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Table of Content
    Volume 29 Issue 5
    30 August 2005
    Research Articles
    WANG Man-Lian, FENG Yu-Long
    Chin J Plan Ecolo. 2005, 29 (5):  697-705.  doi:10.17521/cjpe.2005.0093
    Abstract ( 4132 )   PDF (743KB) ( 1273 )   Save
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    Nitrogen availability is a major determinant of successional patterns in many ecosystems. Increased levels of soil nitrogen, caused by atmospheric nitrogen deposition, continuously fertilize a large (and growing) portion of the terrestrial biosphere. Increased nitrogen deposition onto natural ecosystems is disadvantageous to slow_growing native plants that have adapted to nutrient_poor habitats by creating environments favorable for faster_growing plants, such as grasses. In this paper, two invasive plant species, Ageratina adenophoraand Chromoleana odorata,were studied. Both of them were planted under five soil nitrogen levels for more than four months. By investigating their traits related to morphology, biomass allocation, growth and photosynthesis, we compared their phenotypic responses to nitrogen. Our main objectives were to 1) explore how the two species acclimate to soil nitrogen availability, 2) evaluate which plant traits were associated with the invasiveness of the two species, and 3) determine whether the increased levels of soil nitrogen could facilitate their invasion.  The two species were very plastic in their response to nitrogen availability. They exhibited considerable (nitrogen_acclimation abilities. With an increase in nitrogen levels, their root mass ratio and root mass/crown mass decreased, but their leaf mass ratio (LMR), leaf area ratio and leaf area to root mass ratio increased. At lower nitrogen levels, more biomass was invested into the root system, a nutrient absorbing organ, which could enhance nutrient_capture ability. At higher nitrogen levels, more biomass was invested into the leaves, an assimilative organ, which could increase their carbon accumulation and improve their competitive abilities. A. adenophorum could acclimate better to nitrogen environments than C. odorata.The two invasive plant species could benefit from high nitrogen levels, which were usually excessive and/or harmful for most native species. Under a wide range of nitrogen levels, relative growth rates (RGR), total biomass, branch numbers, leaf area index, maximum net photosynthetic rate, chlorophyll and carotenoid content increased significantly with increasing nitrogen levels, and did not decrease significantly at over_optimal nitrogen levels. The two species could maintain relatively higher RGR in the dry season when native plant species almost stopped growing. Having the ability to use resources at times when native plants could not, their competitive abilities and invasiveness were promoted. Mean leaf area ratio (equal to LMR/SLA (specific leaf area)) and net assimilation rate were coequally important in determining the response of RGR to nitrog en levels in A. adenophora and C. odorata.LMR was a very important dete rminant of RGR, which played the most important role in determining differences in RGRamong nitrogen treatments and between species. With an increase in nitrogen levels, the SLA decreased in A. adenophora whereas it increased in C. odorata.  But under all nitrogen levels, SLA was higher in A. adenophora than in C. odorat a. The higher SLA of A. adenophora compensated this species for its lower LMR and was favorable to its growth. The response trend of SLA to nitrogen levels in A. adenophora was more profitable than in C. odorata. In conclusion, our results indicated that the two invasive plant species were able to acclimate to a wide range of nitrogen environments and could grow better in higher nitrogen environments, suggesting that enhanced soil nitrogen levels might promote their invasion.
    SUN Hai-Long, WU Chu, XU Wen-Jing, WANG Zheng-Quan
    Chin J Plan Ecolo. 2005, 29 (5):  706-712.  doi:10.17521/cjpe.2005.0094
    Abstract ( 3505 )   PDF (624KB) ( 983 )   Save
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    Roots absorb water from the soil and play an essential role in their water balance. In wet soil, the root system is the primary limitation for plant water uptake and can contribute up to approximately 50% of the overall hydraulic resistance of the plant. The root hydraulic conductance (Lp) based on the root surface area has a major influence on the shoot water status, plant growth and development. Recent studies show that radical water transport is an important determinant of root Lp, because aquaporins in the protoplasm and vacuole membranes play a key role in the radial water transport of plant roots, i.e., higher activity of aquaporins favors higher water transport, and Lp is an effective index in determining the relationship between aquaporins and water transport. However, most studies on Lp have used the same uptake solution, (e.g., deionized water), and variation of Lp in solutions of different substances has been ignored. The objectives of this study were to 1) compare differences in Lp in deionized water and various concentrations of NH4NO3 solutions, and 2) examine changes ofLp when treated with deionized water and NH4NO3 solutions after HgCl2 treatment to inhibit aquaporins.  The experiment was conducted in May 2002. Manchurian Ash (Fraxinus mandshurica ) seedlings were grown for 40-45 d on a bench in a greenhouse at the Northeast Forestry University. Greenhouse temperatures were approximately 25 ℃ day maximum and 18 ℃ night minimum, and average photosynthetic photon flux was 500-800 μmol photons•m-2•s-1 during this period. Roots were placed in treatments of deionized water and NH4NO3 solutions with concentrations of 16, 8, 4, and 1 mmol•L-1for 15 min and Lp measured by the pressure_flux approach. Roots were treated with HgCl2 (50μmol•L-1) and then again placed in the different treatment solutions andLp measured.  The results showed that the Lp of roots in deionized water was 22% higher than when treated with HgCl2 and 16% higher than both HgCl2_treated and mercapto ethanol_treated plants. In NH4NO3 solutions of various concentrations, Lp increased with increasing concentrations of NH4NO3, and the maximum Lp occurred at 8 mmol•L-1NH4NO3. The average Lp in NH4NO3 solutions was 77% higher than in deionized water After HgCl2 treatment, Lp still increased from lower to higher NH4NO3 concentrations but was reduced by 68%, which was three folds higher than the 22% reduction in HgCl2_treated roots in deionized water. In contrast to other studies , the percent Lp reduction in nutrient solutions was higher than in deionized water after HgCl2 treatment.  These results suggest that aquaporins are very important in regulating water movement in the roots of Manchurian ash, and nutrient ions can have a significant effect on the activity of aquaporins. In plants not treated by HgCl2, nutrient ions can stimulate aquaporins and increase their activity. After aquaporins were inhibited by the HgCl2 treatment, the Lp of roots in different solutions decreased significantly and inhibition of HgCl2 increased with ion solution concentrat ions. Our results indicate that there are HgCl2_sensitive aquaporins and ion cha nn els in the protoplasm and vacuole membranes of the roots. However, these need to be experimentally tested in the future.

    FAN Da-Yong, ZHANG Wang-Feng, CHEN Zhi-Gang, XIE Zong-Qiang
    Chin J Plan Ecolo. 2005, 29 (5):  713-723.  doi:10.17521/cjpe.2005.0095
    Abstract ( 2047 )   PDF (814KB) ( 729 )   Save
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    A preliminary study was carried out to study the acclimation of saplings of Cathaya argyrophyllato light across a gradient of canopy openness within a typi cal forest gap (≈100 m2) at Jin Fo Shan of Sichuan Province. Photosynthe sis related eco_physiological traits of fully expanded leaves were measured by gas_exchange techniques, the canopy openness of experimental leaves was measu red by a modified fish_eye method, and the current growth status of experimental saplings was investigated. Better acclimation of saplings at the site of gap ed ge was indicated by higher growth status of above_ground parts, higher height growth rate in last year, higher average basal stem growth rate, and higher crown expedition rate in last year than saplings in the understorey or gap center site s. With increasing canopy openness, the maximum net photosynthetic rate per leaf area, the dark respiration rate per leaf area, and specific leaf mass increased showing significant positive correlations to canopy openness. With increasing c anopy openness, leaf area and dark respiration rate per leaf mass decreased slig htly, the maximum net photosynthetic rate per leaf mass and single leaf mass increased slightly, but there were no significant correlations with canopy openness . Across the gradient of canopy openness, the phenotypic plasticity index was slightly lower than the ecophysiological plasticity index, but the differences were not significant.

    XIANG Yan-Ci, PENG Shao-Lin, PENG Xiu-Hua, CAI Xi-An, RAO Xing-Quan
    Chin J Plan Ecolo. 2005, 29 (5):  724-729.  doi:10.17521/cjpe.2005.0096
    Abstract ( 3598 )   PDF (477KB) ( 1065 )   Save
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    Juvenile plants usually compete with the established vegetation within a community. Competition among plants takes place both aboveground and belowground. Aboveground competition primarily involves a single resource, light. In contrast to aboveground competition, plants compete for a broad range of soil resources, including water and many essential mineral nutrients that differ inmolecular size, valence, oxidation state, and mobility within the soil. Of particular interest is the intensity of competition by invasive exotic plants and how this varies among species. We used a field experiment to measure the intensity of competition on juvenile trees in a secondary forest community with infertile soil on an island in South China. We separated competition into above_ and belowground components in the field. We reduced aboveground competition by felling large trees to create a treefall gap and removed belowground competition by trenching. We created three neighborhood treatments: roots of neighbors only, shoots of neighbors only and no neighbors. Seedlings of three tree species were transplanted in to the understory with and without trenches cut around plots (with roots and shoots of neighbors and with shoots of neighbors only), and single treefall gap are a with and without trenches cut around plots (with roots of neighbors only and with no neighbors). The species used included two exotic species, Eucalyptus urophylla and Acacia auriculaeformis, and one native species, Schima super ba. We measured the biomass, net primary productivity (NPP), and total competitive intensity in above_ and belowground components over a two_year period.The biomass and NPP in the plots without root competition were greater than those in the plots with root competition. Comparisons among the three species of seedlings showed that the intensity of total competition was greater for E. urophylla and  A. auriculaeformis and lower for S. superba. Difference in the intensity of aboveg round competition among the three tree species showed the same pattern as that of intensity of total competition. However, the intensity of belowground competition was the greatest for S. superba and the lowest for A. auriculiformis.  Results based on the average of the three species showed that seedlings of the exotic trees, such as E. urophylla, A. auriculiformis, were also influenced by competition with established vegetation within the community during the invasion process . The intensity of total competition on the two exotic species, E. urophylla and A. auriculaeformis, were significantly greater than on the native species,  S. superba. These results suggest that competition might pose a greater impact on the two exotic species when they invade a secondary forest community with a complex structure.

    ZHU Ya-Juan, DONG Ming, HUANG Zhen-Ying
    Chin J Plan Ecolo. 2005, 29 (5):  730-739.  doi:10.17521/cjpe.2005.0097
    Abstract ( 2781 )   PDF (668KB) ( 951 )   Save
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    Sand burial is a common phenomenon in sand dune ecosystems all over the world. Although many studies concerning the effects of sand burial on seed germination and seedling emergence have been conducted in many parts of the world, such studies are rare in China. In the summer of 2003, we studied the effects of sand burial depth (0, 1, 2, 4, 6, 8, 10 and 12 cm) and seed size on seed germination and seedling emergence of Psammochloa villosa, an important psammophyte that establishes on mobile sand dunes in deserts and sandy lands of North China. Seeds were collected in their natural habitat in Mu_Us Sandland, Ordos Plateau, in 2002 and stored at -18 ℃. The average seed mass of P. villosawas (5.51±0.05) mg. Seeds were sorted into 3 groups based on their mass: small (4 - 4.9 mg, mean = (4.489 ± 0.012) mg), medium (5 - 5.9 mg, mean = (5.457 ± 0.012) mg), and large (6 - 6.9 mg, mean = (6.415 ± 0.011) mg). We conducted two experiments , one in the field and the other in a non_heated greenhouse. In the second exper iment, burial depth was 1, 2, 4, 6 and 8 cm. Before burial experiments, all seeds were treated with cold stratification in 5 ℃ for 4 weeks in order to break dormancy.We found that sand burial depth had significant effects on seed germination and seedling emergence of P. villosa. Experiments revealed that the highest percentage of seeds that germinated and seedlings that emerged occurred at 2 cm burial depth. Few seeds lying on the surface of the sand germinated, and, those that did germinate, could not anchor their roots into the soil and seedlings did not establish successfully. The next greatest germination and emergence percentage occurred at the 1 cm burial depth. The timing of emergence was delayed with deeper burial depth. As burial depth increased from 2 to 12 cm, the percentage of seeds that germinated and emerged declined, a higher number of seeds remained dormant, and the number of days before first emergence increased. As sand burial depth increased, some seeds were able to germinate but the seedlings were not able to emerge possibly because the seeds did not have enough energy. Seedlings that could not emerge, etiolated, died and decomposed in the sand. At all depths, the seeds that could not germinate remained dormant. Possible reasons for sustained dormancy include poor aeration, high sand moisture, low soil temperature, poor O2 content, higher CO2 levels in soil and low light intensity. The dormant seeds, which form soil seed banks, are very important for the long_term survival of this species. The dormant seeds can germinate when sand erosion decreases the burial depth and the seeds are exposed to optimal environmental conditions, such as sand depth. However, our experiment revealed that seeds buried at deeper depths had higher mortality rates (15 - 35%). The high mortality might be due to fungal infection, water_soluble germination inhibitors in the seed coat, and an unfavorable microenvironment and low O2 concentrations. Seed size did not affect the percentage of seeds that germinated or remained dormant, but had a remarkable effect on the percentage of seedlings that emerged at different sand burial depths. At greater depths (>6 cm), large seeds had higher rates of seedling emergence than medium and small seeds. From 1 to 6 cm depth, the seedling shoots that originated from large seeds were longer than those from small seeds; also, at 2 to 6 cm depth, seedling shoots that originated from medium sized seeds were longer than those from small seeds. Therefore, we suggest larger seeds have ecological advantages in sand dune ecosystems. Larger seeds have a larger endosperm, contain more energy, and thus can emerge from deeper burial depths. This will benefit seedling growth in early stages and help them to suc cessfully establish under extreme desert conditions. Sand burial might be a sele ctive pressure on seed size; therefore, seed polymorphism as an evolutionary str ategy can benefit the renewal of P. villosa populations. In sand dune habitats, P. villosa produced different sizes of seeds that could emerge from different depths of sand burial. The diversity of seed size could enhance opportunities for seedling establishment in this heterogeneous environment allowing plants to respond to different sand burial depths, enhance their ability to adapt to the chang ing environment, and increase survival.
    LEI Shu-Qing, WANG Hai-Yang, DU Guo-Zhen, PAN Sheng-Wang
    Chin J Plan Ecolo. 2005, 29 (5):  740-746.  doi:10.17521/cjpe.2005.0098
    Abstract ( 2599 )   PDF (576KB) ( 974 )   Save
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    Identifying mechanisms of tolerance to herbivore damage will facilitate attempts to understand the role of tolerance in the evolutionary and ecological dynamics of plants and herbivores. Several external factors, such as water availability, nutrient availability, intensity of damage, and timing of damage, will affect the ability of individual plants to tolerate damage by mediating internal mechanisms. Though interspecific comparisons are useful for identifying possible mechanisms, direct comparisons between tolerance and putative mechanism have been made almost exclusively in interspecies or interpopulation studies. This study compared the compensatory responses of Avena sativaand Brassic acampestris, which belong to different growth forms, to clipping under two fertilization treatments. The results showed that, for Avena sativa,under no fertilization treatment, the biomass, total biomass, fruit weight and number of fruits were greater than in the control, but only some aspects resulted in overcompensation. Under fertilized conditions, clipping treatments did not cause any overcompensation. Whether fertilized or not, the index of compensation during the tillering stage and jointing stage were both higher than during the flowering stage. Thus, under the no fertilization treatment, the low clipping treatment during the vegetative stage was shown to benefit Avena sativa. With respect to Brassica campestris,the index of compensation was greatest in the low clipping treatment durin g the flower bud stage, and was enhanced under fertilization. These results indicate that clipping during the reproductive stage can help compensatory growth in Brassica campestris.The different responses to clipping were attributed to the different growth forms, which had different positions and activities of dormant buds.

    ZHAN Xue-Ming, LI Ling-Hao, LI Xin, CHENG Wei-Xin
    Chin J Plan Ecolo. 2005, 29 (5):  747-752.  doi:10.17521/cjpe.2005.0099
    Abstract ( 2628 )   PDF (538KB) ( 945 )   Save
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    We studied the soil seed bank of a Stipa krylovii steppe community at two sites with different land_use histories: an enclosed pasture, which has been fenced since 2001, and a grazed pasture, which has been continuously grazed for more than 20 years. The experiment was carried out in Duolun County of Xilingol League, Inner Mongolia. Sixteen 12 cm diameter soil cores were collected to a depth of 5 cm at each site on March 30 and 31, 2004. The soil seed bank was estimated by monitoring seedling emergence, a convenient way to compare soils under similar conditions. Vegetation composition was assessed at each site on May and August 2004 by recording the species presented in sixteen 0.5 m×0.5 m quadrats that were distributed randomly alongside the seed bank sampling points.  A total of 1 593 seedlings belonging to 19 species germinated from 32 soil samples. The total seed bank flora at the two sites included 6 grass species and 13 forbs, 18 of which were perennials and 1 annual, 10 monocotyledons and 9 dicotyledons. In the fenced pasture, there were 16 perennials, 6 grasses and 9 monocotyledons from a total of 16 species in the seed bank, whereas 12 perennials, 4 grasses and 5 monocotyledons from a total of 13 species were found in the seed bank of the grazed pasture. The average number of viable seeds per square meter was s ignificantly higher at the fenced site A ((5 139±1 848) seeds•m-2) than that at the grazed site B ((3 664±1 087) seeds•m-2). Sorensen's similarity index was 0.711 1 for the fenced site and 0.611 1 for the grazed site. In total, only fifteen species were found in the established vegetation at both sites. As many as 81.0 % of the seeds recorded belonged to only five species (Artemisia frigida, S. krylovii, Potentilla acaulis, Cleistogenes squarrosa, and Allium bidentatum). Another 5 species accounted for 16.2%of the seeds recorded and the other 9 species accounted for only 2.8% of the total.  Our results showed that the scarcity of seeds of some important steppe species combined with the unbalanced distribution of seeds among species may inhibit the restoration process of the S. krylovii steppe. Therefore, reseeding and other management steps should be used to speed up the restoration process of the degraded S. krylovii steppe.

    GU Zhi-Hui, CHEN Jin, SHI Pei-Jun, XU Ming
    Chin J Plan Ecolo. 2005, 29 (5):  753-765.  doi:10.17521/cjpe.2005.0100
    Abstract ( 2622 )   PDF (1134KB) ( 828 )   Save
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    There is a crucial need in global change studies to understand how terrestrial ecosystems respond to climate systems. It has been demonstrated by many researchers that the Normalized Different Vegetation Index (NDVI) time series from remotely sensed data, which provide effective information of vegetation conditions at large scales with high temporal resolution, are closely correlated with meteorological factors. However, few of these studies have taken the cumulative property of NDVItime series into account. In this study, NDVI difference series was proposed to replace the original NDVI time series to reappraise the relationship between NDVIand meteorological factors. As a proxy of vegetation growing processes, NDVI difference represented net primary productivity (NPP) of vegetation during specific time intervals and under specific environmental conditions . This data replacement eliminates cumulative effects that exist in the original NDVItime series, and thus is more appropriate for understanding how climate systems affect vegetation growth over short time scales. Using correlation analysis, we studied the relationship between NOAA/AVHRR ten_day NDVI difference series and corresponding meteorological data from 1983 to 1999 from 11 meteorological stations located in the Xilingole steppe in Inner Mongolia. Our analyses showed the following results. 1) Meteorological factors were found to be more strongly correlated with NDVI difference at the biomass_increasing phase than during the decreasing phase. 2) The relationship between NDVI difference and climate variables varied with vegetation types. For a typical steppe community dominated by Leymus chinensis, temperature had a higher correlation with NDVIdifference than precipitation and, for a typical steppe community dominated by Stipa krylo vii, the correlation between temperature and NDVI difference was lower than that for precipitation. For a typical steppe dominated by S. grandis, there were no significant differences between the two correlations. Precipitation was the key factor influencing vegetation growth in desert steppe communities, and temperature had a poor correlation with NDVIdifference. 3) The response of NDVI difference to precipitation is fast and almost simultaneous in both typical steppe and desert steppe; however, mean temperature exhibited a time_lag effect, especially in the desert steppe and some typical steppe ecosystems dominated by S. krylovii. 4) The relationship between NDVIdifference and temperature is b ecoming stronger with global warming. 

    LIU Qi-Jing, HU Li-Le, LI Xuan-Ran
    Chin J Plan Ecolo. 2005, 29 (5):  766-774.  doi:10.17521/cjpe.2005.0101
    Abstract ( 2542 )   PDF (789KB) ( 722 )   Save
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    The species diversity of a small watershed was investigated following 20 years treatments. Our results showed that the species dominance index of the upper forest canopy layer was higher than other layers and was higher in the artificial forests than that in secondary forests and Liquidambar formosana stands. As a whole, the dominance index for the shrub layer of the 12 communities investigated was small, averaging 0.17. For the herb layer, the dominance index was high in the Ass. Imperata cylindrica var. major community (0.53) and the Ass. Loropetalum chinense _Bromus remotiflorus community (0.51). The species diversity index was high in the canopy layer of the L. formosana forest and natural Pinus massoniana forest but was low in other communities and, in particular, was lowest in the artificial forests. For the canopy layer, the species diversity index was the highest in the L. formosana forest (2.61). A similar trend was found in the subcanopy layers with the highest value found in the P. massoniana + P. elliottii forest. Also, the species diversity index was higher in the artificial forests than that in the secondary forests except for the P. massoniana + Lits ea cubeba and Castanea mollissima+ L. formosana forests. The richness index was the highest in the shrub layer for all communities and was similarly high in both the canopy and herb layers as the shrub layer in the L. formosana forests. The richness index was higher in secondary forests than that in artificial forests. The evenness index or uniformity index in the herb layer was higher than that in the shrub layer except for the Ass. Ldoropetalum chinense_Bromus remotiflorus and Ass. I. cylindrica var. major stands. For the canopy layer, evenness was as high as 0.76 in the Ass. C. mollissima + L. formosana and Ass. P. massoniana + L. cubeba stands whereas other artificial forests had lower values than that in secondary forests and L. formosana stands. In the shrub layer, evenness was the highest in the artificial forests except for the Castanea+ Liquidambar forest. The diversity indices showed large differences between the shrub layer and other communities, and the L. formosana forest showed properties that can be considered atransitionaltype between artificial and secondary forests.
    PENG Yu, JIANG Gao-Ming, LI Yong-Geng, LIU Mei-Zhen, NIU Shu-Li, YU Shun-Li, YANG Dao-Bin
    Chin J Plan Ecolo. 2005, 29 (5):  775-780.  doi:10.17521/cjpe.2005.0102
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    There are many patches of Ulmus pumila forests distributed in the Hunsh andak Sandland, most of them located in the Zhenglan Banner, Inner Mongolia, China. These forest patches are important for combating desertification and sandstorm, conserving biological diversity and landscape views. In order to protect  Ulmus pumila forests, the establishment of a nature reserve has been planned for the Hunshandak Sandland. The core zone of the planned nature reserve was determined based on principles of island biogeography theory and landscape ecology theory. First, a map of the distribution region of Ulmus pumila forest patches, a map of communication, and a map of the administrative system of Zhenglan Banner were transferred into a computer. Using ArcGIS software, the patch size, patch distribution, patch density, patch distance, patch segmentation and fragmentation were analyzed. We found that the sparse forest patches were mainly distributed in 20 village regions, and each patch was an average of 0.96 km2. For these 20 villages, the patch density ranged from 0.10 to 0.24 km-2, the edge density ranged from 2.50 to 12.50 km-1, and separating degree ranged from 0.4 to 1.20 km. These indices showed that fragmentation is a serious problem and these forests need to be protected. In order to select the proper sites to make up the core zone, A rcGIS buffer analysis was used to determine the minimum distance necessary to unite the patches into one zone. Most patches could be united using a distance of 3 km. By examining changes in the plant community, a 1 km buffer region around each patch had the greatest value for protection. Thus, a 10 km buffer around the patches could guarantee that most objects would be protected and occupy the least amount of area. According to guidelines for nature reserve planning based on principles of island biogeography, the rest of the zones within the buffer lines need to be included into the core zone. Therefore, the planned core zone will be 1 531 km2 and occupy 13% of total Zhenglan Banner nature reserve. We also discuss how to determine the shape of the core zone with respect to edge effects.

    YANG Shu-De, XU Wen-Zhong, LIU Shi-Rui, ZHANG Tong, MA Mi
    Chin J Plan Ecolo. 2005, 29 (5):  781-784.  doi:10.17521/cjpe.2005.0103
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    Unsymmetrical dimethyl_hydrazine (UDMH) is a highly volatile and reactive compound widely used as a liquid rocket propellant fuel in manned and unmanned space and missile systems. Exposure to UDMH is an important occupational health issue, which will increase in significance as space applications increase. Several methods have been tested for removing UDMH from the environment, such as catalytic oxidation, active carbon absorption and ion exchange resins. However, little attention has been given to the application of constructed wetlands to remove UDMH from groundwater even though constructed wetlands are a promising measure for wastewater treatment. In expectation of constructing of an effective wetland by the variant reed lines to deal with UDMH polluted groundwater, attempts were made to screen for UDMH tolerant variant lines of reeds using cellular engineering technology in this study. Seeds of the reed (Phragmites communis) collected from the river banks near by a satellite_launching basement were sterilized with 70% ethanol and 2% hypochlorous acid and then transferred to a MS medium for germination. After germination, hypocotyl seedling segments were used to induce callus on the MS medium supplemented with 1 mg•L-1 2,4_dichlorophenoxyacetic acid. The UDMH_tolerant cell lines were obtained by continuous selection on the UDMH containing medium supplemented with 0.5 mg•L-1 2,4_dichlorophenoxyacetic acid. Three levels of UDMH tolerant cell lines were screened for that contained either 1.63, 3.26, or 8.15 mmol•L-1UDMH after 3-6 times of subculture. Relative growth, measured at the end of a 23 _day incubation, was 90.4%, 84.3%, and 43.4% of the control in the three treatments, respectively, and, at the end of a 43_day incubation, was 95.6%, 91.7%, and 46 .8% of the control, respectively. These results showed that UDMH could inhibit the growth of the callus at 8.15 mmol•L-1. Regeneration of plantlets from the selected cell lines was conducted on UDMH containing MS medium supplemented with 0.1 g•L-16_Benzylaminopurine and 0.01 g•L-11_naphthylacetic acid. However, only the cell lines grown in the lower two treatments could be successfully differentiated, the 8.15 mmol•L-1 UDMH failed to differentiate. These results indicated that the suppressive effect of UDMH on callus differentiation was greater than on callus growth. Plantlets screened under UDMH stress were more tolerant to UDMH than the control, which were plantlets regenerated from callus without UDMH stress. Root growth of the differentiated plantlets was enhanced by transferring them to a hormone_free MS medium supplemented with UDMH for 35 days prior to successful transplantation of the plantlets into the soil. Finally, more than 70% of regenerated plantlets could be successfully transferred to the greenhouse. Further experiments estimating the growing status and remediation efficiency in UDMH polluted water of these variant seed lines were undertaken.

    LI Shan, QIAN Zeng-Qiang, CAI Yu-Liang, ZHAO Gui-Fang
    Chin J Plan Ecolo. 2005, 29 (5):  785-792.  doi:10.17521/cjpe.2005.0104
    Abstract ( 2029 )   PDF (711KB) ( 830 )   Save
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    Dipteroniais an endemic genus to China and includes only two species, Dipteronia sinensis and D. dyeriana. Based on RAPD markers, a comparative study on the genetic diversity and genetic structure of Dipteroniawas performed. Intotal, 128 and 103 loci were detected in 17 D. sinensis populations and 4 D. dyeriana populations, respectively, using 18 random primers. These results showed that the proportion of polymorphic loci for the two species were 92.97% and 81.55%, respectively, indicating that the genetic diversity of D. sinensis was higher than that of D. dyeriana. Analyses based on similarity coefficients, Shannon diversity index and Nei gene diversity index, also confirmed this result. AMOVA analy sis demonstrated that the genetic variation of D. sinensis within and a mong populations accounted for 56.89% and 43.11% of the total variation, respectively, and 57.86% and 42.14%, respectively, of D. dyeriana. The Shannon diversity index and Nei gene diversity index showed similar results. The above_mentioned characteristics indicated that the genetic diversity levels of these two species were extremely similar and that the interpopulational genetic differentiation within both species was relatively high. Analysis of the geneticd istance among populations also supported this conclusion. Low levels of interpop ulational gene flow within both species were believed to be among the leading causes for the above_mentioned phenomenon. The correlation analysis between genetic and geographical distances showed that there existed a remarkably significant correlation between the genetic distance and the longitude difference among popu lations of D. sinensis (p<0.01) while no significant correlation existed between genetic and geographical distances among populations of D. dyeriana . This indicates that genetic distance was correlated with geogra phical distances at large scales other than at small scales. This result may be related to differences in selection pressure exerted on species by their habitats with different distribution ranges. We suggest that in situ conservation efforts should focus on establishing more sites to protect the natural populations an d their habitats. Ex situconservation efforts should focus on enhancing the exchange of seeds and seedlings among populations to facilitate gene exchange and recombination to help conserve genetic diversity.

    PENG Yan-Qiong, YANG Da-Rong, DUAN Zhu-Biao, DENG Xiao-Bao
    Chin J Plan Ecolo. 2005, 29 (5):  793-798.  doi:10.17521/cjpe.2005.0105
    Abstract ( 2011 )   PDF (560KB) ( 1152 )   Save
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    The interaction between Ficus species and their pollinating wasps (Agao nidae) represents a striking example of mutualism. With few exceptions, seed production by fig trees is dependent on a unique fig_pollinating wasp, and the poll inator's offspring feed only on the ovules. We studied the reproductive components of Ficus hispida and its pollinator (Ceratosolen solmsi marchali) in the tropical area of Xishuangbanna. Ficus hispida is functionally dioecious, with male and female functions relegated to separate plants, called gall and seed figs. Gall figs are functionally male because they foster the pollinator larvae that disperse the fig's pollen as adults. Seed figs are functionally female and produce only seeds. When a syconium becomes receptive, pollinators will enter it and lay eggs or pollinate the female flowers inside the syconium cavity. The pollinators trapped inside the syconium cavity and in the ostiole were recorded in the wild. The results showed that mean foundress number was greater in female th an in male syconia (females: 2.72±2.04; males: 2.08±1.65). About 52% of the foundresses could enter the cavity of the female syconia through the ostiole and only 35% for the male syconia. Many foundresses died in the ostiole and consequently failed to lay eggs and pollinate the female syconium. As there were fewer female flowers per syconium in male than in female syconia, and foundress numbers were greater in female than in male syconia, female syconia produced more seeds than male syconia produced pollinators, and the number of seeds (1 891.63 ± 471 .53) was over five times of the number of pollinators (367.20 ± 208.02). In female syconia, there was a significant positive correlation between seeds and female flowers, while there was a significant negative correlation between the number of seeds and aborted female flowers. Male syconia produced pollen and wasps also reproduced inside the male syconia, but many factors influenced pollinator repr oduction. Gall numbers showed the strongest impact on the pollinator population; the secondary factor was aborted galls, which could reduce the pollinator numbers by about 30%; the third factor was the total number of female flowers, which influenced the oviposition ratio of pollinators. Moreover, three species of non_ pollinating wasps, Philotrypesis pilosa, Philotrypesissp., and Apocry pta bakeri, oviposit from outside the syconium into ovaries containing pollinator eggs on the male tree. They co_existed with pollinators inside the male syconia. The non_pollinating wasps had a direct impact on the reproductive success of the pollinator wasp and were able to reduce the number of pollinator individuals per male syconium by 30. They also showed an indirect impact on the host figs.

    ZHANG Yun-Chun, DU Xiao-Jun, ZHANG Qiao-Ying, GAO Xian-Ming, SU Zhi-Xian
    Chin J Plan Ecolo. 2005, 29 (5):  799-806.  doi:10.17521/cjpe.2005.0106
    Abstract ( 2436 )   PDF (771KB) ( 748 )   Save
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    Symplocos laurina is an arboreal clonal plant. Its branches are bi_fun ctional organs that both undertake photosynthesis and produce ramets for propagation. Due to their unique functions, the branches of S. laurina differ in their physical appearance: upper branches grow well and rather strong and function to accumulate and provide photosynthates to the whole plant, whereas the lower branches have slim proximal ends but much sturdier head ends and grow downward in a V shape. Lower branches grow in length rather than in diameter to clone young erramets. When branches switch their roles from photosynthetic structures to ramet producers, they experience much greater germination. Apical dominance and stimulation from root appearance may be the two primary reasons. Germination times are very different between ramets in different habitats but do not differ in control branches indicating that older and stronger trees are not as sensitive as the younger tender ramets to environmental conditions, even though the ramets are still connected to the mother plant. As new organisms, young ramets adopt different strategies to compete for light: they grow hard to increase the number of leaves so as to enlarge the total light_receiving area in horizontal space. In contrast, parent branches increase the area of each leaf to get the largest possible leaf area in vertical space. Because the ramets can germinate more than once a year, they can produce more leaves than their parent branches, which can only germinate once and the number of leaves are predetermined by the buds. However, even in different habitats, no significant differences in total leaf area were observed between ramet_produced branches and control branches. The biomass of leaves, petioles and the total biomass of plant change greatly among habitats, but they don't change with the role switch of the branches as ramets producers. Stem biomass does not respond to different habitats but the role switch of branches does affect stem biomass. Control branches allocate their biomass mainly to photosynthesis modules, such as leaves and petioles; in contrast ramets focus most their biomass on stems. We conclude that branches change their function after they take root in the ground, which brings about correspondent changes in their morphological features. This is probably due to internal hormonal changes as a branch changes its function but more research is needed to better understand this phenomenon.

    LI Yong-Geng, YU Zhen-Wen, LIANG Xiao-Fang, ZHAO Jun-Ye, QIU Xi-Bin
    Chin J Plan Ecolo. 2005, 29 (5):  807-813.  doi:10.17521/cjpe.2005.0107
    Abstract ( 2226 )   PDF (511KB) ( 928 )   Save
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    Changes in the yields and quality, and the physiological response of w heat (Triticum aestivum cv. `Ji'nan17' and `Lumai21') grown under low light intensities (50% PPFD) at different filling stages were studied. Wheat was p lanted in 2.5 m×2.0 m plots. The plots were shaded with white fabric that filtered 50% PPFD during three different filling stages: 1-10, 11-20, and 21-30 days afte r anth esis. Protein accumulation rate, nitrogen accumulation and distribution, kernel weight and yield, test weight, protein composition, protein content, wet gluten content, glutenin macropolymer (GMP) content, starch and protein quality were me asured. The results showed that under 50% PPFD, wheat yield and test weight deceased sharply (p<0.05), less nitrogen was distributed to kernels (p<0 .05), bu t protein content and wet gluten content increased significantly (p<0.05), esp ecially at the early filling stage. At the late filling stage, gliadin and glute nin content, glutenin macropolymer (GMP) content and glutenin/gliadin index incr eased significantly (p<0.05). Thus, farinogram parameters, such as water a bsor ption and dough stability time, improved at low light intensities (p<0.05) . Ho wever, these parameters changed only slightly during the early and middle fillin g stages (p>0.05), which suggested that the late filling stage is an impor tant stage during which light intensity influences protein quality. Shading or lower light intensity improved wheat quality, but yields decreased sharply, especiall y during the early grain filling stage. The two varieties, `Ji'nan17' and `Lumai 2 1', showed similar responses to light levels at the different filling stages. Th erefore, future investigations should be conducted to resolve the conflict betwe en differences in yield and quality.

    BAI Li-Ping, TONG Cheng-Feng, LIN Er-Da, LU Zhi-Guang, RAO Min-Jie
    Chin J Plan Ecolo. 2005, 29 (5):  814-818.  doi:10.17521/cjpe.2005.0108
    Abstract ( 2878 )   PDF (504KB) ( 886 )   Save
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    Elevated CO2 concentrations and temperatures under global climate change scenarios projected for coming decades could impact bread wheat (Triticum aestivum) quality. Hence, there is a need to understand the effects of these environmental factors on crop quality. The objective of this study was to experimentally test the effects of elevated CO2, temperature, and their interactions on wheat grain quality characteristics relevant to its processing, including grain protein content, wet gluten content, sedimentation value, and falling number (alpha_ amylase activity). Bread wheat (`Liaochun 10') was field grown in CO2 and temperature gradient chamber (CTGC) and ambient controlled temperature gradient chamber (TGC). The results indicated that elevated CO2 concentrations (433.3 to 610. 2 mol•mol-1) had negative effects on grain protein content, wet gluten c ontent a nd sedimentation value of bread wheat, but gradual temperature increases (2 ℃ range) had positive effects on these three main quality characteristics. The inter action between gradual increases in CO2 levels from 433.3 to 551.5 mol•mol-1 and temperature increases (+2 ℃ range) showed a similar benefit on grain protein content, wet gluten content and sedimentation value of bread wheat, but when CO2 concentrations approached 610.2 mol•mol-1 and mean daytime temperature increased more than 2 ℃, the interaction of CO2 and temperature had a negative effect on the three bread wheat quality characteristics. In addition, under CO2 conce nt rations of 433.3-610.2 mol•mol-1 and gradually increasing temperatures , CO2 enrichment reduced alpha_amylase activity of bread wheat whereas elevated temperatures and the interaction of elevated CO2 and temperature enhanced alpha_amylas eactivity of bread wheat.

    WANG Wei, CAI Yi-Xia, CAI Kun-Zheng, ZHANG Jian-Hua, YANG Jian-Chang, ZHU Qing-Sen
    Chin J Plan Ecolo. 2005, 29 (5):  819-828.  doi:10.17521/cjpe.2005.0109
    Abstract ( 2406 )   PDF (876KB) ( 1100 )   Save
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    Grain yields of rice in China are often low and vary substantially due to the shortages of available water. Assimilates stored prior to grain filling have been identified as important contributors to grain yield in drought environments, but evaluating their benefit has been hampered by an inadequate understanding of the physiological mechanism of remobilization to grains of stored carbon reserves during grain filling. A moderate soil drying applied at grain filling period can enhance plant senescence and may improve the yield through remobilizing more pre_stored food to grains. The study was designed to test the following hypo thesis: if soil drying is controlled properly at the mid_late stage of grain filling, an early senescence induced by drought stress would accelerate the rate of grain filling by enhanced relocation of carbon stored reserves, and improved use of pre_stored carbon reserves may increase yields where photosynthetic assimilation is decreased by soil drying. Further evaluation of enzyme regulation in the remobilization process will help to understand the physiological mechanisms. In the current study, two rice combinations (cvs `Shanyou63' and `Pc311/Zao') were grown in cement containers and pots at Yangzhou University. Controlled soil water deficit were imposed at 9 d after anthesis in consideration that the division of endosperm cell is sensitive to water deficit.  We found that, compared to well_watered treatments, the remobilization of stored carbohydrates in the rice stem was significantly enhanced under water_deficit conditions, and the increased contribution to grain yields was 1.9-3.0 times as that of rice grown under well_watered conditions. Pronounced effects of water_deficit treatment on carbohydrate metabolism were observed in rice stems. Sugar concentrations and carbon distribution among sugar components were altered under water_deficit conditions. Starch breakdown in rice stems under soil water deficit occurred through action via the hydrolytic pathway: α¬_amylase,β_amylase,α¬_glucosidase and D_enzyme were induced and enhanced by soil drying. Starch phosphorylase activity was decreased under water_deficit conditions, indicating that phosphorylase was not involved in the processes of starch remobilization. Water deficit markedly altered the pattern of sucrose metabolism by shutting down the bypass of carbon flow through the sucrose synthase (SuSy) catalyzed system and enhanced the unidirectional flow through the irreversible sucrose_phosphate synthase (SPS) and acid invertase (AINV) catalyzed pathway. SPS was induced and activated by water deficit, and played a pivotal role in enhancing synthesis of sucrose through the conversion of stored carbon into sucrose. During rapid remobilization of stored sugar, the sucrose content was increased by 10.3% in `Shanyou63' and 20.45% in `Pc/Zao' as compared to the well_watered treatments. It is concluded that the enhanced remobilization of stored non_structural carbohydrates (NSC) in rice stems under soil water deficit was due to induced hydrolytic enzyme activities, in creased SPS activity and activation state, but decreased invertase activity. 

    XU Fu-Xian, XIONG Hong, ZHU Yong-Chuan, XIE Rong, WANG Gui-Xiong
    Chin J Plan Ecolo. 2005, 29 (5):  829-835.  doi:10.17521/cjpe.2005.0110
    Abstract ( 2186 )   PDF (605KB) ( 896 )   Save
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    Many studies on grain quality as affected by rice cultivation density have been conducted on one or two rice varieties, but there are few studies on varieties of hybrid midseason_rice. The hybrid midseason_rice grown in southern and eastern Sichuan Province has very low grain quality, especially the percentage of head milled rice, because of high temperatures and summer droughts. This paper explores the effects of low density cultivation on the percentage of head milled rice and source to sink ratios of varieties of hybrid midseason_rice, and it provides a theoretical and practical basis for improved cultivation. The experiment was conducted with eighteen mid_season rice hybrids grown at two densities, a low density cultivation and a traditional cultivation system, in a randomized block design with 4 replications using two hybrids (a big_panicle type hybrid `Gangyou 22' and a small_panicle type hybrid `II you 7') with 3 different treatments of grain or leaf cut. By using variance, correlation and regre ssion analysis, the results indicated that the percentage of head milled rice in creased in hybrids with small_medium panicles under low density cultivation but not for the hybrids with big_panicles. This was because of the significant decrease in the leaf to grain ratio and grain filling rate of hybrids for small_middle panicle under low density cultivation, as compared to the traditional cultivation system, resulting in an increase in the grain unit weight and percentage of head milled rice. The leaf to grain ratio and the grain filling rate of big panicle hybrids were low in traditional cultivation, and decreased significantly under low density cultivation, causing a decrease in the grain unit weight and percentage of head milled rice. These results were produced because of an imbalance in the leaf to grain ratios. Hybrids with less than 177 spikelets per panicle un der cultivation using traditional densities can adapt to the low density cultivation in the eastern and southern regions of Sichuan Province.

    ZHANG Yun, KONG Zhao-Chen, YAN Shun, YANG Zhen-Jing, NI Jian
    Chin J Plan Ecolo. 2005, 29 (5):  836-844.  doi:10.17521/cjpe.2005.0111
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    Palaeo_biodiversity and environmental characteristics in the Mid_ and Late Holocene were investigated in five profiles from Daxigou, Huashuwozi, Xiaoxi gou, Dongdaohaizi and Sichanghu, which are located at different elevations and in different vegetation zones on the northern piedmont of Tianshan Mountains, Xin jiang. Selected profiles were located at three vegetation zones. The first was the alpine and subalpine meadow vegetation zone (2 700-3 400 m). Thirty_eight genera and families were identified in the Daxigou profile (3 450 m).Pollen Simpson index was 2.72-7.67 with a mean value of 4.84. The forest_steppe ecotone was from 1 200-1 600 m elevation. Forty_two pollen genera and families were identified in the Huashuwozi profile and 39 in the Xiaoxigou profile at this vegetation zone. The pollen Simpson index of Huashuwozi profile was 4.2-12.4 with a mean value of 7.8, and that of Xiaoxigou profile was 4.13-12.06 with a mean value of6. 89. The last two profiles were located in the typical desert zone (400-600 m). Thirty_two pollen genera and families were identified in Dongdaohaizi profile and 35 in Sichanghu profile. The pollen Simpson index of Dongdaohaizi profile was 1 .8 - 18.8 with a mean value of 6.62. The pollen Simpson index of Sichanghu profile was 4.1 - 9.0 with a mean value of 5.55. These results showed that Holocene biodiversity and the pollen Simpson index were the highest in profiles from the desert_steppe vegetation zone. Our analysis also showed that four periods (3600-3200 aB.P., 1700-1400 aB.P., 1000-600 aB.P., and 450 aB.P.) had the highest pollen Simpson index and highest biodiversity on the northern piedmont of Tianshan Mountains in Xinjiang. Thus, the pollen Simpson index reflects climatic and environmental changes to a certain extent. However, biases exist due to the influence of buried pollen assemblages, low precision in pollen identification _ to genera and family levels only, influx of exotic pollen and human disturbances. Pollen that had low representation in the profiles was not taken into account in calculating the biodiversity index, which might underestimate overall palaeo_biodiversity. Due to different abilities o f pollen dispersal and transportation, there is high spatial variation in the number of pollen taxa at different sites within the same research district. In addition, sediment composition, strata chronology and human activities have great impacts on pollen preservation, which also affect our ability to estimate palaeo_ biodiversity. All in all, this research showed that the pollen Simpson index is an important index of palaeo_biodiversity and the paleoenvironment,but more research is needed.

    WANG Zhi-Ping, CHEN Quan-Sheng
    Chin J Plan Ecolo. 2005, 29 (5):  845-850.  doi:10.17521/cjpe.2005.0112
    Abstract ( 3855 )   PDF (515KB) ( 841 )   Save
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    Recently photosynthesized carbon (C) constitutes an important portion of C cycling in plant systems. The quantification of the partitioning of recently photosynthesized C to plant tissues, soils and respirations is essential for understanding the global C cycle. The cycle of recently photosynthesized C is difficult to study due to the rapid turnover of recently metabolized C and that stable organic C compounds are not immediately synthesized. Carbon isotope techniques can be used to study the allocation and turnover of recently photosynthesized C. The following research topics should be addressed in future studies. 1) The fraction of recently photosynthesized C used in respiration of living roots, rhizosphere, and soil organic matter, and photosynthate turnover in the rhizosphere needs to be quantified. 2) The amount of recently photosynthesized C that is lost to atmospheric fluxes of CO2 and CH4 should be quantified. 3) Because of significant difference in the physiologies C3 and C4 plants, global climate change will produce profound effects on the distribution and productivities of C3 and C4 species and further influence the global C cycle pattern. We believe that studies on differences in C allocation and turnover in C3 and C4 species are very valuable. 4) The effects of human activities, such as livestock farming and land use, on the cycle patterns of recently photosynthesized C. In order to further understand C partitioning patterns, it is vital that more ecosystem level studies of C cycling are conducted. Little information in photosynthesized C fluxes in China's grasslands is available and this region should be a research priority.
    SUN Wei, LIN Guang-Hui, CHEN Shi-Ping, HUANG Jian-Hui
    Chin J Plan Ecolo. 2005, 29 (5):  851-862.  doi:10.17521/cjpe.2005.0113
    Abstract ( 2350 )   PDF (856KB) ( 999 )   Save
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    Stable isotopes are used as both natural integrators and tracers of complicated biological, ecological and biogeochemical processes, and their responses to environmental changes at different spatial and temporal scales. In this article, the application of stable isotopes and the Keeling plot approach to carb on and water exchange studies of terrestrial ecosystems were reviewed. We focused mainly on the current applications and potential development of stable isotope techniques and the Keeling plot approach in conjunction with concentration and flux measurements of CO2 and water in terrestrial ecosystems. For these applic at ions it is critical to know the isotopic identities of specific ecosystem compon ents, such as the isotopic compositions of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil_plant_atmosphere continuum. Based on the principle of mass conservation, the Keeli ng plot approach combines measurements of stable isotope ratios and concentrations of CO2, water or other trace gases, and allows the identification of the contributions of various ecosystems, or ecosystem components, to the net exchange fluxes between the terrestrial biosphere and atmosphere, and the estimation of net ecosystem isotopic discrimination and disequilibrium effect. Net ecosystem carb on fluxes can be partitioned into C uptake during photosynthesis and C release during respiration or evapotranspiration into leaf transpiration and soil evaporation by the Keeling plot technique. This approach also allows partitioning urban CO2 sources into gasoline combustion, natural gas combustion and biogenic respiration. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in forest ecosystems. At the global scale, we can estimate relative contributions of terrestrial and ocean ecosystems to the global carbon cycle by combining stable isotope techniques, the Keeling plot approach and terrestrial ecosystem models. However, applications of stable isotope techniques and the Keeling plot approach to ecological research are sometimes constrained by the hete rogeneity of terrestrial ecosystems. In addition, selection of suitable isotopic sampling protocols is another factor that we should consider in its application. Nevertheless, with new improvements in analytic protocols in the near future, stable isotope techniques and the Keeling plot approach will become one of the most effective techniques for understanding carbon and water relationships in terrestrial ecosystems.

    MI Xiang-Cheng, MA Ke-Ping, ZOU Ying-Bin
    Chin J Plan Ecolo. 2005, 29 (5):  863-870.  doi:10.17521/cjpe.2005.0114
    Abstract ( 2430 )   PDF (651KB) ( 699 )   Save
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    Artificial neural network appeals to many ecologists because of the complexity of agroecological systems and its precise predicting ability to simulate vaguely understood and highly uncertain ecosystems. We focus on the introduction of the structure, algorithm and the applications for back_propagation artificial neural network (BPN) in this paper. People usually adopt BPN of a three_layer structure that can approximate functions of any complexity and is not easy to overfit because of its simple structure. The main principle of back_propagation artificial neural network is to adjust weights according to the errors of input neurons. In agroecological research, neural networks are always employed to predict crop yield, biomass yield, relationships between organisms and environmenta l factors and so on. Previous studies indicate that neural networks greatly outper form linear models, while the accuracy of the results produced by neural networks are very similar to that of algorithmic models. Moreover, neural networks can extrapolate to some degree when enough training data are provided. However, neural networks require a large number of samples to guarantee the robustness of its parameterization, which seems unrealistic for complicated networks to collect such large data sets from crop growth experiments involving countless small plot trials over multiple site_years. The requirement of large amount of training data has hindered the application of neural networks. Some techniques, such as early stopping, jittering and metamodels, have been advanced to induce generalization of neural networks, while techniques such as Garson's algorithm, sensitivity analysis and randomization test are advanced to explain the mechanisms of neura l networks. The advantage of neural network lies on its ability to precisely simulate the vaguely understood and uncertain ecosystems, which cannot be realized by traditional approaches. As a nonlinear approximator, artificial neural network is an important tool complementary to comprehensive models.

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