Research On The Formation Mechanisms Of Aolian-Sand-Vegetation Patterns

Desertificaiton has becoming one of the most serious environmental problems in the world.Vegetation patterns are the typical landscape on desertified lands,and they have been observed widely in arid,semi-arid and semi-humid areas in the whole world.The extensive distribution and the heterogeneity of vegetation patterns contributed to draw the attention of substantial amount of geologists and ecologists.Researches on vegetation patterns can help to understand the potential of related ecological processes,to explore the mechanisms of pattern formation,and promote the understanding of desertificaiton and restoration,and to predict the formation and characteristics of vegetation patterns.These researches can be the guidance for desertification control and ecological restoration.Wind induced sand-vegetation patterns have been observed and described for quite a long time,but exploration of mechanisms on vegetation pattern formation in this situation has been,rare.In this research,we considered six ecological processes including,aeolian sand tranportation in the air,deposited sand movements on the ground,vegetation spread through seed dispersal,the flow of soil water,vegetaion growth and aeolian sand deposition.Based on these processes,a temporal and spatial discrete vegetation-sand model is established.Using this model,the mechanism of wind induced vegetation patterns is explored,and the main results are as follows:(1)Turing-type labyrinth patterns are induced by Turing bifurcation when there is no prevailing wind.While Turing-type regularly banded patterns are formed when prevailing wind exists.Irregular patterns are induced by the Neimark-Sacker bifurcation in the absence of prevailing wind.While irregularly banded patterns are induced by Neimark-Sacker bifurcation when prevailing wind exists.Simulated vegetation patterns are basically the same in type as the observed vegetation patterns in reality.(2)Wind-induced vegetation patterns induced by Turing-Neimark-Sacker coupled bifurcation can be classified into the following five types:coupled bifurcation irregular band patterns,coupled bifurcation labyrinth patterns,coupled bifurcation regular band patterns,coupled bifurcation spot patterns coupled bifurcation crescent-shaped strip patterns.Transitional patterns between these pattern types are more similar to the real patterns.(3)Shannon entropy and contagion index are calculated in this research in the formation of vegetation patterns.The results show that Turing patterns have lower Shannon entropy and higher contagion index than Neimark-Sacker patterns.For coupled bifurcation patterns,the coupled bifurcation irregular band,the coupled bifurcation labyrinth,the coupled bifurcation regular band,the values of Shannon entropy and contagion index in the three type patterns are in the middle of Turing patterns and Neimark-Sacker patterns.However,due to the special effects of coupled bifurcation,coupled bifurcation spotted patterns,coupled bifurcation crescent banded patterns,these two types present lower Shannon entropy than the Turing bifurcation and higher contagion index.(4)Based on the banded vegetation patterns,the characteristics of vegetation pattern under different environmental parameters are studied.The larger the advection intensity(reflecting the strength of prevailing wind),the greater the maximum and mean of vegetation coverage in the pattern;the smaller the wavelength of the pattern,and the larger the wave number.The longer the time step(reflecting the frequency and time scale of the wind sand activity),the larger the maximum and the mean of the vegetation cover in the pattern,the smaller the wavelength of the pattern and the larger the wave number.Through the comparison of waves between vegetation bands and sand bands,we can see that there is phase difference between vegetation bands and sand bands.And the phase difference is related to advection intensity.In addition,the migration velotity of band vegetation patterns are related to the relative magnitude between sand and vegetation advections.Simulations show that the direction of migration can be reverse to the prevailing wind direction.(5)The effects of the variations of environmental parameters on the already-formed banded patterns are studied.Starting from an already-formed vegetation pattern(at a relatively low advection intensity),when the advection intensity increased gradually,vegetation bands break down to small patches in the vertical direction of prevailing wind,and then self-organized in other directions to form brand new vegetation bands.The wave number of vegetation bands gradually increases,and the wavelength gradually decreases.When the advection intensity decreases,the wavenumber and wavelength of the pattern remain basically unchanged,but the width of the vegetation bands decreases gradually.However,the wavenumber suddenly increases and the wavelength decreases abruptly only when advection intensity is reduced to 0.It shows that the characteristics of banded vegetation patterns have certain stability to the variations of advection intensity.(6)In this paper,the effects of the variations of the time step(one of the environmental parameters)on the characteristics of banded pattern are studied.The wavenumber of the formed pattern changes with the time step,and the corresponding standard value(in the case of a single time step,the value of wave number of the pattern obtained by simulation is the standard value)is not different from that of each time step.This indicates that the characteristics of banded patterns are reversible with the variations of time step.(7)The effects of prevailing wind direction on the characteristics of pattern are studied.When the prevailing wind seasonal reverse occurs,the phase relationship between vegetation bands and sand bands will vary.The final phase relationship depends on the duration of the opposite direction of prevailing winds.When the prevailing wind direction rotates(keep the advection intensity constant),vegetation bands will not rotate with prevailing wind,but the width of the bands will gradually decrease.When the angle of rotation is very large(nearly 5?/1 2),vegetation bandswill rotate but with a smaller angle.When the angle of rotation reaches ?/2,vegetation bands will rotate completely and perpendicular to new prevailing wind direction.(8)The effects of human activities on the formation of patterns are studied.Overgrazing can lead to the disappearance of vegetation patterns.Deforestation can accelerat the formation process of vegetation patterns.While vegetation restoration didn't increase the number of vegetation zones,but increased the total vegetation coverage in the patterns.This research has successfully revealed the formation mechanism of wind-induced sand-vegetation patterns,enriched the scene of vegetation pattern formation,and meanwhile deepened the understanding of desertification and vegetation restoration.