The role of alluvial fan geomorphology on the density and canopy of vegetation( case study: the southeast alluvial fan of the Shah Ghaib salt dome, Larestan, Fars).

Alluvial fans are important depositional landforms developed around salt diapirs in the Zagros mountains. During time, some surfaces with different ages such as relict, old and young surfaces can be developed on an alluvial fan. Geomorphological processes and forms of the alluvial fan surfaces have great variations. For example, on the relict and old surfaces, channels and interfluves can be developed, wheeas on the young surface, bars and swales are dominant landforms. Dominant Processes on the relict and old fan surfaces are incision on the channel, wheathering and soil development on the interfluves, wheareas aggradation is the main process on the young surface of a fan. Differences in geomorphological forms and processes in the mentioned surfaces of alluvial fans can cause spatial changes in the soil characteristics and vegetation on the surface of alluvial fans. The purpose of this research is to investigate the impact of landforms and processes on the density and canopy of vegetation on the surface of an alluvial fan in the southeast of Shah Ghaib salt diapir in Larestan, Fars province. The mentioned alluvial fan consists of three surfaces including relict, old and young.

In order to calculate the vegetation density and canopy and their relations to the geomorphology, different surfaces of the stdy area alluvial fan including relict, old, and young surfaces, were identified. Aerial images were obtained by a DJI Phantom 4 Pro drone in nine paths in the apexes, mid-fans and toes of the fan surfaces. Based on the arial images prepared by drone, the density and canopy of vegetation cover were measured in 72 plots. Als, the 10 cm precision DEM (Digital Elevation Model) were obtained for 9 sections in the apex, mid-fan and toe of the three surfaces of study area fan. To compare the means of vegetation density and canopy variables in channels and interfluves, and also in bars and swales, the t-test were calculated. The One-way AVOVA test was used to evaluate whether the vegetation density and canopy variables in the relicl, old and young surfaces, as well as in the apex, mid-fan and toes of fan surfaces have a significant difference or not. The linear relationship between the vegetation density and canopy variables also was obtained.

Topographic cross sections of the fan surfaces prepared from the 10 cm precision DEM show that relict and old surfaces especially relict one are crenulated with higher channel incision (wide channels and short interfluves in the relict surface, and v-shaped channels and long interfluves in the old surface), whereas young surface is fairly smooth with bars and wsales landforms. Data show that the mean vegetation density is higher in the young surface (13.9%) compared to the relic and old surfaces, whereas the mean vegetation canopy in higher in the relict surface (10.44% ) than the old and young surfaces. Results reveal that the mean vegetation density is highr in the apexs (11.67%) than the mid-fan (10.22%) and toes (10.19%). Nevertheless, the mean vegetation canopy is lower in the apex (5.16%) than the mid-fan and toe (10.64% and 10.19% respectively) of the fan surfaces. In the relict and old surfaces, the mean vegetation density and canopy are higher in the channels compared to the interfluves. In the young fan surface, the means of both vegetation density and canopy are higher in the bars compared to the swals. Results of t-test show that there is a statistically significant difference between the means of vegetation density in the bars and swals, and also in the channels and interfluves. Also, there is statistically significant difference between the means of vegetation canopy in the bars and swals, and also in the channels and interfluves. Overal, vegetation cover is considerably higher in the channels of the relict and old surfaces of the fan, compared to the interfluves. This situation can be attributed to the higher shadow and moisture in the channels compared to the interfluves. Higher vegetation cover of the bars compared to the swales in the young fan surface can be associated with relatively better stability of bars than swales.

Alluvial fans can have different surfaces including young, old, and relic ones. These surfaces with different ages and also different morphology and processes can result in the variotaions in the vegetation type, density and canopy. In this study, the vegetation density and canopy of the relict, old and young surfaces of an alluvial fan located in the southeast of Shah Ghaib salt diapir were studied. Evaluation of the morphology of alluvial fan shows that the relict surface with deep channels and crenulated morphology is reltively older that the old surface, and the old surface with v-shaped channels is older than the young surface (with fairly smooth surface). Results of this study show that the vegetation cover is higher in channels than the interfluves in the old and relict surfaces of the fan, and also higher in the bars than the swales in the young surface of the fan. Overall, it can be concluded that the morphology and processes of the fan surfaces with different ages strongly control the vegetation density and canopy.