2- Department of Biology, SR.C., Islamic Azad University, Tehran, Iran and Institute for Biology and Environmental Sciences, Carl von Ossietzky-University Oldenburg, Oldenburg, Germany
3- Department of Botany, Research Institute of Forests and Rangelands; Agricultural Research Education and Extension (AREEO), Tehran, Iran ,
4- Department of Forest Sciences and Engineering, Faculty of Natural Resources and Marine Sciences, Tarbiat Modares University, Nur, Mazandaran, Iran
5- Department of Biology, Fal.C., Islamic Azad University, Isfahan, Iran
Background and Objectives: Dry rangelands in Qom Province are characterized by water limitation, soil heterogeneity, and local variation. Despite the medicinal and food value of Gundelia tournefortii L., the roles of climatic aridity and soil chemical conditions in its biomass and species richness remain unclear. This study aimed to examine the relationship between aridity gradient and G. tournefortii biomass, compare biomass and species richness among soil chemical groups, and assess whether soil chemical groups modify the relationship between aridity and biomass.
Methods: The study was conducted across 10 dry-rangeland localities in Qom Province, Iran. A total of 300 plots (2 × 2 m) were surveyed, and soil characteristics were determined from 29 independent soil profiles. Flowering-branch dry weight of G. tournefortii and plot-level species richness were recorded. Soil chemical groups were derived by standardizing organic carbon, available phosphorus, and available potassium, followed by Euclidean-distance clustering using Ward.D2, resulting in three groups. The De Martonne aridity index was calculated from annual temperature and precipitation. Statistical analyses were performed in R version 4.6.0 using mixed-effects models.
Results: Mean flowering-branch dry weight was 18.06 g, while mean species richness was 5.13 species per plot. Soil chemical groups did not significantly affect dry weight (χ² = 0.43, P = 0.806) or species richness (χ² = 0.98, P = 0.614). The relationship between the De Martonne index and dry weight was non-significant (R² = 0.094, P = 0.388). The interaction between aridity and soil chemical groups was not significant (χ² = 0.15, P = 0.930).
Conclusion: Within the conditions examined, annual aridity and soil chemical groups, individually or interactively, did not sufficiently explain variation in biomass or species richness. These findings highlight the importance of local environmental conditions, topography, biotic processes, and temporal variation in water availability when assessing vegetation responses in dry rangelands.
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