Geomorphological controls on vegetation changes: a case study of alluvial fans in southwest of Miami City, Northeastern Iran

Geomorphological processes operating in depositional landforms, such as alluvial fans, play an important role in vegetation characteristics. Proper understanding of the processes, such as erosion, deposition, sediment entrainment, flooding, channel migration, and avulsion, would be helpful in management and conservation of vegetation on the alluvial fans. Therefore, this study was conducted to investigate the effect of processes and morphology of alluvial fans on vegetation type, density, and percent of canopy cover in three alluvial fans in southwest of Miami City (Northeastern Iran). Vegetation type was identified by field works, and large-scale aerial photographs (1:500) prepared by a DJI Phantom 4 Pro drone were used to determine vegetation density and percent of canopy cover. The results showed that vegetation density is higher on inactive surfaces compared to the active ones (52 and 10, respectively). The fans’ apexes had higher values of vegetation density and percent of canopy cover (34 and 34.2, respectively) compared to the fans’ toes (28 and 28.16, respectively). On the active surfaces, bars had higher values of vegetation density and percent of canopy cover (16 and 57.7, respectively), compared to swales (5 and 11.2, respectively), likely due to higher amounts of clay, silt, and phosphorus in the bars. On the inactive surfaces, channels had higher percent of canopy cover but lower vegetation density, which is due to the fact that channels, with relatively moist soils, host larger shrubby plant species with more canopy covers owing to their partly shaded conditions and their proximity to the groundwater. More fertile soils of interfluves, compared to the channels, have resulted in higher vegetation diversity and density. Overall, differences in morphology and processes operating on the fan surfaces control diversity of texture, hydraulic conductivity, moisture, and aeration of soils, and hence, vegetation type and density.