Scale-Dependent Changes in Plant Species Richness and Diversity Indicators: A Case Study in the Plain Rangelands of Northern Iran (Khomam, Gilan)

Vegetation indicators such as abundance, diversity, and richness are susceptible to variations in sampling scale, a factor often overlooked in ecological and plant sociology theories. This study, conducted in the northern plain rangelands of Iran (Khomam, Guilan), introduces concentric plots for the first time in the country to minimize the impact of environmental changes on diversity and abundance measurements. The research aims to analyze changes in vegetation diversity and richness concerning alterations in the sampling scale.

Vegetation sampling was conducted during the spring of 1400 at two sites on the northern slope of the Alborz mountain range. Six sampling scales (plots) ranging from 0.5 to 10 square meters were employed, with each scale centered on the same initial point. A total of 720 samples were collected, with statistical analyses, including mean comparisons and Tukey's test, performed using the R software.

Results indicate a significant effect of sampling scale on diversity and richness indices (p < 0.001). Notably, the scale of 0.5 m2 exhibited the most substantial impact on both diversity and richness, compared to 5 m2 and 10 m2 scales. Scales of 3 m2 and 4 m2 showed no significant differences in variation. The accuracy of variable calculation improved with an increase in scale, reaching a constant level at 2 m2 and 3 m2, followed by a subsequent increase at 5 m2 and 10 m2 scales. The 0.5 m2 scale significantly underestimated measured indicators, yielding inaccurate results.

For accurate sampling and calculation of diversity and species richness indicators in the plains of northern Iran (specifically, grass cover and thorny species), the use of a 3-square meter plot/scale is recommended. Apart from delivering precise results, this scale offers practical advantages in terms of ease of use (carrying and deployment) and the relative uniformity and homogeneity of vegetation within it. However, caution is advised when applying this scale to other areas, especially slopes and higher altitude regions, where environmental gradients may significantly impact plant community composition and characteristics.