Research paperEcological implications of Cousinia Cass. (Asteraceae) persistence through the last two glacial–interglacial cycles in the continental Middle East for the Irano-Turanian flora
Highlights
► Cousinia is used as a system model to understand the Irano-Turanian floral history. ► Changes in pollen of Cousinia are explored in a long pollen record of NW Iran. ► We find that Cousinia was more frequent during glacial than interglacial periods. ► Dampened glaciations in SW Asia reduced the rate of extinction of Cousinia spp. ► Further, long-lasting heterogeneous topography favored allopatric speciation.
Introduction
The Irano-Turanian area is one of the richest floristic regions of the world. It is located at the contact zone of the Mediterranean (W), Saharo-Sindian (S), and Euro-Siberian (N) floristic regions (Fig. 1) and is characterized by high species diversity and endemism, particularly in several large taxonomic groups including the genera Astragalus L. (Fabaceae), Cousinia Cass. (Compositae), Acantholimon Boiss. (Plumbaginaceae), and the families Apiaceae, Brassicaceae, and Chenopodiaceae (e.g. Eig, 1931, Davis et al., 1965–1988, Guest and Al-Rawi, 1966, Zohary, 1973, Couvreur et al., 2010). The evolution and persistence of floristic richness in the face of repeated glacial/interglacial climatic cycles is of interest to biogeography and conservation. What role did glacial/interglacial cycles play in amplifying or dampening species richness in the region? Or is the modern floristic richness a legacy of Tertiary speciation events?
Responses of vegetation to the Quaternary glacial–interglacial cycles in the Irano-Turanian realm have been previously described in a number of studies (van Zeist and Bottema, 1977, Bottema, 1986, Wick et al., 2003, Djamali et al., 2008a). However, none specifically examined floristic changes driven by the climatic cycles. We review late Pleistocene–Holocene pollen records from the Irano-Turanian part of the Middle East, and use a recently published long pollen record from Lake Urmia (NW Iran; Djamali et al., 2008a) to examine the responses of the Irano-Turanian flora to climatic changes during the Quaternary period. Lake Urmia is situated in one of the richest sub-regions of the Irano-Turanian phytogeographical region (IT2 according to White and Léonard, 1991 and Irano-Anatolian according to Zohary, 1973; Fig. 1).
One problem with this approach is that most of the dominant upland Irano-Turanian steppe plants (e.g., Astragalus, Acantholimon, Acanthophyllum Hook & Arn., Cousinia Cass., Heliotropium L., Zygophyllaceae, etc.) and many steppe-forest trees (Amygdalus L., Prunus L., Cerasus Mill., and Pyrus L.) are entomophilous and hence are severely under-represented in the pollen rain (e.g. van Zeist et al., 1970, Moore and Stevenson, 1982, Djamali et al., 2008b). These genera are only sparsely recorded in pollen diagrams. The genus Cousinia is, however, more frequently encountered in pollen diagrams of the Irano-Turanian region (e.g. van Zeist and Bottema, 1977, Wick et al., 2003, Djamali et al., 2008a). Further, the modern geographical range of Cousinia correlates almost perfectly with the Irano-Turanian floristic region suggesting it as a good representative of the Irano-Turanian flora (Figs. 1 and 2). Chenopodiaceae comprise another characteristic Irano-Turanian group that is well-represented in pollen diagrams. However, this pollen may belong to either littoral lowland or upland species (Djamali et al., 2008c), whereas Cousinia spp. most commonly grow in the upland vegetation. This is confirmed by the modern distribution of most Cousinia species, which are concentrated in the mountainous areas of Turkey, Iran, Afghanistan, and Central Asia (Knapp, 1987). This suggests that the variation in the pollen curves of Cousinia can be used as a proxy to understand how the Irano-Turanian upland flora in general may have responded to the Quaternary glacial–interglacial cycles. Although some studies on the pollen morphology of Cousinia exist, (Shtepa, 1976, Saber et al., 2009), current knowledge is insufficient to investigate past changes in species diversity of the genus.
This palaeoecological approach can be complemented by phylogenetic investigations to better reveal plant population changes driven by past climatic changes (e.g. Hu et al., 2008). Recent analyses of the phylogeny and systematics of the Arctium–Cousinia complex (López-Vinyallonga et al., 2009, López-Vinyallonga et al., 2011) have revealed that within this complex only Cousinia subg. Cousinia should be treated as a newly defined genus Cousinia. All pollen considered here belongs to the Cousinia-type characteristic of this lineage. These findings, combined with palaeoecological data, allow us to evaluate the factors driving the diversification of one of the most speciose taxa of the Irano-Turanian flora.
Using Cousinia as a representative taxon, this study aims at (i) reconstructing the variation in distribution and abundance of Cousinia in the Irano-Turanian landscapes of NW Iran during the two most recent glacial–interglacial cycles, (ii) analyzing the relationships between modern species diversity and distribution patterns in Cousinia and the topography of SW Asia, (iii) studying the likely role of the long-term regional tectonic history in shaping this distribution pattern, and (iv) inferring the effects of Quaternary glaciations and pre-Quaternary palaeoclimates in SW Asia on the evolution of the Irano-Turanian flora.
Section snippets
Material and methods
To investigate the relationship between the distribution of Cousinia species and the topography of the Middle East and Central Asia, we analyzed the topographical heterogeneity of each of the four classes of species diversity (i.e. areas with equal number of species) of Cousinia as proposed by Knapp (1987). Fig. 2A shows the geographical distribution of these diversity areas, and Fig. 2B shows a topographical transect along with the specific diversity of Cousinia. In Fig. 3, the spatial
Results
The results of our topographical analysis show a direct relationship between the heterogeneity of topography and Cousinia species diversity (Fig. 3). The highest diversity is observed in regions with the widest range of topographical variations and the highest frequency of elevations above 2000 m (Fig. 3C and D). This is demonstrated spatially in the topographical cross-section of Fig. 2 showing that the major SW Asian mountain ranges including the Zagros Mts (W Iran), Alborz Mts (N Iran), Kopeh
Persistence of Cousinia in glacial landscapes of the Irano-Turanian region
The Cousinia-type pollen recorded in NW Iranian and E Anatolian regions (see pictures in van Zeist and Bottema, 1977) most probably was produced exclusively by species of Cousinia s.str. as defined above (see also Shtepa, 1976, Saber et al., 2009). Cousinia is both insect- and self-pollinated (Funk et al., 2009, López-Vinyallonga et al., 2009) and is therefore under-represented in the modern pollen rain. Although Cousinia is one of the dominant constituents of the Irano-Turanian flora today,
Conclusion
Our current knowledge of the diversification and palaeoecology of the Irano-Turanian flora is very scanty and the hypotheses proposed in this study remain to be tested in future studies. However, long-term variation in Cousinia pollen records from the Irano-Turanian region help to understand the general response of the flora of similar ecology and geographical distribution (e.g. Astragalus, Acantholimon, Acanthophyllum) of this region to Quaternary glaciations. The available palaeoecological
Acknowledgments
This study was supported by funds from the University of Wyoming and the National Science Foundation. Discussions with Professor Hossein Akhani (University of Tehran), Drs. Bruno Fady (INRA Avignon-IMBE) and Jérémy Miglore (IMBE) were valuable in the improvement of paper. We are very grateful to Prof. Suzanne Leroy and two anonymous reviewers for their thoughtful and constructive comments on this manuscript.
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