The Influence Of The Shrub Configuration Features Of Tetraena Mongolica Maxim On The Aeolian Processes And Sedimentary Characteristics

Tetraena mongolica Maxim,as a relict plant in the Mediterranean period,has excellent cold and drought tolerance genes,making it an important constructive plant in the steppe desert region of the Ordos Plateau.Long-term adaptive survival makes Tetraena mongolica show specific configurational characteristics,and the change of configuration also makes stable nebkhas of different sizes formed under the canopy.The deposition and extinction of nebkha in desert will directly affect the growth of shrub plants and the protection of community.At present,it is urgent to clarify the influence mechanism of Tetraena mongolica plant configuration on wind-blown sand process and sedimentary characteristics.This thesis takes the Tetraena mongolica in the steppe desert region of the northwestern Ordos Plateau as the research object.The potential relationship between the Tetraena mongolica configuration and nebkha morphology was explored through field investigation.The effect of Tetraena mongolica on the deposition process of nebkha was determined from the wind erosion mechanism by field wind speed measurement and indoor wind tunnel test.On this basis,the sediments of the nebkha were collected,combined with the indoor measurement to reveal the influence of the formation process of Tetraena mongolica nebkha on the sediment characteristics.The main conclusions are as follows:（1）Tetraena mongolica shrub and nebkha show a semi-ellipsoidal development strategy in which the horizontal growth rate is higher than the vertical growth rate.There is a significant positive correlation between shrub morphology and nebkha morphological parameters,the fitted equations R² of the canopy area to the base area and volume of the nebkha were 0.9382 and 0.8056,respectively.There were significant differences in the silhouette area of the windward side of shrubs with different sizes（P<0.05）.The correlation coefficient between the silhouette area of the 10-30cm height layer and the volume of the nebkha is 0.81.The branching ratio(SBR_3:4)of Tetraena mongolica near-surface branches increased significantly with the increase of shrub size（P<0.05）.The branch diameter ratio(RBD_4:3)of the small shrub and the large shrub near the ground was 2.46 and 1.76,respectively.The branch length of the 4th grade branch and the 3rd grade branch angle also showed an increasing trend with the increase of the shrub volume.The results of the path analysis and the correlation heat map show that,According to the order of decision coefficient,the branch architectures that directly affect the growth of Tetraena mongolica and the development of nebkha are SBR_3:4>4th grade branch length>3rd grade branch angle>RBD_4:3.Among them,the direct effect of the branching rate near the ground layer on the shrub development was the strongest,and the decision coefficient was 0.5154.（2）The overall wind speed profiles in the open field in the study area showed a"J"-shaped distribution.Tetraena mongolica under different branch densities interfered with the transit airflow to different degrees and then affected the wind speed changes around the shrub.When the wind speed at the height of 2m in the open field is 4m/s,the wind speed increases at the height of 10cm and 20cm in the middle of the left side of the shrub are 18.25%and 11.09%,respectively,the acceleration level gradually decreased with the decrease of branch density.The area immediately behind the shrub will create a significant area of reduced wind speed.The decreasing range showed a decreasing trend with the increase of the height of the measuring point,and the area of the wind speed attenuation area behind the shrub gradually decreased with the decrease of the density of the shrub branches.The difference of wind speed variation of each measuring point at the height of 50cm in the shrub was significantly reduced.（3）The flow field around Tetraena mongolica in the wind tunnel is basically the same as that in the field.The increase in shoot density enhanced the sand blocking efficiency.The overall performance of sediment transport in back of shrubs with no branches removed（0%）,25%branches removed and 50%branches removed was CK>50%>25%>0%.Under the wind speed of 12m/s,the total sediment transport from high-density to low-density branch shrubs accounted for 26.52%,59.61%and 88.35%of the CK,respectively.The difference in sediment transport from shrubs with different branch removal intensities gradually decreased with the increase of the measured height.Tetraena mongolica branches blocked the wind-blown sand flow,which resulted in effective sediment accumulation under the shrubs.When the density of shrub branches is high,obvious sand accumulation is formed under the shrub.When the density of shrub branches is high,the volume of sand pile under shrub also increases,under the wind speed of 6m/s-12m/s,the deposition height of the shrubs without removing branches was8%,10%,18%and 32%higher than that of the sand bed,respectively.The stacking height of the sediment under the bushes decreased significantly when the shoot density decreased.（4）With the expansion of Tetraena mongolica shrub and the increase of nebkha height,the sediment texture under the shrub showed a fine grain trend.Among them,the dust content of the 0-10cm,10-20cm and 20-30cm soil layers of the small nebkha were18.4%,20.2%and 23.7%,respectively.The average particle size of the sediments from the top of the nebkha to the outside of the nebkha decreases gradually.Sediment-sensitive components at the top and waist of medium and large nebkha are concentrated around122μm and 322μm.Most of the sensitive components at the edge and outside of the nebkha are fine-grained materials,and the inside of the shrub is more likely to intercept the coarse sand material,which further explains the distribution characteristics of the shrub sediments.With the continuous accumulation of sediments under the shrub,the soil nutrient content gradually forms a law of enrichment.The slopes of the fitted straight line between the content of alkaline hydrolyzable nitrogen（AHN）in the surface layer of large,medium and small nebkha and the canopy index（CI）were-7.9255,-6.3137 and-3.7626,respectively.With the increase of soil depth,the slope of nutrient enrichment showed an increasing trend.The expansion of Tetraena mongolica canopy increased the soil volume water content in nebkha.The volumetric water content of the soil depth of 10-20cm in the vertical direction is the largest.Redundancy analysis found that the main fine sand components（100-250μm）in nebkha sediments and soil volume water content can obviously promote the increase of soil nutrient content.