Climatic determinism in phytogeographic regionalization: A test from the Irano-Turanian region, SW and Central Asia

Abstract

This study investigates the role of climate in determining phytogeographic regions, focusing particularly on the Irano-Turanian floristic region in SW and Central Asia. A set of simple climatic variables and bioclimatic indices were used to prepare climate-space scatter plots and climate diagrams. The climate data were also subjected to multivariate analyses (PCA and Regression tree) in order to develop a bioclimatic characterization of the Irano-Turanian region in comparison with the adjacent Mediterranean, Saharo-Sindian, Euro-Siberian, and Central-Asiatic regions. Phytogeographic regions of SW and Central Asia display distinct bioclimatic spaces with small overlaps. The Irano-Turanian region is differentiated from surrounding regions by continentality, winter temperature, and precipitation seasonality. Continentality is the most important bioclimatic factor in differentiating it from the Mediterranean and Saharo-Sindian regions and is responsible for floristic differences among sub-regions of the Irano-Turanian region. In our case study, the Irano-Turanian region is a nearly independent bioclimatic unit, distinct from its surrounding regions. Hence, it is suggested that the term “Irano-Turanian bioclimate” be used to describe the climate of most of the continental Middle East and Central Asia. Among different sub-regions, the west-central part of this floristic region (“IT2 sub-region”) is a major center of speciation and endemism. Our case study demonstrates that climate is a primary determinant of phytogeographic regionalization. Although modern climate and topography are strong control parameters on the floristic composition and geographical delimitation of the Irano-Turanian region, the complex paleogeographic and paleoclimatic history of SW Asia has also influenced the Tertiary and Quaternary evolution of the Irano-Turanian flora, with additional impacts by the long-lasting historic and present land-use in this region. Many Irano-Turanian montane species are threatened by global warming, and particular conservation measures are needed to protect the Irano-Turanian flora in all sub-regions.

Introduction

Plant geographers have been aware that specific regions of the world are characterized by distinct ensembles of plant species, genera, and families since the early 19th Century (Willdenow, 1811), and have identified specific floristic regions based on these patterns (de Candolle, 1820, Takhtajan, 1986). Plant geographers have also long recognized that geographic variations in climate exert a strong influence on plant distributions (Good, 1964, Humboldt and Bonpland, 1807), and that climate plays a role in determining the placement of floristic regions (Daget, 1977). However, although many studies have tried to compare climatic or bioclimatic boundaries to the modern vegetation (e.g., Lauer and Rafiqpoor, 2002), few floristic regions have been the subject of systematic climate analysis to identify the critical climate variables that characterize them and to assess the relative roles of climate, physiography, and isolation in determining their boundaries (e.g. Daget, 1977, Djamali et al., 2011, Roumieux et al., 2010, Walter and Lieth, 1960–1967, Walter, 1974, Walter, 1976). Such studies can be useful both in understanding the origins of phytogeographic regionalization and in developing geographically grounded strategies for biodiversity conservation. In this paper, we present results of a climate analysis of the species- and endemic-rich Irano-Turanian floristic region of SW Asia (Rechinger, 1963–2010, Takhtajan, 1986).

Eig (1931) and later Zohary (1950; 1973, p. 87) gave a historic background of the development of the concept of the Irano-Turanian phytogeographic region. This region was first identified by de Candolle (1820) as the “oriental region”, defined as “containing the countries bordering upon the Black and Caspian Seas”. It was also described by Boissier (1867) in Flora Orientalis as the oriental region (“Région Orientale proprement dite”) and by Grisebach (1884) as the Steppic region (“Steppenregion”). Eig (1931) suggested replacing these terms with “Irano-Turanian” to describe the commonly accepted geographic extent of this territory which covers the mainland of Persia (present-day Iran) and Turan.

As early as the mid-19th century, Boissier (1867) remarked on the rich and unique flora of the Irano-Turanian Central Asia highlands of Iran, Turkey, and Afghanistan, describing it as the richest flora of the “Near” East. The floristic traits of the Irano-Turanian region meet those proposed by Takhtajan (1986) for the definition of phytochorion rank of “region”: “high amounts of species and generic endemism… and a definite series of families that occupy a predominant position in the region, the quantitative correlations between these dominant families being relatively stable”. Discussions of the Irano-Turanian flora and vegetation (e.g. Boissier, 1867, Davis et al., 1965, Eig, 1931, Guest and Al-Rawi, 1966, Zohary, 1973), identify the following features: (i) scarcity of forest vegetation, (ii) almost total absence of Pinaceae, forming an approximately 2500-km gap between the Mediterranean and Caucasian regions in the west and the Himalayan foothills in the east, (iii) dominance of chamaephytes (mainly dwarf shrubs) and hemicryptophytes (mainly Poaceae), forming steppe vegetation, (iv) development of several specific taxonomic groups, including the “giant” genera Astragalus spp. (Fabaceae), Cousinia (Asteraceae), Acantholimon (Plumbaginaceae), Centaurea (Asteraceae), Allium (Alliaceae), Heliotropium (Boraginaceae), and Salvia (Lamiaceae), (v) high species richness (>12,000 species estimated in Boissier's Flora Orientalis; 9977 species have been reported in Flora Iranica which covers Iran, Afghanistan and parts of N Iraq and E Pakistan (Akhani, 2006)), (vi) high endemism (exceeding 25%), (vii) high diversity and complex specialization of Chenopodiaceae, especially in desertic and sub-desertic areas mainly in the Turanian region (e.g. presence of chenopod trees such as Haloxylon ammodendron and many C4 chenopods): Akhani et al. (1997), and (viii) very high “species irradiation” (i.e., the penetration of its floristic elements into neighboring regions). This last characteristic led Zohary (1973) to name the Irano-Turanian region as a “donor” of species to the “recipient” neighboring regions of the Saharo-Sindian and Mediterranean. Many infiltrations of the Irano-Turanian elements have been reported in the floristic analyses of the Saharo-Sindian and Mediterranean regions (e.g. Al-Nafie, 2008, Davis et al., 1965, Eig, 1931, Guest and Al-Rawi, 1966, Léonard, 1993, Zohary, 1973). In southern Iran, such infiltrations are so important that Akhani (2007) proposes the extension of the Irano-Turanian region further to the south. Here a broad overlapping with species from the Sahara-Sindian realm is observed, e.g. date-palms are found up to Torud, Eastern Iran (Breckle, 2004). According to the delineation of White and Léonard (1991), the Irano-Turanian floristic region comprises some 6,730,000 km².

In spite of extensive floristic investigations in different parts of the Irano-Turanian region, the causal factors of its establishment and maintenance remain obscure. These factors include climate and the physiographic setting of the region, which probably interacted to influence floristic development during the geological history of SW Asia. This paper is aimed at the following questions: (1) Does the Irano-Turanian region stand out uniquely in climate phase-space compared to surrounding floristic regions? If so, what unique combinations of ecologically relevant climate variables distinguish the Irano-Turanian from surrounding regions? (2) Can the Irano-Turanian bioclimatic and phytogeographic regions be used interchangeably? (3) Are there synoptic-scale climate patterns that dictate the position of the Irano-Turanian region? (4) What are the relative roles of climate and topography in accounting for the Irano-Turanian region? (5) Are the boundaries of the Irano-Turanian region stable through space and time, or have they shifted in response to global climatic oscillations of the Quaternary period? (6) What lessons can be drawn for conservation of Irano-Turanian biodiversity in a changing world, especially concerning the strong land-use change?