Natural Hybridization of Woody Plants in Siberia and Promises of Use of Natural Hybrids in Introduction and Selection

Igor Koropachinsky, Siberian Botanical Garden, RAS, Zolotodolinskaya, 101 Novosibirsk, 630090, Russia

root@botgard.nsk.su / gbs@aha.ru

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Natural hybridization of plants has been known for a long time and well described in special literature. However, up to now no serious attention has been paid to the study of spontaneous hybrids and to introduction of them. We often have to encounter paradoxical phenomena: for the purpose of breeding, scores of hybrid plants are artificially developed with great difficulty, while millions of hybrids in large areas in the wild are disregarded.

In this presentation I would like to bring to the attention of the specialists the latter source of obtaining valuable forms of woody plants for introduction.

Hybrids are of special importance in the regions with severe climate where potentials of species assortment enlargement for introduction are limited, but there is need for it. In particular, Siberia, whose territory is about 11 million square kilometers, is such a large area in Eurasia.

Introduction of woody plants in Siberia is an extremely complicated task as the harsh continental climate does not make it possible to grow many Eurasian and North American species. Only plants of Alaska and some northern regions of Canada are promising for introduction in these conditions. Meanwhile an intensive development of Siberia requires a wide range of species for urban plantings, forest shelterbelts and forestry. Besides a small number of plant species introduced from different regions, assume great importance local species and their valuable forms existing in the wild, including spontaneous hybrids among which there are plants with a very good combination of valuable characters of the initial parental forms and sometimes with those unavailable in parents.

Siberian dendroflora contains 375 species of woody plants belonging to 122 genera and 40 families. Their distribution over the territory is very uneven. The greatest number of them occur in the southern mountain regions and considerably less of them occur northwards and to the north-east (Table 1). The main part of plants consists of bushes, dwarf shrubs and dwarf semishrubs - 83 % (Table 2); as regards types of habitat, the richest species composition of the arboriflora is in forests (140 species) and mountain steppes (129 species) (Table 3).

What is the extent of natural hybridization of woody plants in Siberia and how great are the real potentials of the enlargement of the assortment of species and forms suitable for introduction due to spontaneous hybrids? An analysis of the published data and my own study of many years allow to maintain that a minimum 11 families, 20 genera and 90 species participate in hybridization, which makes up 27.5 %, 16.4 %, and 24.0 %, respectively.

Intensive hybridization is registered, first of all, at the juncture of large floristic areas and in the mountain regions with more pronounced vertical zonality.

Intensity of hybridization in the wild is determined by a series of factors, including biological properties of species, nature of formation of the plant cover of one or other region, peculiarities of the physiographic conditions of the territory, etc. Of particular importance are anthropogenic factors which often account for the beginning of intensive hybridization processes. Conditions required for hybridization are also absence of geographical, physiological and ecological isolation of species, phenological compatibility, combining ability and that of formation of viable seeds and hybrid generations. It is necessary to take into account that hybridization could also take place in the past. In such cases complete replacement of certain species by others or formation of hybrid populations in the absence of the initial species, especially when these populations are isolated by different types of vegetation or some physiographic barriers, are possible. If so, especially strong variation of species is observed in different habitats as only in a part of areals they obtain a series of features non-specific for them, moreover often unstable, due to hybridization. Not by chance herbarium collections in certain hybrid populations often served as a basis for description of new species. In Siberia there are enough such species, in inverted commas: Larix czakonovskii, Populus canescens, Betula kelleriana, B. gmelinii and others.

It is difficult to bring all types of hybridization into some well-constructed system as it depends on many factors and the character of its manifestation is rather different. Random crossings of usually non-hybridizing species are found in Siberia. Such hybrids are replaced with time by one of the initial species or rarely form clone thickets and survive for a long time occupying small sites. Widespread is introgressive hybridization which takes place in a large territory in the regions where areals of hybridizing species contact, e. g., between Larix sibirica, L. gmelinii and L. cajanderi. Of special note are processes of hybridization at the juncture of altitudinal limits of species in mountain regions.

To characterize different types and extent of hybridization, including those in Siberia, to evaluate objectively expediency and possibilities of study, selection and introduction of valuable hybrid forms, a series of examples is worth consideration.

Hybridization between Larix sibirica Ledeb. and L. gmelinii (Rupr.) occurs along the whole line of contact of these species areals, from the border with Mongolia to the arctic tundra; a belt of hybrid forms is about 450 km wide in the region of Lake Baikal, in the basin of the Chikoi River and to the north-east from the mouth of the Podkamennaya Tunguska River (Fig. 1). In due time these hybrids were described by Szafer as an independent species - L. szekanowskii Szaf. No less considerable in its extent is hybridization between L. gmelinii and L. cajanderi whose belt of hybrid forms at the juncture of the areals is 600 km wide, and the total area occupied by hybrids encompasses 350 thousand square kilometers by the most conservative estimate. The general area of hybrid populations at the junctures of L. sibirica, L. gmelinii and L. cajanderi areals amounts to 870 thousand square kilometers and occupies about 11 % of the total area of areals (7.8 mln square kilometers) of Siberian larches. It should be noted that in the mountain regions the belt of hybrid forms is usually wider than in plain ones. This is due to different ecological peculiarities of the hybridizing species. In particular, L. cajanderi, more resistant to frozen soil and low winter temperatures, spreads far to the west along the upper limit of the forest, and L. gmelinii does so to the east in the lower mountain belts. The same phenomenon was also described when studying hybrid populations in the belt of contact of L. sibirica and L. gmelinii.

Sometimes hybridizing species relationship becomes more complicated because of human activity. Forest cutting, ploughing of lands divide hybrid populations into small sites, which strongly affects the total picture of variation of features. If hybridization takes place in a large area, at the same time in different sites pure populations of parental species, those consisting of various variants of hybrid forms, and isolated hybrid populations which appeared as a result of replacement of one of the hybridizing species are observed. An example of such a complicated relationship between two species is found in hybridization between Betula pendula Roth. and B. microphylla Bunge. It encompasses the large territory along the border with Mongolia and China, from Pribaikalye to Jungarsky Alatau, and possibly also to the west. The hybrid forms of these species were described as independent species - B. kelleriana Sukacz., B. rezniczenkoana (Litv.) Schischk., B. gmelinii Bunge and others - at different times (Fig. 2).

These hybrids are of great interest for introduction as among them there are some forms with a combination of valuable features characteristic of parental species. In particular, B. microphylla Bunge is a valuable timber tree, very drought and heat tolerant and B. pedula is a fast-growing tree of larger size. A small number of hybrid plants selected by us in Tuva are now used in greening of Novosibirsk.

Hybridization between Betula pendula and B. ermanii Cham. is rather peculiar in the large area from Lake Baikal to the coast of the Okhotsk Sea (Fig. 3). However, the hybridization between these species is possible only in those regions where the lower limit of B. ermanii distribution is in contact with the upper limit of B. pendula. In so doing, the belts of hybrid forms, enveloping mountain massifs by horizontal circles, are formed. Among these hybrids there are some forms with wood of high value which is characteristic of B. ermanii and at the same time fast-growing and with a straight trunk which is characteristic of B. pendula. Therefore, these hybrids can also be of interest for forest breeding and introduction.

Hybrids of woody plants are often characterized by higher quality of seeds and distinguished by better viability in comparison with the parental species. For instance, hybrids between Salix alba L. and S. fragilis L., Populus alba L. and P. tremula L., Salix pentandra and S. pseudopentandra L. (B. Floder) B. Floder, between different species of the genera Ribes, Rosa, Cotoneaster and others. Sometimes hybrid forms occupy larger areas than the initial species do.

The purpose of my presentation is not to show all known cases of hybridization between woody plants in Siberia, even those well described in special literature. I would like only to emphasize that up to now the extent of this phenomenon has been underestimated. Meanwhile spontaneous hybrids found in large areas in Siberia represent a nice natural laboratory that can give a lot of valuable forms and species of woody plants for selection and introduction.

However, the foregoing is connected with the necessity of conducting research both in nature and in introducing plants.

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Fig. 1. Areals and regions of introgressive hybridization of the Siberian species of Larix Mill.

Fig.2. Areals of: Betula pendula Roth.

Fig. 3. Areals of Betula ermanii Cham.

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Table 1 : Area and species diversity of the arboriflora of different macroprovinces in Siberia

Number of
Macroprovince Area of the macroprovince Families genera species species per 1000 km2
Arctic 1170.0 13 29 70 0.06
West Siberian 2302.5 31 84 174 0.08
Altai-Sayan 476.5 33 103 266 0.56
Middle Siberian 1988.6 30 66 164 0.08
Circum Lena 1092.9 21 50 124 0.11
Yano-Kolyma 1378.8 19 46 111 0.08
Zabaikalye 1208.5 29 68 184 0.15
Dauro-Mongolian 357.0 29 67 159 0.44


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Table 2 Life forms of Siberian woody plants, % of the total number of species in each of the macroprovinces

Macroprovince T1-T2 T3-B B2-B4 Ds Ss L
Arctic 5.7 14.3 38.6 31.4 10.0 -
West Siberian 9.8 9.2 43.1 13.8 23.6 0.5
Altai-Sayan 6.4 6.0 43.2 17.3 25.6 1.5
Middle Siberian 11.0 10.4 43.3 17.1 17.1 1.2
Circum Lena 10.5 12.9 40.3 22.6 12.1 1.6
Yano-Kolyma 8.1 10.8 38.7 27.0 13.5 1.9
Zabaikalye 10.4 11.5 45.9 16.9 13.1 2.2
Dauro-Mongolian 12.7 11.4 48.1 11.4 14.6 1.8
T=trees, B=bushes, Ds=dwaf shrubs, Ss=subshrubs, L=lianas


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Table 3 Number of Siberian woody plant species occurring in various habitats

Habitats Total % Habitats Total %
Tundra 65 17.3

Rocks and stone fields (above the limit, in tundra and forest tundra)



Rocks and stone fields (in the steppe and forest regions and belts)


70




115
18.6




30.6
Forest 140 37.3
Wetland 41 10.9
Steppe 129 34.4
Solanchaks 25 6.7
Sands 17 4.5
River banks, islands, meadows 137 36.5


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