Quantification of the Effects of the Invasion of Mesquite (Prosopis Juliflora) on Native Vegetation in Southern Iran

One of the most important invasive plant species in Iran is the American mesquite tree (Prosopis juliflora) which has taken the control of a large area of ​​south and southeast Iran. The invasive effect of this species is profound such that it is the dominant species in some areas and the only tree species in most coastal ecosystems of southern Iran. Regardless of draining underground water as the only source of drinking water for local communities and destroying livestock feed, simplifying the ecosystem to only one tree species is a serious alarm of losing the ecosystem balance and causing serious economic and social effects. Considering the importance of the management of American mesquite trees in the south of Iran, this research investigated the spread of American mesquite trees using satellite image processing.

Among the 45 identified species of the genus Proposis, few of them, especially the American mesquite, are known as invasive species. This plant is known as the most threatening invasive plant in Europe. In the red list of the International Union for Environmental Protection, American mesquite is mentioned as one of the world’s 100 invasive species threatening. In this study, an area of ​​1850 km2 between the two cities of Sirik and old Jask was investigated. Pixel-based processing of Landsat 8 satellite images (OLI sensor) was used to extract the tree cover map of the area. The land units (areas covered with similar soil and vegetation characteristics) were produced using the object-oriented processing of Landsat 8 satellite images. Finally, the dominance of American mesquite compared to other trees was calculated in each unit and their canopy density changes were investigated and analyzed using the MODIS vegetation index in a 20-year period. 

According to the results, 13417 ha of the study area is covered by various trees. The American mesquite tree class was the second largest tree class (3991 ha, 29% of the total tree cover) with a heterogeneous distribution and is often located in areas with light and salty soils along the streams. Areas with a strong predominance of mesquite (more than 60%) accounted for 11% of the area which was mostly located in the central parts of the region. 

The results of the image classification showed that the American mesquite heavily occupied areas are mostly located along the main rivers and channels, extending from the eastern highlands towards the waters of the Strait of Hormuz. Thus, proximity to rivers and waterways can be considered as one of the most important environmental factors affecting the spread of these trees. In areas with a strong dominance of American mesquite trees, it was expected that the plant biomass will increase significantly as observed by their high NDVI values. However, the noteworthy point is the increasing rate of the average values ​​of the NDVI index during the last 20 years in the most dominant units, which is now increasing at a high rate with an average NDVI exceeding 0.5 since 2017. This indicates the increasing spread of the American mesquite in the region and even in the areas that have hosted this invasive species for decades.