Limited information exists on synchronized nutrient release, especially of nitrogen, to meet the nutrient demand of plants in the Xilin River Basin, Inner Mongolia. We conducted a field experiment to examine soil nitrogen fractions during the first two years of grassland restoration following 11 years of consecutive grazing under four different stocking rates. The seasonal variation of different soil nitrogen fractions in different soil layers was investigated. Soil and plant samples were randomly taken from each block on May 12, June 23, August 3, and September 13, 2002 at depths of 0-10 cm, 10-20 cm and 20-40 cm. The concentrations of NO－3-N, NH+4-N, inorganic-N (the sum of NO-3N and NH+4-N) and total Kjeldahl-N in the different soil layers were determined. At the same time, we determined the microbial biomass N of the top 10 cm soil using the chloroform fumigation-extraction method. Our results showed that the total nitrogen in different soil layers had no significant seasonal change under all treatments. This indicated that grazing rates had no significant effects on the pool size of total soil N. However, there were significant seasonal change patterns of the soil NO－3-N, NH+4-N, inorganic-N and microbial biomass N. During the growing season, the NO－3-N decreased and was negatively correlated with aboveground green phytobiomass, suggesting that soil NO－3-N concentrations were controlled primarily by synchronized N uptake by plants although other N transformation processes such as microbial immobilization, denitrification and leaching also can exert some control over the NO－3-N pools. Microbial biomass N could explain 22.3% of the variation in inorganic nitrogen concentrations whereas NH+4-N was negatively correlated with microbial biomass N (p<0.01) indicating that soil microbial organisms can have an important impact on soil N transformation processes. Total soil inorganic nitrogen showed an inconsistent pattern during the growing season, but was significantly negatively correlated with aboveground green phytobiomass. We also found that NH+4-N pools were relatively constant in the top 10 cm of soil from June to September, but NO－3-N fluctuated throughout the year and was almost undetectable by the end of plant growing season. After plant senescence in September, inorganic nitrogen concentrations increased again. In general, the amounts of different soil nitrogen fractions decreased with soil depths.Following a two-year exclosure period after 11 consecutive years of grazing at four different stocking rates, the different soil N fractions showed a differential response. Total nitrogen concentrations were not affected by previous stocking rates while the soil microbial biomass nitrogen differed significantly among treatments during the growing season. Soil microbial biomass N was the highest in the previously overgrazed and moderately grazed treatments followed by ungrazed and lightly grazed treatments. The responses of soil inorganic nitrogen, NH+4-N and NO－3-N to two years of no grazing were complicated. In general, soil inorganic N, NH+4-N and NO－3-N concentrations were higher in the moderately grazed and overgrazed treatments. After two years of restoration, there were no significant differences in total aboveground green phytobiomass (maximum biomass in August) between the previously ungrazed and overgrazed treatments.

The concept of nitrogen use efficiency (NUE) offers a powerful tool to study plant strategies with respect to nutrient limitation. We studied the NUE of an annual herb, Chenopodium album, in a dense monospecific stand using the concept introduced by Berendse and Aerts. Larger individuals absorbed more N in greater proportions relative to their size, suggesting that the competition for soil nitrogen was asymmetric (one-sided) among individual plants in the stand. Nitrogen loss from individuals also increased with plant size. Nitrogen influx (rin, the rate of N uptake per unit aboveground N) was greater in larger individuals while nitrogen outflux (rout, the rate of N loss per unit aboveground N) was the reverse. Therefore, the relative rate of nitrogen increment (rin-rout) was greater in larger individuals whereas it was around zero in the smallest plants. Larger individuals decreased their N concentration with time while smaller individuals showed little change in N concentration. These results suggested that the growth of smaller individuals was limited by light availability rather than by N availability, and N limitations were greater in larger individuals. Individual plants in this dense stand of C. album differed in their N economy. NUE and its components, i.e., MRT and NP, were different among individuals in the stand. Both NP and MRT were positively related to plant size. Larger individuals had longer MRT and higher NP, both of which contributed to higher NUE, than the smaller individuals. No trade-off relationship between NP and MRT was found at the intraspecific level. This study showed that the concept of NUE defined by Berendse and Aerts offered a powerful tool in studing plant strategies within species as well as among species.

Drought and high temperature often occur simultaneously in arid and semiarid regions, but few investigators have studied the interactions between these two stresses. Our objective was to compare the effects of soil moisture, temperature and their interactions on the photosynthesis and growth of Leymus chinensis. Plants were subjected to different soil moisture regimes (relative soil water content ranged from 25% to 80%) at temperatures of 20, 23, 26, 29 and 32 ℃ in a controlled environment. Under sufficient soil moisture conditions, both net photosynthetic rates and water use efficiency decreased with increasing temperature whereas both stomatal conductance and transpiration rates increased. There were no significant effects of soil moisture alone on net photosynthetic rates but significant soil moisture-temperature interactive effects were observed; net photosynthetic rates increased under conditions of moderate soil drought at 20-26 ℃ but were significantly reduced under drought conditions at extremely high temperature (32 ℃). Water use efficiency showed a similar response to changes in temperature and soil moisture as net photosynthetic rates. Soil moisture did not significantly affect leaf stomatal conductance or transpiration rates indicating that higher adaptive ability to soil drought may be exhibited under these experimental conditions. However, under all soil moisture conditions, both net photosynthetic rates and water use efficiency always decreased with increasing temperature whereas both stomatal conductance and transpiration rates always increased with increasing temperature. Leaf biomass also responded to changes in soil moisture and temperature. The leaf biomass was greatest under conditions of light to moderate soil drought at temperatures from 20-26 ℃ but decreased with decreasing soil moisture at higher temperatures of 29 ℃ and 32 ℃. Leaf biomass of plants grown at 26 ℃ was greatest under sufficient soil moisture and light drought conditions, but the leaf biomass at 23 ℃ was greatest under moderate to high drought conditions. These results suggest that the optimal growing temperature might be lowered under droughty conditions. Leaf biomass was reduced in plants grown at higher temperatures (29 ℃ and 32 ℃) under all soil moisture regimes. Interactions between water stress and temperature were highly significant for several physiological processes examined. Leaf gas exchange characteristics and growth were more impacted by drought conditions at high temperatures than at low temperatures. Similarly, the productivity of L. chinensis was reduced more by the combined stresses of drought and high temperature than by either stress alone and much of the effect was on photosynthetic processes. Our research suggests that the decreased precipitation and increased temperatures forecasted for this semi-arid region due to global climate change could adversely affect the distribution and abundance of L. chinensis. To conserve this species, future research should focus on ways to enhance the drought tolerance of L. chinensis at high temperatures.

Caragana opulens belongs to the Caragana genus of legume, and it is found across a vast area of the Inner Mongolian Plateau from Daqing Mountain and Erlianhaote (112° E) in the east to Alashan (105° E) in the west. There have been many reports about its distribution, floristic composition, growth habits, anatomy and morphology. However, there have not been any reports about its physiological and biochemical characteristics. The adaptation of plants to their environment is determined by their genetic potential, but light energy and water metabolism are ready measurable indicators. The characteristics of photosynthesis and water metabolism of C. opulens populations which are found in Helinger (a semi-arid/partial humid region with lower temperature, lower light intensity and shorter day length) and Alashan (a very droughty region with higher temperature, higher light intensity and longer day length) were compared in this paper in order to understand the adaptative mechanisms of the species to its habitat. The results indicated that the light compensation point (<500 μmol proton·m－2·s－1), light saturation point (1 200 μmol proton·m－2·s－1) and optimum temperature for photosynthesis (26 ℃) in the Helinger population were all lower than those in the Alashan population (700-800 μmol proton·m－2·s－1, 1 500 μmol proton·m－2·s－1 and 28－29 ℃, respectively). The Helinger population exhibited a higher photosynthetic rate at lower temperature and light intensity; i.e. the Helinger population exhibited better adaptations to lower temperature and light radiation, while the Alashan population was better adapted to higher temperature and more intensive light radiation. The Helinger population needed higher relative humidity to maintain its higher net photosynthesis rate than the Alashan population. The Helinger population was characterized by higher transpiration rates, higher photosynthetic rates and lower WUE, whereas the Alashan population exhibited water-saving strategies with lower photosynthetic rates and lower transpiration rates. These results suggest that the different populations have adapted physiologically to local conditions of light, temperature and humidity allowing them to photosynthesize most efficiently in their native habitats. Considering that water is a key factor for plant growth and development in the Inner Mongolia Plateau and that there is a difference in annual precipitation, soil water content and plant water status between the Helinger and Alashan regions, it is suggested that water shortage was the key driving factor responsible for the physiological differences in the net photosynthetic rate, transpiration rate, light use efficiency, and water use efficiency between the two populations.

We present a soil carbon budget for a grazed Leymus chinensis steppe community in the Xilin River basin of Inner Mongolia based on historical data and current field measurements. The study site was situated in the southern part of the basin, approximately 1 265 m above sea level. The climate is temperate and semi-arid with a mean annual rainfall of 350 mm and annual mean temperature of 0.3 ℃. The dark chestnut soil profile is about 1 m in depth with a 10-20 cm thick humus layer, a pH of 7.4 to 8.3, and an organic C content of 1.53%-1.37% in the upper 0-20 cm layer. A grazed stand (43°32′58″ N, 116°40′34″ E) outside the L. chinensis permanent plot of the Inner Mongolian Grassland Ecosystem Research Station was chosen as a sampling plot. Vegetation consisted mainly of L. chinensis, Stipa grandis, Agropyron cristatum, and Cleistogenes squarrosa, which accounted for 80% of the total aboveground biomass. The pasture has been grazed continuously for more than 40 years at a moderate stocking rate with about 2/3 of the above-ground biomass consumed annually.Soil respiration, and aboveground biomass, and root biomass measurements were initiated May 31, 1998 and measured every ten days over two growing seasons until October 14, 1999. The alkali absorption method was used to measure soil respiration. Root respiration was evaluated indirectly by relating the amount of root biomass under the chamber to rates of CO2 evolution. Aboveground biomass was measured in 10-25 cm diameter circular plots using the harvest method. The maximum biomass value measured during the growing season was taken as the annual net primary productivity (NPP). Root biomass was measured to 30 cm depth, and the root productivity was estimated using the annual increment of growth, as defined by Dahlman and Kucera. Food consumption by insects was determined using the cage-feeding method by using a series of cages with different insect population densities.The annual average carbon input from aboveground biomass production was 78.2 gC·m-2·a-1, and inputs from root biomass to 30 cm depth averaged 322.5 gC·m-2·a-1. The summed mean annual carbon input of shoot and root materials was approximately 400.7 gC·m-2·a-1. The annual amount of aboveground biomass consumed by insects averaged 14.7 gC·m-2·a-1, and the carbon output by leaching or light-chemical oxidation was 3.2 gC·m-2·a-1. The annual evolution rate of CO2 from net soil respiration averaged 343.7 gC·m-2·a-1. Cattle consumption was 49.7 gC·m-2·a-1. The summed mean annual output was approximately 411.3 gC·m-2·a-1 and a net carbon release of about 10.6 gC·m-2·a-1 was calculated. Based on the soil organic carbon density of the field, the turnover rate of soil carbon in 0-30 cm depth was 6.2%, with a turnover time of 16 years.Carbon budgets for each of the two years, 1998 and 1999, were quite different. A net carbon release of 75.1 gC·m-2·a-1 was detected in 1999, while a net carbon accumulation of 54.1 gC·m-2·a-1 occurred in 1998. By comparing carbon cycling patterns between 1998 and 1999, it was evident that the difference between years arose primarily from differences in root production and soil respiration between the two years. Severe drought in 1999 and record high rainfall in 1998 caused a difference in root NPP by as much as 220.5 gC·m-2·a-1. The root NPP accounted for about 73%～85% of the total carbon input in the community, indicating the importance of the belowground biomass in estimating the carbon budget for this ecosystem. Soil respiration seemed to be less sensitive to drought, with only about a 22.5% decrease in 1999 whereas root NPP decreased by about one half in 1999. Our results indicated that drought, a frequent occurrence in this region, could change this ecosystem from a sink to a source for atmospheric CO2. The degree to which this grassland could become a source of CO2 appeared to depend also on the stocking rate.

Soil respiration is one of the key components in the carbon cycle of terrestrial ecosystems. Many studies on methodologies for measuring soil respiration have been conducted. However, soil respiration is difficult to measure accurately due to the uncertainty associated with the various methods, and the large spatial and temporal variability that is inherent in soil respiration due to many biotic and abiotic factors. In this study, we used the closed-chamber IRGA method to determine the diurnal (24 h) dynamic pattern of soil respiration at the maximum biomass period, and daily soil respiration rates calculated by this method were compared to those measured by the alkali absorption (AA) method for ten different plant communities in the agro-pastoral ecotone. For the IRGA method, we used the LI-6400 system (LI-COR, Lincoln, NE, USA) and for the AA method, we used NaOH solution for absorption of CO2. The major results were as follows: 1) The diurnal fluctuations in soil respiration rate for the ten communities were remarkable, all of them showed a single-peaked curve that was driven primarily by soil temperature. The combined influence of other meteorological factors such as temporary rainfall (thus soil moisture), wind speed and clouds were non-negligible. Therefore, the diurnal patterns for the ten communities differed; 2) The daily soil respiration rate for these communities varied from 394 to 894 mg C·m-2·d-1 as estimated by the alkali absorption method, and from 313 to 2 043 mg C·m-2·d-1 by the closed-chamber method, with the AA method averaging 67.5% of that measured by the closed chamber method; (is this what is meant? It wasn’t clear from the previous wording) 3) The results measured by the alkali absorption method compared well with those measured by the closed-chamber IRGA method, and the correlation between them was highly significant (R2=0.873 9). Of particular note was that when soil respiration rates were low, the measured values by the two methods were similar, whereas when soil respiration rates were high, the values measured by the closed-chamber method were significantly higher than that by the alkali absorption method. In general, the variation in measured values by the two methods was regular and systematic providing a reliable basis for correcting our past measurements by the alkali absorption method in the region.

Changes in the membrane protective system and Ca2+ homeostasis in young grape (Vitis vinifera) plants during the enhancement of chill resistance with heat acclimation and heat resistance with cold acclimation were examined to explore the mechanism of cross adaptation to temperature stress. MDA content in heat (cold) acclimated leaves by cold (heat) stress was less than that in the non-acclimated control plants which showed that heat (cold) acclimation could alleviate the damages of cell membrane caused by cold (heat) stress. Higher leaf levels of glutathione (GSH) and ascorbate acid (AsA) also were found with pre-cold (heat) acclimation under heat (cold) stress than in the non-acclimated control plants. Cold and heat stresses have been shown to cause transient elevated levels of cytosolic free Ca2+, but there is little evidence showing that Ca2+ homeostasis was maintained in cross adaptation, a crucial factor for confirmation of the existence of cross adaptation and the elucidation of the mechanisms of cross adaptation. Ca2+-homeostasis was demonstrated by examining Ca2+ distribution using the cytochemical method of the calcium antimonite precipitation. The results revealed calcium was distributed mainly within the vacuole and intercellular spaces at the optimum growth temperature. When the plants were treated by either heat or cold acclimation for 3 h, calcium levels in the cytoplasm increased dramatically. Meanwhile, deposits in the vacuole concentrated in the tonoplast and deposits in the intercellular space decreased after heat acclimation whereas, after cold acclimation, the deposits in vacuole decreased significantly but did not change in the intercellular space. These results indicate that heat acclimation mostly pumped Ca2+ out of intercellular spaces whereas cold acclimation pumped Ca2+ out of vacuoles. These results revealed two physiological phenomena: on one hand, heat (cold) acclimation increased Ca2+ in cytoplasm to increase the integrity of membranes under cold (heat) stress; on the other hand, increased Ca2+ in the cytoplasm induced the expression of GSH and AsA genes increasing GSH and AsA levels.When non-acclimated plants were stressed at 44 ℃ or -3 ℃ for 3 h, compared to control plants, more Ca2+ particles were located in the inner side of the plasma membrane, fewer Ca2+ particles were in the intercellular spaces and vacuole, the plasma membrane and chloroplast showed signs of damage, and chloroplast ultrastructure was destroyed. However, when heat acclimated plants were exposed to chilling stress at -3 ℃ for 3 h and cold-acclimated plants were exposed to heat stress at 44 ℃ for 3 h, more Ca2+ particles were found in the intercellular spaces and in vacuoles and few Ca2+ particles were on the inner side of the plasma membrane. Cell membranes and chloroplasts were not destroyed. These results indicate that too much intracelluar Ca2+ can be pumped off in heat-acclimated cells under cold stress or cold-acclimated cells under heat stress but cannot be pumped off in non-acclimated cells, suggesting that calcium homeostasis is very important in grape mesophyll cells during cross adaptation to temperature stress.

We conducted a field investigation of the floral syndrome and breeding system of Disanthus cercidifolius Maxim. var. longipes H. T. Chang, an endangered plant species, in both artificial and natural populations in Mt. Jinggang, a National Reserve in Jiangxi, China. We describe our findings below.There are two inflorescences, often on opposite axillaries at the same node, each inflorescence with two opposite bisexual flowers with no pedicels. Flowering lasted 5 to 6 days. On the day of anthesis, the styles are longer than the filaments; the length between anthers and stigmas is about 1.02 mm. The color of petals changes from light red to brown. The stigma changes from light green to pale yellow, to brown, and lastly to black. The anthers dehisce in order of priority. Two of the anthers whose dehiscence pattern is longitudinal and synclinal upward always dehisce first, followed by the others. The pollen forms an obvious “pollen circle" surrounding the stigma when the anthers all dehisce.The flowering span among populations is 49-55 days. The flowering process for one flower of this species can be divided into four periods by the flower morphology and dehiscence: “Pre-dehiscence" in which two filaments stretch out with no dehiscence; “Initial dehiscence"—after two days of flowering, one or two anthers dehisce; “Full dehiscence" is between the third and fifth day when three to five anthers dehisce, and the color of the stigma changes to yellow; and the last period is “Withering period", that occurs from the sixth to seventh day, when all anthers have dehisced, some have begun to wither, and the color of some stigmata changes to brown or black yellow. The floral diameter is ca. 15 mm. There are both temporal and spatial isolations of male and female organs within the same flower. It is protandrous with an outcrossing index of 4. According to criteria put forward by Dafni, the breeding system of this species can be determined as outcrossing with partly self-compatible and needs pollinators during the pollination. The pollen-ovule ratio (P/O) is 1 250. Based on Cruden's criterion, the breeding system would be termed xenogamy. Based on the results of emasculation, bagging, and artificial pollination studies, the inflorescences of this species produced seeds differently. There are no seeds when the inflorescences are emasculated, bagged and not pollinated and few seeds when unemasculated, bagged and free pollinated. In the treatments where flowers were emasculated, unbagged and free pollinated, or unemasculated, unbagged and free pollinated, or emasculated, bagged and hand self-pollinated, the inflorescences were able to produce some seeds. In the treatments of emasculation, bagging and hand geitonogamy or hand cross-pollination, the inflorescences was able to produce more seeds, but its production ratio was still low, ranging from 28.50% to 45.01%. There was no agamospermy, and outcrossing was the main form of breeding system.This species maintains a relatively high level of genetic variation as compared to an average species. The proportion of polymorphic loci (P) is 62.70%, the average number of alleles per locus (A) is 1.63, and the mean effective number of alleles per locus is 1.55. The Gst is only 0.09. The results show that outcrossing is predominant in the breeding system of this species. We conclude that pollen competition may be the major factor leading to the endangered status of D. cercidifolius Maxim. var. longipes H. T. Chang.

Minjiang fir (Abies faxoniana) (MJF) is a dominant tree species of sub-alpine forests on the eastern edge of Qinghai-Tibetan Plateau, and is mainly distributed over the upper reaches of the Minjiang, Dadu and Bailong Rivers. The population structure and spatial pattern of MJF was studied in a naturally occurring stand. In a 100 m×60 m plot, the location of every tree was mapped, and the diameter at breast height (DBH), height and canopy area of each individual recorded. Trees were divided into five size classes: seedlings, height <0.33 m; saplings, height ≥ 0.33 m, and DBH<2.5 cm; small trees, 2.5 cm ≤ DBH<7.5 cm; medium Trees, 7.5 cm ≤ DBH <22.5 cm; and big trees, DBH ≥ 22.5 cm. The spatial pattern of MJF was analyzed using four independent methods: the Morisita index (Iδ), variance to mean ratio (V/m), the congregation index (m*/m) and the spatial point pattern analysis (SPPA) (Ripley's second-order- analysis method). The results revealed that MJF was a stable population with an inverse J-shaped size structure indicating good natural regeneration. Seedlings and saplings were very abundant, with densities of 2 217·hm-2 and 2 683·hm-2, respectively. Irregularities in the size structure histogram reflected past disturbances. The spatial analyses revealed that seedlings, saplings and small trees were clumped at most spatial scales studied which ranged from 1 m to 30 m, whereas the medium-sized trees and big trees were randomly distributed. The intensity of assemblage (IA) varied with scale. The first three methods indicated that IA decreased with increasing scale, but the SPPA method showed that the IA of seedlings, saplings and small trees first increased with increasing scale, and then declined at greater scales. We conclude that the spatial pattern of MJF in this subalpine forest resulted from long-term interactions between the MJF and its natural environment and mechanisms of natural regeneration that vary among species. The four different methods were very similar on the whole in their abilities to discriminate spatial patterns, but SPPA was superior in its ability to detect changes of IA with scale. Thus, we recommend SPPA for analyzing spatial patterns of populations. However, a limitation to using SPPA relates to the complexity of sampling and calculation requised and some refinements in Ripley's second-order-analysis are needed in order to better as it detect gaps.

The species composition, plant diversity, and the ecological and physiognomic characteristics of a tropical montane rain forest in Mengsong, southern Yunnan, were studied. Two main forest community types were recognized based on their habitat, species composition and forest profiles: a Mastixia euonymoides-Phoebe megacalyx community and a Parachmeria yunnanensis-Gymnanthes remota community. The montane rain forest is characterized by evergreen mesophanerophytes and microphanerophytes with simple, leathery and entire mesophyllous leaves, a relatively high abundance of woody lianas and epiphytes, abundant herbaceous phanerophytes, few buttressed trees and little or no cauliflory. Compared to the seasonal rain forests of the region and equatorial lowland rain forests, the montane rain forest has a lower abundance of mega- and mesophanerophytes and big woody lianas but a higher abundance of microphanerophytes, nanophanerophytes and herbaceous phanerophytes, and more plants with simple, leathery and non-entire leaves and microphyllous leaves. Compared with the dominant monsoon evergreen broad-leaved forests common in southern China, epiphytes and herbaceous phanerophytes are more abundant in the montane rain forest but have fewer plants with non-entire and microphyllous leaves. The montane rain forest in Mengsong can be classified as a wet tropical montane forest similar to the lower montane rain forest from tropical Asia. The montane rain forest in Mengsong has similar species diversity to the seasonal rain forest but higher species diversity than the monsoon evergreen broad-leaved forest of the region.

The Helan Mountains (38°10′－39°30′　N and 105°45′－106°45′　E) is situated on the eastern edge of the Alashan Plateau and the western edge of the Yinchuan Plain and extend about 270 km from north to south and about 20－40 km east to west. Its general altitude ranges from 2 000 m to 3 000 m with the highest summit at 3 556 m above sea level and relative elevations are 1 500－2 000 m. The Helan Mountains form an important boundary of climate and vegetation in northwest China: the eastern side of the Helan Mountains belongs to steppe climate and steppe vegetation and desert climate and desert vegetation characterize the west resulting in two different biomes. The mountains are a core area of the Alashan-Ordos biodiversity center that is among the top eight centers of biodiversity in China. It is a rich source of plants for the arid west and is an important pivotal point that connects the floras of the Qinghai-Xizang Plateau, the Mongolian Plateau and North China. Hence, it is very important to study the biodiversity of the Helan Mountains. The diversity and spatial distributional characteristics of plant communities are discussed in this paper. Based on our observations and research over many years, we have classified 11 vegetation types, and 55 formations in the Helan Mountains. The vertical zonation of the vegetation is strongly developed: vegetation belts can be divided into the desert belt (below 1 600 m asl), the steppe belt (1 600-1 900 m asl), the coniferous forest belt (1 900-3 100 m asl), and the alpine shrub or alpine meadow belt in the alpine or sub-alpine zone (>3 100 m asl). There also is a strong differentiation of vegetation on sunny and shady slopes. In the steppe belt of low hillsides, steppe communities inhabit sunny slopes but mesophilous shrub occur on shaded slopes. In the coniferous forest belt in the mid-elevation zone, the community of Picea crassifolia is distributed widely on shaded slopes but open forests of Ulmus glaucescens and Juniperus rigida or other mesophilous shrub occur on the sunny slopes. At 3 000 m and upwards, the vegetation of sunny and shade slopes is similar. The vegetation also is differentiated in an east-west and north-south direction resulting in some unique communities. The climate is warm and dry on the east side of the Helan Mountains and some thermophilic plants such as Zizyphus jujuba var. spinosa and Ostryopsis davidiana are distributed on the eastern side only. On the western side, the climate is cool and wet and there is a greater proportion of forests. The mid-elevation zone is the main body of the Helan Mountains, and the vegetation comprised primarily of forests and mesophilous shrub. The degree of desertification is very distinct in both the north and south segments of the mountains, but the communities are different. In the north, Ammopiptanthus mongolicus, Salsola laricifolia and Tetraena mongolica are dominant whereas Ephedra rhytidosperma, Syringa pinnatifolia var. alashanensis are dominant in the south. Furthermore, there are four endemic communities with Syringa pinnatifolia var. alashanensis, Ephedra rhytidosperma, Leptodermis ordosica and Hippolytia alashanensis in the Helan Mountains.

Environmental change of oases in the arid regions of China is a subject of recent concern. Although many of the changes have been recorded qualitatively through the use of comparative photography and historical reports, little quantitative information has been available on regional scale. Geographical information systems (GIS) and remote sensing (RS) technologies provide the basis for developing landscape composition and pattern indicators as sensitive measures of large-scale environmental change and thus may provide an effective and economical method for evaluating the condition of oases in relation to human and natural disturbances. This paper discusses the dynamic changes of the Qira oasis over a 40 year period by using 3 types of imagery data: aerial photographs (1956), TM (1990) and SPOT (1998). Remote sensing data were interpreted and classified using ARC/INFO and ERDAS software and landscape indices of satellite imagery were calculated using FRAGSTATS 3.0 software. The change in the spatial extent of landscape forms and vegetation types around Qira oasis was analyzed by comparing SPOT satellite image from 1998 with aerial photographs from 1956. The results showed that the area covered by vegetation of the Qira oasis increased by nearly 20% from 1956 to 1998, but the distribution of vegetation distribution changed greatly and the river bed shifted about 400 m. Dune and desert encroachment was successfully combated near the oasis border but increased in extent at the outward border of the surrounding vegetation. The area covered with Populus trees was smaller in 1956 than today due to some new forests established in 1977 north of the oasis. Subfossil wood and leaf remnants of Populus euphratica found in the surrounding areas must have originated from forests destroyed before 1956. The main Qira river has shifted its bed significantly northward and developed a new furcation with a large new bed in 1986. The natural river dynamics are not only an important factor forming the oasis landscape pattern but also provides the only new regeneration sites for plant species.Quantitative analysis of the Qira oasis, the oasis-desert ecotone, desert and dunes from 1990 to 1998 showed that the area of the oasis increased by 6 km2 but patch decreased 256 blocks; the ecotone decreased in size by 2.5 km2 but patch increased 1 055 blocks; the area of desert decreased by 3.4 km2 but patch increased 608 blocks; and the area of dune did not change but patch increased 671 blocks. At the class level, Patch Density (PD), Edge Density (ED), Interspersion and Juxtaposition Index (IJI), Clumpiness Index (CLUMPY) and Aggregation Index (AI) were all calculated. At the landscape level, PD, ED, IJI, AI, Contagion Index (CONTAG), Diversity Index and Evenness Index were calculated. The indices of PD and ED increased, IJI, CONTAG and AI decreased. Diversity Index and Evenness Index changed slightly. The Qira oasis has changed considerably over the last 40 years due to natural flooding and vegetation degradation by human overexploitation. The trend of a decrease in the width of the ecotone (indigenous vegetation belt), which resulted from the advancing desert and expansion of arable land, is particularly alarming, because a decreased protective function against shifting sand can be expected in the future.

This study was carried out in the evergreen broad-leaved forest of Tiantong National Forest Park, Zhejiang Province, China. We identified and measured all trees in quadrats established in the forest to quantify the community structure of the forest. The population structures of the different tree species were categorized into five regeneration types based on their size-class frequency distribution patterns: Unimodal, Sporadic (multimodal), Inverse-J, L and Unibar type. The population size structure of a species reflects the biological and ecological characteristics of that species. The forest community was comprised of 69 tree species with six co-dominant species. Unimodal type species, such as Pinus massoniana, Liquidambar formosana, Sassafras tzumu etc., are shade intolerant, pioneer species or long-lived pioneer emergent trees which only regenerate on the bare ground following severe disturbances or in very large canopy gaps. Sporadic (multimodal) type species, such as Schima superba, Symplocos heishanensis, Machilus thunbergii etc., were late seral stage species that regenerate in large gaps and have intermediate shade-tolerance. Inverse-J type species, such as Castanopsis carlesii, C. fargesii, Lithocarpus harlandii etc., are shade tolerant, climax forest species that can regenerate through seedling bank or sprouting under the closed forest canopy. L type species, such as Neolitsea aurata var.　chekiangensis, Ternstroemia gymnanthera etc., and Unibar type species, such as Camellia fraternna, Symplocos stellaris etc., are understory, shade tolerant shrubs and treelets. The successional stage of the forest was classified as a late seral stage community that would develop into a climax community dominated by Castanopsis carlesii and C. fargesii.

Biological invasions are a pervasive environmental problem and are an important focus of ecological research and environmental management. Research on the function of biodiversity has stimulated an interest in the mechanisms underlying the invasibility of plant communities. Although many theoretical and observational studies suggest that diverse communities are more resistant to invasion by exotic species than less diverse communities, results of experimental studies are not conclusive and this remains a highly debated topic.

In this experiment, a series of manipulated grassland communities with different levels of species diversity and different species functional groups (16 species belong to C3 grasses, C4 plants, forbs and legume respectively) were established to test Elton's diversity-invasibility hypothesis by studying the pattern and process of invasion by Alternanthera philoxeroides. Total biomass of the invasive species, an index of community invasibility, was recorded in each community type. Our results showed that in communities with higher functional group diversity, the biomass of A. philoxeroides was significantly lower due to decreased niche opportunity whereas species diversity alone did not show any significant effects on the biomass of the invasive species. The results showed that community invasibility was negatively correlated with functional group diversity suggesting that the diversity of characteristics of species rather than species diversity itself was important. Niche opportunity for invasive species in communities might be a key determinant influencing its invasibility. The characteristics of functional groups also influenced the success of the invasion. Annual grasses with short life spans and nitrogen-fixing legumes were more susceptible to invasion. Moreover, A． sessilis, which belongs to the same morphological and functional group as A． philoxeroides, caused a significant decrease in establishment of the invader species. This suggests that competition might be more intense within functional groups than across functional groups. Because community invasibility is influenced by many factors and their interactions, the pattern and mechanisms of community invasibility are likely to be more complicated than we have acknowledged so far. More experimental work coupled with theoretical modeling studies are needed to better understand the characteristics of community invasibility.

Since shallow groundwater is the main source of vegetation growth in arid zones, groundwater level is one of the important eco-environmental factors affecting natural vegetation in the middle and lower reaches of the Tarim River in Xinjiang of China. Therefore, it is important to understand the relationship between the groundwater level and the vegetation. Accordingly, 12 monitoring sections with 58 monitoring wells and 58 vegetation sample plots were selected and established. Based on several years of monitoring data, we describe the influence of different levels of groundwater level on soil moisture content, vegetation (species and coverage) and the proline (Pro) and abscisic acid (ABA) content in the leaves of Populus euphratica. Regression models on groundwater level-vegetation coverage (Y=159.32e-0.314 8X, R2=0.819 3, p<0.01) and groundwater level-species (Y=9.113e-0.162 3X, R2=0.606 7, p<0.01) were both significant. The effect of groundwater level on vegetation function is expressed through soil moisture content, and ground water level also was significantly related to soil moisture content by the following regression models: Y=64.898e-0.515X, R2=0.727, p<0.01 (when the groundwater levels are between 1 to 4 m of the soil surface); and, Y=21.147e-0.178X, R2=0.658, p<0.01 (when groundwater levels are between 4 to 12 m below the surface). The soil moisture content changes significantly when the groundwater level drops to 3.5-4.0 m below the surface, henceforth, 3.5 m is regarded as the lowest groundwater level acceptable for restoration of the natural meadow vegetation. By analyzing the changes of the Pro and ABA content in the leaves of P. euphratica, we determined that water stress develops in these populations when groundwater levels drop below 5.0 m depth and that the ABA content is a more sensitive indicator of water stress than Pro content in the leaves. This relationship is described by the following equation: ABA content=0.703 5e0.408X, R2=0.830 4, p<0.01. Our results indicate that ground water level is a critical ecological factor controlling the vegetation in the lower reaches of the Tarim River. Some xerophytic trees, meadows and bushes were restored by changing groundwater levels; hence, any vegetation restoration efforts in this region will need to manage ground water levels to be successful.

From May to September 1998, we measured the seasonal near-ground spectral reflectance characteristics and biological parameters of non-degraded, moderately (or lightly) degraded and heavily degraded grassland communities of Leymus chinensis steppe and Stipa grandis steppe in Inner Mongolia, China. The results of Duncan multi-variance analysis indicated that the near-ground spectral reflectance varied with the degree of degradation, growing season and grassland type. For Leymus chinensis steppe grazing gradient, differences in the near-ground spectral reflectance among degraded communities were most distinct when measured at the end of July and were least at the end of May. For the Stipa grandis steppe grazing gradient, the spectral response was most distinct at the end of July and August and was least distinct at the end of May and June. No distinct differences in the Normalized Difference Vegetation Index (NDVI) among degraded community types were observed at the end of May, June and September. At the end of July and August, differences were observed but the degree of difference of the NDVI was less than that of the near-ground spectral reflectance. PCA and Pattern-Recognition Method, used to develop discriminate functions, indicated that the blue, red and near-infrared reflectance captured at the end of June had the greatest ability to discriminate among degrees of degradation along the L. chinensis grazing gradient ( mean probability of error was 0.7%) and had the poorest discriminatory power at the end of May (the mean probability of error was 12%). For S. grandis steppe grazing gradient, the discrimination ability was highest at the end of June and August when composed of blue, green and near-infrared reflectance (mean probability of error was lower than 4%) and lowest at the end of September (mean probability of error was about 10%).

Long-distance dispersal (LDD) of plant propagules has significant ecological and evolutionary implications for plant species migrations, biological invasions, conservation biology and many other fields of research. Although seeds can disperse by many different processes, the seeds of many herbaceous species of open grasslands and tree species in temperate and tropic zones are anemochoric, i.e., wind dispersed. Modeling the dispersal of anemochoric seeds, particularly their long distance dispersal, is a major research field because of the importance of this ecological process for understanding such phenomena as the spread of invasive alien plants and gene flow among meta-populations in fragmented landscapes. Our search results indicated that there are no synthesis papers on the LDD of anemochoric seeds. This paper discusses the background and significance of long-distance dispersal of air-borne seeds, analyzes the basic formulas and structures of models of seed dispersal by wind, summarizes recent advances in phenomenological and mechanistic models, and presents future research directions in this field. LDD models of anemochoric seeds are categorized into two major classes: phenomenological models and mechanistic models. Phenomenological models include short-tailed dispersal kernels (SDK), Leptokurtic fat-tailed kernels (LFK) and mixed dispersal kernels (MDK). The LFK and MDK models are most promising for simulating long-distance dispersal of seeds. Mechanical models are categorized into Eulerian advection-diffusion models (EADM) and Lagrangian stochastic models (LSM). The mechanisms of LDD and the major parameters of these two classes of models are a major focus of this paper. Important mechanisms of LDD include synchronization of seed release with suitable weather conditions. and updrafts that occur at forest edges and on the ground surface. Also, gradients of wind speed that form during storms was speculated as being an important mechanism of LDD. Operative factors in wind LDD models include biological, meteorological and topographical factors. We introduce and evaluate a number of models that have been used successfully to model LDD by wind, including tilted plumed model (TPM), advection-diffusion-deposition model (ADDM), no-shelter model (NSM), background model (BM), WINDISPER, modified WINDISPER (MWINDISPER) and PAPPUS. Lastly, the current status of wind LDD model construction is analyzed and some gaps are pointed out. The authors advocate that more effort should be made to construct models for herbaceous species since currently there are many more wind LDD models for tree species. There are many challenges and needs for modelers to link models with empirical field data of fragmented landscapes. Finally, collaborative approaches among researches from different fields are encouraged in order to improve LDD model forecasting especially with regard to increasing the precision of inputs of attributes of the physical environment.

Tree root respiration is a major contributor to soil CO2 pools, and 2/3 of total forest soil respiration is from root respiration. Information about root respiration is important for understanding the implications of environmental change on soil carbon cycling and carbon sequestration, and for the development of carbon models of forest ecosystem dynamics. Furthermore, the global CO2 flux from root respiration is estimated to be about 18 Pg C·a-1, which is an order of magnitude larger than that produced by anthropogenic sources of CO2. Therefore, changes in forest tree root respiration could have a significant impact on the future global carbon balance. Ecological research on tree root respiration is relatively recent and still very little research on this topic has been conducted in China. In this paper, We review the characteristics, methodologies and factors affecting tree root respiration for the purpose of stimulating new, domestic research on this topic. Tree root respiration is composed of maintenance respiration and growth respiration. Maintenance respiration, the dominant component of total of total root respiration, is used to maintain the living biomass, and growth respiration, which is used to construct new biomass, is proportional to the amount of new dry matter synthesized. Root respiration rates vary significantly among forest types. The proportion of the total soil carbon flux that is attributable to live root respiration appears to be very high in cold, northern biomes, ranging from 50% to 93% in the arctic tundra and from 62% to 89% in boreal forests. In temperate zones, estimated proportions of the total soil respiration flux that is derived from live root respiration range from 33% to 50% in broad-leaved forests and from 35% to 62% in pine forests. Root respiration typically is 40%-60% of total soil respiration in most forests. Tree root respiration has significant seasonal dynamics with respiration greatest during the growing season and lowest during the dormant periods of the year. . Methods used to measure tree root respiration include root exclusion methods, in vitro root techniques, stable or radioactive isotope methods and in situ cuvette methods. Each approach has advantages and disadvantages. The first two methods are relatively simple and inexpensive and are commonly used in forest ecosystems. Isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots, but the complexity of the experimental setup and the high costs associated with the analysis of radioactive or stable C isotopes are major disadvantages. The in situ cuvette method is considered an important method for future studies. Critical factors influencing rates of tree root respiration include soil temperature, root diameter size, ambient CO2 concentration, soil moisture, and nutrient availability. . Despite intensive research in recent years, many uncertainties remain in this dynamic and important field of research. Topics of particular importance include: 1) Discussion and comparison of the appropriate methods for accurately measuring tree root respiration; 2) Application of effective methods for separating root respiration and rhizosphere respiration in the field; 3) Long-term research on the dynamics of tree root respiration in forest ecosystems; 4) Studies of tree root respiration in different ecosystems and climatic zones; 5) How to scale up from small chambers to the stand level, ecosystem level, regional or global level; 6) Understanding the mechanistic response of tree root respiration to global climate change; and, 7) Inter-disciplinary research on tree root respiration to understand its role in the global carbon cycle.