Alireza Naqinezhad
email: a.naqinezhad@umz.ac.ir

Elias Ramezani

Morteza Djamali

Annik Schnitzler

Claire Arnold

Wild grapevine (Vitis vinifera subsp. sylvestris) in the Hyrcanian relict forests of northern Iran: an overview of current taxonomy, ecology and palaeorecords

Keywords: Domestication, Grapevine, Hyrcanian forests, Pollen, Vitis

Due to severe anthropogenic impacts on lowland and submontane zones of the Hyrcanian forests of northern Iran, wild grapevine (Vitis vinifera subsp. sylvestris), a sporadically distributed woody liana, is currently considered an endangered species. Using data from the literature and 34 studied populations, herbarium assessments and nine palynological sites, we provide an overview of its taxonomy, distribution and ecology in the first part of the investigation. The separation of the two subspecies, namely V. sylvestris subsp. anebophylla and V. sylvestris subsp. trichophylla (sensu Flora Iranica), based on their leaf indumentums, could not be confirmed by our examination of herbarium materials and field observations. Indumentum of the leaves is a result of leaf polymorphism in different Vitis specimens and can be strongly influenced by environmental conditions. Vitis vinifera subsp. sylvestris grows in a wide range of habitats including wetlands, seasonal stream sides in closed forests, alluvial beds of large rivers, sand dune shrublands and forested wetlands (alder forests). Parrotia persica and Carpinus betulus were the most frequent host species in the studied populations. In the Hyrcanian region, no pollen record of Vitis older than the Mid-or Late-Holocene has yet been established. Despite the intrinsic problem in pollen identification by normal (i.e., light) microscopy of wild from cultivated grapevines, the significant values and persistent occurrence of Vitis pollen since the Mid-Holocene (before the Bronze Age) in the Hyrcanian pollen records may imply the onset of viticulture in low-to mid-elevation sites in the region. This represents an argument to consider the Hyrcanian region as a possible domestication center for V. vinifera. However, the question of wild versus cultivated origin of grapevines in the Hyrcanian pollen records and the possible date of its domestication and/or cultivation will remain open until further palynological studies are undertaken.

Introduction

Since several wild and naturally occurring Asian and American Vitis species are now recognized as valuable sources of resistant genes against diseases and environmental stresses [START_REF] Ruel | Resistance to Pierce's disease in Muscadiniarotundifolia and other native grapevine species[END_REF][START_REF] Wan | The ecogeographic distribution of wild grapevine germplasm in China[END_REF][START_REF] Zecca | The timing and the mode of evolution of wild grapevines (Vitis)[END_REF], scientists have dedicated great attention to the study and development of cultivars of Vitis vinifera L. subsp. vinifera, and to the study of the wild form of the cultivated grapevine, V. vinifera subsp. sylvestris (C. C. Gmel.) Hegi (e.g., This et al. 2006). Habitat loss, and the spread of pests and diseases, (particularly Phylloxera), have caused the extinction of most wild populations during the last century [START_REF] Arrigo | Naturalised Vitis rootstocks in Europe and consequences to native wild grapevine[END_REF][START_REF] De Andres | Genetic diversity of wild grapevine populations in Spain and their genetic relationships with cultivated grapevine[END_REF][START_REF] Schneider | Identification and characterization of Vitis vinifera subsp sylvestris populations in north-western Italy[END_REF].

The genus Vitis L. (Vitaceae family) includes about 70 woody lianas distributed mostly in the temperate regions of the Northern Hemisphere [START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF][START_REF] Zecca | The timing and the mode of evolution of wild grapevines (Vitis)[END_REF]. They include 34 American and 37 Asian species which are geographically separated (GRIN 2016), as well as the European-Middle Asian wild grapevine (V. vinifera subsp. sylvestris), believed to be the living ancestor of modern grapevine cultivars [START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF][START_REF] Terral | Evolution and history of grapevine (Vitis vinifera) under domestication: new morphometric perspectives to understand seed domestication syndrome and reveal origins of ancient European cultivars[END_REF].

Wild grapevine (V. vinifera subsp. sylvestris) is a tendrillar woody, dioecious liana, indigenous to a wide portion of Eurasia extending from Portugal to Turkmenistan, and from Rhine riversides to the northern forests of Tunisia [START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF]). It has entire, trilobate deciduous leaves, often hairy on the under surface. Female plants produce bunch of grapes, that appear as black berries of 1 cm in diameter, containing up to four round seeds [START_REF] Hegi | Vitis[END_REF]. The diversity centre of the subspecies seems to be located in the Middle East, from the Anatolian Peninsula to the Transcaucasian regions and the Hyrcanian area [START_REF] Grassi | Historical isolation and Quaternary range expansion of divergent lineages in wild grapevine[END_REF][START_REF] Zecca | The timing and the mode of evolution of wild grapevines (Vitis)[END_REF], where there exists the oldest living Eurasian Vitis species [START_REF] Zecca | The timing and the mode of evolution of wild grapevines (Vitis)[END_REF].

The Hyrcanian area in the north of Iran, as well as the plains around the Black Sea, have acted as the main sources for conservation of grapevine germplasm during the Quaternary glaciations [START_REF] Leroy | Final Pliocene macro and micro floras of the palaeovalley of Bernasso (Escandorgue, France)[END_REF][START_REF] Leroy | Glacial refugia for summer-green trees in Europe and south-west Asia as proposed by ECHAM3 time-slice atmospheric model simulations[END_REF], and probably the only regions of continuous presence of the genus since the Tertiary period. Regions close to the Caspian and the Black seas also acted as refugia during the Last Glacial Maximum (LGM, 23,000-18,000 years before present) for the wild grapevine as well as for several thermophilous forest species (species growing in higher temperatures), such as Parrotia persica (DC.) C. A. Mey., Gleditsia caspia Desf. Zelkova carpinifolia (Pall.) K. Koch, and Pterocarya fraxinifolia (Poir.) Spach [START_REF] Van Zeist | Palynological investigations in western Iran[END_REF][START_REF] Grassi | Historical isolation and Quaternary range expansion of divergent lineages in wild grapevine[END_REF][START_REF] Akhani | Plant biodiversity of Hyrcanian relict forests, north Iran: an overview of the flora, vegetation, palaeoecology and conservation[END_REF].

Despite a number of genetic, ecological and taxonomic studies on wild grapevine (e.g., [START_REF] Zecca | The timing and the mode of evolution of wild grapevines (Vitis)[END_REF][START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF][START_REF] Rubio | Ecological and sanitary characteristics of the Eurasian wild grapevine (Vitisvinifera L. ssp. sylvestris (Gmelin) Hegi) in Georgia (Caucasian region)[END_REF][START_REF] Ekhvaia | Genetic diversity of Vitis vinifera in Georgia: relationships between local cultivars and wild grapevine, V. vinifera subsp. sylvestris[END_REF] and a few genetic studies on wild and cultivated grapevines of western Iran [START_REF] Ramezani | Evaluation of genetic diversity of Iranian grapevine accessions using microsatellite markers[END_REF][START_REF] Doulaty-Baneh | The use of AFLP and morphological markers to study Iranian grapevine germplasm to avoid genetic erosion[END_REF][START_REF] Ramezani | Palynological reconstruction of late-Holocene vegetation, climate, and human impact in Kelardasht (Mazandaran Province, N Iran)[END_REF], 2014;[START_REF] Karatas | Genetic diversity of wild and cultivated grapevine accessions from southeast Turkey[END_REF], specific studies on habitat characteristics, taxonomical features and palaeoecological evidence of the occurrence of this species in the Hyrcanian area are lacking. This paper deals with taxonomical delimitation, habitats, palaeorecords and current distribution of V. vinifera subsp. sylvestris in the relict Hyrcanian area of northern Iran.

Materials and methods

Study area

The study area includes the south Caspian lowlands and plains along with the northern slopes of the Alborz Mountains, extending over 800 km from the forests of the Golestan National Park (longitude 56°E) near the Turkmenistan border to the forests close to the border of Iran and the Azerbaijan Republic (longitude 48°E) (Fig. 1). The region is bioclimatically different from other parts of Iran [START_REF] Akhani | Plant biodiversity of Hyrcanian relict forests, north Iran: an overview of the flora, vegetation, palaeoecology and conservation[END_REF]Djamali et al. 2011b), benefiting from a mild climate that has harboured the relict thermo-mesophilous Hyrcanian forest during the Quaternary glacial climates [START_REF] Zohary | Geobotanical foundations of the Middle East[END_REF][START_REF] Frey | A synopsis of the vegetation of Iran[END_REF].

The study area has an annual average rainfall that ranges from 2045 mm in the west to 866 mm in the east (forest area of Golestan National Park), and average minimum temperatures of the coldest month higher than the freezing point ( 0°C) [START_REF] Akhani | The illustrated flora of Golestan National Park, Iran[END_REF][START_REF] Akhani | Plant biodiversity of Hyrcanian relict forests, north Iran: an overview of the flora, vegetation, palaeoecology and conservation[END_REF]. Considering the spatial analysis of precipitation seasonality, there are different precipitation regimes across the Hyrcanian region [START_REF] Domoers | An analysis of regional and intra-annual precipitation variability over Iran using multivariate statistical methods[END_REF]. According to recent bioclimatic classification of Iran based on Rivas-Martı ´nez system, the Hyrcanian region is mainly affected by temperate oceanic climates followed by Mediterranean pluviseasonal oceanic, Mediterranean xeric oceanic and Mediterranean pluviseasonal continental bioclimates (Djamali et al. 2011b).

The study area is predominantly deciduous closed forests (Hyrcanian forests) restricted to the south Caspian lowlands up to the adjoining northern slopes of the Alborz mountain range, one of the largest mountain ranges in Iran [START_REF] Frey | A synopsis of the vegetation of Iran[END_REF]. The forests contain relicts of the Arcto-Tertiary and Indo-Malesian floras and are divided altitudinally into lowland and montane zones. Lowland forests include important evergreen broad-leaved species and range up to 1000 m a.s.

Data collection and analyses

To understand the habitat and taxonomic characteristics of wild grapevine, 34 populations were studied across the Hyrcanian area, from the Golestan National Park (eastern Hyrcanian area) to Asalem road (western Hyrcanian area, Guilan) in June 2014 (Appendix S1, see Supplemental Data with online version of this article). Using standard protocols provided by [START_REF] Schnitzer | Supplemental protocol for liana censuses[END_REF] and [START_REF] Gerwing | A standard protocol for liana census[END_REF], the morphological descriptors include: the area covered by grapevine, the ratio of foliage cover to host canopy cover, height in metres and diameter at breast height (DBH). Ecological descriptors include: altitude, openness of habitat (degree of landscape openness: 0 closed forest to 3 open), moisture (degree of moisture across the landscape: 0 dry and semi-dry to 4 wetlands) and distance to the nearest river (0 less than 50 m from the river, 1 more than 50 m from the river, 2 no river). Among all ampelographic characters of the populations (e.g., [START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF], only some morphological descriptors are correlated with effective environmental features. Despite the high polymorphic status of foliar and flower morphology of wild grapevines [START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF][START_REF] Rubio | Ecological and sanitary characteristics of the Eurasian wild grapevine (Vitisvinifera L. ssp. sylvestris (Gmelin) Hegi) in Georgia (Caucasian region)[END_REF], the amount of foliage cover and stem diameters and heights are considered as morphological responses to habitat requirements. Due to the presence of ordinal and quantitative variables in the dataset, we used Spearman correlation coefficient to study inter-correlation of environmental and morphological variables using SPSS software (version 21).

All materials were examined with a stereomicroscope (NIKON-SMZ1) and compared with samples in the University of Mazandaran Herbarium (HUMZ), the University of Tehran Herbarium (TUH), the herbarium of the Iranian Research Institute of Plant Protection (IRAN), the herbarium of Conservatoire et Jardin botanique de la Ville de Gene `ve (G), the Museum National d'Histoire Naturelle, Paris (P), the Universite ´Montpellier Herbarium (MPU), and the herbarium of Botanical Garden, Munich (M). All special collections of the Geneva herbarium, (including Boissier's Flora Orientalis collections and those collected by P. Aellen, K. H. Rechinger and de Candolle), were also examined. Nomenclature of the plants is according to Flora Iranica (Rechinger 1963(Rechinger -2012) ) and Flora of Iran (Assadi et al. 1988(Assadi et al. -2014)). Data from herbarium and literature surveys were regrouped in a map showing the distribution points of wild grapevine since the 19th century (Fig. 2). Palynological data of Vitis were obtained from published works [START_REF] Ramezani | The late-Holocene vegetation history of the Central Caspian (Hyrcanian) forests of northern Iran[END_REF][START_REF] Ramezani | Between the mountains and the sea: late Holocene Caspian Sea level fluctuations and vegetation history of the lowland forests of Northern Iran[END_REF][START_REF] Leroy | Late Little Ice Age palaeoenvironmental records from the Anzali and Amirkola Lagoons (south Caspian Sea): vegetation and sea level changes[END_REF][START_REF] Ramezani | Palynological reconstruction of late-Holocene vegetation, climate, and human impact in Kelardasht (Mazandaran Province, N Iran)[END_REF][START_REF] Shumilovskikh | Landscape evolution and agro-sylvo-pastoral activities on the Gorgan Plain (NE Iran) in the last 6000 years[END_REF]) and from unpublished pollen diagrams. Calendar ages (presented as cal. yr BP) were determined using the available age-depth models or by linear interpolation between radiocarbon ages.

Results and discussion

Taxonomic remarks and herbarium studies Despite being classified as a subspecies under V. vinifera, wild grapevine was considered as a separate species, V. sylvestris Gmelin in the Flora Iranica and other local floras [START_REF] Sosnovskii | Vitacee[END_REF]Vassilchenko 1970;[START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF][START_REF] Mozaffarian | Trees and shrubs of Iran[END_REF]). This historical separation into species and subspecies had been based on morphological differences but today this distinction is invalid because differences between wild and cultivated plants are likely the result of domestication over time rather than geographical isolation [START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF]. During domestication, several crucial changes in biology occurred to yield greater sugar content in the berries and more regular production. During this process, both vegetative (berry and branch sizes) and reproductive parts of the plant (seed morphology and the change from dioecious wild plants to hermaphrodite cultivated plants) underwent substantial changes [START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF].

In Flora of USSR [START_REF] Sosnovskii | Vitacee[END_REF]), V. hyrcanica Vass. was characterized with tomentose indumentums on the under surface of the leaves. It was, however, later synonymised under V. sylvestris (Vassilchenko 1970). Concerning the taxonomical wealth of the genus Vitis in the Flora Iranica realm, two other wild taxa occur, V. nuristanica Vassil. and V. hissarica Vassil. (the latter subdivided into subsp. hissarica and subsp. rechingeri Vassil.). The latter taxon was recently recorded from a wetland in Khuzestan province, southwest Iran [START_REF] Dinarvand | A new record of the genus Vitis (Vitaceae) for the flora of Iran[END_REF]. The American species V. labrusca L. has also been listed as another cultivated species from Guilan and Mazandaran provinces [START_REF] Mozaffarian | Trees and shrubs of Iran[END_REF].

Besides describing wild grapevine as a separate species in several localities from northern and western parts of Iran in the Flora Iranica, two subspecies (V. sylvestris subsp. anebophylla Kolen. and subsp. trichophylla Kolen.) were proposed within the taxon on the basis of differences in indumentum morphology of the lower surfaces of the leaves (Vassilchenko 1970). In the subsp. anebophylla, the undersides are covered with long arachnoid, appressed hairs (trichomes) which are shed at the end of the growing season, while in subsp. trichophylla, the undersides are covered by small cellular hairs (Vassilchenko 1970). The presence of two different taxa within the V. sylvestris group has not been described elsewhere in its distribution area. However, Vitis trichophylla (Kolen.) Vass. was proposed as a separate species in the Flora of USSR among 12 species of V. vinifera sensu lato [START_REF] Sosnovskii | Vitacee[END_REF].

During several herbarium visits in Paris (P), Geneva (G), Montpellier (MPU) and Munich (M), there were few specimens of V. vinifera subsp. sylvestris from Asia and the Caucasus. We found four materials from the Hyrcanian region: (1) 57 km NE Gonbad-e Ghaboos, 340 m, Pabot-7702 (G!); (2) Golestan National Park, Tang-e Gol, 720 m, There were five specimens of wild grapevine in these herbaria collected from Kurdistan, Luristan, Zanjan, west Azerbaijan and Kerman.

Vitis sylvestris subsp. trichophylla was determined based on one specimen collected from Mazandaran . The exact locality is "Between Kinj and Dasht-e Nazir 800-1300 m" (G!, M!, IRAN!). It is included in the Flora Iranica and corresponds to our sites no. S21-S24 (Fig. 1). This specimen was originally named under V. sylvestris subsp. torulosa Vass. and considered as TYPUS specimen in the Geneva and Munich herbaria (Fig. 3). However, although the description of the taxon has never been published, it is mentioned as the only specimen of subsp. trichophylla in the Flora Iranica. Moreover, there is a specimen in the Geneva herbarium under the name of Vitis sylvestris subsp. anebophylla as determined by the author of the Flora Iranica. The specimen was collected in the coastal forests of Bandar-e Gaz, Golestan Province, Rechinger-5524 (cited in Flora Iranica) (Fig. 4). The author also listed five other localities for the subsp. anebophylla which were examined in the herbarium IRAN.

We checked the original descriptions of the two subspecies provided by A. Kolenati in Bull. Soc. Natur. Mosco. [START_REF] Kolenati | Versuch einer systematischen Anordnung der in Grusinien einheimischen Reben nebst einem oekonomischtechnischen Anhange[END_REF] under his classification of wild forms of Vitis vinifera. These were based on morphological variations, particularly those related to indumentums of the leaves. While these two forms of the plant were collected in more or less the same original localities, the authors of Flora Iranica proposed these two forms under the synonymies of V. sylvestris, and at the same time as two separate subspecies of V. sylvestris. It seems that the separation of subspecies anebophylla and trichophylla was not completely certain for the author of Flora Iranica as he gave localities for Vitis sylvestris at first, and then only based on a few specimens, two subspecies were recognized. This is particularly the case of subsp. trichophylla which is nomenclaturally based on only one specimen. The specimen is not morphologically different from another subspecies except for the indumentum features of the lower surface of the leaves. This finer classification was not followed by other researchers in Iran [START_REF] Akhani | Plant biodiversity of Golestan National Park, Iran[END_REF].

According to diagnostic characters in Flora Iranica, all material of 34 populations was carefully checked. They have mostly the features of those described for subsp. anebophylla, i.e., long arachnoid hairs on the lower surface of their leaves. Only specimens from sites S22 and S23 (Mazandaran, Chalus road, Hassanabad, 2 km after deviation toward Kandalous village between Kines and Hassanabad, on the road to Kojour) showed some similarities with the description provided for subsp. trichophylla. However, it was not entirely certain because two forms of the leaves were observed in neighboring populations and also even on the same plant. It would appear that environmental features, especially climate, affect leaf morphological variations of the polymorphic Vitis specimens [START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF]. The occurrence of both forms of the leaves (i.e., both subspecies) in almost the same area is consistent with the localities given in Flora Iranica (Vassilchenko 1970) and observed in the surrounding areas [START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF].

Morphological peculiarities, habitat characteristics and strategies

The tallest grapevine (30 m) was found in the closed forests and shrublands of Golestan National Park with Carpinus betulus and Prunus divaricata Ledeb. as dominant species, and the shortest plant (2 m) was in Miankaleh Biosphere Reserve with Punica granatum L. and Mespilus germanica L. as dominant species. The largest grapevine (DBH: 24.5 cm) grew in the shady closed forests of Shastkola Research Forest with Parrotia persica as the dominant species (Fig. 5), while the smallest trunk diameter (\2 cm) was in the Miankaleh Biosphere Reserve and in Hassanabad populations in open habitats. In valleys at higher Fig. 3 Vitis sylvestris subsp. trichophylla (sensu Flora Iranica) in the herbarium Geneva (G!) proposed to be a new subspecies (i.e. subsp. torulosa) which was never published and then considered as subsp. trichophylla Wild grapevine (Vitis vinifera subsp. sylvestris) in the Hyrcanian relict forests of northern… 123 altitudes, plants have thicker leaf densities than those along the sea. This may be due to a more humid environment. DBH, plant height and openness of the landscape were statistically correlated as the grapevines tended to be taller and stronger inside the closed forests. Individuals with larger foliage mostly belonged to plants with higher DBHs but not necessarily to those with taller stems (Table 1). Sites located further from the river are mostly at lower altitudes. It may be interpreted as a sub-tropical lowland environment benefiting from higher rainfalls and temperature [START_REF] Frey | A synopsis of the vegetation of Iran[END_REF].

According to our field observations, V. vinifera subsp. sylvestris grows in a wide range of habitats, including wetlands, along seasonal rivers and streams, in shady closed forests, on alluvial beds of large rivers, in sand dune shrublands, and forested wetlands (alder forests). In Golestan National Park, north east of Iran, plants grow in different alluvial habitats, in secondary closed shrublands and on river banks along the Khandushan River, alluvial habitats of the Mezalli river bed, and in open shrublands along seasonal rivers near Golzar. It has also been recorded to be a riparian species in Juniperus woodlands of the Park [START_REF] Akhani | Plant biodiversity of Golestan National Park, Iran[END_REF].

Except for azonal vegetation of sand dune shrublands and a few wetland and ruderal habitats, all other habitats are classified as lowland and submontane (river basin) forest vegetation below 1000 m (sensu [START_REF] Frey | A synopsis of the vegetation of Iran[END_REF][START_REF] Akhani | Plant biodiversity of Golestan National Park, Iran[END_REF][START_REF] Akhani | Plant biodiversity of Hyrcanian relict forests, north Iran: an overview of the flora, vegetation, palaeoecology and conservation[END_REF]. They are largely represented in two phytosociological alliances Parrotio-Carpinion and Alnion subcordatae (e.g., [START_REF] Zohary | Geobotanical foundations of the Middle East[END_REF][START_REF] Assadollahi | Etude phytosociologique et bioge ´ographique des fore ˆts hyrcaniennes[END_REF][START_REF] Djazirei | Contribution a `l ˇe ´tude des fore ˆts primaires de la Caspienne[END_REF][START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF][START_REF] Hamzeh'ee | Phytosociological survey of remnant Alnus glutinosa subsp. barbata communities in the lowland Caspian forests of northern Iran[END_REF]. Moreover, we observed in some of our study sites populations of wild grapevine that had escaped from the closed forest to climb remote trees and shrubs in the forest margin in more favourable habitats. All the sites in this investigation ranged from below zero to 936 m.a.s.l. The highest occurrence of this species was recorded from 2300 m.a.s.l. in Gorgan [START_REF] Vassilczenko | Vitaceae[END_REF][START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF], not observed in the current investigation partly because of the higher destruction situation today in the upper altitudes of the area. Nevertheless, our personal observation over the whole Hyrcanian area indicates that the species is very rare and mainly confined to elevations lower than 500 m. The latter, which is classified as lowland forest, is characterized by sub-tropical, humid closed forest [START_REF] Zohary | Geobotanical foundations of the Middle East[END_REF][START_REF] Frey | A synopsis of the vegetation of Iran[END_REF][START_REF] Naqinezhad | Vegetation-environment relationships in the alderwood communities of Caspian lowlands, N. Iran (toward an ecological classification)[END_REF]). Based on previous investigations, V. vinifera subsp. sylvestris has been reported to be accompanied by other lianas such as Hedera pastuchovii, Smilax excelsa, Jasminum officinale L., Humulus lupulus L. (extremely rare), Periploca graeca L., Dioscorea communis, Calystegia sepium (L.) R. Br. [START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF][START_REF] Browicz | Chorology of trees and shrubs in south-west Asia and adjacent regions[END_REF]). It should be noted that wild grapevine expands its distribution towards west Azerbaijan and Lorestan provinces [START_REF] Vassilczenko | Vitaceae[END_REF]) (Fig. 2).

Wild grapevine grows on a large variety of host species (trees, shrubs or other lianas). Parrotia persica and Carpinus betulus were, however, the most frequent in the studied populations (Appendix S1). Parrotia persica forests of submontane zones have previously been reported as habitat for this liana [START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF].

Wild grapevine has also shown various strategies for establishment. On riversides of Golestan National Park, north east of Iran, we have noted an interesting reproductive strategy. Plants easily germinate on the first alluvial terrace where floods regularly destroy other more competitive plants (Fig. 6). By layering, they colonize the second terrace and climb up to the canopy of both terraces, leading to impressive sizes (up to 20 m long and 25 cm DBH). They also form numerous secondary axes and abundant foliage. In the Shastkola alluvial forests, wild grapevines have colonised both forest edges and forest interior at least 100 m from the river. These individuals have often reached impressive size such as in the Golestan National Park, especially near gaps where possibly old channels occurred and are now recovered with closed forest.

Strategies for growth and survival of the wild grapevine are different in the sand dune shrublands of Miankaleh Biosphere along the coast of the Caspian Sea. The vegetation forms scattered patches of small-diameter bushes of Punica granatum, Rhamnus pallassii Fisch. & C. A. Mey. and Mespilus germanica. The wild grapevine here has small diameters and germinates at the edges of spiny bushes, spreading leaves on the surface of the bush. Large amounts of fruits are produced in the early summer.

Conservation

In Iran, wild grapevine has been recognized to be at risk of local extinction in the Zagros Mountains and in northwestern Iran (Doulati-Baneh et al. 2007, 2014). A similar situation exists in the lowland Hyrcanian forests where many sites have been destroyed or drained and replaced with agricultural fields [START_REF] Akhani | Plant biodiversity of Golestan National Park, Iran[END_REF][START_REF] Siadati | Botanical diversity of Hyrcanian forests: a case study of a transect in the Kheyrud protected lowland mountain forests in northern Iran[END_REF][START_REF] Naqinezhad | A phytosociological survey of two lowland Caspian (Hyrcanian) remnant forests, Northern Iran, for validation of some forest syntaxa[END_REF][START_REF] Naqinezhad | A floristic survey of the Hyrcanian forests in Northern Iran, using two lowland-mountain transects[END_REF]. The presence of endangered wild grapevine in some sites is an important observation, given the high conservation value of the sites for birds. Some of these sites are part of Iran's nine Biosphere Reserves of the Biosphere Programme. It is a stopover and wintering area for songbirds and birds of prey, and a suitable zone for nesting owing to the presence of shrubs such P. granatum. Their morphology provides protection, No record of occurrence was found of the subspecies in old phytosociological surveys in different places of the Hyrcanian area (e.g. [START_REF] Djazirei | Contribution a `l ˇe ´tude des fore ˆts primaires de la Caspienne[END_REF][START_REF] Dorostkar | Contribution a `l ˇe ´tude des fore ˆts du district Hyrcanien oriental (Chaine de Gorgan)[END_REF][START_REF] Assadollahi | Etude phytosociologique et bioge ´ographique des fore ˆts hyrcaniennes[END_REF]. Moreover, except for those cited in Flora Iranica, no herbarium records were recorded for this species in the virtual catalogue of International Herbaria. The situation appears similar in the eastern part of its range south of the Caucasus in Georgia [START_REF] Ramishvili | Dikorastushchii vinograd Zakavkazia (Wild grape of the South Caucasus)[END_REF][START_REF] Ekhvaia | Morphological variation and relationships of Georgian populations of Vitis vinifera L. subsp. sylvestris (C.C. Gmel.) Hegi[END_REF][START_REF] Maghradze | Ecological and sanitary characteristics status of the Eurasian wild grapevine in the South Caucasian region[END_REF].

In the western part of the wild grapevine range in Europe, the main habitats are forest gaps and river edges of flood-prone areas [START_REF] Arnold | Ecologie de la vigne sauvage (Vitis vinifera L. subsp. sylvestris (Gmelin) Hegi.) dans les fore ˆts alluviales et colluviales d'Europe[END_REF]. As a woody liana, individuals can live several decades and reach large sizes (i.e., 30 cm DBH and lengths of several tens of meters). On alluvial floodplains, wild grapevine populations compete with other woody vines such as Hedera sp., Smilax sp., Clematis vitalba L. and Humulus lupulus [START_REF] Schnitzler | Contribution des lianes a `la biodiversite ´me ´diterrane ´enne[END_REF]. Currently wild grapevine in Europe is present only in remote mountain sites, screes, floodplain forests and islands [START_REF] Arnold | Ecologie de la vigne sauvage (Vitis vinifera L. subsp. sylvestris (Gmelin) Hegi.) dans les fore ˆts alluviales et colluviales d'Europe[END_REF][START_REF] Arnold | Historical reconstruction of a relictual population of wild grapevine (Vitis vinifera subsp. sylvestris) in a floodplain forest of the upper Seine valley, France[END_REF][START_REF] Schneider | Identification and characterization of Vitis vinifera subsp sylvestris populations in north-western Italy[END_REF]. The species was added to the IUCN (International Union for Conservation of Nature) list of endangered European plants in the 1980s and was listed as a critically endangered species in 2000. The plant has been strictly protected in some countries, including France, the Czech Republic, Spain and Italy.

The Holocene history of Vitis in the Hyrcanian region

Vitis pollen has been frequently reported in Holocene (the last about 11,700 years) pollen records of the South Caspian region. Characteristics of the palynological sites along with the approximate date of the earliest records of Vitis pollen are presented in Table 2.

In pollen identification under a light microscope, the distinction between wild and cultivated species is not possible [START_REF] Bottema | Anthropogenic indicators in the pollen record of the eastern Mediterranean[END_REF]. This is the main reason hampering the inference of viticulture in fossil pollen records from the sites located within the natural distribution such as the South Caspian region [START_REF] Turner | Vitis pollen dispersal in and from organic vineyards: I. Pollen trap and soil pollen data[END_REF][START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]. However, in southwest Asia, Vitis cultivation can be more certain when additional arboricultural pollen indicators, such as Olea, Castanea, Fraxinus, Cornus, Juglans, and Platanus occur in the pollen records of a given area [START_REF] Behre | Some reflections on anthropogenic indicators and the record of prehistoric occupation phases in pollen diagrams from the Near East[END_REF][START_REF] Bottema | Anthropogenic indicators in the pollen record of the eastern Mediterranean[END_REF][START_REF] Djamali | A late Holocene pollen record from Lake Almalou in NW Iran: evidence for changing land-use in relation to some historical events during the last 3700 years[END_REF][START_REF] Connor | A Promethean Legacy: late Quaternary vegetation history of Southern Georgia, the Caucasus[END_REF]. In some cases, the inadequacy of pollen records may also be compensated with seed morphology analysis in archaeobotanical materials as the latter may help to differentiate material of wild versus cultivated grapevine [START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]. For example, seeds of domesticated grapevines dating back to 8000 BP (6th millennium BC) have been found in Georgia and Turkey [START_REF] Terral | Evolution and history of grapevine (Vitis vinifera) under domestication: new morphometric perspectives to understand seed domestication syndrome and reveal origins of ancient European cultivars[END_REF].

On the other hand, considering the entomophilous (insect distributed) mode of pollination in Vitis, along with its low pollen dispersal, it is possible to assess the local presence of this plant even with percentage values as low as 0.1-0.2% in fossil records [START_REF] Bottema | Anthropogenic indicators in the pollen record of the eastern Mediterranean[END_REF][START_REF] Turner | Vitis pollen dispersal in and from organic vineyards: I. Pollen trap and soil pollen data[END_REF]. [START_REF] Turner | Vitis pollen dispersal in and from organic vineyards: I. Pollen trap and soil pollen data[END_REF] have shown that the on-site presence of a vineyard can be strongly evidenced by values as low as 2%.

The Holocene history of wild grapevine in the Hyrcanian region of northern Iran is based on available palynological records. In Iran, the oldest pollen records were found in the catchment area of Lake Urmia in northwest Iran dating back to the last interglacial period (ca. 130 to 115 9 10 3 years ago) and the following stadial/interstadial intervals from ca 115 to 75 9 10 3 years ago [START_REF] Djamali | A late Pleistocene long pollen record from Lake Urmia, NW Iran[END_REF]. This finding suggests that the Lake Urmia region is probably located within the natural range of wild grapevine as suggested by [START_REF] Zohary | Domestication of plants in the Old World[END_REF]. Vitis pollen appear in pollen diagrams from inside the vine distribution area since the beginning of the Holocene (e.g. Lake Ghab, Lake Van, and Lake Urmia), and can be attributed to the wild grapevine [START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]. However, outside its natural range, the vine pollen indicates the spread of viticulture.

In the south Caspian region located within the natural distribution area of Vitis, no pollen record older than the Mid-Holocene has been established (Table 2). However, low values of Vitis pollen in the oldest pollen records, i.e., Pay Hassal and Kongor [START_REF] Shumilovskikh | Landscape evolution and agro-sylvo-pastoral activities on the Gorgan Plain (NE Iran) in the last 6000 years[END_REF], sites suggest either the sporadic incidence of wild grapevines in the immediate surrounding forest communities or the cultivation of its producers (mother plants) somewhere in the Caspian lowlands. Both cases seem quite plausible considering the steep altitudinal gradient from the Caspian lowlands to the Alborz highlands, and the dominant northerly winds which can easily transport pollen grains upslope [START_REF] Markgraf | Pollen dispersal in a mountain area[END_REF][START_REF] Ramezani | Pollen-vegetation relationships in the central Caspian (Hyrcanian) forests of northern Iran[END_REF]). This upslope transport is not uncommon as typical Hyrcanian tree pollen in the central Iranian deserts may be found showing that they have crossed the [ 3000 m Alborz Mountains [START_REF] Moore | Pollen studies in dry environments with particular reference to Turan[END_REF]. The present altitudinal distribution of wild grapevine is mainly confined to lowto mid-elevation (below 1000 m) sites (Vassilchenko 1970;[START_REF] Sabeti | Forests, trees, and shrubs of Iran[END_REF]. In line with this interpretation, the appearance and persistent values of Vitis pollen in the Pay Hassal pollen record since the Mid-Holocene may imply the onset of viticulture in the central Hyrcanian region (Table 2). In the eastern Hyrcanian region, the occurrence of sporadic Vitis pollen in Mid-Holocene may indicate its presence in the wild while its increase since the transition from the late Bronze Age to early Iron Age, in combination with other arboricultural

Wild grapevine (Vitis vinifera subsp. sylvestris) in the Hyrcanian relict forests of northern… species (Juglans and Platanus), strongly suggests its cultivated origin in the southeast Caspian lowlands in Gorgan Plain [START_REF] Shumilovskikh | Landscape evolution and agro-sylvo-pastoral activities on the Gorgan Plain (NE Iran) in the last 6000 years[END_REF]). This increase in Vitis pollen in association with other cultivated species during the Bronze and Iron Ages is not an exclusive feature of the Hyrcanian area but has been documented in other parts of the Irano-Anatolian region [START_REF] Bottema | Anthropogenic indicators in the pollen record of the eastern Mediterranean[END_REF]Djamali et al. 2011a).

The presence of Vitis pollen before the Bronze Age, even if sporadic, suggests that this area may be one of the domestication centers for Vitis or an area with co-occurrence of both wild and cultivated species. However, it should be noted that the cultivation and domestication of wild grapevine appears to have occurred between the seventh and the fourth millennia BC, somewhere in a vast geographical area between the Black Sea and northwest Iran [START_REF] Mcgovern | Neolithic resinated wine[END_REF][START_REF] This | Historical origins and genetic diversity of wine grapevines[END_REF][START_REF] Myles | Wild grapevine (Vitis vinifera subsp. sylvestris) in the Hyrcanian relict forests of northern… Genetic structure and domestication history of the grapevine[END_REF], and that one of the earliest evidence of wine production was found in Iran at the Hajji Firuz Tepe site in the northern Zagros mountains ca. 7400-7000 cal yr BP [START_REF] Mcgovern | Ancient wine: the search for the origins of viniculture[END_REF][START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]. The exact time and place of grapevine domestication in western Asia is, however, not known due to the incompleteness of archaeobotanical and palynological data from the region [START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]. It seems that the first use of grapes from wild species dates back to the 8 th millennium BC, the first evidence of wine production in the 6th millennium BC, but the real domestication and planting of grapevine dates back to the Early Bronze Age [START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF].

Pollen evidence can more reliably be used as indicating viticulture in areas located outside the natural distribution of Vitis, e.g., the southern Zagros Mountains in Iran [START_REF] Miller | Sweeter than wine? The use of the grapevine in early western Asia[END_REF]Djamali et al. 2011a). However, this does not exclude the fact that the South Caspian region may be considered as a domestication center and/or important viticulture area since at least the Bronze Age. The natural presence of the wild species increases the possibility that domesticated grapevines were selected and vegetatively propagated by Bronze Age people in the Caspian region.

Conclusion

Our study indicates that wild grapevine (V. vinifera subsp. sylvestris) is sporadically distributed in the Hyrcanian region, the eastern edge of its global distribution, and as a result, important for biodiversity conservation. Thirty-four natural populations from below sea level up to 1000 m.a.s.l. were examined. Two variants of anebophylla and trichophylla (within the subspecies sylvestris) in the Flora Iranica were separated only by the indumentums of the leaves and not confirmed by herbarium specimens and material collected in the study sites. Despite to its sporadic nature, V. vinifera subsp. sylvestris grows in a wide range of habitats, including wetlands, along seasonal rivers in closed forests, alluvial beds of large rivers, sand dune shrublands and forested wetlands (alder forests). Does the available pollen data reveal further significant spatio-temporal patterns of Vitis populations in the Hyrcanian region over the Mid-to Late-Holocene? Due to the young age of all pollen diagrams from the South Caspian region and the overall sporadic nature of Vitis pollen in most of these records, this cannot be interpreted with certainty. Even the constant and relatively high values throughout the Mazgah pollen record do not reflect changes in the frequencies of its producer in the Caspian lowlands (Table 2). The question of wild versus cultivated grapevine in the Hyrcanian pollen records and the possible date of its domestication and/or cultivation will remain open until further palynological and archaeobotanical studies are undertaken in the area. The natural and/or anthropogenic origins of the Late-Holocene populations of Vitis require older pollen records covering at least the entire Holocene with a higher spatial resolution.

  l and are characterized by Acer velutinum Boiss., Carpinus betulus L., Pterocarya fraxinifolia, Quercus castaneifolia C. A. Mey., Zelkova carpinifolia, Albizia juilibrissin Durazz., Buxus hyrcana Pojark., Diospyros lotus L., Gleditsia caspia, Ilex spinigera (Loes.) Loes., Parrotia persica, Ruscus hyrcanus Woronow, Hedera pastuchovii Woronow, Smilax excelsa L., V. vinifera subsp. sylvestris and Dioscorea communis (L.) Cassick and Wilkin. Montane forests are located mainly from 1000 up to 2000 m.a.s.l. and are characterized by Fagus orientalis Lipsky and Carpinus betulus. Moreover, open woodlands of Quercus macranthera Fisch. and C. A. Mey. ex Hohen., Acer campestre L., Acer hyrcanum Fisch. and C. A. Mey. and Carpinus orientalis Mill. prevail in some upper altitude vegetation.

Fig. 1

 1 Fig. 1 Distribution of Vitis vinifera subsp. sylvestris populations (S, black filled circle) with reported and unreported pollen sites (PS black filled square) across the Hyrcanian region, N. Iran

Fig. 2

 2 Fig. 2 Records of distribution of wild grapevine populations in Iran since the 19th century based on literature and data from herbarium specimens. Dots represent localities. Mountains are in grey. Seas are in dark

Fig. 4

 4 Fig. 4 Vitis sylvestris subsp. anebophylla (sensu Flora Iranica) in the herbarium Geneva (G!)

Table 1

 1 Spearman correlation coefficients between the environmental variables across 34 sites of Vitis vinifera subsp. sylvestris populations Vitis vinifera subsp. sylvestris) in the Hyrcanian relict forests of northern… humidity, and shade during the reproductive season and a source of fruit (V. vinifera subsp. sylvestris, Rubus sanctus Schreb. and Lonicera floribunda) that these birds disseminate after consumption.

	Variables	Moisture	Openness	Distance (m)	Foliage	Plant	DBH (m)	Ratio of foliage cover of
					cover (m 2 )	height (m)		grapevine to host
								canopy cover (%)
	Altitude	0.35*	-0.19	-0.69**	0.40*	0.28	0.02	-0.05
	Moisture		-0.9**	-0.53**	0.23	0.62**	0.47**	-0.13
	Openness			0.16	-0.17	-0.39*	-0.40*	0.37*
	Distance				-0.49**	-0.44**	-0.28	-0.15
	Foliage cover					0.29	0.37*	0.44**
	Plant height						0.49**	0.11
	DBH							0.15
	*P \ 0.05, **P \ 0.01

Fig. 6 The habitat of Vitis vinifera subsp. sylvestris in Golestan National Park, north east of Iran Wild grapevine (

Table 2

 2 Site characteristics, the earliest dates of Vitis pollen and the temporal coverage of available palynological records in the Hyrcanian region. Unless otherwise indicated, the most abundant and/or dominant tree species occurring in or around the site are typed bold italic

	Site	Site name	Geographical	Altitude	Temporal	Earliest date	Percentage	Woody taxa	References
	no.		coordinates	(m. a.s.	coverage	of Vitis pollen	values of
				l.)	(cal. yr	(cal. year BP)	Vitis pollen
					BP)
	1	Pay Hassal 36˚31.217′N	1314	Entire	5000	0.8 (0.3-	Fagus orientalis, Alnus subcordata,	Ramezani et al.
			51˚25.017′E		Holocene		1.5)	Carpinus betulus, Acer velutinum, A. cappadocicum, Cerasus avium,	(unpublished data)
								Ulmus glabra, Sorbus torminalis,
								Quercus castaneifolia, Fraxinus
								excelsior subsp. coriariifolia, Tilia
								platyphyllos, Salix aegyptiaca,
								Mespilus germainca, Crataegus sp.
	2	Muzidarbon 36˚33′N	550	[ 1000	ca. 950	0.3 (0.05-	Carpinus betulus, Parrotia persica,	Ramezani et al.
			51˚29′E				1)	Pterocarya fraxinifolia, Diospyros Acer velutinum, Alnus subcordata,	(2008)
								lotus, Quercus castaneifolia, Tilia
								platyphyllos, Acer cappadocicum,
								Ulmus glabra, Fagus orientalis
	3	Djamand	36˚30.117′N	1550	650	550	0.3 (0.1-	Fagus orientalis, Carpinus betulus,	Ramezani et al.
			51˚30.417′E				0.5)	Alnus subcordata, Acer velutinum, A. cappadocicum, Cerasus avium,	(unpublished data)
								Fraxinus excelsior subsp.
								coriariifolia, Vaccinium
								arctostaphyllos, Laurucerasus
								officinalis, Ilex spinigera
	4	Tepe Kelar 36˚31.202′N	1080	900	ca. 700	2 (0.6-4.2) Scattered individuals of Populus alba,	Ramezani
			51˚11.992′E					Salix aegyptiaca, Salix alba, Crataegus spp., Lonicera floribunda, Prunus divaricata, Cornus australis, Rhamnus pallasii	(2013) Khalili et al. (unpublished data)
	5	Mazgah	36˚36.283′N	-22	2600-2700 2600-2700	4.6 (0.4-	Remnants of Alnus glutinosa and	Ramezani et al.
			51˚36.35′E				11.8)	occurrences of Acer velutinum, Pterocarya fraxinifolia, Scattered	(2016)
								Gleditsia caspia, Diospyros lotus,
								Prunus divaricata, Morus alba,
								Punica granatum, Ficus carica, Vitis
								vinifera, Lonicera sp., Hedera
								pastuchovii, Smilax excelsa
	6	Kannoo	36°31.45′N	1330	600	600	0.3 (0.2-	Fagus orientalis, Alnus subcordata,	Ramezani et al.
		Hassal	51°22.8′E				0.6)	Carpinus betulus, Cerasus avium, Quercus castaneifolia, Pterocarya	(unpublished data)
								fraxinifolia, Ulmus glabra, Acer
								cappadocicum, Tilia platyphyllos,
								Salix aegyptiaca, Frangula alnus
	7	Tuska	37°11.65′N	1040	2050	2000	0.71 (0.4-	Quercus castaneifolia, Carpinus	De Beaulieu
		Tchal	55°31.633′E				2.5)	betulus, Alnus subcordata, Salix sp., Pterocarya fraxinifolia, Ulmus	et al. (unpublished
								glabra, Juglans regia, Fagus	data)
								orientalis
	8	Kongor	37°21.35′N	93	6100	5800		Open landscape with background	Shumilovskikh
			55°23.45′E					Carpinus betulus, Alnus subcordata pollen: Quercus castaneifolia,	et al. (2016)
	9	Amirkola	37°21.427′N	-20	Before	At least	\ %1	Alnus glutinosa, Carpinus betulus,	Leroy et al.
			50°11.703′E		171-223	slightly before 223		Ulmaceae, Fagus orientalis, Quercus castaneifolia

Acknowledgements This research was supported by the Centre for International Scientific Studies and Collaboration (CISSC) and the French Embassy in Tehran under the "Gundishapour" project. We thank Pierre Lhote of the French Embassy for his encouragement during the study, and Nicolas Fumeaux (Geneva), Florian Jabbour (Paris) and Caroline Loupe (Montpellier) for their herbarium assistance. We also thank V. Kheirabadi and M. Madadi and other staff of the Golestan Nationa Park, R. Esmailnejad, Astaneh Ashrafieh, M. Naseri, Asalem, for all their support during field studies. This study was carried out with permission from the Department of Environment for the Golestan National Park and the Miankaleh Biosphere Reserve under permission 93/10330, and by the protection of the Department of Forestry, Gorgan University for the Shastkola forest under permission 93/24/17431.

Project Funding: This research was supported by the Centre for International Scientific Studies and Collaboration (CISSC) and the French Embassy in Tehran under the "Gundishapour" project.

The online version is available at http://www.springerlink.


Kailang Liu

Isabelle Berbezier

Thomas David

Luc Favre

Marco Abbarchi

Antoine Ronda

Peter Voorhees

Jean-Noël Aqua
email: aqua@insp.jussieu.fr

Nucleation-vs-instability race in strained lms

Keywords: self-organization, heteroepitaxy, quantum dots, instability, nucleation

Under the generic term Stranski-Krastanov are grouped two dierent growth mechanisms of SiGe quantum dots.

They result from a self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. We elucidate here the puzzling dierence between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the nucleation time scale and ATG characteristic time, we show that the former exhibits a strong exponential divergence at low strain while the latter behaves only algebraically. Consequently, at high/low strain, nucleation/instability occurs faster and inhibits the alternate evolution. The cross-over between the nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50%.

Quantum dots are nowadays extensively grown by different techniques and used in a broad range of applications, from high-performance broadband photodiodes 1 to quantum information processing, 2 quantum cryptog- raphy with photon quantum bits (Qbits), 3,4 In Si 1-x Ge x lms on Si, one can taylor the amount of strain by varying the mean Ge concentration x. At low x, see Fig. 1, one nds an instability in connection with self-organized phenomena where dynamics builds long-range structures. 15,16 This instability is nucleationless 17,18 and leads after some coarsening to anisotropic quantum dots. 19 At high x conversely, see Fig. 1, the dots nucleate quickly and randomly without any long range order. 20,21 We develop here a ki- netic model to rationalize the competition between the two mechanisms and to evaluate the cross-over concentration x c that separates them. It incorporates the main ingredients that rule island growth, i.e. the driv- In Fig. 2(a), the morphology roughens on the whole surface as described by the ATG instability. 17,18 The wavelength of this corrugation is conveniently extracted from a ring-like Fourier Transform image, in good agreement with the experimental results for this instability. 17 is

∆E = ∆E surf + ∆E ed + ∆E el , (1)
with the surface energy contribution ∆E surf , the edge energy ∆E ed and the elastic relaxation ∆E el . Mass conservation enforces the balance h w = h 0 -ρ V /a where a is the lattice parameter (a = 0.27 nm in SiGe) and 1/ρ, the surface available for each island.

∆E surf = γ (001) Ge η L 2 , (2)
with the capillary number

η = γ (105) Ge γ (001) Ge 1 cos θ -1 , (3)
that describes the stability of the (001) surface with respect to faceting to (105). When η > 0, the creation of a (105) facet is overall a cost in energy, so that capillarity is a resistant force. 34 35 We consider in the fol- lowing γ

(001) Ge = 60.5 meV/Å 2 , see 36,37, while γ (105) Ge is given by η as discussed below. Finally, we also include edge energy 25,28,38 to describe the pyramidal shape with a mean edge energy σ ed for the pyramid and pyramid/wetting layer angles, so that

∆E ed = 4H tan θ 2 + 2 + tan 2 θ σ ed . (4)
As regards elasticity, mechanical equilibrium equations may be solved exactly in the systems under investigation that display small slopes [at most 11 • for the (105) facets]. In the small slope approximation, a lm with a free surface z = h(r) has an elastic energy 39

E el = E 0 dr {h(r) -ζh(r)H ii [h(r)]} , (5)
with the energy density

E 0 = Y f m 2 /(1 -ν f ) and coe- cient ζ = Y f (1 -ν 2 s )/Y s (1 -ν f )
, where Y and ν are the Young's modulus and Poisson's ratio while subscripts

∆E el = -ζ p E 0 V , (6)
for a square base pyramid, with

p = 4( √ 2 -1)[1 + ln(1 + √ 2)] tan θ/π . (7)
As a whole, the energy barrier ∆E reduces to the

� (�� � ) � � * � * � �� ���
dρ n dt = m [f m,n ρ m -f n,m ρ n ] , (9)
that relates the densities ρ n of clusters with n atoms, with the frequencies f m,n of the transitions from m to n-atoms clusters. When only one-atom events occur, only the frequencies f n = f n,n+1 matter. They may be estimated as

f n = γ n αD s ρ 1 , (10)
with the attachment coecient γ n , capture coecient α, adatom density ρ 1 (t) and surface diusion coecient D s (T ). In the following, we will use α 1, γ n 1, D s = a 2 ν 0 e -βE dif f with ν 0 10 13 s -1 and the diusion barrier E dif f = 0.83 eV in Si. 42 We also make the approximation ρ 1 = 1 a 2 e -βE2 with the attachment energy E 2 0.3 eV. The master equation has a stationary solution characterized by a ux of nucleation per unit time and surface 23,41

J st = Zf n * ρ n * , ( 11
)
with f n * , the growth frequency of a critical cluster with n * atoms (corresponding to the critical volume V * ), and the critical cluster density

ρ n * = ρ 1 e -β∆E * . (12)
In (11), the Zeldovich factor is given by 23,41

Z = - ∂ 2 ∆E ∂n 2 n * 1 2πk B T , (13)
that reduces here to

Z = 2a 3 √ β tan θ √ πσ 2 γ -γ2 + 3σ p γ2 + 3σ p 1/4 . ( 14
)
The ux J st is associated with the typical time scale for nucleation

τ nuc = 1 J st λ 2 , ( 15
)
where we choose to consider nucleation over the typical island zone λ 2 dened by the experimental density λ = 1/ √ ρ, with ρ 10 13 m -2 .

With this time scale in hand, we turn to the ATG morphological instability. 43,44 It may be captured by the continuum description of surface diusion governed by ∂h/∂t = D∆ s µ with the diusion coecient D, surface Laplacian ∆ s and chemical potential µ. 42 The latter includes the capillary term γ∆ s h and the elastic energy density E 0 so that dimensional analysis leads to the instability space and time scales 45

l AT G = γ f 2ζE 0 and t AT G = l 4 AT G Dγ f , (16)
with the surface energy γ f = γ (001) Ge and the elastic energy density given above that is proportional to m 2 = (0.042 x) 2 . Hence, t AT G is proportional to 1/x 8 . ‡ ‡ We plot in Figure 5 the typical time scales of the ATG instability and of nucleation. The nucleation time τ nuc displays a strong exponential increase at low x, overshooting the rather slow varying t AT G . Hence, we argue

� ��� τ ��� τ ��� ��� ��� ��� ��� ��� ��� ��� ��� ��� � �� -� � �� � �� � �� �� �� �� �� �� � (�)
Figure 5: Typical time scales for nucleation τ nuc (red dark grey line) and ATG instability t AT G (greenlight grey line) as a function of the lm Ge composition x for η = 0.003 and σ ed = 3.3 meV/Å. The red dotted line corresponds to the analytic approximation (17).

that the two pathways (instability vs nucleation) are dictated by kinetics: for large enough x, τ nuc t AT G so that nucleation occurs rst, relaxes partially the elastic strain and prevents the occurrence of the ATG instability. On the contrary, for low enough x, τ nuc t AT G and only the instability has time to occur.

The cross-over between the two time scales may be rationalized by the strong decrease in the critical clusters density ρ n * when x decreases. Indeed, when x decreases, the surface energy contribution is constant while the amplitude of the elastic relaxation decreases as E 0 ∝ x 2 so that the maximum for ∆E, ∆E * , increases. Because this energy barrier enters in a Boltzmann factor in ρ n * , the nucleation rate exponentially decreases with x. To quantify this eect, one may simplify the expression of J st by performing a small-x expansion of ∆E * , with the result

τ nuc ≈ τ nuc 0 e β b γ 3 x 4 + c γσ ed x 2 , (17)
with some constants τ nuc 0 , b and c. This approximation is shown in Fig. 5 and does indeed match the exact result at low x. With this approximation, it is clear that the capillary-vs-elasticity balance leads to a strong exp(1/x 4 ) divergence of τ nuc at low-x that quickly overshoots the ATG time scale that `only' behaves as 1/x 8 .

The system under study may include extra eects such as: alloying (intermixing, segregation, surface inhomogeneities), surface reconstruction inhomogeneity and evolution, wetting eects, surface stress, inhomogeneous nucleation etc. However, we argue that the main scenario ruling the cross-over between the ATG instability ‡ ‡ we use in the following D = βD 0 c Ω, with the vacancy surface concentration c 1/a 2 , Ω = a 3 and D 0 = 8.45 10 -10 e -βE d m 2 s -1 with E d = 0.83 eV 42 and nucleation is contained in this when x decreases, the elastic driving force decreases, the energy barrier increases so that nucleation occurs over an exponentially divergent time scale.

The data in Fig. 5 are computed with parameters typical of SiGe systems as described above ‡ ‡ . The in- tersection between the two time-scales occurs around x c 0.55 that is consistent with the experimental results but that is dependent on the model parameters as described below. The classical nucleation theory includes dierent parameters that are only approximate (capture zone, etc) but which precise values are not relevant for the existence of the cross-over. The two parameters that prove to be quantitatively important for x c are the capillary number η and edge energy σ ed .

We consider here a positive but small capillary number η = 0.003 that corresponds to γ (105) Ge = 59.5 meV/Å 2 , only 1 meV lower than γ (001) Ge . This value leads to a cross-over x c in the vicinity of 0.55 for σ ed = 3.3 meV/Å. The latter edge energy is lower but comparable with the atomistic estimation in Ref. 46 that is 10 meV/Å. Given the dierent uncertainties of the model, the comparison between theory and experiment is satisfactory.

To go further, we characterize the parameter dependence of the model. The more crucial parameters are η and σ ed . For σ ed =3 and 4 meV/Å, one nds x c = 0.50 and 0.71 respectively for η = 0.003. Also, for η = 0.0046 and 0.0013 (that correspond to γ The surface composition proves also to be important.

By changing x s to 0.9, we nd x c = 0.8 using a Vegard's law for the surface energy with γ Si = 90 meV/Å 2 both for (001) and (105) orientations 36,37 note that in this case η changes signicantly to 0.0053. We also changed the geometric pathway by computing numerically the elastic energy for a truncated pyramid that typically leads to an decrease in ∆E * of 0.1 eV. As a conclusion, given the uncertainties in the dierent parameters (surface concentration, reconstruction, alloying ...), the choice of parameters here is plausible and validates the overall scenario with a cross-over expected to be around 0.5.

As a conclusion, we performed a joint experimental and theoretical work to rationalize the competition between two growth modes in strained lms : the nucleation of islands and the ATG morphological instability, that both eventually lead to quantum dots. We show experimentally in SiGe systems that the instability occurs for a Ge concentration x 0.5 while quantum dots stochastically nucleate at higher x 0.6. We computed the nucleation barrier and time scale τ nuc from rate theory. We show that τ nuc diverges exponentially at low ‡ ‡ As regards elasticity, ν Si = 0.279, ν Ge = 0.273, Y Si = 1.30 10 11 J/m 3 , Y Ge = 1.03 J/m 3 , a = 0.27 nm

x, with a Boltzmann factor exp[α/k B T x 4 ] while the instability time scale evolves only as 1/x 8 . Consequently, the competition between the nucleation and instability pathways is ruled by kinetics : nucleation occurs at large strain but is frozen at small strain, allowing time for the instability to occur. Additional eects such as alloying 47 or patterning 48 could be investigated in future work, e.g. using kinetic Monte-Carlo simulations. 49

Acknowledgement Kailang Liu thanks CSC fornancial support 1, The silicon attachment energy (0.3eV) comes from which reference? 2, The last comment of Peter in the discussion seems not so clear. 3, The heterogeneous nucleation mentioned in the very beginning of modeling introduction

  light emit- ting diodes with photonic QDs crystals in microcavity, 5 QDs transistors, 6 QDs solar cells, 7,8 etc. Many eorts have been devoted to circumvent the low quantum eciency of Si and Ge QDs associated to indirect bandgap issue. Band folding in strained heterostructures was expected to create quasi-direct band structure 911 and to increase radiative recombination. 12 Various con- gurations of self-assembled Si/Ge multiple quantum dots(MQDs), 13 nanopatterned microdisks, 10 nanopat- terned superlattices pyramidal QDs 14 have been elabo- rated to adjust the band structure. Complex design of QDs devices allows to mix dierent signals such as spins and carriers or photons and carriers with a large variety of QDs per chip with multiple functions, whose placement and homogeneity commonly request a combination of nanotechnology and self-organization steps. Devices such as single (or some) electron transistors are also congured with one, two or three QDs closely packed on laterally conned active areas. Nevertheless, most QDs systems fabricated by nanotechnological tools are limited by their intrinsic lack of homogeneity, which reduces to only a small number of relevant dots to be achieved per chip. At the opposite, MOSFET devices require perfectly at, Ge-rich, free of defects and fully strained 2D thin lms epitaxially deposited on ultra-small transistor gate. Whatever the end-use application, ultimate fabrication of devices necessitates a perfect control of the island formation and evolution behavior over a large range of composition. Despite the large number of studies dedicated to the SiGe system, an unied quantitative evolution of the QDs in dierent experimental conditions is still lacking. The outstanding challenge is to allow QDs control and scalability to engineer quantum devices based on QDs located at will. Since the QDs growth signicantly diers in rather similar experiments and is a matter of confusion or controversy by a combination of theory and experiments we give here quantitative insights on the very rst steps of this evolution.

Figure 1 :

 1 Figure 1: Graphical representation and TEM plane view images of the two growth modes at work in SiGe strained lms ; a) at low strain for a typical Si 0.7 Ge 0.3 lm on Si(001) and b) at high strain for a typical Ge lm on Si(001).

Figure 2 :

 2 Figure 2: AFM images of (a) 2nm Si 0.5 Ge 0.5 and (b) 1.3nm Si 0.4 Ge 0.6 lms deposited on Si(001). The im- age sizes are 2µm×2µm and their vertical scale is 7 nm. Images (c) and (d) are lines proles corresponding respectively to (a) and (b).

Figure 3 :

 3 Figure 3: Geometry of the system, each island with a square-base pyramidal geometry, grows on a capture zone 1/ρ on top of a wetting layer of thickness h w .

  The surface energies of the (001) and (105) facets, γ (001) and γ(105) , depend on dierent parameters such as the lm thickness, composition, etc. The surface composition of a SiGe lm deposited on Si is still a matter of experimental investigation. One knows that Ge segregates so that the surface is enriched in Ge. 32 As a simplication, we consider the limiting case of a surface composition x s = 1. It corresponds to experimental results that indicate a full enrichment of the surface in Ge even in deposited alloys. 33 Moreover, we consider lms above their Stranski-Krastanov critical thickness so that wetting interactions do not enter signicantly in the energy barrier. For a pyramid with facet angle θ, volume V and base length L = αV 1/3 [with α = (6/ tan θ) 1/3 ], see Fig.3, one nds

f

  and s refer to the lm and substrate. The elastic contribution to the nucleation barrier can be computed exactly, 40 with the result

Figure 4 :

 4 Figure 4: Nucleation barrier ∆E as a function of the pyramid volume V with the parameters described below.

2 )

 2 , one nds x c = 0.73 and 0.37 for σ ed =3.3 meV/Å. Furthermore, by changing the temperature to T = 650 • C, we get x c 0.48. At higher temperatures, intermixing is supposed to play a significant role 19 and will decrease the elastic driving force.

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