This study examined the mechanisms for improving the adhesion performance of the asphalt-aggregate interface with two anti-stripping agents and two coupling agents.The investigation of contact behavior between various...This study examined the mechanisms for improving the adhesion performance of the asphalt-aggregate interface with two anti-stripping agents and two coupling agents.The investigation of contact behavior between various asphalt-aggregate surfaces was conducted using molecular dynamics(MD)simulations.The interaction energy and the relative concentration distribution were employed as the parameters to analyze the enhancement mechanisms of anti-stripping agents and coupling agents on the asphalt-aggregate interface.Results indicated that the adhesion at the asphalt-aggregate interface could be strengthened by both anti-stripping agents and coupling agents.Anti-stripping agents primarily improve adhesion through the reinforcement of electrostatic attraction,while coupling agents primarily upgrade adhesion by strengthening the van der Waals.Hence,the molecular dynamics modeling and calculation techniques presented in this study can be utilized to elucidate the development mechanism of the asphalt-aggregate interface through the use of anti-stripping agents and coupling agents.展开更多
Sixteen different vegetation types of grassland and shrubland were selected to study the component and diversity of plant species of riparian plant communities along main channel in the Three-Gorges areas. Species ric...Sixteen different vegetation types of grassland and shrubland were selected to study the component and diversity of plant species of riparian plant communities along main channel in the Three-Gorges areas. Species richness (s), Simpson index (D), and Shannon-Weiner index (H) were used to study the biodiversity and the hierarchical classification was carried out by the methods of TWINSPAN and DCA ordination. The results showed that the components of flora were complex and dominated by the temperate type in the riparian plant communities. Species diversity was not different between the communities, but Shannon-Weiner indexes of different layers in some grassland were significantly different. TWINSPAN and DCA indicated that riparian plant communities distributed along the gradient of moisture.展开更多
The effects of nitrogen （N） availability and tree species on the dynamics of carbon and nitrogen at early stage of decomposition of forest litter were studied in a 13-week laboratory incubation experiment. Fresh lit...The effects of nitrogen （N） availability and tree species on the dynamics of carbon and nitrogen at early stage of decomposition of forest litter were studied in a 13-week laboratory incubation experiment. Fresh litter samples including needle litter （Pinus koraiensis） and two types of broadleaf litters （Quercus mongolica and Tilia amurensis） were collected from a broadleaf-korean pine mixed forest in the northern slope of Changbai Mountain （China）. Different doses of N （equal to 0, 30 and 50 kg.ha^-1yr^-1, respectively, as NH4NO3） were added to litter during the experiment period. The litter decomposition rate expressed as mass loss and respiration rate increased significantly with increasing N availability. The mass loss and cumulative CO2-C emission were higher in leaf litter compared to that in needle litter. The dis- solved organic Carbon （DOC） concentrations in litter leachate varied widely between the species, but were not greatly affected by N treatments. Regardless of the N addition rate, both N treatments and species had no significant effect on dissolved organic N （DON） concentrations in litter leachate. About 52-78% of added N was retained in the litter. The percentage of N retention was positively correlated （R^2=0.9 1, p〈0.05） with the litter mass loss. This suggested that a forest floor with easily decomposed litter might have higher potential N sink strength than that with more slowly decomposed litter.展开更多
Investigations on charcoal in the soil, fire-scarred trees, stand composition, forest structure as well as regeneration status were carried out in the natural broad-leaved/Korean pine (Pinus koraiensis) forest after f...Investigations on charcoal in the soil, fire-scarred trees, stand composition, forest structure as well as regeneration status were carried out in the natural broad-leaved/Korean pine (Pinus koraiensis) forest after fire disturbance at Liangshui Nature Reserve on the mid-north of Xiaoxingan Mountains from 1990 to 1992, and the ecological effects of fire disturbance on the formation and succession of this kind of forest were analyzed according to the survey results. The average depth of charcoal in the soil was related to the timing of the fire. According to the characteristic of fire-scarred trees, the dynamic map of the fire behavior was drawn onto the topographic map. It showed that the dimension and extent of the fire disturbance was closely related with site conditions. Fire disturbance only led to a significant difference in stand composition and diameter class structure for the stands at different locations, rather than completely destroying the forest. After fire disturbance, the horizontal community structure was a mosaic of different patches, which were made up of different deciduous species or different sizes of Korean pines, and the succession trend of each patch was also different. In the sites with the heavy fire disturbance, the intolerant hardwood species were dominant, and there were a large number of regenerative Korean pine saplings under the canopy. In the moderate -disturbed sites, the tolerant hardwood species were dominant, and a small number of large size Korean pines still survived. In the light-disturbed sites, large size Korean pines were dominant.展开更多
Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), a fast-growing, ever-green conifer tree with high yield and excellent quality, is the most important tree species of timber plantations in subtropical China. We inv...Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), a fast-growing, ever-green conifer tree with high yield and excellent quality, is the most important tree species of timber plantations in subtropical China. We investigated the characteristics of biomass, litterfall and nutrient fluxes in the 8, 14 and 24 year-old stands, representing the young, middle-aged and mature stands. The results showed that Chinese fir plantations in central Fujian province had high productivity, and the proportion of stem mass in total biomass was between 50%-70%. Chinese fir was low nutrient-return tree species with litterfall. Nutrient withdrawal from senescing needles was a strong age-dependence for nitrogen, phosphorous and potassium in Chinese fir. With a management system of such short-rotation and continuously pure-crop planting, harvesting timber can lead to great nutrient loss, which may be one of the causes for site degradation.展开更多
A study was conducted to evaluate the cultivable filamentous fungal diversity in organic layers (L, F, and H layers) and A1 layer of two main forest types, Pinus massoniana and Liguidambar formasana mixed forest and Q...A study was conducted to evaluate the cultivable filamentous fungal diversity in organic layers (L, F, and H layers) and A1 layer of two main forest types, Pinus massoniana and Liguidambar formasana mixed forest and Quercus variabilis forest, in Zijin Mountain(325?N, 11848?E), Nanjing, China. A total of 67 taxa comprising 56 Deuteromycetes, 3 Zygomycetes, 5 Asco-mycetes and 3 unidentified fungi were recognized from samples from the forest floor of the two forest types. The most abundant group was Deuteromycetes. The dominant genera in both forests were Alternaria sp., Aspergillus sp., Cladosporium sp., Mucor sp., Penicillium sp., Rhizopus sp., Gliocladium sp. and Trichoderma spp. The fungal diversity was higher in the mixed forest than that in Q. variabilis forest. For both forest types, the maximum fungal diversity was found in layer F and there existed significantly different in fungal diversity between layer F and layer L. In the mixed forest, richness of fungi isolated from needle litter (P. massoniana) was lower than that from leaf litter (L. formasana). The richness of fungi from needle litter increased with the in-crease of forest floor depth, but for leaf litter, the fungal diversity decreased with the depth of forest floor. The co-species of fungi from the two forest types, as well as from two kinds of litters in mixed forest, increased with the depth of the forest floor. The succession of fungi along with the process of decomposition was discussed here. The results also showed that litter quality was a critical factor affecting fungal diversity.展开更多
During January to February in 2008,severe ice-snows occurred on 2.09×10^6 km^2 of south China and caused tremendous damage to the forests.Stem damage and crown debris input from the ice-snow in a Cunninghamia lan...During January to February in 2008,severe ice-snows occurred on 2.09×10^6 km^2 of south China and caused tremendous damage to the forests.Stem damage and crown debris input from the ice-snow in a Cunninghamia lanceolata stand in northern Guangdong Province,China,were investigated to determine the extent of ice-snow damage to trees and the nutrient distribution characteristics on woodland.Icicles were produced on branches and leaves of the C.lanceolata trees by sleets,resulting in all live trees were damaged by stem breakage.A strong positive correlation was found between the broken position of trees and diameter at breast height（DBH）.Nutrient concentrations varied according to components of crown debris.Total nutrient concentration graded in the following order：leaves 〉stembark 〉branches 〉stemwood.Crown debris input from ice-snows was 19.11 t·ha^-2,and branches,stemwood,leaves and stembark accounted for 37%,28%,27% and 8%,respectively.Nutrient distributions between components of crown debris differed drastically in 2008.The contribution of leaves to total nutrient accumulation of crown debris was 70%,whereas branches,stemwood and stembark contributed 13%,7% and 10%,respectively.Accumulations of N,P and K by the woodland were 105 067.9 t·ha^-2 in 2008,and the nutrients of crown debris,litter and the stand soil accounted for 0.18%,0.03% and 99.79%,respectively.The order of nutrient accumulation in leaves,branches,stembark,stemwood of crown debris and litter was N〉K〉P,but the nutrients stored in the soil were in the order of K 〉N 〉P.The N and P concentrations of litter in 2009 were greater than those in 2008,whereas its K concentration was smaller than that of the latter.N and P concentrations of stemwood and stembark in 2009 were slightly greater than those in 2009,whereas their K concentration was smaller than the latter.The N and P accumulations of stemwood and stembark of crown debris in 2009 were close to those in 2008,whereas their K accumulation was slightly smaller than the latter.The N,P and K accumulations of litter in 2009 was greater than those in 2008.展开更多
The stand growth and yield dynamic models for Larch in Jilin Province were developed based on the forest growth theories with the forest continuous inventory data. The results indicated that the developed models had h...The stand growth and yield dynamic models for Larch in Jilin Province were developed based on the forest growth theories with the forest continuous inventory data. The results indicated that the developed models had high precision, and they could be used for the updating data of inventory of planning and designing and optimal decision of forest management.展开更多
According to the survey of Guangdong mangrove resource in 2001 and authors field investigations in the past 5 years, the total mature mangrove land area of Guangdong Province was estimated at 9084 hm2, accounting for ...According to the survey of Guangdong mangrove resource in 2001 and authors field investigations in the past 5 years, the total mature mangrove land area of Guangdong Province was estimated at 9084 hm2, accounting for 41.4% of total mangrove land area of China. These mangrove forests totally consisted of 50 species belonging to 28 families, distributed at more than 100 locations along the coastlines of Guangdong and most of them present a high dense and dwarf appearance. The Ass. Avicennia marina is the most dominant mangrove association. The investigation results showed that the mangrove forests with coverage rates above 0.7 accounted for 68.0% of the total mangrove land area of the province and 77.8% of mangrove forests was less than 2 m in tree height. Since 1950, 54.6% of mangrove forests have disappeared due to paddy rice reclaim, aquaculture and city constructions. Derivational conservation efforts are still weakly empowered and should be strengthened intensively. The author suggested that mangrove laws and regulations should be enforced, mangrove scientific research should be strengthened; and mangrove conservation awareness of local community should be raised on conservation of mangroves.展开更多
The top-coal failing ability is a key factor to analyze for the application of coal mining with top-coal caving. Based on a hard-and-thick strata, which acts both as the floor of the upper coal seam and as the roof of...The top-coal failing ability is a key factor to analyze for the application of coal mining with top-coal caving. Based on a hard-and-thick strata, which acts both as the floor of the upper coal seam and as the roof of the lower coal seam, nine mining projects wcrc put forward to examine the mining of upper and lower coal seams, and a numerical simulation was used to study in detail the corresponding top-coal compressed volume of the lower coal seam. By the simulation effects of different layouts of coalface, the rational mining method was determined to bc the staggered layout of coalface in the upper and the lower coal seam. This can ensure the successful use of fully-mechanized coalfacc with top--coal caving in the lower coal seam.展开更多
Combined with a digital bored photography system and in-situ statistics concerning the joints and fissures of both ore-body and surrounding rock,a 2D discrete model was constructed using UDEC.The stress field and disp...Combined with a digital bored photography system and in-situ statistics concerning the joints and fissures of both ore-body and surrounding rock,a 2D discrete model was constructed using UDEC.The stress field and displacement field changes of different sublevel stoping systems were also studied.Changes in the overlying rock strata settlement pattern has been analyzed and validated by in-situ monitoring data.The results show that:in the caving process,there exists an obvious delay and jump for the overlying rock strata displacement over time,and a stable arch can be formed in the process of caving,which leads to hidden goafs.Disturbed by the mining activity,a stress increase occurred in both the hanging wall and the foot wall,demonstrating a hump-shaped distribution pattern.From the comparison between simulation results and in-situ monitoring results,land subsidence shows a slow-development,suddenfailure,slow-development cycle pattern,which leads eventually to a stable state.This pattern validates the existence of balanced arch and hidden goafs.展开更多
Biodiversity of arbuscular mycorrhizal colonization and spore population was investigated in different agroforestry trees and crop species collected from different locations of Dinajpur district of Bangladesh. Roots a...Biodiversity of arbuscular mycorrhizal colonization and spore population was investigated in different agroforestry trees and crop species collected from different locations of Dinajpur district of Bangladesh. Roots and rhizosphere soils of Albizia procera Benth., Capsicum frutescens L, Curcuma domestica Vahl., Dalbergia sissoo Roxb. and Swietenia macrophylla King. from Dashmail; C. domestica, D. sissoo, Eucalyptus camaldulensis Dehnn., Gmelina arborea （Roxb） DC and Oryza sativa L. from Kantaji and C. domestica, D. sissoo, Litchi chinensis Sonn. and O. sativa from Ramsagar were collected. Roots and soils were assessed following standard methods. The range of AM colonization was recorded 36%-79% from Dashmail. The highest AM colonization was recorded in C. frutescens （79%） and the lowest was in C. domestica （36%）. The range of colonization was recorded as 33%-70% from Kantaji. The highest AM colonization was recorded in G. arborea （70%） and the lowest was in O. sativa （33%）. The range of AM colonization was recorded as 35%-70% from Ramsagar. The highest AM colonization was recorded in D. sissoo （70%） and the lowest was in O. sativa （35%）. Arbuscular mycorrhizal spore population varied from 54 to 140/100g dry soil in the soils from Dashmail. The highest was in the soils of D. sissoo （140） and the lowest was in C. domestica （54）. The spore population varied from 63 to 221 in Kantaji. The highest was in G. arborea （221） and the lowest was in O. sativa （63）. The range population in Ramsagar varied from 69 to 160. The highest was recorded in D. sissoo （160） and the lowest was in L. chinensis （69）. No significant relationship of soil pH and soil OM with AM colonization and with spore population was observed. Simpson＇s index of diversity （Ds） and Shannon＇s index of diversity- （Hs） were highest in the soil of D. sissoo from Kantaji and the lowest in the soils of O. sativa from Ramsagar. Biodiversity of AM colonization, spore population and the distribution of AM fungi in the rhizosphere soils of different agroforestry plants indicated the occurrence of AM fungi, mycotrophic nature of the trees and crop species, contribution and necessity of AM fungi and the AM dependence of the agroforestry plants growing in Dinajpur district of Bangladesh.展开更多
In the passive turning state,the helicopter turns through the tail rotor force and the friction of the ground to the tire.In practice,it is found that the helicopter will turn difficultly under low aircraft ground spe...In the passive turning state,the helicopter turns through the tail rotor force and the friction of the ground to the tire.In practice,it is found that the helicopter will turn difficultly under low aircraft ground speed or static state.This paper takes a certain type of helicopter as the research object,and establishes the dynamic model of helicopter ground turning motion based on the basic theory of dynamics.This model takes into account the six-degree-of-freedom motion model of the helicopter body,the motion model of the landing gear buffer,the tire mechanics model and the friction characteristics of the strut friction disc.The dynamic simulation of the helicopter right angle turn and static turn is carried out,and the influence of parameters such as tail rotor pull,taxi speed,tail wheel stability distance on the dynamic response of the turn is studied.The results show that under the same ground taxing speed,the tail wheel angle increases with the increase of tail rotor force.When the tail rotor force is the same,the tail wheel angle increases with the increase of ground taxing speed.When the helicopter is completely static,it is the most difficult to turn,which requires much bigger force of the tail rotor to turn.In addition,the change of the stability distance of the tail wheel has an obvious influence on the turning.When the stability distance is doubled,the tail rotor force will be reduced by 30%to the same angle of the tail wheel.展开更多
An exact average symbol error rate analysis for the distributed dual-hop relay cooperative network with multiple relays in a Nakagami-m fading environment is presented.In the derivation of the moment generation functi...An exact average symbol error rate analysis for the distributed dual-hop relay cooperative network with multiple relays in a Nakagami-m fading environment is presented.In the derivation of the moment generation function of receiver Signal-to-Noise Ratio(SNR),the sectional integral method is used,instead of the cumulative density function method which is ordinarily used by the deduction of the outage probability of S-R-D link.The accurate symbol error rate of a dual-hop relay cooperative network is obtained with the closed-form Moment Genoration Function (MGF) expression.The correctness of the symbol error rate is verified through numerical simulations and is compared with other analytical methods.These deductions clearly show that the distributed cooperative diversity network presented has strong superiorities in overcoming severe fading and can achieve full diversity order.展开更多
Most resource allocation algorithms are based on interference power constraint in cognitive radio networks.Instead of using conventional primary user interference constraint,we give a new criterion called allowable si...Most resource allocation algorithms are based on interference power constraint in cognitive radio networks.Instead of using conventional primary user interference constraint,we give a new criterion called allowable signal to interference plus noise ratio(SINR) loss constraint in cognitive transmission to protect primary users.Considering power allocation problem for cognitive users over flat fading channels,in order to maximize throughput of cognitive users subject to the allowable SINR loss constraint and maximum transmit power for each cognitive user,we propose a new power allocation algorithm.The comparison of computer simulation between our proposed algorithm and the algorithm based on interference power constraint is provided to show that it gets more throughput and provides stability to cognitive radio networks.展开更多
文摘This study examined the mechanisms for improving the adhesion performance of the asphalt-aggregate interface with two anti-stripping agents and two coupling agents.The investigation of contact behavior between various asphalt-aggregate surfaces was conducted using molecular dynamics(MD)simulations.The interaction energy and the relative concentration distribution were employed as the parameters to analyze the enhancement mechanisms of anti-stripping agents and coupling agents on the asphalt-aggregate interface.Results indicated that the adhesion at the asphalt-aggregate interface could be strengthened by both anti-stripping agents and coupling agents.Anti-stripping agents primarily improve adhesion through the reinforcement of electrostatic attraction,while coupling agents primarily upgrade adhesion by strengthening the van der Waals.Hence,the molecular dynamics modeling and calculation techniques presented in this study can be utilized to elucidate the development mechanism of the asphalt-aggregate interface through the use of anti-stripping agents and coupling agents.
基金This study was supported by the Chinese Academy of Sciences (A grant KZCX2-406) the National Natural Science Foundation of China (NSFC39970123) and Changbai Mountain Open Research Station.
文摘Sixteen different vegetation types of grassland and shrubland were selected to study the component and diversity of plant species of riparian plant communities along main channel in the Three-Gorges areas. Species richness (s), Simpson index (D), and Shannon-Weiner index (H) were used to study the biodiversity and the hierarchical classification was carried out by the methods of TWINSPAN and DCA ordination. The results showed that the components of flora were complex and dominated by the temperate type in the riparian plant communities. Species diversity was not different between the communities, but Shannon-Weiner indexes of different layers in some grassland were significantly different. TWINSPAN and DCA indicated that riparian plant communities distributed along the gradient of moisture.
基金Foundation project： This work was supported by the Knowledge Innova- tion Project of the Chinese Academy of Sciences （KZCX2-YW-416） and the National Natural Science Foundation （90411020）
文摘The effects of nitrogen （N） availability and tree species on the dynamics of carbon and nitrogen at early stage of decomposition of forest litter were studied in a 13-week laboratory incubation experiment. Fresh litter samples including needle litter （Pinus koraiensis） and two types of broadleaf litters （Quercus mongolica and Tilia amurensis） were collected from a broadleaf-korean pine mixed forest in the northern slope of Changbai Mountain （China）. Different doses of N （equal to 0, 30 and 50 kg.ha^-1yr^-1, respectively, as NH4NO3） were added to litter during the experiment period. The litter decomposition rate expressed as mass loss and respiration rate increased significantly with increasing N availability. The mass loss and cumulative CO2-C emission were higher in leaf litter compared to that in needle litter. The dis- solved organic Carbon （DOC） concentrations in litter leachate varied widely between the species, but were not greatly affected by N treatments. Regardless of the N addition rate, both N treatments and species had no significant effect on dissolved organic N （DON） concentrations in litter leachate. About 52-78% of added N was retained in the litter. The percentage of N retention was positively correlated （R^2=0.9 1, p〈0.05） with the litter mass loss. This suggested that a forest floor with easily decomposed litter might have higher potential N sink strength than that with more slowly decomposed litter.
基金Doctor Foundation of Ministry of Education (2000002716)
文摘Investigations on charcoal in the soil, fire-scarred trees, stand composition, forest structure as well as regeneration status were carried out in the natural broad-leaved/Korean pine (Pinus koraiensis) forest after fire disturbance at Liangshui Nature Reserve on the mid-north of Xiaoxingan Mountains from 1990 to 1992, and the ecological effects of fire disturbance on the formation and succession of this kind of forest were analyzed according to the survey results. The average depth of charcoal in the soil was related to the timing of the fire. According to the characteristic of fire-scarred trees, the dynamic map of the fire behavior was drawn onto the topographic map. It showed that the dimension and extent of the fire disturbance was closely related with site conditions. Fire disturbance only led to a significant difference in stand composition and diameter class structure for the stands at different locations, rather than completely destroying the forest. After fire disturbance, the horizontal community structure was a mosaic of different patches, which were made up of different deciduous species or different sizes of Korean pines, and the succession trend of each patch was also different. In the sites with the heavy fire disturbance, the intolerant hardwood species were dominant, and there were a large number of regenerative Korean pine saplings under the canopy. In the moderate -disturbed sites, the tolerant hardwood species were dominant, and a small number of large size Korean pines still survived. In the light-disturbed sites, large size Korean pines were dominant.
基金This research was sponsored by National Natural Science Foundation of China (Grant No. 39630240 and 3000132).
文摘Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), a fast-growing, ever-green conifer tree with high yield and excellent quality, is the most important tree species of timber plantations in subtropical China. We investigated the characteristics of biomass, litterfall and nutrient fluxes in the 8, 14 and 24 year-old stands, representing the young, middle-aged and mature stands. The results showed that Chinese fir plantations in central Fujian province had high productivity, and the proportion of stem mass in total biomass was between 50%-70%. Chinese fir was low nutrient-return tree species with litterfall. Nutrient withdrawal from senescing needles was a strong age-dependence for nitrogen, phosphorous and potassium in Chinese fir. With a management system of such short-rotation and continuously pure-crop planting, harvesting timber can lead to great nutrient loss, which may be one of the causes for site degradation.
基金This paper was supported by Chinese Program for High Technology Research and Development (2003AA209030) Scien-tific Research Foundation for doctoral supervising laboratory State Education Ministry (20030284044) and National Natural Sc
文摘A study was conducted to evaluate the cultivable filamentous fungal diversity in organic layers (L, F, and H layers) and A1 layer of two main forest types, Pinus massoniana and Liguidambar formasana mixed forest and Quercus variabilis forest, in Zijin Mountain(325?N, 11848?E), Nanjing, China. A total of 67 taxa comprising 56 Deuteromycetes, 3 Zygomycetes, 5 Asco-mycetes and 3 unidentified fungi were recognized from samples from the forest floor of the two forest types. The most abundant group was Deuteromycetes. The dominant genera in both forests were Alternaria sp., Aspergillus sp., Cladosporium sp., Mucor sp., Penicillium sp., Rhizopus sp., Gliocladium sp. and Trichoderma spp. The fungal diversity was higher in the mixed forest than that in Q. variabilis forest. For both forest types, the maximum fungal diversity was found in layer F and there existed significantly different in fungal diversity between layer F and layer L. In the mixed forest, richness of fungi isolated from needle litter (P. massoniana) was lower than that from leaf litter (L. formasana). The richness of fungi from needle litter increased with the in-crease of forest floor depth, but for leaf litter, the fungal diversity decreased with the depth of forest floor. The co-species of fungi from the two forest types, as well as from two kinds of litters in mixed forest, increased with the depth of the forest floor. The succession of fungi along with the process of decomposition was discussed here. The results also showed that litter quality was a critical factor affecting fungal diversity.
文摘During January to February in 2008,severe ice-snows occurred on 2.09×10^6 km^2 of south China and caused tremendous damage to the forests.Stem damage and crown debris input from the ice-snow in a Cunninghamia lanceolata stand in northern Guangdong Province,China,were investigated to determine the extent of ice-snow damage to trees and the nutrient distribution characteristics on woodland.Icicles were produced on branches and leaves of the C.lanceolata trees by sleets,resulting in all live trees were damaged by stem breakage.A strong positive correlation was found between the broken position of trees and diameter at breast height（DBH）.Nutrient concentrations varied according to components of crown debris.Total nutrient concentration graded in the following order：leaves 〉stembark 〉branches 〉stemwood.Crown debris input from ice-snows was 19.11 t·ha^-2,and branches,stemwood,leaves and stembark accounted for 37%,28%,27% and 8%,respectively.Nutrient distributions between components of crown debris differed drastically in 2008.The contribution of leaves to total nutrient accumulation of crown debris was 70%,whereas branches,stemwood and stembark contributed 13%,7% and 10%,respectively.Accumulations of N,P and K by the woodland were 105 067.9 t·ha^-2 in 2008,and the nutrients of crown debris,litter and the stand soil accounted for 0.18%,0.03% and 99.79%,respectively.The order of nutrient accumulation in leaves,branches,stembark,stemwood of crown debris and litter was N〉K〉P,but the nutrients stored in the soil were in the order of K 〉N 〉P.The N and P concentrations of litter in 2009 were greater than those in 2008,whereas its K concentration was smaller than that of the latter.N and P concentrations of stemwood and stembark in 2009 were slightly greater than those in 2009,whereas their K concentration was smaller than the latter.The N and P accumulations of stemwood and stembark of crown debris in 2009 were close to those in 2008,whereas their K accumulation was slightly smaller than the latter.The N,P and K accumulations of litter in 2009 was greater than those in 2008.
文摘The stand growth and yield dynamic models for Larch in Jilin Province were developed based on the forest growth theories with the forest continuous inventory data. The results indicated that the developed models had high precision, and they could be used for the updating data of inventory of planning and designing and optimal decision of forest management.
文摘According to the survey of Guangdong mangrove resource in 2001 and authors field investigations in the past 5 years, the total mature mangrove land area of Guangdong Province was estimated at 9084 hm2, accounting for 41.4% of total mangrove land area of China. These mangrove forests totally consisted of 50 species belonging to 28 families, distributed at more than 100 locations along the coastlines of Guangdong and most of them present a high dense and dwarf appearance. The Ass. Avicennia marina is the most dominant mangrove association. The investigation results showed that the mangrove forests with coverage rates above 0.7 accounted for 68.0% of the total mangrove land area of the province and 77.8% of mangrove forests was less than 2 m in tree height. Since 1950, 54.6% of mangrove forests have disappeared due to paddy rice reclaim, aquaculture and city constructions. Derivational conservation efforts are still weakly empowered and should be strengthened intensively. The author suggested that mangrove laws and regulations should be enforced, mangrove scientific research should be strengthened; and mangrove conservation awareness of local community should be raised on conservation of mangroves.
基金Projects 50374065 supported by National Natural Science Foundation of China
文摘The top-coal failing ability is a key factor to analyze for the application of coal mining with top-coal caving. Based on a hard-and-thick strata, which acts both as the floor of the upper coal seam and as the roof of the lower coal seam, nine mining projects wcrc put forward to examine the mining of upper and lower coal seams, and a numerical simulation was used to study in detail the corresponding top-coal compressed volume of the lower coal seam. By the simulation effects of different layouts of coalface, the rational mining method was determined to bc the staggered layout of coalface in the upper and the lower coal seam. This can ensure the successful use of fully-mechanized coalfacc with top--coal caving in the lower coal seam.
基金financially supported by the National Natural Science Foundation of China(No.51374033)the Doctoral Program of Higher Education Research Fund(No.20120006110022)the Chenchao Iron Mine and the technical support of Itasca
文摘Combined with a digital bored photography system and in-situ statistics concerning the joints and fissures of both ore-body and surrounding rock,a 2D discrete model was constructed using UDEC.The stress field and displacement field changes of different sublevel stoping systems were also studied.Changes in the overlying rock strata settlement pattern has been analyzed and validated by in-situ monitoring data.The results show that:in the caving process,there exists an obvious delay and jump for the overlying rock strata displacement over time,and a stable arch can be formed in the process of caving,which leads to hidden goafs.Disturbed by the mining activity,a stress increase occurred in both the hanging wall and the foot wall,demonstrating a hump-shaped distribution pattern.From the comparison between simulation results and in-situ monitoring results,land subsidence shows a slow-development,suddenfailure,slow-development cycle pattern,which leads eventually to a stable state.This pattern validates the existence of balanced arch and hidden goafs.
文摘Biodiversity of arbuscular mycorrhizal colonization and spore population was investigated in different agroforestry trees and crop species collected from different locations of Dinajpur district of Bangladesh. Roots and rhizosphere soils of Albizia procera Benth., Capsicum frutescens L, Curcuma domestica Vahl., Dalbergia sissoo Roxb. and Swietenia macrophylla King. from Dashmail; C. domestica, D. sissoo, Eucalyptus camaldulensis Dehnn., Gmelina arborea （Roxb） DC and Oryza sativa L. from Kantaji and C. domestica, D. sissoo, Litchi chinensis Sonn. and O. sativa from Ramsagar were collected. Roots and soils were assessed following standard methods. The range of AM colonization was recorded 36%-79% from Dashmail. The highest AM colonization was recorded in C. frutescens （79%） and the lowest was in C. domestica （36%）. The range of colonization was recorded as 33%-70% from Kantaji. The highest AM colonization was recorded in G. arborea （70%） and the lowest was in O. sativa （33%）. The range of AM colonization was recorded as 35%-70% from Ramsagar. The highest AM colonization was recorded in D. sissoo （70%） and the lowest was in O. sativa （35%）. Arbuscular mycorrhizal spore population varied from 54 to 140/100g dry soil in the soils from Dashmail. The highest was in the soils of D. sissoo （140） and the lowest was in C. domestica （54）. The spore population varied from 63 to 221 in Kantaji. The highest was in G. arborea （221） and the lowest was in O. sativa （63）. The range population in Ramsagar varied from 69 to 160. The highest was recorded in D. sissoo （160） and the lowest was in L. chinensis （69）. No significant relationship of soil pH and soil OM with AM colonization and with spore population was observed. Simpson＇s index of diversity （Ds） and Shannon＇s index of diversity- （Hs） were highest in the soil of D. sissoo from Kantaji and the lowest in the soils of O. sativa from Ramsagar. Biodiversity of AM colonization, spore population and the distribution of AM fungi in the rhizosphere soils of different agroforestry plants indicated the occurrence of AM fungi, mycotrophic nature of the trees and crop species, contribution and necessity of AM fungi and the AM dependence of the agroforestry plants growing in Dinajpur district of Bangladesh.
基金supported in part by the Fundamental Research Funds for the Central Universi-ties(No.NP2022416)the Aeronautical Science Founda⁃tion of China(No.20180627210315).
文摘In the passive turning state,the helicopter turns through the tail rotor force and the friction of the ground to the tire.In practice,it is found that the helicopter will turn difficultly under low aircraft ground speed or static state.This paper takes a certain type of helicopter as the research object,and establishes the dynamic model of helicopter ground turning motion based on the basic theory of dynamics.This model takes into account the six-degree-of-freedom motion model of the helicopter body,the motion model of the landing gear buffer,the tire mechanics model and the friction characteristics of the strut friction disc.The dynamic simulation of the helicopter right angle turn and static turn is carried out,and the influence of parameters such as tail rotor pull,taxi speed,tail wheel stability distance on the dynamic response of the turn is studied.The results show that under the same ground taxing speed,the tail wheel angle increases with the increase of tail rotor force.When the tail rotor force is the same,the tail wheel angle increases with the increase of ground taxing speed.When the helicopter is completely static,it is the most difficult to turn,which requires much bigger force of the tail rotor to turn.In addition,the change of the stability distance of the tail wheel has an obvious influence on the turning.When the stability distance is doubled,the tail rotor force will be reduced by 30%to the same angle of the tail wheel.
基金supported by Important National Science ＆ Technology Specific Projects under Grant No.CX01011the Important National Science ＆ Technology Specific Projects under Grant No.4101002+2 种基金the National Natural Science Foundation of China under Grants No.61002014,No.60972017,No.60972018the Excellent Young Teachers Program of MOE,PRC under Grant No.2009110120028the Research Fund for the Doctoral Program of Higher Education under Grants No.20091101110019,No.20070007019
文摘An exact average symbol error rate analysis for the distributed dual-hop relay cooperative network with multiple relays in a Nakagami-m fading environment is presented.In the derivation of the moment generation function of receiver Signal-to-Noise Ratio(SNR),the sectional integral method is used,instead of the cumulative density function method which is ordinarily used by the deduction of the outage probability of S-R-D link.The accurate symbol error rate of a dual-hop relay cooperative network is obtained with the closed-form Moment Genoration Function (MGF) expression.The correctness of the symbol error rate is verified through numerical simulations and is compared with other analytical methods.These deductions clearly show that the distributed cooperative diversity network presented has strong superiorities in overcoming severe fading and can achieve full diversity order.
基金ACKNOWLEDGEMENTS This work is supported by National Natural Science Foundation of China （No. 61171079）. The authors would like to thank the editors and the anonymous reviewers for their detailed constructive comments that helped to improve the presentation of this paper.
文摘Most resource allocation algorithms are based on interference power constraint in cognitive radio networks.Instead of using conventional primary user interference constraint,we give a new criterion called allowable signal to interference plus noise ratio(SINR) loss constraint in cognitive transmission to protect primary users.Considering power allocation problem for cognitive users over flat fading channels,in order to maximize throughput of cognitive users subject to the allowable SINR loss constraint and maximum transmit power for each cognitive user,we propose a new power allocation algorithm.The comparison of computer simulation between our proposed algorithm and the algorithm based on interference power constraint is provided to show that it gets more throughput and provides stability to cognitive radio networks.
