Changes in the Spread of Sand Dunes and Surface Cover under the Influence of Wind Speed near Mangrove Ecosystems

Dry and ultra-dry conditions, prevailing in a large part of Iran with less than 710 mm of rainfall per year, have caused about 40 million hectares of the country to cover desert areas, sand dunes and areas with low vegetation (Refahi, 2004). Nebkas are very important in stabilizing mobile sands in arid and semi-arid areas and protecting human settlements and facilities to some extent from the onslaught of wind sands. They are found and usually formed in semi-arid, hot and dry and hot and humid areas (Amini et al., 2011, quoted by Thomas and Tousar). Nebka plays a very important role in the stability of ecosystems in arid and semi-arid areas. One of the species that has high resistance to wind and its roots and stems settle large volumes of aerosols and wind sediments and fights against wind erosion is Choug species that can stabilize quick sands in the south of the country. No studies have been conducted on this species. Therefore, this study aimed to investigate sediment changes in relation to vegetation in sediments and its comparative analysis against wind erosion to evaluate the process of stabilization of quick sands.

 In this research, one-dimensional sampling method and longitudinal unit have been used. This method allows random sampling of nebkas throughout the study area. After identifying the study areas and determining the scope of development of nebkas, by field reference, sampling and measurement of morphometric components of nebkas were performed. Sampling was done with 5 representative areas, 10 transects of 1000 meters with a distance of 500 meters from each other and perpendicular to each other. To determine the starting point of transect and the beginning of sampling, transects were selected so that they transversely cover the study areas. In each linear transect, 5 plots with dimensions of 4 * 4 meters were placed equal to 50 plots at each transect. The size of plot was determined by minimal area method. A total of 10 transects and 50 plots were used in each area; vegetation measurement was performed on totally 250 plots put at the region. Then, to start the sampling, the points were selected by GPS as an indicator at equal distances from the start of the Nebka landscape in the 5 areas under the study. Two Landsat satellite images from 1990 and 2020 were also used to determine a mount of morphometric changes in the sampled nebulae over a 30-year period. Then, using Envi 4.7 software, the region's nebkas and other existing uses were determined. Finally, after preparing land use maps, the area in the region was surveyed. 3-1: Measurement of morphometric properties of nebkas At this stage, the morphometric characteristics of Nebka such as sediment volume, dune height, dune base diameter, nebka slope, plant height and canopy diameter were measured. The basis for measuring the morphometric components of Nebka is as follows:In order to measure the height of Nebka, the height of Nebka peak to the level of its base and for the diameter of the base of Nebka, the average diameter of the base were measured by a tape measure. The slope of the Nebka cone is calculated by means of a slope gauge and the volume of the Nebka cone is calculated by Equation (1). Also, in order to obtain the average height of nebkas in the main and larger habitat, after many rounds of forest, the total height of nebkas was measured in all sample plots of one hectare that were systematically randomly distributed in the habitat (Zobeiri, 2009). Then, the ratio of canopy area to nebka area (in percentage) and the volume of each nebka were estimated using Equation (1).

 The relationship between plant traits and Nebka morphometric traits was estimated based on correlation analysis and multivariate regression analysis. In order to compare and evaluate the measured parameters of sediment and plant, to investigate the relationships between them and to perform the mean comparison test to evaluate the differences between the measured parameters in the study site, EXCEL 2013 and SPSS16.0 software were used. In order to compare the indicators studied in the study, Pearson correlation test with 95% confidence level was used to investigate the trend of their changes. In order to investigate the correlation coefficient and the explanatory relationship between plant morphological indices and sediment morphometric indices, after determining the non-normality of the data, the data were normalized by multiplying each data by 2. After normalizing the data, considering the indices measured from Nebka as a dependent variable and the indices measured from the plant as an independent variable, the regression relationship was examined and also the degree of correlation between the different measured indices was computed. 4- Findings (Results) 4-1 Results of correlation study between variables The results of correlation between sediment and plant variables in the Sirik region showed that there is a significant correlation between all measured variables except the canopy diameter of the plant at 95 and 99% levels. 4-2 Multivariate linear regression

The results of multivariate linear regression using independent variables of plant height and canopy diameter in the Sirik region showed a significant relationship for estimating dune height and sediment volume. Based on the results, plant height was the most important and effective independent variable in estimating the height and volume of dune sediment in the old Sirik region. 4-3: The rate morphometric changes of the measured nebulae during a period of 30 years The results of the study and classification of Nebka in the Sirik region during the 30-year period showed that the rangeland use has undergone the most changes and the least amount of changes were related to areas with Nebka. In addition, the study area has been affected by wind erosion over time, which has reduced the area of ​​nebkas in the study area by 8 hectares.

The results of multivariate linear regression using independent variables of plant height and plant canopy diameter in the study areas showed that there is a significant relationship between sediment height and nebka volume only in the Sirik region. Based on these results, plant height was the most important and effective independent variable in estimating the height of sand dune and volume of Nebka in the old Sirik region. These results were in line with the findings of Glaze et al. (2014), who studied sediment transport recovery in Nebka vegetation and uncovered Nebka vegetation and wind speed influences. Examination of sediment relations between flowing dunes and plant species in southern Thailand showed that sediment in windmills in the direction of wind and at the foot of phanrophite plant species was more than hemicryptophyte and criptophyte species and in the opposite direction of wind. Considering this fact, the sediments at the foot of Salvadora persica phanrophytic species in this study, along with the species, was more than Caparis decidua or Alhaji camelorum; this fidning confirmed a similarity in the results of these two studies. The results of Bobenzer (2020) in Egypt confirmed that the level of sand stabilization with a linear pattern has decreased, which is in line with the findings of this study. Valentini (2020) also stated that the changes in the use of Nebka habitat near the coast in the northern part are decreasing and in the southern part are increasing, so that the level of Nebka habitat is generally growing; these findings are not in line with the findings of this study. Also, the results of Hogan Holtz (2012) showed that the areas stabilized by quick sands, especially active nebulae, are in an increasing process and have deposited more sediment and quicksand than the last four decades; in fact, their findings contradicted the findings of this study.