Folia Geobotanica

, Volume 48, Issue 1, pp 7–22

Species Diversity and Life-Form Patterns in Steppe Vegetation along a 3000 m Altitudinal Gradient in the Alborz Mountains, Iran

Authors

  • Parastoo Mahdavi
    • Department of Plant Sciences, School of BiologyUniversity of Tehran
    • Department of Plant Sciences, School of BiologyUniversity of Tehran
  • Eddy Van der Maarel
    • Community and Conservation Ecology GroupUniversity of Groningen
Article

DOI: 10.1007/s12224-012-9133-1

Cite this article as:
Mahdavi, P., Akhani, H. & Van der Maarel, E. Folia Geobot (2013) 48: 7. doi:10.1007/s12224-012-9133-1

Abstract

Biodiversity pattern and life-form spectra were studied along a 3,000 m altitudinal gradient from a semi-desert area to the alpine peak of Tochal Mountain. The gradient is located on the southern slopes of Central Alborz with a Mediterranean continental climate. DCA ordination was applied to 1,069 relevés and 7 quantitative variables to discover the relation of diversity and altitude. A biodiversity pattern was obtained by relating values for species richness and Shannon-Wiener’s index to 100-m altitudinal sections. Altitude was determined as the major ecological gradient. Both diversity indices are negatively correlated with altitude and show a decreasing trend beyond a peak in species richness at 1,800–1,900 m a.s.l. towards a very low diversity in the high alpine zone. The biodiversity peak does not match with the potential tree line in the area (2,500–3,000 m a.s.l.). The high diversity in foothills can be related to habitat heterogeneity, longer suitable climatic conditions, and diverse disturbance factors, while unfavorable conditions at high-altitude alpine and low-altitude desert areas reduce the number of species at both extremes. Life-form patterns clearly change along altitudinal gradient. Annuals with decreasing trend, and hemicryptophytes and chamaephytes with increasing trend along the altitudinal gradient are notable patterns of life form in the area. Temperature, soil moisture and nutrients are the main factors that explain the ecological influence of altitude on species diversity and life-form patterns in the semi-arid steppe vegetation of the area.

Keywords

Altitudinal gradientBiodiversity patternCentral AlborzDCA ordinationLife formSteppe vegetation

Plant nomenclature Rechinger (1963–2010)

Introduction

Biological changes along altitudinal gradients are very pronounced because of the large environmental variation within a small geographical area (Körner 2000; Lomolino 2001). Many studies have been published on the effect of altitude on the biodiversity of different taxonomical groups, including birds (Kaboli et al. 2006), mammals (McCain 2005), insects (Sanders 2002), lichens (Grytnes et al. 2006), bryophytes (Grau et al. 2007) and vascular plants. Altitudinal studies on vascular plants have been devoted to vegetation distribution and composition (Wang et al. 2006; Gould et al. 2006), endemism (Kessler 2000; Vetaas and Grytnes 2002), life-form spectra (Pavón et al. 2000; Klimeš 2003), and also patterns of plant species richness (e.g. Wang et al. 2002; Grytnes and Beaman 2006).

There are two main patterns of species richness in relation to increasing altitude: i) a monotonic decrease, and ii) a unimodal relation with peak of richness somewhere in the middle of the gradient. A monotonic increase is also found in a few studies (e.g., Gutiérrez et al. 1998). Rahbek (1995), in his literature review, showed that in about half of the studies species richness peaked at mid-altitude (e.g., Kessler 2000; Grytnes and Vetaas 2002; Grau et al. 2007) while some studies reported a monotonic decrease along altitude (Ohlemüller and Wilson 2000).

Biodiversity is an important concept, especially with regard to climate and land use. Species diversity plays a vital role in the persistence of ecosystem processes and services in relation to severe land-use pressure and changing environmental conditions (Hooper and Vitousek 1997). There is increasing evidence of the impact of plant diversity on ecosystem function (Díaz and Cabido 2001). Notably, reduction in biological diversity will cause a reduction in ecosystem-level processes (Zobel et al. 2006).

Plant life forms are functional types that have been used to describe plant adaptation to certain ecological conditions (Odland 2009). Plants with a similar life form are assumed to have a similar effect on the dominant ecosystem processes (Pausas and Austin 2001). Therefore these morpho-ecological types can be used to indicate particular climate properties, biogeographic regions, major biomes of the world (e.g., Raunkiǽr 1934) and other environmental differences especially in regions with a seasonal climate (Klimeš 2003). Variation in life forms along altitudinal gradients has been used for a better interpretation of vegetation and species richness patterns in relation to environmental gradients.

Most studies on biodiversity patterns have been conducted on tropical, subtropical and temperate mountains with a clear tree limit but data on montane steppe vegetation especially in semi-arid regions is lacking. Because several ecological and biogeographical factors affect richness patterns, studying different climatic regions would be helpful to better understand global plant biodiversity patterns.

The Alborz Mountains are the highest mountain range in the Iran Plateau. This high-altitude mountain stretches about 650 km from west to east and is interrupted in many places by deep valleys and rock cliffs. The summit of Mt. Damavand at 5,670 m a.s.l. is the highest point. This complex system has been formed during a pre-Cambrian orogeny and the Mesozoic-Tertiary alpine orogeny (Stöklin 1974). The humid conditions on the northern slopes of the Alborz Mountains result in a Hyrcanian deciduous closed forest along the southern shores of the Caspian Sea. The climate of this area is mesic and warm, with rainy summers and mild winters (Akhani et al. 2010). The southern slopes are influenced by the drier climates of the central parts dominated by Irano-Turanian floristic elements. Because of the habitat heterogeneity and the climatic gradient, a wide range of steppe, montane and alpine vegetation types have developed on the southern slope.

Floristic and vegetation studies on the Alborz Mountains date back to Kotschy (1861), Buhse (1899) and Gilli (1939). Modern studies include Zohary (1973), Frey and Probst (1986) and Naqinezhad et al. (2009). A recent phytosociological study was done by Klein (2001) in high altitude zones of central Alborz. However, the vegetation and biodiversity patterns of the Alborz Mountains have been insufficiently investigated.

The present paper concentrates on the species diversity pattern and life-form spectrum along the altitudinal gradient in the steppe vegetation of central Alborz. The study focuses on the dominant (semi-) natural steppe vegetation of the area and therefore hygrophytes are excluded. We aim to i) analyze the correlation of species richness and diversity with altitude, ii) define the plant biodiversity pattern on the southern slopes of central Alborz, iii) study the distribution of plant life forms along the altitudinal gradient and iv) interpret the effect of ecological factors on the observed patterns.

Methods

Study Area

The study area is situated on the southern slopes of central Alborz in Tehran province (35°20–55′ N, 51°15–26′ E). We focus on vegetation sampling along an altitudinal transect from south of Tehran (1,000 m a.s.l.) to the alpine peak of Tochal (Tuchal) mountain (3,966 m a.s.l.) in N Tehran (Fig. 1). The Tehran area is an alluvial plain with a gentle slope to the south. In the northern part, the altitude suddenly increases from 1,400 m a.s.l. in the city to 1,800 m a.s.l. at the foothills and from there it goes up to 3,966 m a.s.l. in less than 10 km from the city center. Geologically the rocks consist of volcanic and sedimentary layers from the Eocene with basaltic and andesitic elements, dacite lava, shale and tuffs that are cut in some places by calcite and silicate layers. In higher parts of the mountain, soils are largely derived from Eocene volcanic tuff. Towards the south, Eocene volcanic rocks and tuffs decrease and are replaced by alluvial deposits. Generally, the area is characterized by an alkaline soil. Furthermore, salinity increases to the south because of the dry weather, higher evaporation and gentler slope leading to an accumulation of salt in low-altitude plains to the South of Tehran (Emami et al. 1993)
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Fig. 1

Location of the investigation area in Iran (above) and N Tehran (below, left) showing the altitudinal transect. Topographic cross section of the study area specifies the main vegetation zone and their altitudinal range (below, right). Gaps in the cross section show urban areas along the transect

According to the available data, the area has a semi-arid climate with conspicuous differences between day and night temperatures characteristic of a Mediterranean continental climate (more information in Djamali et al. 2011). Precipitation mainly occurs in late autumn, winter and early spring. A large amount of snow falls during winter and snow may persist up to mid-summer in the alpine zone. The summer is arid, hot and sunny with intensive radiation most of the time. The available climate data of the meteorological stations at 1,000, 1,191, 1,548 and 2,465 m a.s.l. (Table 1) show an increasing trend for mean annual precipitation (from 152 mm to 525 mm) and decreasing trend for mean annual temperature (from 16.6 °C to 8.4 °C) with increasing altitude. Cool and wet weather is predictable for the alpine and sub-alpine zones of Tochal Mountain, while the semi-arid area of south Tehran has hot and dry weather.
Table 1

Comparison of meteorological data derived from the nearest stations at different altitude along the transect, with its climate type, showing mean annual temperature and precipitation, mean January and July temperature. MDC – Mediterranean Desert Continental, MXC – Mediterranean Xeric Continental, MPC – Mediterranean Pluviseasonal Continental. The bioclimate types follow Djamali et al. (2011). The first three stations are close to our transect but the Abali station is located ca. 50 km E of the area

Station

Altitude (m a.s.l.)

Bioclimate type

Mean annual temperature (°C)

Mean annual precipitation (mm)

January temp. (°C)

July temp. (°C)

Varamin

1,000

MDC

16.6

151.7

4

29

Tehran (Mehrabad)

1,191

MXC

17

230

4

30

N Tehran (Tajrish)

1,548

MPC

15.4

422

2

27

Abali

2,465

MPC

8.4

525

-4

20

Geomorphologically the area can be classified into three parts (Fig. 1):
  1. 1)

    Arad Mountain (35°20–30′ N, 51°15–20′ E) is located ca. 20 km south of Tehran, near Hasan-abad (1,000–1,428 m a.s.l). This area is characterized by semi-desert steppes consisting of xerophytic communities, mostly dominated by Artemisia sieberi.

     
  2. 2)

    Pardisan Nature Park (35°44–45′ N, 51°20–24′ E) is located in the northwest of Tehran (1,300–1,450 m a.s.l.). This is an alluvial undulating gentle slope consisting mainly of steppe vegetation (including Astragalus microcephalus and Stipa hohenackeriana) and riverside vegetation along the Farahzad valley. Water availability plays an important role in the vegetation physiognomy of this area. This type of vegetation has been largely destroyed outside the Park. The Farahzad valley has recently been devastated after the field work of this study was completed.

     
  3. 3)

    Tochal Mountain (35°49–55′ N, 51°22–26′ E) in N Tehran with an altitudinal range of 1,800–3,966 m a.s.l. This is a steep mountain interrupted in many places by deep valleys and rock cliffs. The average slope is 25°–30° with a maximum of 90° in the vertical rocks. The area’s vegetation is diverse as a result of extensive heterogeneity in topography, disturbance, slope direction, slope steepness and substrate.

     

Studied Vegetation

Hemicryptophytes are the dominant life form in the area’s steppe vegetation. There is no tree line along the gradient but some scattered trees and shrubs occur, including Amygdalus lycioides, Cotoneaster nummularius, Cerasus microcarpa, Crataegus spp. and Berberis integerrima; they can be seen in rocky places and moist slopes, while some hygrophilous trees (e.g., Salix acmophylla) occur along the water streams in valleys.

In total, 732 species of vascular plants (excluding hygrophytes) have been recorded in the study area. Most species belong to Irano-Turanian elements. The physiognomy of the area is characterized by steppe vegetation dominated mostly by dwarf shrubs, thorn cushions, herbs and rarely shrubby elements that are adapted to different habitats.

A synopsis of plant communities and their altitudinal range are discussed in Akhani et al. (2013). A brief summary of vegetation types is presented here:
  1. 1)

    Semi-desert steppe (950–1,400 m a.s.l.), located at the lower altitudes in the semi-arid area of south Tehran with a Mediterranean xeric continental bioclimate. This zone is mostly characterized by an Artemisia sieberi steppe, mixed with xerophytic elements including Caroxylon orientalis, Salsola kerneri, S. annua (=Anabasis setifera), Pteropyrum aucheri and Amygdalus eburnea.

     
  2. 2)

    Stipa grassland [1,400–1,800(–2,000) m a.s.l], including Stipa hohenackeriana and Astragalus microcephalus communities on the alluvial hills of Pardisan and Tochal foothills.

     
  3. 3)

    Montane steppe and grassland (1,900–2,900 m a.s.l.), covering large parts of Tochal Mountain with diverse habitat types and microhabitats on finer soils in the lower parts of this zone and on rocky and gravely substrates at higher altitudes. This zone’s physiognomy is characterized both by tall grasses such as Stipa arabica, Festuca sclerophylla, Psathyrostachys fragilis, Dactylis glomerata, Elymus spp. and Melica persica, and tall herbaceous species such as Prangos uloptera, Hypericum scabrum, Ferula spp., Eryngium billardieri and Verbascum cheiranthifolium. The cushion-forming species Acantholimon spp., Acanthophyllum spp., Astragalus spp., Dianthus orientalis, Aethionema grandiflorum and Asperula glomerata occur in several communities of this zone.

     
  4. 4)

    Thorny cushion formations [(2,700)2,900–3,500] m a.s.l.], dominate the subalpine zone especially on windswept slopes. Large parts of this area are covered by Onobrychis cornuta communities associated with Acantholimon demavendicum, Astragalus spp., Tanacetum polycephalum, Minuartia lineata and Silene commelinifolia, which are all adapted to cool and windy conditions.

     
  5. 5)

    Alpine steppes and meadows [(3,200)3,500–3,980 m a.s.l.], usually consisting of small hemicryptophyte or compact cushion-forming species in moist depressions or on gravely outcrops. Cousinia multiloba, Polygonum serpyllaceum, Ranunculus crymophilus, Catabrosa parviflora, Jurinella frigida, Trifolium radicosum, Rumex elbursensis, Vicia ciceroidea and Arenaria insignis are some of the common species in this zone. Noroozi et al. (2010) provides a phytosociological analysis of this zone.

     

Data Collection

The vegetation was sampled phytosociologically between 2000 and 2006 in the spring and summer according to Braun-Blanquet (1964). From the lower steppes at 1,000 m a.s.l. to the Peak of Tochal Mountain at 3,966 m a.s.l, 1,069 relevés were recorded. The hygrophyte communities were excluded from this study because our study’s focus is on steppe vegetation. Also, the distribution of hygrophytes does not change very much with altitude. Mostly 25-m2 relevés were placed in homogeneous and (semi-) natural steppe vegetation. Relevés were distributed almost evenly along the altitudinal gradient, except in urban and disturbed areas and high vertical cliffs. In each relevé total cover, cover-abundance and height of the occurring vascular plant species were recorded. In addition, environmental data (latitude, longitude, altitude, aspect, slope angle, bedrock, topographical situation), and anthropogenic activities were recorded. A commercial GPS (Garmin 12 channel) is used to record altitude and geographical position of each relevé. Vascular plants were collected and identified in the laboratory. Plant nomenclature mainly follows Flora Iranica (Rechinger 1963–2010) but also recent taxonomic literature is used to update some species names. Voucher specimens and the original relevés are preserved in the School of Biology, University of Tehran (Hb. Akhani).

Two indices were chosen to estimate diversity: (1) species richness (SR), i.e., the total number of plant species recorded in a sample plot, and (2) Shannon-Wiener’s Index of diversity \( \left( {H\prime = - \sum\limits_{i = 1}^s { {p_i}\;ln\;{p_i}} } \right) \), where S is the number of species and pi is the proportion of the individual species cover relative to the total cover. This index takes into account the amount of equitability of the species distribution. SR and H' represent the α-diversity level of the vegetation in relation to the size of the relevé, i.e., 25 m2.

Plant life forms were defined according to Raunkiǽr’s classification (1934), based on the position of renewing buds in relation to the soil surface (phanerophytes, chamaephytes, hemicryptophytes, geophytes, annuals). Two additional growth forms, perennial grasses and thorny cushion species, which are common and important in the mountainous area of Iran and show adaptations to the specific climatic conditions of the semi-arid area were also included. A list of species and their life forms are provided in Table S1 in Electronic Supplementary Material.

Data Analysis

Field data were incorporated in a local TURBOVEG data base (Hennekens and Schaminée 2001) and analyzed using ordination methods from the PC-ORD program, version 4 (McCune and Mefford 1999).

To analyze the correlation between vegetation and major environmental factors, a DCA (Detrended Correspondence Analysis) was conducted (Fig. 2). Two matrices were used: (1) 1069 relevés × 732 species and (2) 1069 relevés × 6 quantitative variables (altitude, slope, aspect, total cover, species richness and Shannon-Wiener’s index). The default settings of the program were used. Rare species were downweighted and the scatter plot was applied to show DCA result and then second matrix overlaid with main matrix in the graph.
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Fig. 2

Distribution of main vegetation types in DCA ordination and their correlation with external variables (altitude and diversity indices). Eigenvalue for the first axis is 0.90. 1 – Semi-desert steppe, 2 – Stipa grassland, 3 – Montane steppe and grassland, 4 – Thorny cushion formations, 5 – Alpine steppes and meadows

The 3,000-m altitudinal gradient was divided into 100-m intervals to find out how diversity will change along altitude. The diversity indices were calculated for each relevé while mean values of relevés were applied to the altitudinal belts (Fig. 3).
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Fig. 3

Decreasing trend of species richness a and Shannon-Wiener diversity index b along 100-m altitudinal intervals. The vertical lines indicate standard deviation (s.d.) for each altitudinal zone. The gap in the diagram at altitudes 1,500–1,800 m a.s.l. is related to the urbanization in this zone

The biodiversity pattern was obtained by plotting species richness for all relevés against altitude (Fig. 4). An equal number of relevés (25) was used for each altitudinal zone. The relevés were selected randomly in zones with more than 25 relevés available. The continuity of transect is interrupted between 1,500 and 1,800 m a.s.l. because of urban expansion. This resulted in a gap in the diagrams of species diversity. The small gaps at some altitudes in Fig. 4 especially around 2,300 m a.s.l. are mostly the result of human activities (mainly road construction) and also of vertical rock cliffs in some parts. The pattern of coverage has also been calculated along the altitude by using mean value for intervals (Fig. 5).
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Fig. 4

Predicting pattern of species richness in the whole study area from low to high altitude using a polynomial regression trend

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Fig. 5

Variation of total cover along the altitude with mean values for each altitudinal zone. Standard deviation is shown for each interval

The life-form pattern was based on the total number of species in each 100-m altitudinal interval (with 25 relevés), recorded over a width of ca. 500 m and assigned to one of the five main plant life-form categories. Subsequently, the percentage of life forms in each interval was determined and the related diagrams were drawn (Fig. 6). Additionally the percentages of two growth forms (thorny cushions and perennial grasses) have also been calculated.
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Fig. 6

Life-form pattern along the altitudinal gradient. Percentage share in the total species composition of a annuals; b hemicryptophytes; c chamaephytes; d perennial grasses; e thorny cushion species; f geophytes

Results

DCA Ordination and Species Diversity

The DCA ordination clearly shows a wide distribution of relevés along the main axis with an eigenvalue of 0.90 (Fig. 2). Altitude is determined as the major ecological gradient of the whole study area. The main five community groups are shown in the DCA graph. The classification was supported by TWINSPAN and cluster analysis (not shown, see Akhani et al. 2013), and inspection of the vegetation physiognomy in the field. The species richness and diversity index of the studied zones indicate that the plant communities in Artemisia semi-desert steppe show a moderate diversity with an average species richness of 22.5 and an average Shannon-Wiener diversity index of 2.8. Both values increase by altitude in the Stipa grassland and montane steppe and grassland zones with an average of 28.3 species and a Shannon-Wiener diversity index of 3 in the former and an average of 29.3 and a diversity index of 3.2 in the latter. The thorny cushion zone shows a moderate species richness of 23.5 and Shannon-Wiener diversity index of 2.7. Finally there is a clear decreasing of both values in the alpine zone with an average species richness of 10.5 and a Shannon-Wiener diversity index of 1.7.

The DCA graph (Fig. 2) also indicates that altitude is significantly negatively correlated with plant species diversity. The most diverse groups are concentrated in the foothills of the montane and steppe zone whereas the alpine zone has the lowest species diversity. Slope, aspect and total cover did not show a significant trend in the DCA analysis.

Biodiversity Patterns

Both species richness and the Shannon-Wiener diversity index show a decreasing trend with increasing altitude from about 1,800–1,900 m a.s.l. in the Tochal Mountain, but an increasing trend from the lowest altitudes covered by this study up to intermediate altitude (Fig. 3a,b). Species diversity peaks at 1,800–1,900 m a.s.l. with an average value of 39 for species richness (SD = 6) and 3.5 for the Shannon-Wiener diversity index (SD = 0.2). In the highest zone between 3,900 and 4,000 m a.s.l. the average species richness was only 5 (SD = 2.3) and the Shannon-Wiener diversity index was 1.2 (SD = 0.5). The lowest altitude range (1,000–1,400 m a.s.l.) shows a moderate diversity (average SR = 24.9). Furthermore, the data from Mount Arad, from 1,000 to 1,400 m a.s.l., show a decreasing trend, similar to Tochal Mountain but with a smaller range (Fig. 3a,b). Unfortunately, the lack of data from 1,500–1,800 m a.s.l. prevents us from presenting a more definitive conclusion about the nature of the relation between diversity and altitude. The incomplete data nevertheless suggest a unimodal relation even with decreasing trend in Mount Tochal. This is confirmed by Fig. 4, showing the individual values in relation to altitude according to a polynomial regression.

Figure 5 shows how the total cover changes with altitude. Although there is not a specific trend, there is a decreasing trend from 1,900 to 2,700 m a.s.l. and also from 3,000 to 3,500 m a.s.l. Percentage of total cover is obviously less than 45 % for most altitudinal zones up to 2,800 m a.s.l. but above this altitude the cover value is more than 45 %. There was not a real relation between species richness and total cover but the interaction of these two parameters is interesting where at high altitude there is a high coverage but low species richness and vice versa in low altitude.

Life-Form Patterns

The percentage of species belonging to each life-form category relative to the total number of species in each altitudinal interval is presented in Fig. 6. Annuals with 28 % on overall average show a sharp decreasing trend with altitude reaching from 65 % at 1,000 m a.s.l. to the minimum of 6 % at 3,900 m a.s.l. (Fig. 6a). Hemicryptophytes (43 %) are the dominant life form in the entire transect and together with chamaephytes with an average of 19 % show an increasing trend with altitude (Fig. 6b,c). Perennial grasses also show an increasing trend towards high altitude with a clear peak between 2,200–3,000 m a.s.l. and then above 3,800 m a.s.l. (Fig. 6d). Thorny-cushion species have slightly increasing trend with an obvious peak between 3,200 and 3,700 m a.s.l. (Fig. 6e). The geophytes (9 %) are almost constant through the altitude range of 2,400–3,400 m a.s.l. but decrease below and above this range (Fig. 6f). There are very few phanerophytes (0.7 %) in the study area; they occur between 1,300–1,500 m and 2,000–2,300 m a.s.l. and are mostly concentrated in the valleys or occur as scattered shrubs on the foothills and rocky outcrops (not shown). Above 2,500 m a.s.l. no phanerophytes are found.

Discussion

Altitude is a complex environmental gradient associated with variation in several ecological factors (Odland 2009). However, it seems that severe environmental conditions simplify this complexity to some extent and reduce the number of factors that control species diversity. This study’s results indicate that altitude is the main factor influencing both biodiversity and vegetation structure. The results presented in this paper suggest a unimodal relation of species diversity against altitude (Fig. 4) with high richness at the foothills of the montane steppe zone (1,800–1,900 m a.s.l.), and a very low diversity in the high-altitude alpine zone (Fig. 3a,b).

An important ecological feature of this study area is the lack of a tree line. There are only a few scattered shrubs in rocky outcrops and thus there is no tree zone in the Tochal Mountain and on the southern slopes of Central Alborz. Long-term land-use and anthropogenic impacts are probably important reasons for the disappearance of shrubs and trees in the area (Noroozi et al. 2008). This is likely because of the proximity of human settlements to the Tochal Mountains. The foothills of Tochal Mountains with rich water resources provided suitable areas for agriculture and human settlements before Tehran was designated as the Capital of Persia at the end of the 18th century. In the last two centuries the rapid increase of the population size in Tehran Province (now ca. 13 million) causes much vegetation disturbance, particularly the removal of phanerophytes in the area. The remnants of the previous vegetation can be seen for example in Dizin, 3 km east of the area where Juniperus excelsa stands can be found (Akhani et al. 2013). The potential timberline in the area lies between ca. 2,500–3,000 m a.s.l. The peak of diversity in the area (1,800–1,900 m a.s.l.) does not match with the potential tree zone. The absence of a tree line in the area may be the reason for the absence of a peak above the potential tree zone. The tree zone has an important effect on species diversity where the ecotone effect may enhance species richness (Lomolino 2001; Grytnes 2003a). However, the montane steppe vegetation of the study area changes gradually with altitude without a change in physiognomy of the vegetation. Consequently there is no significant transition zone and the peak in species richness does not occur at the tree line as in most studies with unimodal pattern (Rahbek 1995).

The relatively high coverage above 2,800 m a.s.l. (Fig. 5) can be related to the availability of soil moisture provided by snow. The ground is more homogenous in higher than the lower altitudes. In the montane zone there are many rocky outcrops and the slopes are steeper (up to 2,800 m a.s.l.) in comparison to the alpine zone with its more flat areas. The steepness acts as a barrier for growth of plants and prevents them to become dominant in a given area. Therefore despite of potential of the area to have variety of species (high richness), these species do not occupy all the available space which results in low total cover. Plant species in the alpine zone favour the damp flat areas and cover the ground (more than 45 %) but with less diversity with the harsh alpine conditions reducing the number of species to six. Soil analysis was not included in this investigation but soil nutrition and structure definitely play an important role in vegetation distribution. The well-developed fine soils at low mountain altitudes resulted in high species richness.

The low diversity in both extremes can be related to the severe environmental conditions at either end of the gradient that limit plant growth and productivity. This is in agreement with the “Hard boundary hypothesis” (Grytnes and Vetaas 2002; Grytnes 2003b). The upward shift to the alpine zone is generally characterized by a decrease in temperature, growing season, water holding capacity, surface area, nutrient availability, and increase in snow cover and solar radiation (Körner 2007). However, high temperature, little precipitation, high evaporation, and high salinity are the main environmental constraints that plants face in the arid area. These ecophysiological stress factors (especially temperature) explain the low species richness at both high and low mountain altitudes.

Area is another factor that may affect the biodiversity pattern in different ways, in particular through habitat heterogeneity and resource availability (Romdal and Grytnes 2007). Generally it is expected that in a larger area there is higher habitat heterogeneity and a better chance for survival of larger populations of the species. Local species extinction events can threaten the persistence of species growing in small areas. However, this is a complex issue possibly related to the scale of study and the type of ecosystems. For instance, in species-rich limestone grasslands Van der Maarel and Sykes (1993) found very high small-scale species richness combined with a very high species turnover. Usually the surface area of the mountain will decrease with increasing altitude and consequently a linear decrease in species diversity is expected.

Disturbance is an important factor that has an impact on all plant communities in the study area. Road construction, grazing, plantations and trampling are common disturbance factors influencing the biodiversity here. These disturbances result in soil erosion along steep slopes and the occurrence of many ruderals and ephemerals, in particular at low altitudes, where the anthropogenic activities are more intense. Although disturbance may initially enhance species richness this is a temporary effect. As a result of long-term disturbance, perennial species of the original vegetation will disappear causing a decrease in biodiversity. Therefore, disturbance in semi-arid, open habitats can not only reduce biodiversity, but also act as a threat for vegetation, especially for endangered and/or endemic species. As it is reported by Noroozi et al. (2008), about 58 % of the alpine flora of Iran is endemic or subendemic. This is an important aspect that should be considered in conservation management of arid and semi-arid mountains regions.

Because our transect is interrupted between 1,500 and 1,800 m a.s.l. (because of urbanization) and because there are no comparable data from below 1,000 m, we carried out a polynomial regression of all plots presented in Fig. 4 to predict the trend for the desert zone. Based on the given trend, a unimodal pattern was suggested for the relation between diversity and altitude. This could be further supported when the altitudinal gradient is extended to the poorly vegetated desert area (e.g., the coast of the salt lake in Central Kavir, 800 m a.s.l.). Our unpublished relevés outside the area supports such a decreasing trend.

Life Forms

Life form can be considered a plant strategy in response to the hydrothermic gradient (Wang et al. 2002). As is supported by the available meteorological data (Table 1) precipitation shows an increasing trend with altitude while temperature shows a decreasing trend. The short-lived life form of annuals is an advantage in arid and semi-arid zones. In our area most annuals occur from 1,000–1,300 m a.s.l., while their number decreases towards higher altitudes and drastically drops in the subnival zone (Fig. 6a). The Artemisia semi-desert steppes are rich in annual species during spring. Many of these species are ephemerals that complete their life cycle just before June. Also the predominance of annuals at low altitudes partly can be explained by soil disturbance, which makes a suitable place for ruderals at this zone. The limited growing season and early frost favor perennial occurrence. Hemicryptophytes and chamaephytes, with an increasing trend towards higher altitudes, are the dominant life form in the alpine zone (Fig. 6b,c). The survival of hemicryptophytes is strongly related to the soil moisture in the upper soil layers. The large representation of hemicryptophytes (43 %) confirms the preponderance of steppe vegetation in the study area. Of hemicryptophytes, 8.4 % are perennial grasses that usually form a tussock and have higher richness in montane zones (Fig. 6d). In the Iranian plateau, dwarf-shrubs (chamaephytes) are the most typical steppe vegetation. Many of the diverse Irano-Turanian genera exhibit this life form, e.g. Artemisia, Astragalus, Acantholimon, and Acanthophyllum. They are competitive at higher altitudes because they survive heavy grazing and unsuitable climatic and edaphic conditions such as snow cover and soil erosion. Wind-swept habitats at altitudes of 2,900 to 3,700 m a.s.l. are characteristically occupied by thorny-cushion vegetation (Fig. 6e). Geophytes usually bloom in the early spring and are not apparent in the other seasons. Therefore it was not possible to trace them all the time during the sampling period from mid-spring to late summer. The other life forms do not show a remarkable trend in the area.

Acknowledgments

The authors would like to thank Ms. V. Zarrinpour and Mr. J. Noroozi, for collecting part of the data during their master studies and Prof. E. Bergmeier (Göttingen) and one anonymous referee for giving valuable comments, and Dr. M. Djamali for his help in providing Fig. 1. This paper is a result from the Research project “Geobotanical Studies in Different Parts of Iran, I-IV” supported partly by the Research Council of the University of Tehran, and additionally by a small studentship grant for P.M.

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© Institute of Botany, Academy of Sciences of the Czech Republic 2012