Qualitative Study of Vegetation Resilience in Response to Long-Term Precipitation Changes (Case Study: Part of the Namakzar-e khaf Watershed Basin, South Khorasan Province)

Vegetation is one of the most important ecosystem components that is constantly being adapted or altered to the final balance under the influence of environmental factors. One of the most important signs of the health of the vegetation and the consequent ecosystem health is its ability to resilience after the removal of environmental turbulences. Although vegetation in arid and semi-arid areas has different methods for dealing with destructive forces, the continuity of these factors can disrupt the ability of vegetation to return to its original state after eliminating the destructive factors. The result will be the disease and, ultimately, the destruction of the ecosystem. Therefore, detection, prediction of ecosystem changes over time and management of such changes is of great importance. This research was carried out in the Namakzare Khaf watershed basin in South Khorasan Province with the aim of studying and qualitative determining of ecosystem resilience vegetation to severe changes in long-term precipitation and drought as environmental turbulences. For this purpose, the standardization of annual rainfall during the thirty-year period and the determination of the duration and severity of dry, wet and normal periods was performed with SPI method and then the ecological resilience of the vegetation was examined qualitatively based on the mean plot and Transformed Normalized Difference Vegetation Index (TNDVI) variance. These changes were studied using landsat satellite images in the final growth season of vegetation. Results indicate the mean ability to resilience and restore vegetation in the study area after the elimination of severe to medium droughts. According to the results, the ecosystem health of the studied area was confirmed on the basis of vegetation resilience, despite being located in dry land and facing severe droughts with continuity of two years and continuation of rainfall reduction compared to the mean for 6 years.
Extended Abstract              
1-Introduction
Understanding the concept of ecosystem health, the indicators involved in it, and the interpretation of effective interactions in environmental sustainability are crucial for the proper management of renewable natural resources. Vegetation is one of the most important ecosystem components that is constantly being adapted or altered to the final balance under the influence of environmental factors. One of the most important signs of the health of the vegetation and the consequent ecosystem health is its ability to resilience after the removal of environmental turbulences. Although the vegetation of arid and semi-arid areas has different ways to deal with destructive forces, the continuity of these factors can disrupt the ability of vegetation to return to its original state after eliminating the destructive factors. The result will be the disease and, ultimately, the destruction of the ecosystem. Therefore, detection, prediction of ecosystem changes over time and management of such changes is of great importance. Using seasonal and annual time series, vegetation indices such as NDVI and multilateral analyzes can determine the decline, increase or stability in vegetation in relation to environmental changes and human impacts.
2-Materials and Methods
This research was carried out in Namakzar-e khaf watershed in the northeast of South Khorasan Province with the aim of studying the qualitative of ecosystem reversibility and the response of natural vegetation to severe changes in long rainfall and drought as perturbation. For this purpose, firstly, using rainfall data of 15 meteorological stations around the study area, annual rainfall was extracted during thirty years (1986- 2015). Then, the data were interpolated in software ArcGIS for the study area using Inverse Distance Weighted method (IDW) being determined by standard precipitation index (SPI) dry, wet and normal years. From the Landsat 5 & 7 satellite imagery archives in the thirty years of study period, an image was created for each year between June 15 and July 15, with permanent coverage at the best of growth. Following the necessary corrections for satellite images, Normalized Difference Vegetation Index (NDVI) and Transformed Normalized Difference Vegetation Index (TNDVI) of each image were prepared and the ecological regression of vegetation was studied based on the plot of the mean-TNDVI variance.
3- Results and Discussion
The results reveal the resilience of vegetation in the study area after the elimination of severe to medium droughts. According to the results of this study, the ecosystem health of the study area was confirmed based on the vegetative resilience, despite being in dry climates and exposing severe droughts within two years of duration and reducing rainfall compared to the average for 6 consecutive years. The results of the study confirmed the belief that dry ecosystems, despite apparent brittleness, have a high potential for environmental degradation and specific methods for adaptation and confrontation with turbulence. The results are consistent with the performance of the mean-variance graphical method for vegetation regeneration with the findings of Washington Allen et al. (2008) and Suu et al. (2013). It worths noting that the study was conducted in a climate that was unprecedented in previous studies. Because the study of Sue et al. (2013) has been conducted in a region of a Savannah ecosystem in South Africa, which is more climate-friendly than the present study area. Rainfall conditions in the study area of ​​Washington Allen et al, despite the introduction of the climate of the region as a dry climate, are different from the rainfall conditions of this study. The average annual precipitation in the Washington Allen study was 402 mm, and vegetation classes are composed from grass species to native plots. Therefore, the results of previous studies cannot be generalized to dry climatic conditions in most of the continental regions of Asia. Therefore, the present study is carried out for the first time in Asia and the continent of Iran in a completely different climate with more sensitivity to the type and density of vegetation and annual average precipitation is a unique example and a model for demonstrating the success of the method in these areas.
4- Conclusion
The sustainability of the vegetation of the studied area against severe rainfall changes is so obvious that it challenges the notion of desertification in such areas. But it is likely that rainfall reduction of the 60-mm limit (the minimum annual rainfall in the study area during the thirty years of period) and its continuity in a few years would disrupt the vegetative resilience and cause desertification of the area and eventually the deterioration of the ecosystem. This study also confirmed the effect of the plot-mean-variance plot method for qualitative study of vegetation regeneration in a dry region in Central Asia (Iran) for the first time. In addition, by proving the health of the ecosystem, it can be planned for proper management and enhancement of indigenous vegetation, which has shown good resistance to climate turbulence.