Determination of Ecological Thresholds in Saline Habitats onWestern Shore of Lake Urmia, Iran

In temporal and spatial patterns, the critical (ecological) thresholds have rapid changes in ecological gradient. Determination of thresholds along the environmental gradients can help to control destructive factors and therefore, the success in restoration projects can be guaranteed. This study was conducted to evaluate the trend of changes in functional and structural indices along soil salinity gradient and to determine the ecological thresholds in saline habitats of Uremia Lake, Iran, in the growing season of 2019 and 2020. Landscape Function Analyses (LFA) guideline was used to evaluate the sites. The values of the indices were fitted with the pattern diagrams (S-shaped curves). The habitat restoration process after five years of range management and development projects was evaluated and ecological thresholds were determined. The results showed that the values of the functional and structural indices decrease along the salinity gradient and the lowest values were observed at the end points of the gradient. As a result, the success of range development projects was lower at salinity center. The results of the S-shaped curve models showed that the regression relationship between landscape organization index and nutrient cycle with eleven soil indices along the salinity gradient was not significant and their determinant coefficients were less than 50%. On the other hand, there was a strong relationship between the index of stability and permeability with the salinity gradient, with determinant coefficients of 83 and 63%, respectively so that the values of these two indices decreased significantly when approaching the salinity center. Regarding the PCA analysis, the first component justified 39% of the changes in the properties, and the values of Sand, Mg, and Ca increased as they approached the salinity center, and on the other hand, as they moved away from the salinity center, the values of Clay, N, OM, CEC, p, and Silt increased. The participation of the most soil parameters in the extraction of the first component of the decomposition of the physical and chemical characteristics of the soil along the salinity gradient and the separation of the sites from each other indicate the good ability of the LFA method in showing the ecological processes of nature. In this regard, to fit the data by the S-shaped curve, the distance of 1000 m far from the salinity hotspot was defined as an ecological threshold, through which sudden changes occur. At these thresholds, the functional differences of the habitat were more than the surrounding areas, so for controlling the dust centers, different projects such as drainage, windbreaks or planting dense vegetation cover are suggested to stabilize the thresholds. In such a case, the soil seed bank in the distance between the two thresholds could be regenerated and prevent the soil surface from eroding if grazing is prevented. Identifying thresholds as a transition zone and surrounding zones as state zones in this study will be a scientific and appropriate alternative to the conventional guidelines in executive organizations and will help them to zoning ecosystems according to nature and selection and prioritization of restoration sites in saline rangelands.