Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear wh...Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization,and determines species diversity.In this study,the impacts on clonal grasses were studied in a field experiment employing two frequencies(twice a year vs.monthly)crossing with nine N addition rates in a temperate grassland,China.We found that the N addition decreased species frequency and increased extinction probability,but did not change the species colonization probability.A low frequency of N addition decreased species frequency and colonization probability,but increased extinction probability.Moreover,we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions.The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity,suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition.Overall,this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.展开更多
Nitrogen(N)deposition decreases the temporal stability of ecosystem aboveground biomass production(ecosystem stability).However,little is known about how the responses of ecosystem stability differ based on seasonal N...Nitrogen(N)deposition decreases the temporal stability of ecosystem aboveground biomass production(ecosystem stability).However,little is known about how the responses of ecosystem stability differ based on seasonal N enrichment.By adding N in autumn,winter,or growing season,from October 2014 to May 2020,in a temperate grassland in northern China,we found that only N addition in autumn resulted in a significantly positive correlation between ecosystem mean aboveground net primary productivity(ANPP)and its standard deviation and significantly reduced ecosystem stability.Autumn N-induced reduction in ecosystem stability was associated with the vanished negative effect of community-wide species asynchrony(asynchronous dynamics among populations to environmental perturbations)on the standard deviation of ecosystem ANPP in combination with the emerged positive effect of dominance(Simpson's dominance index that indicates the relative weight of dominant species in a community).Our findings indicate that autumn N addition might overestimate the negative effect of annual atmospheric N deposition on ecosystem stability,suggesting that to better evaluate the influence of N deposition in temperate grasslands,both field experiments and global modeling should consider not only the annual N load but also its seasonal dynamics.Moreover,further studies should pay more attention to the alteration in the ecosystem temporal deviations,which might be more sensitive to human-induced environmental changes.展开更多
As a kind of popular smart materials, shape memory polymers （SMPs） have a great potential for applications in deployable aerospace structures and other engineering struc- tures. However, the vibration analysis of sh...As a kind of popular smart materials, shape memory polymers （SMPs） have a great potential for applications in deployable aerospace structures and other engineering struc- tures. However, the vibration analysis of shape memory polymer structures, which would play an important role in engineering, has not gained much attention. In this study, we propose a dynamic model and establish the governing equations for characterizing the dynamic behavior of a shape memory polymer membrane subjected to time-dependent forces. The derivation of governing equations is based on a well-developed constitutive model of SMPs combined with the Euler-Lagrange equation. With the proposed model, two different loading cases are stud- ied： the equal-biaxial sinusoidal force and the uniaxial sinusoidal force. To analyze the dynamic response of a shape memory polymer membrane and find some effective ways to control vibra- tion, the isothermal amplitude-frequency response, the time-dependent behavior of vibration and the vibration in a variable temperature process are investigated in the numerical simulation. It is observed that temperature, mechanical force and heating rate have significant effects on the dynamic performances of a shape memory polymer membrane. We also investigate the shape memory behavior of SMP membrane involving the dynamic response. The influence of dynamics on shape fixation and shape recovery is discussed. These results and discussion may provide guidance for exploring the vibration and dynamic performances of shape memory polymer in deployable aerospace structures.展开更多
基金supported by a grant from the National Natural Science Foundation of China(grant no.32071603 and 32122055)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDA26020101)。
文摘Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization,and determines species diversity.In this study,the impacts on clonal grasses were studied in a field experiment employing two frequencies(twice a year vs.monthly)crossing with nine N addition rates in a temperate grassland,China.We found that the N addition decreased species frequency and increased extinction probability,but did not change the species colonization probability.A low frequency of N addition decreased species frequency and colonization probability,but increased extinction probability.Moreover,we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions.The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity,suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition.Overall,this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.
基金This work was supported by the National Natural Science Foundation of China(32122055 and 32071603)the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(ZDBS-LY-DQC034).
基金funded by the National Natural Science Foundation of China (32122055 and 32071603)the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA26020101).
文摘Nitrogen(N)deposition decreases the temporal stability of ecosystem aboveground biomass production(ecosystem stability).However,little is known about how the responses of ecosystem stability differ based on seasonal N enrichment.By adding N in autumn,winter,or growing season,from October 2014 to May 2020,in a temperate grassland in northern China,we found that only N addition in autumn resulted in a significantly positive correlation between ecosystem mean aboveground net primary productivity(ANPP)and its standard deviation and significantly reduced ecosystem stability.Autumn N-induced reduction in ecosystem stability was associated with the vanished negative effect of community-wide species asynchrony(asynchronous dynamics among populations to environmental perturbations)on the standard deviation of ecosystem ANPP in combination with the emerged positive effect of dominance(Simpson's dominance index that indicates the relative weight of dominant species in a community).Our findings indicate that autumn N addition might overestimate the negative effect of annual atmospheric N deposition on ecosystem stability,suggesting that to better evaluate the influence of N deposition in temperate grasslands,both field experiments and global modeling should consider not only the annual N load but also its seasonal dynamics.Moreover,further studies should pay more attention to the alteration in the ecosystem temporal deviations,which might be more sensitive to human-induced environmental changes.
基金Authors are grateful for the support from the National Natural Science Foundation of China through Grant numbers 11572236, 11372236.
文摘As a kind of popular smart materials, shape memory polymers （SMPs） have a great potential for applications in deployable aerospace structures and other engineering struc- tures. However, the vibration analysis of shape memory polymer structures, which would play an important role in engineering, has not gained much attention. In this study, we propose a dynamic model and establish the governing equations for characterizing the dynamic behavior of a shape memory polymer membrane subjected to time-dependent forces. The derivation of governing equations is based on a well-developed constitutive model of SMPs combined with the Euler-Lagrange equation. With the proposed model, two different loading cases are stud- ied： the equal-biaxial sinusoidal force and the uniaxial sinusoidal force. To analyze the dynamic response of a shape memory polymer membrane and find some effective ways to control vibra- tion, the isothermal amplitude-frequency response, the time-dependent behavior of vibration and the vibration in a variable temperature process are investigated in the numerical simulation. It is observed that temperature, mechanical force and heating rate have significant effects on the dynamic performances of a shape memory polymer membrane. We also investigate the shape memory behavior of SMP membrane involving the dynamic response. The influence of dynamics on shape fixation and shape recovery is discussed. These results and discussion may provide guidance for exploring the vibration and dynamic performances of shape memory polymer in deployable aerospace structures.
