Trend analysis of MODIS-NDVI Time series and its relationship with land use changes in Golestan province

Abstract

Introduction: Human activities and climate change directly affect the earth's surface cover. In the northern part of the country, land use changes have emerged as a significant factor contributing to the destruction of vegetation in Hyrcanian forests. Unfortunately, this destruction has persisted over the past few decades due to human activities. Golestan province, known for its diverse climate and surface cover, has experienced noticeable changes in vegetation, illustrating the impacts of these activities. Therefore, it is crucial to monitor the dynamics of vegetation in order to gain a better understanding of how vegetation respond to human pressures. This knowledge is essential for preserving the Hyrcanian forests.

Materials and methods: The investigation and monitoring of vegetation cover in Golestan province involved the use of the Normalized Difference Vegetation Index (NDVI). To analyze vegetation trends over a 20-year period, 16-day combined MODIS-NDVI time series data (MOD13Q1) with a spatial resolution of 250 meters, were utilized. The research began with the acquisition of raw NDVI images in HDF format from the NASA website. These 920 images, from the 16-day time series, were then analyzed statistically. To standardize the images and facilitate further analysis, they were converted into 460 images through mosaicing and scaling between -1 and +1. The evaluation of serial trends was performed in IDRISI TerrSet software, where the images underwent a preprocessing step to remove seasonal anomalies. The changes in vegetation activity and their significance were subsequently analyzed using the non-parametric Mann-Kendall method.

Results and discussion: The results indicate that 20% of the studied area experienced a decrease in vegetation, while 80% exhibited an increase. Out of these, 5% had a significant decrease, and 50% showed a significant increase in vegetation, while the remaining had no discernible trend. Over the past 20 years, a total of 4088 square kilometers of vegetation has been lost. To analyze the impact of human activities on these changes, location maps of cities and main routes were utilized. The findings revealed that the northern plains of Golestan exhibited a greater reduction in vegetation due to easier land access for human activities, changes in land use, and urban development. The cities of Kordkoy, Bandar Gaz, Aqqala, Gorgan, Azadshahr, and Ramian experienced the most significant increase in vegetation cover, whereas the cities of Bandar Turkman, Gonbadkavoos, Minoodasht, and Kalaleh witnessed the most significant decrease. The main routes leading to Golestan province, including Gorgan-Bujnoord, Gorgan-Sari, Azadshahr-Semnan, Incheh Brun West Road, and Incheh Brun East Road, all showed a decline in vegetation in their vicinity. Due to the significant climatic diversity in Golestan province, surface cover changes were examined at specific locations where a substantial reduction in vegetation cover was expected.

Conclusion: Based on the research conducted from 2001 to 2020, it can be concluded that the highlands of Golestan province, particularly the western highlands, have witnessed an increase in vegetation. This phenomenon is believed to be correlated with the rise in temperature resulting from global warming, which has created more favorable conditions for plant growth in these highland areas. In contrast, low-altitude regions such as plains, coastal areas, lower elevations, and especially areas surrounding cities and roads, have experienced a decline in vegetation cover. This decline can be attributed to human activities, including changes in land use and urbanization in the northern plains of Golestan. These changes signify the loss of extensive vegetation and the exacerbation of detrimental impacts from both human activities and natural factors. Although the reduction in vegetation in the highlands has been relatively slower compared to the lower regions, concerns arise regarding the destruction of vegetation in pristine forest areas. These developments highlight the urgent need for immediate attention and implementation of measures for sustainable management of natural resources and adaptation to climate change.