تعیین کمّی سلامت پوشش گیاهی بر اساس قابلیت بازگشت‌پذیری

The correct management in natural ecosystems is not possible without knowledge of the health in its sectors. Vegetation is the most significant sector in ecosystem that has important role in its health. Resilience is one of the defining features of health vegetation The term resilience was first introduced in the study of ecological systems and demonstrates the ability of the ecosystem to maintain its performance in the face of environmental disorders. A resilience-based system is not only equipped with a disorder adjustment mechanism but also has the potential to benefit from changes in a way that lead to creating an opportunity for development, innovation, and updating. Therefore, when a change occurs, the resilience provides the needed conditions for restarting and reorganization. If this goes beyond disturbing forces, the system will have the power to return to the maximum vegetation density with the least erosion effects, otherwise the system will be vulnerable to the change that was created and could already be controlled. This research was done in part of North east of South Khorasan province (arid climate) with the aim of quantifying vegetative resilience on behalf of ecosystem health in response to drought occurrences and long-term precipitation changes, as environmental disturbances. Therefore first, using daily precipitation data from 15 meteorological stations around the study area, their annual precipitation was extracted and was standardized by Standard Precipitation Index (SPI) over the course of thirty years (1986 - 2015). Then, the SPI index data in 15 stations were interpolated by ArcGIS software based on Inverse Distance Weighted (IDW) method and dry, wet and normal years was estimated in the study region for each year. On the other hand, from archive of satellite images of Landsat 5 and Landsat 7, an image was created for each year in study period, between 15 June and 15 July, with permanent coverage at the best of growth. Following the necessary corrections for satellite images, the average Transformed Normalized Difference Vegetation Index (TNDVI) was obtained of each image by ENVI software. Finally, effected of precipitation changes on mean TNDVI was assessed and vegetation resilience was stabilized whit selected of sever time period samples based on four effective parameters (Amplitude, Malleability, Damping and Hysteresis). Comparison of annual precipitation variations in the thirty-year time series (1986 -2015) indicated two approximate wet and dry periods in study area. The wet period is related to the first fourteen years of the time series (1986-1999) and the dry period is related to the last sixteen years (2000-2015). In this term, severe precipitation incidents with different intensities were occurred in the study area including one case of very intense precipitation (1986), one case of intense precipitation (1991) and two cases of moderate precipitation (1996 and 1992). Also, four drought incidents were occurred including one case of intense drought (2001) and three cases of moderate drought (1987, 2006 and 2008). All precipitations (wet years) are related to the first half and most droughts are related to the second half of the studied period. In this study for fixing of vegetation resilience in study area and for calculating of its parameters, In addition to the thirty-year time series selected sever time sections. in the whole study series (1986 - 2015), maximum of mean TNDVI (49.37 %) was in 1986 (reference), the lowest mean TNDVI (43.58%) was in 2010, The year effect of the decrease precipitation and drought, and mean TNDVI in 2015 was 44.28 %. Amount of parameters amplitude, malleability and damping are respectively 5.79, 0.7 and 5.09, and hysteresis was zero (%). The result of this case showed that the vegetation has moved towards the reference state (Resilience) but has not reached to amount of reference vegetation. The most specific cases for vegetation resilience happened from 1986 to 1996 (wet period) and 2003- 2009 time sector (dry period). In the first time section amount of amplitude and malleability were 0.64 %, damping was zero and hysteresis was 0.25%. The result of this case showed that not only the vegetation was returned to the reference state but also was increased to the reference (Cross reference).So despite the reduced rainfall and occurrence of sever occurrences of drought in dry period, hysteresis parameter (0.05 %) observed in 2003- 2009 time sector too that confirmed clearly vegetation health in study area whit dry climate. Awareness of the health status of the vegetation and its response to long-term precipitation changes and environmental disorders, such as drought occurrence, ensure the success of the managerial plans for renewable natural resources. The present study is the second study on quantifying the vegetation resilience and the first study under dry climatic conditions in Asia (an average annual precipitation of 160 mm) conducted in Iran by calculating four factors related to resilience, and is the first study that has presented the factor hysteresis in the calculations. Despite continuous of difficult condition, the native vegetation of the study area has been able to return the reference state not only by resolving the disorder relatively, but also it has experienced hysteresis stage. A set of quantitative calculations showed despite reduced annual precipitation and drought events, vegetation has been able to maintain its resilience, which indicates the health of the vegetation in the studied ecosystem. With the presence of such amazing protective and consistent mechanisms in the vegetation of arid regions, it is possible to maintain and restore these regions by proper managerial plans.