بررسي تراكم، ژيومورفولوژي و پهنه‌بندي ارتفاعي نبكاهاي حاشيه غربي دشت لوت و تاثيرات پوشش گياهي بر مورفولوژي آنها

Density survey, Geomorphology and Elevation zoning of Nebkhas in Western Margin of the Lut Desert and Vegetation Effects on the Morphology

نبكاها به‎دليل تركيب دو علم اكولوژي و ژيومورفولوژي (پوشش گياهي و تپّه ماسه‎اي)، همواره مورد توجّه انديشمندان اين علوم هستند. اين پژوهش به بررسي تراكم، ژيومورفولوژي و پهنه‌بندي ارتفاعي نبكاهاي غرب دشت لوت پرداخته است. پژوهش از نوع توصيفي، تحليلي، برپايه روش‎هاي ميداني، آماري و كتابخانه اي است، به اين‎گونه كه پس از شناسايي پهنه مطالعاتي با استفاده از تصاوير ماهواره اي و عكس‎هاي هوايي، با بررسي ميداني منطقه و از طريق ترانسكت و پلات، نبكاهاي مورد نظر انتخاب و ويژگي‎هاي مورفومتري نبكاها و نوع گياه در هر پلات اندازه گيري شده و با استفاده از نرم‎افزار اس.پي.اس.اس. تحليل آماري آنها شامل تحليل همبستگي و فراواني انجام شده و با يكديگر مقايسه شدند. در مرحله بعد با استفاده از نرم‎افزار ARC GIS توزيع و پراكندگي نبكاهاي هر يك از گونه‎ها نقشه‎سازي شده و به پهنه‎بندي ارتفاعي نبكاها در منطقه پرداخته شد. نتايج به‎دست آمده از تحليل همبستگي عناصر مورفومتري نبكاها نشان مي‎دهد كه نبكاهاي هر گونه گياهي در دو پلات مجاور، ويژگي‎هاي متفاوتي دارند كه اين موضوع تاثيرات متفاوت عوامل تاثيرگذار نبكاها، از جمله اندازه پوشش گياهي و فرآيندهاي بادي بر قسمت‎هاي مختلف منطقه را نشان مي دهد. پراكندگي نبكاهاي هر يك از گونه‎هاي گياهي از الگوي خاصي پيروي مي كند، بدين صورت كه بعضي گونه‎ها مانند خارشتر، فقط در قسمت‎هاي جنوبي منطقه مشاهده شدند. همچنين از لحاظ پهنه‎بندي ارتفاعي نبكاها، پلات‎هاي مورد نظر با يكديگر تفاوت دارند، به‎طوري كه برخي گونه‎ها در شمال منطقه نبكاهاي بلندتري را تشكيل داده و برخي ديگر در قسمت‎هاي مركز و جنوبي متمركز شده اند.

Introduction Nebkais one of the accumulations winds landforms that are under the effect of vegetation in arid and semi-arid regions. In this research we try to assess the echogeomorphology characteristic of kinds of Tamarix and Prosopis koelziana nebkas in the west of Lut Plain in southeastern of Iran. Vegetation has a major role in dynamic and morphology of sand dunes in desert area. Vegetation affects the movement of sands and trapped the sands transported by wind. Lut is a desert area with many especial landforms such as Yardang, Barkhan, Star dunes and other desert landforms with 175000 sq. kilometers area. Methodology In this research we used descriptive and analytical methods for study of Nebkas, and also by using of field study we measured morphometric characteristic of selected nebkas and formasion of nebkhas. Also the samples of sediments were taken and measuring the components of land forms was measured. After identification of study area by using the aerial photos and satellite images, with field study we take the samples, and then continued our studies. We identified 4 transects and selected some Nebkas in transects and plot then we started the measuring of morphometric characteristic and recognizing the kind of vegetation and then sampling from those ones. At the end of field work we calculate statistical parameters of samples by using SPSS software. Then using the shaker we applied granulometry method for analyzing of grain size. For analyze of grain size we used Gradistat software. This software calculates all statistical parameters of grain size taken from Nebkas sediments. Results and Discussion Results show that among 105 selected Nebkas’ samples, there are 5 types of vegetation with different morphometric characteristic. Results from correlation coefficient analysis of nebkas morphometric characteristics (height of dunes, height of plant), dunes area, width and perimeter) show that correlation between Tamarix aphylla and Prosopis koelzianas, in 99 percent, meaning level is high and forTamarix kotschyi and Tamarix Floridas in level of 95 and 99 percent meaning level is high. Mean height of Nebkas in Tamarix Aphylla is7.8 meters and mean height of vegetable is 11.09 meters. The mean in other kinds of Nebkas is lower. Moreover in view point of area, perimeter and width’s mean, the highest data belongs to Tamarix aphylla with 322.39, 68, 3 and 18,24 meter. Also results from granulometry show that coarser grain belongs to Tamarix aphylla and other kind located at next ranks. Conclusion The results from statistical analysis of the morphometric characteristic of Nebkas show that the increase in height of the crest of vegetation leads to increase in Nebkas height and the consequent is increase in area of Nebkas. Also increase of height lead to increase in perimeter and width of Nebkas. This condition is identical for all Nebkas. Tamarix aphylla with higher height in compare with other kinds of Nebkas vegetable has bigger area, perimeter and width. In reverse situation, Tamarix kotschyi has lower height and in relation to these characteristics, are smaller than other kind of vegetable in study area. Also the result from mean of area, perimeter and width of Nebkas show that the bigger one belongs to Tamarix aphylla with 322.39, 68,3 and 18.24 meters and the secondrank belongs to Tamarix florida and nextranks belong to Tamarix aphylla, Prosopis koelziana and Tamarix kotschyi. The maximum standard deviation, variance, kurtosis and skewness belong to Tamarix aphylla, Prosopis and Tamarix kotschyi. Moreover, between all Nebkas, the maximum area, perimeter and width belong to Tamarix aphylla with 565.7, 93 and 30.5 meter and the minimum area and perimeter belong to Tamarix florida with 11.84 and 11 meters. In spite of this situation, the minimum width belongs to Tamarix kotschyi with 2.9 meter width. Results from granulometry shows that the coarser sand belongs to sediments of Tamarix aphylla and the other kinds of vegetable located at lower rank of Nebkas. Nebkhas In the southern of area are higher density and concentrated in the southeastern region of highest Nebkhas tamarix aphyla.