بررسي الگوي پراكنش مكاني نبكا (مطالعة موردي: دشت صوفيكم، استان گلستان)

Investigating the Spatial Distribution Pattern of Nebaka (Case Study: Sufikam Plain, Golestan Province, Iran)

نبكا نقشهاي متعددي در پايداري اكوسيستمهاي مناطق خشك و فراخشك دارد؛ مهمترين نقش آن حفظ پوشش گياهي و تثبيت ماسههاي روان است. تحقيق حاضر بر اساس روش ميداني شمارش 147 نبكا در دشت صوفيكم استان گلستان انجام شده است. بهمنظور بررسي تعيين نوع الگوي پراكنش نبكاها و عوامل تأثيرگذار در پراكنش مكاني آنها، خصوصيات مورفومتريك نبكاها (طول، عرض، و ارتفاع نبكاها) در عرصه تعيين شد و موقعيت مكاني آنها ثبت گرديد. همچنين، براي تعيين خصوصيات فيزيكي و شيميايي خاك از رأس نبكاها خاك نمونهبردار شد و سپس تحليل مكاني نبكاها با استفاده از توابع k رايپلي، g تكمتغيره، و تابع همبستگي نشاندار انجام شد. پراكنش مكاني نبكاها در فاصلههاي بين 0 تا 50 متر بهصورت كپهاي تعيين شد. همچنين، تحليل تابع همبستگي نشاندار با احتمال 95درصد و در سطح 5درصد براي پارامترهاي طول نبكا، كجشدگي رسوبات، اسيديته، و مادة آلي خاك نبكاها بيانگر تأثير اين عوامل در الگوي پراكنش بود و ساير پارامترهاي مورد مطالعه تأثيري نداشتند. با توجه به نقش مثبت نبكا در كاهش اثر فرسايش بادي، بررسي پراكنش مكاني و عوامل مؤثر در پراكنش و توسعة نبكاها و ضرورت آگاهي از فرايندهاي طبيعي آنها ميتواند در مديريت بهتر بهمنظور كاهش ميزان فرسايش بادي كاربردي باشد.

Introduction Nebkhas are single dunes formed by accumulation of aeolian sediments on shrubs and single trees. They play an essential role in the stability of arid and hyper-arid ecosystems, mostly by preserving the vegetation. Moreover, they are very important in stabilization of moving sand in deserts and semi-arid areas, as well as in protecting human settlements and facilities from sand infestation. The spatial pattern analysis of natural or manmade effects provides useful information about the effects of competition between environmental conditions. The spatial point pattern analysis of every phenomenon and their impacts on environment is being considered as a basic criterion for decision making, especially in risk management. For instance, spatial pattern of plants is a context in ecological theory. A data-set consisting of a series of mapped point locations is a main example of a spatial point pattern. The methods for quantifying these characteristics of point pattern are known as summary statistics. From a statistical point of view, the methods are also appropriate for phenomena that represent various types of geomorphologic features. Thus, to better understand the factors controlling the pattern of Nebakas, univariate and bivariate summary statistics have been used. Moreover, a more promising approach in such analyses is to complement point position with quantitative size attributes and use mark point patterns (i.e., Nebakas with a quantitative characteristic such as physical and chemical properties of the soil). In fact, the spatial distribution of Nebakas in the semi-arid regions can be described and modeled by point pattern processes where the points are given by locations of the Nebakas. Given the importance of spatial statistics in obtaining useful information for better management in all fields, there are not remarkable investigations about the spatial analysis of the Nebka dunes. Materials and methods In the present study, we have selected an area of 197 hectare and produced a maps related to 147 Nebkas in the Sufikam plain, northwest of Golestan province, Iran. In order to study the distribution pattern of the Nebkas and also the factors influencing their spatial distribution, we have recorded morphometric characteristics of the Nebkas including length, width and height. The physical and chemical properties of the soil have also been assessed and the spatial pattern of Nebkas analyzed using R and Programita software based on Ripley’s K and G univariate functions and Mark Correlation Function (MCF). Results and discussion Results showed that by using RipleyK function, the spatial distribution of Nebkas in Sufikam plain is clustered and most of them are located in the north part of the area of interest. Density of Nebkas and also their mean spatial distance (distance between Nebkas) was 0.75 per hectare and 33m in turn. Regarding the results of g(r) in Sufikam plain, the significant aggregation of Nebkas was observed at all scales (0–50 m). As the function failed to be in the bounds of respective 95% simulation envelope, the aggregation of Nebkas in all of the scales was a significant departure from random labeling at the significance level of 0.05 (Fig. 8). Furthermore, regarding the results of mark correlation function (MCF), Nebkas and parameters of the length of the Nebka, sediment deposition, pH and organic matter of the Nebka soil, were highly correlated. Moreover, the parameters of width, volume and height of the Nebkas, EC, SAR, ESP, sorting and mean diameter of sediments did not affect their distribution pattern. Conclusion The results of the spatial analysis of the Nebkas helped us identify the factors affecting the distribution of these facies. In this study, it was also found that soil acidity has a significant effect on the distribution pattern of the Nebkas. In addition, greater organic matter content has a greater effect on the growth of the plant species, which can also affect the distribution pattern. Soil textures such as clay, silt and sand did not affect the neighboring Nebkas in the study area. The sediment deposition and the distribution of the Nebkas indicates a positive correlation; which means that the larger its value (or the symmetry of the sediments towards fine particles), the denser the distribution of the points. The results have indicated that the of Nebka landform play a positive role in reducing the effect of wind erosion. The investigation of their spatial dispersion and factors affecting their dispersion, development and natural processes, can be a guide for natural resource managers for controlling and reducing the wind erosion.