Effects Of Wind Erosion Stress On Population Dynamics Of Juvenile And Middle-aged Haloxylon Ammodendron

Purpose:Haloxylon ammodendron is an important shrub widely distributed in arid and semi-arid desert areas.It plays a crucial role in windbreak,sand fixation,and maintaining the ecological balance of desert environments.Due to the scarcity of water resources and nutrients in desert surfaces,H.ammodendron has evolved highly developed root systems to absorb underground water and nutrients.At the top of the dune with serious wind erosion,It forms clustered patterns on the wind-eroded tops of sand dunes to resist strong wind erosion.Within H.ammodendron shrubs,there exists a special of interactions that promote the growth dynamics of the shrubs.Therefore,this study aims to investigate the survival patterns among individual H.ammodendron plants under wind erosion stress,providing a theoretical basis for the restoration and protection of H.ammodendron in severely wind-eroded areas.Methods:This study focused on H.ammodendron growing on the wind-eroded tops of sand dunes in the Gurbantunggut desert.The degree of wind erosion stress was determined according to the wind speed at the top of the dune.Through field surveys,the number of surviving and dead of juvenile and middle-aged H.ammodendron individuals in the shrub clusters on the sand dune tops was recorded.Basal diameter and tree height were measured.The Taylor power law was used to analyze the pattern of juvenile H.ammodendron individuals,while the size inequality equation was employed to study the variation in tree height and biomass among juvenile and middle-aged the shrub individuals.Additionally,the Zone of Influence(ZOI)model was employed to investigate: The impact of initial density on the growth of juvenile and middle-aged H.ammodendron shrubs;The effects of biotic and abiotic factors on the growth dynamics and self-thinning trajectory of juvenile and middle-aged H.ammodendron.Main results:(1)Field survey data showed that the juvenile density of H.ammodendron shrubs under three different wind erosion stresses followed a similar trend as they aged.Initially,the density increased rapidly(1～2 years)and then increased slowly.With increased wind erosion stress,the density within H.ammodendron shrubs became more clustered,and Taylor’s index(b)were greater than 1.The competition index(HI)between individuals decreased with the increase of stress intensity,while the mortality in shrub increased with the increase of stress.The mortality of juvenile H.ammodendron individuals under three wind erosion stresses was greater than 20%.The results also revealed that size inequality initially increased with age(1～6 years)and then decreased,resembling a left-skewed hump-shaped pattern.(2)Field investigation found a special relationship between initial density and wind erosion stress.As the wind erosion stress increased,the initial density also increased,showing significant differences between low,medium,and high densities(P < 0.5).Using the ZOI model,it was found that under low wind erosion stress,low initial density had a greater impact on aboveground biomass.However,under high wind erosion stress,high initial density had a greater influence on aboveground biomass in H.ammodendron shrubs.(3)Through the ZOI model,simulations of individual growth conditions within H.ammodendron shrubs under different conditions were conducted.The results showed that wind erosion stress reduced the aboveground biomass growth of the shrubs,while symmetric facilitation promoted an increase in aboveground biomass.When simulating no stress and no neighboring effects,the aboveground biomass of the shrubs tended to exhibit a "J"-shaped growth trend.When simulating stress and competition together,under high wind erosion stress,the growth between individuals tended to follow size symmetrical competition.Simulated stress,competition and facilitation together in H.ammodendron shrubs.When H.ammodendron grows at the end of middle-aged growth stage,the highest aboveground biomass was observed under high stress(S=0.8),allometric asymmetric competition(C-AA),and size symmetric facilitation(SS)modes.(4)During the juvenile growth stage of H.ammodendron,wind erosion stress increased the self-thinning intercept and decreased the self-thinning slope,resulting in a flatter self-thinning line.In the middle-aged growth stage,wind erosion stress reduced the self-thinning intercept.Different competition modes had varying effects on the self-thinning line of H.ammodendron.Specifically,the self-thinning slope(absolute value)was greater under the allometric symmetric competition mode,resulting in a steeper self-thinning line.Under high stress,allometric asymmetric facilitation can slow down the self-thinning phenomenon of individuals in H.ammodendron shrubs,and the self-thinning intercept also increases with it.In the combination mode of allometric asymmetric facilitation by the combination symmetric and asymmetric competition modes,the self-thinning slope(absolute value)and intercept increase with the increase of wind erosion stress.Main conclusions:This study’s field surveys revealed that as the severity of wind erosion stress increased,the clustering degree among H.ammodendron shrubs’ individuals also increased,resulting in higher initial density and lower competition index and size inequality.This suggests that wind erosion stress can alleviate competition and differentiation among individuals within H.ammodendron shrubs.Additionally,the combination of biotic and abiotic factors revealed that under greater wind erosion stress,the allometric asymmetric competition and the size symmetric facilitation modes were more favorable for the growth of middle-aged H.ammodendron shrubs.For self-thinning,during the juvenile growth stage,regardless of the interaction mode,the self-thinning slope was positive.This is because the newly recruited individuals within the shrubs exceed the number of deceased individuals,resulting in a net positive change in density,this phenomenon is unique to H.ammodendron shrubs on severely wind-eroded sand dune tops.In the middle-aged growth stage,under high wind erosion stress,the combination of allometric asymmetric competition and size symmetry facilitation modes yielded the smallest self-thinning slope(absolute value).The transition of interaction patterns within H.ammodendron shrubs demonstrates adaptability for growth under wind erosion stress,as individual plants adjust their growth rates to sustain their survival under stronger wind erosion stress.