Introduction and Objective: Herbaceous plant species diversity is an important component of forest community structure and is influenced by physiographic factors and environmental conditions. In the Zagros forests, elevational heterogeneity and differences in slope aspect can create contrasting conditions in terms of temperature, moisture, and solar radiation, thereby affecting the structure of the herbaceous layer. This study aimed to investigate variations in species richness, the Shannon–Wiener diversity index, and Pielou’s indices along an elevational gradient and across northern- and southern-facing slopes in the northern Zagros forests of Baneh County, Iran.
Materials and Methods: Sampling was conducted across four elevation classes, including 1100–1400, 1400–1700, 1700–2000, and 2000–2300 m a.s.l., on northern- and southern-facing slopes. A total of 81 sampling plots were established, and five 1-m² subplots were surveyed within each plot to record herbaceous plant species. Species richness, the Shannon–Wiener diversity index and Pielou evenness indices were calculated. One-way analysis of variance (ANOVA) followed by Tukey’s HSD test was used to compare elevation classes, while independent-samples t-tests were applied to compare the two slope aspects within each elevation class. A General Linear Model (GLM) was also employed to evaluate the effects of elevation, slope aspect, and their interaction. In addition, rarefaction curves were generated to compare species richness and assess sampling adequacy.
Results: The mean species richness increased from 18.94 species in the 1100–1400 m elevation class to 29.11 species in the 2000–2300 m class, and the differences among elevation classes were statistically significant (F = 6.68, P < 0.01). Similarly, the mean Shannon–Wiener index increased from 1.33 at the lowest elevation class to 1.90 at 2000–2300 m, with a significant effect of elevation class on this index (F = 3.82, P < 0.05). In contrast, Pielou’s evenness did not show a consistent trend along the elevation gradient; however, differences among elevation classes were statistically significant (F = 4.56, P < 0.01). Furthermore, all three biodiversity indices—species richness, Shannon–Wiener index, and Pielou’s evenness—were significantly higher on north-facing slopes than on south-facing slopes (P = 0.032, P = 0.001, and P = 0.004, respectively). The results of the generalized linear model (GLM) analysis showed that elevation above sea level and slope aspect had significant effects on the biodiversity indices. In addition, the interaction effect between elevation and slope aspect (elevation × aspect) was significant only for Pielou’s evenness index.
Conclusion: The findings demonstrated that both elevation and slope aspect influence variations in the diversity and structure of herbaceous plant communities in the northern Zagros forests; however, the responses of individual diversity indices were not consistent. Northern-facing slopes exhibited higher species richness across most of the elevational gradient, whereas community evenness showed a stronger response to the interaction between elevation and slope aspect. Therefore, simultaneous consideration of elevation and slope aspect is necessary for a more accurate understanding of herbaceous plant diversity patterns in the northern Zagros forests. These patterns highlight the importance of topographic conditions in shaping plant species diversity in the region.