Vegetation–environment relationships in the alderwood communities of Caspian lowlands, N. Iran (toward an ecological classification)

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Flora 203 (2008) 567–577 www.elsevier.de/flora

Vegetation–environment relationships in the alderwood communities of Caspian lowlands, N. Iran (toward an ecological classification) Alireza Naqinezhada,b,c,, Behnam Hamzeh’eed, Farideh Attarb a

Department of Biology, Faculty of Science, University of Mazandaran, Babolsar, Iran Central Herbarium of Tehran University, Department of Plant Science, School of Biology, College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran c Department of Animal & Plant Science, Sheffield University, Western Bank, Sheffield S10 2TN, UK d Department of Botany, Research Institute of Forests and Rangelands, PO Box 13185-16, Tehran, Iran b

Received 10 August 2007; accepted 23 September 2007

Abstract Hyrcanian (Caspian) lowland forests (northern Iran) include alderwood communities, dominated by Alnus glutinosa ssp. barbata. A data set of these alderwoods, including floristic releve´s and environmental variables (groundwater level, soil physical and chemical properties from two depths) was analyzed in order to describe the relationships between floristic composition and environmental variables. Classification of releve´s using two-way indicator species analysis (TWINSPAN) and a clustering technique displayed five clear vegetation groups of A. glutinosa ssp. barbata, each with specific indicator species. Principal component analysis (PCA) displayed two major gradients in environmental variables, namely a gradient of acidity-related variables and one of groundwater level-related variables. The five vegetation groups were confirmed by detrended correspondence analysis (DCA) and can be interpreted with these two major environmental gradients and also life form data which were passively projected on the diagram. Hydrophytes and helophytes were mostly found in swampy or wet groups where they influenced the first axis, while other life forms were mostly concentrated in the drier groups on the second axis. The results of both direct canonical correspondence analysis (CCA) and indirect (DCA) analyses of vegetation–environmental data were almost the same. The main environmental variables controlling the separation of these vegetation groups on the first two axes are groundwater level and acidity. There is a little difference between environmental variables analysis by PCA and vegetation–environment analysis by DCA and CCA mainly on different effects of CaCO3 on two first axes. Comparisons between habitat ecology of European alderwoods (stands of A. glutinosa ssp. glutinosa) and the measured environmental variables in the Hyrcanian alderwoods indicate some similar trends of variation of pH and C/N over these habitats in both areas. Three major types of A. glutinosa ssp. barbata habitats in Hyrcanian lowlands are distinguished mainly based on groundwater regime and geomorphology. These major types are compared with similar alderwoods in Europe. r 2008 Elsevier GmbH. All rights reserved. Keywords: Acidity; Alnus glutinosa ssp. barbata; Canocical correspondence analysis; Groundwater level; Iran; Lowland Hyrcanian forest

Corresponding author at: Department of Animal & Plant Science, Sheffield University, Sheffield, UK.

E-mail addresses: bo4an@sheffield.ac.uk, [email protected] (A. Naqinezhad). 0367-2530/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.flora.2007.09.007

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Introduction The effects of environmental variables on plant communities have been the subject of many ecological studies in recent years (e.g. Bragazza et al., 2005; Lyon and Gross, 2005; Pinto et al., 2006; Ramirez et al., 2007). Research focusing on the relationship between plant communities and environmental variables such as soils and water has become increasingly important in understanding the ecology of forest communities and especially their ground-layer vegetation (Huebner et al., 1995; Olano et al., 1998; Pregitzer and Barnes, 1982; Smith, 1995). The Hyrcanian (Caspian) district of northern Iran possesses a closed-canopy deciduous forest, unlike the arid to semi-arid landscape throughout most of Iran. The elevation range of this area is from below sea level up to 2700 m; this promotes to the development of different formations in these forests from ‘resistant to cold’ up to ‘sensitive to cold’ (Bobeck, 1951; Frey and Probst, 1986). The lowland zone of the Hyrcanian forests located adjacent to the Caspian Sea is sensitive to cold. These forests include some isolated remnants of formerly more widespread alderwood communities, dominated by Alnus glutinosa (L.) Gaertn. ssp. barbata (C.A.Mey.) Yaltirik – an element of the hygrophilous Euxino–Hyrcanian flora. This species is fairly common throughout swampy lowland forests and along riverbanks in Iran and rarely penetrates upper altitudes to 1000 m (Hamzeh’ee, 1994; Zohary, 1973). Recognized as a dominant species, A. glutinosa ssp. barbata coexists with other lowland species in some distinct communities (Ghahreman et al., 2006; Hamzehe´e et al., 2008; Rastin, 1980, 1983). Comparative studies on the ecology of forest communities in the Hyrcanian forests are scarce. Assadollahi (1980), Djazirei (1964, 1965), Dorostkar and Noirfalise (1976) and Hamzeh’ee (1994) presented some information about the soils of mainly upper mountain forests at the relation of some high mountain arboreal communities. The first papers about the lowland vegetation and habitat conditions (general overview only) were published by Mossadegh (1981), Tabari et al. (2002), Tregubov (1967), Tregubov and Mobayen (1970) and Zohary (1973) followed by more specific studies conducted by Rastin (1980, 1983). The latter author described 19 soil profiles of lowland areas and used them to explain the separation of some vegetation types. Since there are no vegetation ecology data about A. glutinosa forests available from the Caspian lowlands, an international significance of the present paper is to provide a basis for comparison of alderwood habitat ecology between various areas of their patchy occurrence in the Euxino–Hyrcanian area. The Caspian alderwoods are also ecologically comparable with European swampy alderwoods dominated by another subspecies, subsp.

glutinosa (e.g. Do¨ring-Mederake, 1990; Fremstad, 1983; Kollar, 2001; Marek, 1965; Prieditis, 1997a, b, 1999). This paper evaluates the influence of some environmental variables on the alderwood communities and discusses which variables have the main role in establishing different alder communities. These ecological data can form the basis for future management strategies on the restoration of such threatened patchy ecosystems.

Study area Five remnant protected sites (designed by Department of Environment and Natural Resources Centre, Iran) dominated by A. glutinosa ssp. barbata were selected. These sites distribute over lowland forests from west to east along the Caspian shore, Gilan and Mazandaran provinces. (Fig. 1 and Table 1). All the investigated areas have obvious homogeneity in geology and climate. The Caspian lowland is characterized by Quarternary formations consisting of alluvial deposits, marine terraces, sandy shores and mudflats distributed throughout the study areas (Anonymous, 1977, 1978). Most study sites have climatic stations within or nearby with more or less similar climate data (Anonymous, 1950–2000). Among these stations, Lahijan (adjacent to Am, Ln sites) and Khorramabad (adjacent to Ch, Kh, Kl sites) were selected for each partial study area (see Fig. 1 and Table 1). Most precipitation occurs from early autumn to early spring. Average annual precipitation ranges from 1080.7 mm in Khorramabad to 1425.9 mm in Lahijan. Mean monthly temperatures are 15 and 16.7 1C in Khorramabad and Lahijan, respectively. The lowest and highest absolute temperatures are 7.5 and 39.5 1C in Lahijan and 8.5 and 38 1C in Khorramabad. Using Gaussen’s method of climate classification, temperate axeric climate presents in the area (Sabeti, 1969).

Methods Data collection All quantitative data were collected between 2002 and 2003. The species data comprise 133 classical vegetation releve´s collected on a syntaxonomical purpose (Hamzehe´e et al., 2008) using the Braun-Blanquet scale (BraunBlanquet, 1964). The data represent an ordinal transformation of the Braun-Blanquet scale, i.e. the r, +, and 1–5 values replaced by values 1–7 in our analysis matrix. The studied releve´s are 100–400 m2 in size, based on the minimal areas (Cain and Castro, 1959) that were taken in each studied area. In a subsample of 20 randomly selected releve´s from different plant communities

ARTICLE IN PRESS A. Naqinezhad et al. / Flora 203 (2008) 567–577 Lahijan (10 m) (26) 37° 12'N, 50°00 'E 39.5° °C 30° 125

16.74°C, 1425.9 mm mm Mon 250

Khorramabad (10m) (40) 50°52 'E, 36°47 'N °C 38° 150 30.4°

569

15°C, 1080.7 mm mm Mon 300

100

200

120

240

75

150

90

180

50

100

60

120

25

50

30

60

1.9° -7.5°

0 J F M A M J J A S O N D

2.1° -8.5°

0

0 J F M A M J J A S O N D

Fig. 1. Study sites including two climatic curves on the northern part of vegetation map of Iran as well as schematic diagram of vegetation ranges in Elborz/Talish Mts. after Tregubov & Mobayen (1970) [Am ¼ Amirkelayeh, Ch ¼ Chaldarreh, Kh ¼ Khoshkedaran, Kl ¼ Kelarabad, Ln ¼ Langerud].

Table 1.

Study sites in the Caspian lowland forests, Iran

Name of site (abbreviation)

Locality

Altitude (m asl.)

Estimated surface (ha)

Amirkelayeh (Am)

Gilan province: Lahijan, Titiprizad, along the western margin of Amirkelayeh lagoon, 371200 371250 N, 501100 501120 E

22

250

Langerud (Ln)

Gilan province: around Salekuyeh village, Salekuyeh alderwood, 371100 371120 N, 50190 501120 E

15

100

Khoshkedaran (Kh)

Mazandaran province: Tonekabon, Nashtarud, Khoshkedaran National Natural Monument, 361420 361450 N, 51120 51150 E

20

264

Kelarabad (Kl)

Mazandaran province: Tonekabon, Kelarabad, Kelarabad educational forest, 361410 361450 N, 511150 511160 E

20

60

Chaldarreh (Ch)

Mazandaran province: Tonekabon, on the way of Dohezar river, Park-e Jangaliy-e Chaldarreh

200–300

100

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recognized by total syntaxonomical analysis (Hamzehe´e et al., 2008; Rastin, 1980, 1983), three soil cores per releve´ were randomly collected and separated into two standardized soil depths namely, a and b (a ¼ 0–15 cm depth, b ¼ 15–50 cm depth). Soil samples of each depth were mixed before analysis to reduce soil heterogeneity. Measured soil variables included physical and chemical properties. Soil texture (the proportions of sand, clay and silt) were determined by the hydrometer method (Bouyoucos, 1951); pH in a saturation extract was determined by pH meter and glass electrode within a suspension of 1:2.5 soil:water ratio (Anonymous, 1980); organic matter was estimated by the Walkley and Black’s method (Nelson and Sommers, 1996); total carbon was estimated by the method of Allison (1965) and the proportion of CaCO3 was measured by the Calsimeter method (Allison and Moode, 1965). Organic nitrogen content was determined on a Kjeltec System Instrument (TECATOR) (Anonymous, 1990). Within each releve´ an investigation well was drilled to determine the depth to the groundwater level. To omit the negative values of groundwater level in some releve´s, 100 were added to all. Life form spectrum was assigned to all vascular plants per releve´ according to the definitions of Raunkiaer (1934).

Data analysis Floristic analysis was performed with PC-ORD, V. 4.17 package (McCune and Mefford, 1999). Data were subjected to cluster analysis using Euclidian distance and the Ward group linking method. In addition, to identify species with particular diagnostic value and to confirm clustering results, the floristic data were classified with the two-way indicator species analysis (TWINSPAN) (Hill, 1979). The program Canoco 4.5 for Windows (ter Braak and Sˇmilauer, 2002) was used for ordination analysis. Principal component analysis (PCA) was applied to search for a general pattern in the measured environmental variables. Two types of species-data ordination were applied. First, unconstrained ordination (detrended correspondence analysis, DCA) was used to find major gradients in species composition and thus describe the general pattern in species distribution along the gradients. The DCA diagram was subsequently passively projected with both environmental variables and life form percentages to show their variations across the species data. Constrained ordination (canonical correspondence analysis, CCA) was applied to assess the relative importance of first and second major gradients of environmental variables in explaining the species distribution patterns (Lepsˇ and Sˇmilauer, 2003; ter Braak, 1987). Seventeen constrained soil variables from both depths were involved in the analysis. Species weight and species fit on both canonical

axes were used for the selection of the species shown in the resulting biplot. The patterns in species and releve´ distribution along the main gradients resulting from these two ordination procedures were compared. The effect of all separate variables and whole variables on species composition in CCA were tested using global Monte-Carlo permutation test and forward selection in Canoco (Lepsˇ and Sˇmilauer, 2003; ter Braak and Sˇmilauer, 2002). SPSS 14 and Minitab 14 software were used for some univariate statistics. The Kolmogorov–Smirnov test confirmed the normal distribution of measured variables. The Pearson correlation coefficient was used to test relationships between values of one variable determined at different soil depths. The differences between these values obtained from different depths were tested with paired-samples t-tests.

Results Major trends in variation of environmental variables Most of the values determined at different depths have similar importance in PCA and are strongly intercorrelated. t-Values for CaCO3 and C/N ratio were not significant (Table 3). According to the results of PCA, the variables were classified into two major groups: (i) groundwater level and soil texture-related variables, (ii) related variables of acidity (see Table 2). The first axis of PCA of environmental variables is clearly identified as the axis of soil physical properties, i.e. groundwater level and soil texture (Fig. 2, Table 2). Soil texture features also include soil organic matter content and organic nitrogen content (the more groundwater level, the more organic matter and organic nitrogen content and the less proportion of clay, sand and silt). The second axis is the axis of soil acidity. pH and proportion of CaCO3 are clearly related to the second axis, especially in the topmost layer (Fig. 2, Table 2). High values of pH are strongly correlated with the amount of CaCO3 (the higher proportion of CaCO3, the higher pH value). The C/N ratio shows a negative correlation with acidity and is categorized as an acidity-related variable in our studied environmental variables (the lower pH value, the greater C/N ratio).

Major gradients in the alderwoods vegetation The results of TWINSPAN and cluster analysis allow for a floristically and ecologically sound scheme of five main vegetation groups of studied releve´s with specific floristic composition (Fig. 3). The first division of the clustering dendrogram generated a group of releve´s from rather dry alderwoods (group V). This group was also generated by the second level of division in

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Table 2. Descriptive statistics for the studied variables and the classification of variables into groups (related variables) as well as correlations between the environmental matrix and the first two axes of the PCA ordination Variable

Min

Max

Mean

sd

PCA (axis 1). eig. ¼ 7.469

PCA (axis 2) eig. ¼ 2.534

Related variable

pHa pHb CaCO3a% CaCO3b% C/Na C/Nb Groundwater level: GW (cm) Claya% Clayb% Silta% Siltb% Sanda% Sandb% Organic matter (OMa) % Organic matter (OMb) % Organic nitrogen (Na) % Organic nitrogen (Nb) %

3.30 5.50 1.04 0.21 2.91 7.00 30.00

7.40 7.70 16.67 27.08 22.52 14.30 155.00

6.07 6.97 4.35 5.82 12.63 10.62 63.50

1.00 0.70 4.41 6.46 4.55 1.64 37.28

0.0675 0.2129 0.1170 0.1270 0.0080 0.0016 0.3081

0.5058 0.2769 0.4192 0.3467 0.2546 0.2149 0.1503

Acidity Acidity Acidity Acidity Acidity Acidity Groundwater level and texture

0.00 0.00 0.00 0.00 0.00 0.00 4.30 0.60 0.40 0.05

24.16 34.16 52.72 62.72 85.12 83.12 47.73 46.10 2.58 2.41

5.74 13.84 21.91 33.31 57.35 37.85 23.40 9.29 1.08 0.48

5.40 11.35 15.18 19.14 29.07 23.51 14.11 15.75 0.68 0.82

0.2261 0.1807 0.2715 0.2987 0.2573 0.2148 0.3064 0.3523 0.3522 0.3524

0.2571 0.0288 0.0592 0.0034 0.2746 0.2255 0.0784 0.1195 0.0683 0.1273

Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture Groundwater level and texture

eig. ¼ eigenvalue, sd ¼ standard deviation, depth a ¼ 0–15 cm, depth b ¼ 15–50 cm. The amounts of groundwater level are presented after the addition with 100 (see Methods section).

0.6 C/N-a C/N-b Sand-b Silt-b Silt-a

OM-a

Sand-a

OM-b N-a N-b

Clay-b

Clay-a

GW pH-b

-0.8 -1.5

CaCO3-a CaCO3-b

pH-a

1.5

Fig. 2. PCA ordination of environmental variables (full names of variables in Table 2), a ¼ 0–15 cm, b ¼ 15–50 cm.

TWINSPAN characterized by the indicator fern species Pteris dentata ssp. flabellata. The third division generated a group with three releve´s in swampy sites (group I). This group was also generated by the first level of division in TWINSPAN, characterized by the indicator fern species Thelypteris limbosperma. The four-releve´ cluster in the lowland area (group II) was generated by the third division in the dendrogram and by the third level of division of TWINSPAN with presence of the indicator species Ulmus minor. In addition, the third division of the dendrogram generated a group of four releve´s from higher altitudes (250 m a.s.l.) (group IV). This group was also generated by the fourth level of division in TWINSPAN by preferential occurrence of A. subcordata. Moreover, the third division of the dendrogram generated a group of three releve´s (group III),

which was also generated by the fourth level of division in TWINSPAN, characterized by the indicator species Lindelophia kandovanensis (Fig. 3). Group I (three releve´s) is characterized by the presence of Galium elongatum and T. limbosperma. This group shows the highest proportion of aquatic or hygrophyte species and is mostly confined to swampy forests. Group II (three releve´s) is characterized with U. minor as the dominant and indicator species. The occurrence of Ficus carica and some hygrophyte herbaceous species can differentiate this group from the other groups. Group III (three releve´s) is characterized by L. kandovanensis as indicator species. This group is relatively isolated from the rest by a rather definite floristic composition. Carex sylvatica, Microstegium vimineum, Oplismenus undulatifolius and Viola siehiana

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Fig. 3. Dendrogram of the cluster grouping of the study releve´s. Grouping was performed using Euclidian distance and the Ward method. Indicator species of each cluster are according to TWINSPAN results.

are the most important preferential species in the separation of this group. Group IV (four releve´s) is characterized by A. subcordata. The occurrence of some preferential species such as Glyceria caspica, Salvia glutinosa, Symphyandra odontosepala and Vincetoxicum scandens differentiate this group from the rest. Group V (six releve´s) is characterized by P. dentata ssp. flabellata as indicator species. The occurrence of some Hyrcanian endemic species, i.e. Buxus hyrcana, Ilex spinigera, Parrotia persica and P. dentata ssp. flabellata characterized this group as well. Also this group differs from the rest showing the presence of Arum maculatum, Cyclamen coum, Mercurialis perennis, Ophioglossum vulgatum, Ornithogalum sp. and Viola alba. Indirect gradient analysis (DCA) of 20 releve´s revealed two major gradients in species data. The gradient lengths of the two first axes were 3.848 and 2.506, respectively. This length of the gradient is reasonable to perform a constrained analysis with CCA (Lepsˇ and Sˇmilauer, 2003). The first axis is closely related to the gradient from swampy alderwoods (group I) toward wet or rather dry alderwoods (Fig. 4). This gradient of alderwood structure correlates significantly with the soil texture and groundwater level and more or less with CaCO3. The retaining capacity of water decreases on the first axis of DCA from right to left. This decrease is corresponding with a decrease of soil organic matter as well as an increase of mineral soil structure. Therefore, soil texture elements show a negative correlation with organic matter and groundwater level. The second gradient dealing with the separation of other groups is related to acidity. The vegetation groups that were determined using classification methods can be clearly recognized in the DCA ordination (Fig. 4). The proportions of helophytes and hydrophytes increase in groups I and II on first axis, while along the second axis separation occurs of

therophytes and hemicryptophytes from geophytes and phanerophytes. Group IV demonstrates a high proportion of therophytes and hemicryptyophytes. Geophytes and phanerophytes are found more in groups III and V. The eigenvalues of the first two CCA axes (Fig. 5) were 0.580 and 0.380, respectively (explaining 28.8% of total species data variances), indicating a moderately long gradient for the first axis. A total of 87.7% of total inertia in species data can be explained by studied environmental variables, of which 32.8% can be explained by the first two axes. The Monte-Carlo global test indicates statistically significant eigenvalues for the two first axes (po0.05). The relationships of selected environmental variables with the first and second axes were strongly significant (p ¼ 0.002). The first canonical axis is mostly correlated with groundwater level, organic matter, nitrogen content and soil physical properties (proportion of silta,b, sanda,b and claya,b) (related variable of groundwater level and soil texture), followed by CaCO3a and C/Na. The second canonical axis is more strongly influenced by pHa and C/ Nb followed by CaCO3b (related variables of acidity) while the other variables yielded very low correlations with the second axis (Fig. 5). Two groups, II and III were positioned in the centre of the CCA diagram and can be separated on the first axis of the diagram. The proportion of soil sands together with proportion of clayb, CaCO3a,b and pHb were correlated with group III, while the amount of groundwater level and soil surface pH (pHa) were correlated with group II. The amount of C/Nb and proportion of soil silt and claya influence the separation of group V (Fig. 5).

Discussion Lowland Hyrcanian forests have hygrophilous and thermophilous vegetation distributed in remnant

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4

(Hem)

(Thr)

(Cha)

Sand-b

Sand-a

pH-a

CaCO3-b

GW CaCO3-a C/N-a

Clay-b

OM-a

pH-b Silt-a

(Hyd) N-b OM-b N-a

Clay-a C/N-b

(Hel)

Silt-b (Geo)

(Pha)

-1 -1

4

Fig. 4. DCA biplot of study releve´s and environmental variables. Symbols are according to Fig. 3. a ¼ 0–15 cm, b ¼ 15–50 cm. (full names of variables in Table 2). Life forms in bracket (Cha ¼ Chamaeophyte, Geo ¼ Geophyte, Hel ¼ Helophyte, Hem ¼ Hemicryptophyte, Hyd ¼ Hydrophyte, Pha ¼ Phanerophyte).

patches on the coastal areas along the Caspian Sea. Although one of the studied forest sites (releve´s of group IV) is located in an altitude above the normal distribution range of lowland forests (200–300 m a.s.l.), this altitudinal range has been considered as lowland parts by some authors (e.g. Bobeck, 1951). Moreover, the occurrence of protected stands of A. glutinosa ssp. barbata, was the main reason to include the releve´ data of this site in the current study. Although all alderwood sites had a strong dominance by A. glutinosa ssp. barbata, they showed substantial structural and floristic heterogeneity and a complex relationship with the local environment. There is no significant climatic difference between the six western–eastern alderwood sites along the Caspian coast (Ghahreman et al., 2006), which indicates that other ecological variables such as soil and groundwater level properties can control the separation of different alder communities in Caspian lowland forests. Classification and ordination methods generated results that were complementary and ecologically meaningful. Since the majority of inertia in species data (more than 85%) has been explained by our studied environ-

mental variables, the results of both constrained and unconstrained ordinations indicated almost similar patterns of variation of environmental variables along the axes and somewhat represented the main pattern of variation in PCA. The comparison of some environmental variables indicated that the most significant differences occur between the soils due to differences in groundwater level and amount of soil mineralization. In surface groundwater condition, the soil undergoes a low mineralization rate and thus represents a high value of concentration of organic matter and nitrogen content and therefore produces an organic texture. Consequently, the first axis can separate two types of soil texture, i.e. organic and mineral, based on increasing or decreasing of the groundwater level, respectively. Rastin (1983) also demonstrated that physical properties, especially groundwater regime in the lowland soils, are the most important factors affecting the separation of plant communities. The effects of CaCO3 and also pHb on the first axis of CCA and DCA are more than the effects of these variables on the first axis of PCA. These variables were classified as acidity-related variables and considered to relate to the second axis of PCA.

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0.8

Mer pere Oph vulg

Silt-b

Silt-a

Sand-a Sand-b

Orn sp. Cyc coum

Ile spin Rus hyrc Pte dent Vio alba Aru macu Car tene Par pers C/N-b Ace velu Bux hyrc Lin kand Pol worn Ath fili Sam ebul Cir lute Cra micr Car betu Clay-a Rum sang Smi exce C/N-a Aln glut CaCO3-a Car stri pH-b Fic cari Sol pers Dio lotu Opl undu Ulm mino Vio sieh Lyc euro Clay-b Pte frax Car sylv CaCO3-b Mic vimi Eup amyg Hyp ando Sal glut

N-a

OM-a

OM-b N-b Gal elon The limb Car ripa Lyt sali

GW

Aln subc Gly casp Bra sylv Vin scan Cli umbr pH-a Sym odon

-0.8 -1.0

1.0

Fig. 5. CCA triplot of species, releve´s and environmental variables. Only the most important species have been given. (full names of variables in Table 2). Abbreviated species of diagram: Acer velutinum, Alnus glutinosa ssb. barbata, Alnus subcordata, Arum maculatum, Athyrium filix-femina, Brachypodium sylvaticum, Buxus hyrcana, Cardamine tenera, Carex riparia, Carex strigosa, Carex sylvatica, Carpinus betulus, Circaea lutetiana, Clinopodium umbrosum, Crataegus microphylla, Cyclamen coum ssp. caucasicum, Diospyrus lotus, Euphorbia amygdaloides, Ficus carica, Galium elongatum, Glyceria caspica, Hypericum androsaemum, Ilex spinigera, Lindelophia kandovanensis, Lycopus europaeus, Lythrum salicaria, Mercurialis perennis, Microstegium vimineum, Ophioglossum vulgatum, Oplismenus undulatifolius, Ornithogalum sp., Parrotia persica, Polystichum woronowii, Pteris dentate ssp. flabellata, Pterocarya fraxinifolia, Rumex sanguineus, Ruscus hyrcanus, Salvia glutinosa, Sambacus ebulus, Smilax excelsa, Solanum persicum, Symphyndra odontosepala, Thelypteris limbosperma, Ulmus minor, Vincetoxicum scandens, Viola alba, Viola siehiana.

Although CaCO3 is correlated with acidity in Hyrcanian forests (Assadollahi, 1980; Djazirei, 1964, 1965; Hamzeh’ee, 1994), it can also be considered as an element of soil texture and thus a gradient from organic texture to mineral texture can be accompanied by accumulation of CaCO3 especially in the upper surface soil (Rastin, 1983). The accumulation of CaCO3 in alderwood soils in the direction from the upper stratum to the lower stratum affects directly vegetation grouping. This primary effect of CaCO3a on separation of vegetation groups on the first axis can be extended by further influence of CaCO3b on the second axis (Fig. 5). This result is consistent with insignificancy of t-test for CaCO3 amounts of two depths (see Table 3). We conclude that both axes can separately explain the acidity but the second axis has a stronger role in this related variable. Group II dominated by U. minor occurs on a groundwater level gradient between group I and the

rest. U. minor is an element of damp flood plains and grows best on fertile alluvial soils and in marshy places (Browicz, 1982). The releve´s of group V indicate a remarkably acidic state and a high ratio of C/Nb. Acidic situation in group V is responsible for decrease in soil mineralization and then increase the C/N ratio especially in the lower soil depth. Although a low C/N ratio has been recorded in an acidic Hyrcanian forest soil (Ejtehadi et al., 2004), we found that C/N ratios were decreased with an increase in soil pH (Fig. 5) being consistent with findings by Schuster and Diekmann (2005). Nevertheless, there is no strict trend to correlate between these two variables at the level of releve´ studies as can be observed in many studies on European alderwood vegetation (Prieditis, 1997a). In a wide sense, C/N ratio decreases but pH increases from oligotrophic towards the eutrophic sites in European A. glutinosa stands (Prieditis, 1997a; Szczepanski, 1990). This pattern has been considered as a general trend which may give a

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delusion if only single releve´s from various localities are compared (Prieditis, 1997a). Although the comprehensive habitat comparison between European and Hyrcanian alderwoods is out of the scope of this paper, here a relatively similar ecological trend can be observed, i.e. pHb and C/Na increase from group I (relatively oligotrophic) towards eutrophic groups (groups II–V). In a wide sense, based on groundwater regime and geomorphology, three major A. glutinosa habitat groups in Hyrcanian lowlands can be distinguished: 1. Swampy alderwoods at the margin of marshlands with a rather mesotrophic to oligotrophic state (group I). Here the groundwater movement is almost absent and the substrate is permanently highly saturated with water. In flooded condition, the soil mineralization is decreased and therefore the proportion of organic matter is increased. This habitat group can be considered as analogous with one of habitat groups of European A. glutinosa, which is ascribed as Carici elongatae–Alnetum Schwick. 33 and Sphagno squarrosi–Alnetum Sol.-Gorn. ex Pried. 96 from Alnion glutinosae (Malc. 1929) Meijer Drees 1936 (Prieditis, 1997a, 1993). Although some habitat similarities can be found for this group between these two areas, there are rather different sets of the floristic structure and characteristic species (Hamzehe´e et al., 2008). Due to higher content of organic matter, the amounts of organic nitrogen content in the soils of these releve´s are high (Table 2). Hydrophytes and helophytes are the most dominant life forms in these alderwoods as the majority of their flora is affected by wetland species (Fig. 5). 2. Wet eutrophic mineral alder forests within more inland Hyrcanian forests. This habitat includes vegetation groups II, III, V and possesses species Table 3. Comparison of environmental variables determined at two soil depths Variable

r

t-Value

CaCO3 Organic matter Nitrogen C/N pH Clay Silt Sand

0.833 0.735 0.932 n.s. 0.491 n.s. 0.729 0.722

n.s. 5.747 8.598 2.132 4.435 3.493 3.875 4.300

The first column presents the Pearson correlation coefficient of correlation between the upper (0–15 cm) and lower (15–50 cm) soil depths, the second column presents the results of a t-test comparing values from these two depths. Significance level po0.05 is used for both analyses. n.s. ¼ not significant.

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with lower adaptation to swampy places. This main habitat is analogous with European AlnoUlmion Br.-Bl. et Tx. ex Tchou 1948 forest habitats (Prieditis, 1997a, b). The proportion of phanerophytes and geophytes are increased toward these groups indicating an increase of both arboreal and herbal diversity, which are less adapted to the wetland habitat. 3. Temporarily flooded alderwoods along the submountain rivers with some brooks. The root system penetrates to mineral and calcareous eutrophic soils. This main habitat constitutes vegetation group IV which occurs at higher altitudes (200–300 m a.s.l.) and belongs to riverine ecosystems dominated by A. glutinosa ssp. barbata and A. subcordata. Forests including A. subcordata stands constitute the Alnion subcordatae (Djazirei, 1965) which is mostly distributed on alluvial deposits of Hyrcanian mountain and submountain forests (Assadollahi, 1980; Assadollahi et al., 1982; Djazirei 1964, 1965; Dorostkar and Noirfalise, 1976). This group of releve´s was found where an increasing water level can be locally observed; they are dominated by A. glutinosa ssp. barbata. Here, the groundwater level is relatively high and sometimes affected by surface water. The soil of this group is similar to hydromorphic soil (pseudogley) studied in the submountain forest of Bandar-e Gaz area (Zarrinkafsh, 2002). CCA analysis indicated that group IV is ecologically and floristically close to group III (Figs. 4 and 5) being in consistent with the results of Djazirei (1964, 1965) and Rastin (1983). The high proportion of hemicryptophytes in group IV (Fig. 4), can be explained by altitudinal affects. Moreover, the high percentage of therophytes in this vegetation group is because of penetration of some annual weeds after a temporary human destruction on these riverine forests. Some investigations have been carried out both on the succession from open water to alderwoods of Alnion glutinosae, which is generally considered as a final successional stage, and also on the further succession towards more drier Alno–Ulmion communities (Fremstad, 1983; Fukarek, 1961; Kollar, 2001; Marek, 1965; McVean, 1956; Neuha¨usl, 1992; Prieditis, 1997a; Szczepanski, 1990). Both routes are initiated by decline in substrate wetness. Although the specific successional survey is out of the scope of this paper, a similar pattern can be outlined for the Hyrcanian alderwoods. Group I (swampy alderwoods) is usually located at the margin of open water lagoons or marshlands and often connected to them with surface water. Moreover, other main habitats (groups II–V) can be considered as later successional stages after a long decline in wetness and mineralization as well as establishment of more inland species in group I.

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Acknowledgments Authors are very grateful to Dr. Normunds Prieditis, Latvia, for different aspects of his valuable comments and providing some key papers. We also thank Dr. Sue Shaw, Sheffield University, UK and Prof. Dr. David Charlet, Community College Southern Nevada, USA, for the valuable comments on the first draft version of the text. Prof. Dr. Ahmad Ghahreman, University of Tehran, Dr. Mostafa Assadi and Dr. Manuchehr Amani, Research Institute of Forests and Rangelands, Tehran, are thanked for their guidance during the study project.

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