Patterns of vegetation diversity and soil carbon storage along a transect from Al-Madinah Al-Munawwarah to Al-Rayis in the Hijaz Mountains, Saudi Arabia

Vegetation surveys along ecological transects are widely used to capture changes in species composition and environmental gradients across landscapes. This method is particularly valuable in arid regions, where vegetation patterns respond sensitively to soil and climatic variation. This study aims to assess the floristic composition, vegetation diversity, their environmental relationships, and soil carbon stocks along the Madinah-Rayis transect. The study covered 33 sites along the transect during the period from December 2023 to March 2023. Plant species and floral diversity were documented, and soil samples were collected and analyzed. Multivariate analysis using TWINSPAN, DECORANA, and CANACO programs was performed to determine plant communities and their relationship with environmental factors. For each community, Pearson correlation, diversity index (α,Υ), Simpson index, and Shannon index were calculated. This study also assessed the vertical distribution of soil organic carbon (SOC) content, soil bulk density (SBD), and SOC density in soil of all habitats along the transect, and carbon storage was estimated. The study region recorded 180 plant species across 112 genera from 33 different families. Five plant communities were characterized after application of TWINSPAN - VGI: Zygophyllum qatarense - Halocnemum strobilaceum , VGII: Cenchrus ciliaris - Dipterygium glaucum, VGIII: Vachellia flava - Zygophyllum simplex , VGIV: Lycium shawii - Aizoon canariense, and VGV: Aerva javanica -Aizoon canariense . Soil organic carbon storage varied significantly across habitats and vegetation groups. Abandoned farms exhibited the highest SOC content, likely due to organic inputs from litter and roots, while sand dunes had the lowest. Soil organic carbon (SOC) had shown a significant correlation with Simpson index (R 2 = 0.728, p ≤ 0.05). The findings of this study are crucial for preserving the plant community diversity within the transect ecosystem in Madinah.