Scale-dependent patterns and drivers of plant diversity in steppe grasslands of the Central Alborz Mts., Iran

Prepared by Amir Talebi, Farideh Attar, Alireza Naqinezhad, Iwona Dembicz & Jürgen Dengler

A thorn-cushion community in the alpine zone of Mt. Damavand at 3400 m a.s.l., dominated by Onobrychis cornuta. Photo credit by Amir Talebi.

More than half of the Iranian plateau is covered by mountains, of which the highest point reaches more than 5000 m a.s.l. in the Damavand peak, a dormant volcanic cone. Located in a biodiversity hotspot in the southwest Asia and one of the centres of endemism in Iran, the Central Alborz Mts. harbor a rich plant diversity and has been attractive for many botanists and ecologists. With almost 700 vascular species on the southern slopes, the area is among the richest in Iran. While fine-scale plant diversity patterns and their underlying drivers are generally poorly studied in Iran, the wide elevational range and diverse types of vegetation units made this region particularly suitable for studying this topic.

In this study, we applied a modified version of the multi-scale sampling methodology developed by the Eurasian Dry Grassland Group (EDGG). While most data collected with this methodology comes from the temperate zonobiome in Europe, we provide one of the first studies that applied it in an arid climate in West Asia. On the southern slopes of Mt. Damavand, in elevations between 1500 to 4800 m a.s.l., we distributed 23 series of nested plots, each with grain sizes from 0.0001 to 1000 m², and additionally 334 25-m² plots. We found a total of 437 vascular plant species and recorded soil properties, topography and management regimes.

Southern slopes of the Damavand summit in the central Alborz Mts. Photo credit by Amir Talebi.

Similar to other studies, the pattern of biodiversity was scale-dependent. Mean annual temperature (highly negatively correlated with elevational gradient) was the most important driver of species richness (total species richness and richness of life forms and functional groups) and in all scales except for the two smallest ones. The relationship between this driver and species richness was uni-modal in 25 and 100 m2. Mean annual temperature, along with precipitation of the driest month and mean soil depth, were also the important factors influencing beta diversity. While most of the factors were more important at larger grain sizes, precipitation of the driest month and gravel cover were influential drivers at the smallest grain sizes.

The findings of our study were partly in agreement with theoretical expectations and previous studies, partly not. For example, contrary to our expectations, precipitation of the driest month as a climatic driver was more influential at small scales (0.0001 and 0.001 m2), and some of the soil drivers (CaCO3, the proportion of sand and mean soil depth) were influential at large scales (1-1000 m2). These discrepancies, together with the generally relatively low explanatory power of our models, indicate that in the arid grasslands of Iran, to some extent, other principles apply than in the systems so far mainly studied with the multi-scale sampling approach. This calls for more sampling in other arid regions and the consideration of additional environmental variables.

This is a plain language summary for the paper of Talebi et al., 2021, published in the Journal of Vegetation Science (