau and for sustaining and restoring the local ecosystem.
Methods The Abbe refractometer method, the natural dehydration method and the PV technique were used to analyze the hydrological characteristic of P. mongolica.
Important findings The relative water content, saturation water content, critical saturation deficit and water potential of P. mongolica seedling leaves were 69%, 117%, 48% and -0.85 MPa, respectively. The relative water content, critical water content and leaf water potential of P. mongolica seedlings exposed to drought stress decreased to 48%, 39% and -1.97 MPa, respectively, but the saturation water content and the ratio of bound water/free water increased to 187% and 11.94, respectively. The natural dehydration experiment indicated that P. mongolica seedlings reached equilibrium after 102 h in normal condition, but the dehydration rate decreased and the dehydration equilibrium time was delayed after 3 d of drought stress. This proved that the water-holding ability of P. mongolica increased after drought stress. In comparing the dehydration rates of the xerophytic P. mongolica and the mesophytic P. pedunculata, we discovered that the dehydration tolerance capacity of P. mongolica was higher than that of P. pedunculata. Analysis of PV curve showed that osmotic potential (Ψπ¹⁰⁰) of water saturation and osmotic potential (Ψπ⁰) at turgor loss point of P. mongolica seedling were -2.49 MPa and -3.11 MPa, respectively. The fact that the difference of Ψπ ¹⁰⁰ and Ψπ ⁰ was very large and the values very low showed that P. mongolica had strong ability to regulate turgor pressure. The ratio of bound water/free water was relatively high (7.76), and the modulus elasticity of cell wall was 4.18 MPa. The low leaf water potential and low osmotic potential of P. mongolica were beneficial for absorption of water from soil depths. Our results indicated that P. mongolica was a strong xerophyte in terms of hydrology. Its high ratio of bound water and free water, low osmotic potential at saturation water content, osmotic potential at turgor loss point and low modulus elasticity of cell wall were the physiological fundamentals of its dehydration tolerance.

Aims The difference in efficiency and mechanism of absorption of dissolved inorganic carbon (DIC) by two common species in Taihu Lake, Potamogeton crispus and P. malaianus, has rarely been studied. We ask: 1) do the two species differ in DIC absorption efficiency, and 2) what are the implications of any difference, especially in terms of succession in summer?
Methods We used a pH drift experiment to assess abilities of the two species to use DIC. Matching the natural inorganic carbon in Taihu Lake, we used three different alkalinities (1 200, 1 600, and 2 000 μmol·L^-1), pH values and ${HCO_{3}}^{-}$ concentrations at the beginning of the experiments. Simultaneously, we measured the activity of carbonic anhydrase of both species.
Important findings Both species can utilize ${HCO_{3}}^{-}$ and adapt to low inorganic carbon conditions. Both species used ${HCO_{3}}^{-}$ more efficiently at high alkalinity, so the efficiency of ${HCO_{3}}^{-}$ use depends on the concentration of this ion. However, different efficiencies in the use of ${HCO_{3}}^{-}$ were evident in terms of the photosynthetic rates and pH values attained at the end of the experiments. Photosynthesis rates and pH values of P. malaianus were higher than those of P. crispus at all alkalinities. Photosynthetic rates-CO₂ curves of the two species showed that P. malaianus uses CO₂ more efficiently than P. crispus when the pH value of water is higher, i.e., when the concentration of CO₂ is low. Furthermore, the carbonic anhydrase activity of P. malaianus was distinctly higher than that of P. crispus. This result showed P. malaianus could transfer ${HCO_{3}}^{-}$ to CO₂ more quickly than P. crispus, which was correlated with the different efficiencies of use of inorganic carbon. This study suggests the pH of Taihu Lake in summer may be advantageous to the colonization of P. malaianus, compared to P. crispus. Furthermore, the different efficiencies correlate with the successional patterns of the two submerged macrophytes.

Aims Mangrove plants are usually categorized as true mangrove plants and semi-mangrove plants on the basis of their distribution in inter-tidal regions. However, the identification of some fringe mangrove species found mainly on the landward transitional zones is controversial. Specific leaf area (SLA, leaf area per unit dry mass) and mass- and area-based leaf nitrogen concentrations (N_mass and N_area) are important leaf traits for plants, but relevant comparative research on true and semi-mangrove plants is unavailable. Our objective was to determine differences between the two groups and to classify the controversial species according to their leaf traits. Ultimately, this will assist in the management, protection and utilization of mangrove forest.
Methods Three individuals in similar growth sites were chosen for each species from Hainan Island. Fully expanded mature leaves were sampled from the upper canopy of all plants. Succulence (water content per unit leaf area), SLA, N_mass and N_area of mature leaves were studied for 33 species, representing all but three of the mangrove species in China.
Important findings True mangrove plants accumulated more Cl and water per unit leaf area than semi-mangrove plants, except for Pemphis acidula, Hernandia sonora and Clerodendrum inerme. Cl and water content per unit leaf area of true mangrove plants were generally >2.5 mg·cm^-2 and >2.4 g·dm^-2, respectively. Cl concentrations were positively related to succulence for all mangrove species. True mangrove plants had low SLA (<100 cm ²·g^-1) and high N_area; however, semi-mangrove plants had high SLA (mean of 160.4 cm ²·g^-1). Pemphis acidula had much lower SLA than other semi-mangrove species. Our study suggested that there are significant differences between true mangrove plants and semi-mangrove plants in leaf Cl concentration, succulence, SLA, N_mass and N_area. Heritiera littoralis, Excoecaria agallocha, Acrostichum aureum and Acrostichum speciosum are better classified as semi-mangrove plants, while Acanthus ilicifolius and Acanthus ebrecteatus are classified as true mangrove plants, and Xylocarpus granatum needs further research.

Aims Gracilaria lemaneiformis is a commercially important marine red alga from which agar is extracted for use in food chemistry and in production of tissue culture media. It has been transplanted from Qingdao, Shandong Province to Nan'ao Island, Guangdong Province and other places in South China. Understanding the relationship between its photosynthesis and environmental factors has theoretical and practical importance for growing the macroalga more rationally, understanding its biological functions and increasing its utilization.
Methods We cultured G. lemaneiformis seaweeds in Qing'ao Bay of Nan'ao Island, South Sea, China. The silky samples (immersed in seawater in plastic casks) were taken into a laboratory and provided oxygen by air pumps. We studied their physiological responses to six days of growth in seawater eutrophicated to different degrees by nitrogen and phosphorus alone and in combination.
Important findings Contents of phycobiliprotein and chlorophyll a and total antioxidative ability were decreased significantly in mild phosphorus eutrophic seawater (P 0.2 mg·L^-1) and in severe nigrogen+phosphorus eutrophic seawater (P 10 mg·L^-1, N 55 mg·L^-1). Chlorophyll fluorescence kinetic parameters (F_v/F_m, F_v/F_o, q_P, q_N, Φ_PSⅡ, q_o) also showed the harmful effect of such treatments to PSⅡ function. However, no obvious changes in these parameters were found with mild nitrogen (N 0.9 mg·L^-1) and mild nitrogen+phosphorus (P 0.2 mg·L^-1, N 0.9 mg·L^-1) eutrophic seawater. Gracilaria lemaneiformis did not accumulate more nitrogen or phosphorus in vivo in mild eutrophic seawater than in the control seawater. Because G. lemaneiformis is tolerant to mild N and mild N+P pollution, this species may be used to purify lightly eutrophicated seawater.

Aims Use of saline land has become a major agricultural issue around the world. China has 27 millions kilometer saline land, and the area is increasing. Our objective was to compare the germination ability of the saline-tolerant Bromus stamineus from the USA and B. inermis cv. Xilinguole under saline conditions.
Methods We applied Na₂SO₄, NaCl and MgCl₂ at concentrations of 0.5%, 1.0% and 1.5% for salt stress treatments and recorded germination percentage, germination energy, germination index, germination value etc. We put seeds of the two plants in culture dishes covered with a filter paper and a glass bell jar with a small hole. Each culture dish was put on a beaker containing different salt solution, with gauze connecting the solution and the filter paper. The gauze extended outside the bell jar to evaporate and avoid salt accumulation on the filter paper, keeping the germination environment constant.
Important findings Germination percentage, germination index and germination value for both species declined with increased salinity, with values reduced more for B. inermis cv. Xilinguole than for B. stamineus. Times for seeds to germinate and complete their germination were prolonged under high salinity stress. NaCl had greater effects than Na₂SO₄ and MgCl₂ on both species. Overall, B. stamineus exhibited greater germination ability with salt stress than B. inermis cv. Xilinguole.

Aims Dichondra repens is a warm-season lawn substitute that remains green for long periods and reproduces strongly. Dichondra repens forms short, dense cover and is important in conservation of water and soil. It retains its green color during winter to -8 ℃ with only slight leaf browning and is resistant to diseases and heavy metals contamination. Little is known about the drought tolerance of wild and cultivated D. repens. Our objectives were to determine the drought-tolerant characteristics of wild and cultivated D. repens and explore the physiological foundations for drought tolerance in both.
Methods We watered wild and cultivated D. repens every 3, 7 and 15 days from March 1 to May 1, 2004 and made growth and development measurements after water treatment. As soil drought conditions developed, we measured changes in superoxide dismutase activity (SOD), peroxidase activity (POD), nitrate reductase activity, soluble sugar content, free proline content, soluble protein, nitrate and nitrite (${NO_{2}}^{-}$ /${NO_{3}}^{-}$) content and DNA fragments of leaves.
Important findings Under drought stress, the content of free proline, soluble sugar and ${NO_{2}}^{-}$ /${NO_{3}}^{-}$ in wild and cultivated D. repens leaves was enhanced. Activities of SOD and POD were regulated, protecting leaf cells of wild and cultivated D. repens from oxidative damage caused by drought stress. There were marked differences in drought resistance between the wild and cultivated D. repens. Antioxidant enzyme activities and osmotic adjustment substance content in wild D. repens leaves were higher than in cultivated D. repens leaves, while damage to DNA of wild D. repens was less serious than in cultivated D. repens. ${NO_{2}}^{-}$ /${NO_{3}}^{-}$ content and nitrate reductase activity in wild D. repens leaves were significantly higher than in cultivated D. repens. Maximum ${NO_{2}}^{-}$ /${NO_{3}}^{-}$ content and nitrate reductase activity in wild D. repens leaves were 10 and 2.2 times larger, respectively, than that in cultivated D. repens leaves. Wild D. repens had more antioxidant response to drought stress than cultivated D. repens. Under drought stress, change of ${NO_{2}}^{-}$ /${NO_{3}}^{-}$ content may be associated with endogenous NO concentrations, which probably cause the drought tolerance of wild D. repens to be greater than that of cultivated D. repens.

Aims Knowledge of floral syndrome and breeding system is important for understanding reproduction and population dynamics of plants, especially rare plants. Our objective was to determine the floral syndrome, flowering course and dynamics and functional floral morphology of the rare plant Rhodiola dumulosa in Dongling Mountain, Beijing, China, to determine its breeding system.
Methods For individual flower, flowering lasted 5 to 6 days, the flowering process for one flower of this species can be divided into 6 periods according to the flower morphology and anther dehiscence: “Budding period"—petals are visible but not open, two rings of anthers are red and no dehiscence; “Inside-dehiscence"—petals are open in a way, and the inside anthers dehisce; “Post-inside-dehiscence"—after 1-2 days of flowering, the inside anthers over the pollen exposure and the color changes to black; “Outside -dehiscence"—the outside anthers dehisce, ovary becomes bigger and then split, stigma stretches out; “Post-outside-dehiscence"—petals are open well, all anthers over the pollen exposure and the color changes to black or brown; “Withering period"—after 5-6 days of flowering, the color of filaments and ovary changes to red, petals start wilting.
Important findings In the process of flowering, styles become long and curved slowly. The anthers dehisce in order of priority. The inside anthers accounting for 50% of whole anthers whose dehiscence pattern is longitudinal always dehisce first, followed by the outside others. The flowering span among populations is about 30-40 days. The flowering course of every observation plots is different, the observation plot B whose altitude is 2 202 m lagged behind other observation plots, and we presume that different weather condition of every observation plot and disturbances of tourists should be the great factors. The species is protandrous with an outcrossing index of 4 bases. According to criteria of Dafni (1992), the breeding system is outcrossing with partial self-compatibility, needing pollinators. The pollen-ovule ratio (P/O) is about 700-1 000. Based on Cruden's criterion (1977), the breeding system is facultative xenogamy.

Aims Soil drying is an important factor threatening rice production in China, and there has been much recent research on the development of grain yield and quality in rice grown under shortages of water. The objective of this study is to examine the effects of soil moisture and N supply levels on rice grain N accumulation and nutrient quality.
Methods We conducted a pot study using three rice cultivars and varying in N content to examine physiological characteristics for N accumulation and distribution to grains during the grain-filling period in response to water stress and N supply exposure at heading stage.
Important findings The activities of glutamine synthetase (GS) and glutamate synthase (GOGAT) involved in N metabolism were enhanced by mild water stress at 10-20 DAA (days after anthesis) under normal N level, and abilities to synthesize amino acids were improved, which promoted the accumulation of N and elevated the protein content in grains. However, the activities of GS and GOGAT were negatively affected by mild water stress with exposure to high N level, which resulted in decreased synthesis of amino acids. The dynamic changes of the ratio of protein to grain weight presented a “V" type during grain-filling. Water stress improved the ratio of protein to grain weight from 15 DAA to harvest under normal N level, but the ratio was decreased by water stress under higher N level when compared with well-watered treatments. Changes of four component parts of protein subjected to water stress varied with nitrogen levels and cultivars. Compared to well-watered treatment, the water-stressed treatment had higher contents of prolamine and glutelin under normal N level, but the contents of prolamine and glutelin subjected to water stress were significantly less than those in the well-watered treatment under higher N level. Water stress affected the lysine content of grains, and varied with cultivars and N supplies. Lysine contents of `Shanyou63' under two N levels were decreased by water stress, but were increased in `Yangdao6'. The variation observed in enzymes involved in N metabolism and partitioning at grain filling to grains of the rice imposed by water stress at heading indicated that plant genotypes influence N accumulation and protein components of grains and subsequent nutrient quality.