Areas of endemism (AoEs) are considered the basic units for historical biogeographic studies as well as for identifying priority areas for biodiversity conservation (Fjeldså, 1993; Myers et al., 2000; Crisci et al., 2003; Lamoreux et al., 2006; Gomes-da-Silva et al., 2017). Endemism is the state of a taxon that is restricted to a particular geographical location (Anderson, 1994). AoEs harbor endemic taxa and are indicative of the evolutionary history of flora in each geographical area (Morrone, 2008). They are identified by two or taxa more co-occurring in the same area and represent a common biogeographic history and paleo-ecological events (Szumik & Goloboff, 2004; Morrone, 2008). Therefore, recognizing and delimiting the areas of endemism has always been an important and interesting topic for biogeographers and conservation biologists (Axelius, 1991; Laffan & Crisp, 2003; Jetz et al., 2004; Feng et al., 2016). To date, different methods for identifying areas of endemism have been described, including parsimony analysis of endemism (PAE) using grids (Morrone, 1994), cladistic analysis of distributions and endemism (CADE) (Porzecanski & Cracraft, 2005), endemicity analysis (EA) (Szumik et al., 2002), analysis of biotic elements (BE) (Hausdorf & Hennig, 2003), categorical analysis of neo- and paleo-endemism (CANAPE) (Mishler et al., 2014), and analysis of geographical interpolation endemism (GIE) (Oliveira et al., 2015). Previously, these approaches have been used as influential methods for mapping the centers of endemism (CoEs) and areas of endemism (AoEs) of many taxa worldwide.

The study area is Iran, one of the major biodiversity hotspots in the Middle East. The Iranian ecosystems are home to 8000 to 8112 species of plants, of which 2597 are endemic (Noroozi et al., 2019). The most important reasons for Iran’s high plant diversity are geographical isolation, climatological and edaphic differences throughout the territory, and vegetation history (Hedge & Wendelbo, 1978; Frey & Probst, 1986). Iran is known as one of the centers of endemism and speciation of many plant genera such as Astragalus L. (Fabaceae), Allium L. (Amaryllidaceae), Cousinia Cass. (Asteraceae), Acantholimon Boiss. (Plumbaginaceae), and Onosma L. (Boraginaceae) in the world (Maassoumi, 1993, 1998, 2000, 2003, 2005; Assadi, 2006; Fritsch & Abbasi, 2013; Khajoei Nasab & Mehrabian, 2022). The genus Astragalus (Fabaceae) is the biggest among flowering plant genera (Frodin, 2004). There are about 2748 species in 155 distinct sections in the Old World. Most of the endemic species in Southwest Asia are restricted to Iran with 885 species and 589 endemic species, Afghanistan with 319 species and 144 endemic species, and Turkey with 435 species and 257 endemic species. In total, in these three countries there are about 1459 taxa with 1021 endemics, which means the highest rate of endemism and is the main center of speciation in the Old World. Previous studies have investigated the geographic distribution patterns and species richness of Astragalus in Iran (Maassoumi, 1993; Mahmoodi et al., 2009, 2012; Bagheri et al., 2019; Maassoumi & Ashouri, 2022), but unfortunately, no comprehensive studies have been performed to identify areas of endemism using multiple GIS-based approaches. Hence, AoEs of this genus in Iran are not well known and they need to be accurately assessed using modern techniques. The present study aims to identify spatial endemism patterns of Astragalus in Iran using GIS-based mapping. Therefore, the main objectives of this study are: (1) identifying the hotspots of species richness of Astragalus species endemic to Iran; (2) estimating the number of Astragalus endemics per phytogeographical province and province of the country; and (3) delimiting AoEs in the study area using PAE and GIE approaches.

This is one of the first detailed studies to identify the areas of endemism and hotspots of the endemic richness of Astragalus in Iran. According to our results, Esfahan province had the highest numbers of endemic Astragalus species in the country. This province with 201 species belonging to 42 Astragalus sections is one of the important centers of diversity and speciation of this genus in Iran (Mahmoodi et al., 2012; Akhavan Roofigar et al., 2019). Due to its geographic location, the presence of important mountain ranges such as the Zagros Mountains and Karkas Mountains, and the various predominant climatic zones, the number of endemic species is highest. The province of Esfahan is also located between the Kurdo-Zagrosian and Central Iran provinces, so the combination of these factors makes it a suitable place for the richness of Astragalus endemics. In addition, Atropatanean had the highest endemic richness among the phytogeographical provinces of Iran. Atropatanean is an important center of Iran’s plant diversity and endemism (Assadi, 2006; Noroozi et al., 2018; Khajoei Nasab & Khosravi, 2020; Khajoei Nasab & Mehrabian, 2022). Most parts of the area cover northwest Iran at the intersection of the Alborz, Zagros, and the Caucasus Mountains. The collision of these mountains and the presence of two different climatic zones (Dajmali et al., 2011) have created habitats suitable for many endemic Astragalus species, especially local and narrow endemic species. Previous phytogeographical studies by Mahmoodi et al. (2009, 2012) identified the province as one of the important centers of endemism of this genus in Iran.

On the other hand, the three hotspots of species richness were identified by the circular neighborhood method. The first hotspot was in the Alborz ecosystems (in the central Alborz region). The Alborz Mountains have formed a heterogeneous macroecosystem for the diversity and endemicity of many plant groups such as Astragalus, Acantholimon, Onosma, etc. (Hedge & Wendelbo, 1978; Noroozi et al., 2008, 2018; Khajoei Nasab & Khosravi, 2020). The second hotspot was located at the confluence of the Alborz and Zagros Mountain ranges and the Azerbaijan plateau. The existence of a cold and mountainous climate along with different soil compositions and specific topography has provided the conditions for the high endemic species richness of this genus in this region. The third hotspot was in the central Zagros, a place that has become a harbor for many Iranian animal and plant species such as the endemic Astragalus due to its relatively good annual rainfall, diverse geological structure, and high edaphic and climatic diversity (Ghorbani, 2013). Our results show that Astragalus endemic richness does not follow a latitudinal diversity gradient (LDG) and the richness does not decrease from low to higher latitudes in the provinces of Iran. The latitudinal diversity gradient (LDG) is one of the most recognized patterns in macroecology and biogeography, and based on that, species richness increases from poles to the mid-latitudes in the tropics (Hillebrand, 2004; Mittelbach et al., 2007). But this principle does not apply to the plant genus we are studying, nor to certain groups of plants in the flora of North America (Weiser et al., 2018). Latitude patterns of species richness are not the same for all taxa and are related to ecological variables such as climate factors and solar energy (Willig & Presley, 2018).

Using GIE, seven AoEs were identified in Iran based on spatial data of the endemic Astragalus species. AoE1 was in the Khorassan-Kopet Dagh floristic province in the northeast of Iran. This area well corresponds to the distribution pattern of Khorassan endemism of the montane/alpine regions of the Irano-Turanian region (Hedge & Wendelbo, 1978), N. Khorassan (Assadi, 2006) as well as the endemic patterns of Acantholimon (Khajoei Nasab & Khosravi, 2020). Khorassan-Kopet Dagh is in a transitional position between the different phytogeographical geographical units of the Irano-Turanian and Euro-Siberian regions, which makes it a major area of endemism and diversity of plant species in Iran. AoE6 was located in the Hezar and Lalezar mountains in the Kerman province. These high mountains of the Yazd–Kerman massif are parts of the Irano-Turanian region, which include an ecotone zone on its southern border to the Saharo-Sindian region and provide habitats suitable for many endemic species of many plant genera such as Astragalus, Echinops L., Cousinia, Acantholimon, Nepeta L., etc. (Zohary, 1973; Léonard, 1981; Doostmohammadi et al., 2018; Noroozi et al., 2018; Khajoei Nasab & Khosravi, 2020; Khajoei Nasab et al., 2022). Finally, Mount Taftan in southeastern Iran is home to several endemic species of Astragalus. This mountain has provided a suitable place for the growth of mountain species endemic to southern Iran. Therefore, it could be concluded that mountain ranges play an important role in the diversity and endemism of different plant species in Iran.

According to the results of the PAE analysis, 87% of the total area of can be considered as the areas of endemism (AoEs) of the genus Astragalus. This suggests that in addition to highland ecosystems, the plains and lowlands are also important areas of endemism of Astragalus in Iran, so its AoEs are not limited to the highlands. A recent study of the distribution patterns of Astragalus confirms the existence of high species richness of some sections (i.e. Leucocercis, Bucerates, Eremophysa, Ammodendron) in the plains, salt marshes, and lowlands of Iran (Mahmoodi et al., 2012). Large parts of the four AoEs detected in this study were in the prominent mountains of Iran, which is consistent with previous studies in Iran (Hedge & Wendelbo, 1978; Assadi, 2006; Mahmoodi et al., 2012; Mehrabian, 2015; Bagueri & Maassoumi, 2019; Noroozi et al., 2019; Khajoei Nasab & Khosravi, 2020; Mehrabian et al., 2021). The northern and western mountains of Iran, i.e. Alborz and Zagros, are among the Irano-Anatolian biodiversity hotspots and are known to be the 20th global hotspot area (Myers et al., 2000; Mittermeier et al., 2005).

Endemism patterns of Astragalus in Iran were analyzed by identifying hotspots of richness and AoEs. We found that identifying endemism patterns using multiple approaches such as PAE, GIE, and species richness could outperform one approach to identify centers with conservation priority. The results of the three approaches were in good agreement with each other. However, it seems that PAE has been able to operate much more successfully than the other two approaches and cover more the areas of endemism of this genus in Iran. Thus, this study shows that the pattern of endemism of plant taxa in Iran does not follow the same pattern, and that the center of speciation and endemism of all plant groups does not occur only in the mountains. Mountains play an important role in endemism, but plains and rangelands are also home to many endemic species, so they should be prioritized. As these areas are more at risk of extinction due to destructive human activities such as overgrazing, fires, and droughts, as well as climate change, it is important to determine a strategy to protect most of the areas identified in this study.