Ecological and morphological studies in the hybrid zone between Pinus sibirica and Pinus pumila

Abstract

In the Baikal Region, there is no phenological isolation between Siberian stone pine (P. sibirica Du Tour) and Siberian dwarf stone pine (P. pumila (Pall.) Regel) since the timing of their 'flowering' coincides. Morphologically intermediate individuals, supposedly natural hybrids,occur not very often. In the west half of Stanovoye upland area four regions were investigated: Barguzinskiy, Baikalskiy, Ver- chneangarskiy and the Severo-Myiskiy mountain ridges. Interspecific natural hybridization was found to take place in several overlapping regions of the species' ranges; however there are some differences in frequency of natural hybrid occurrence between regions as well as within each region. Great numbers of natural hybrids are found only in a specific habitat which occurs rarely and occupies a relatively small area. At the north-east coast of Lake Baikal the lakeside zone is occupied by Siber- ian stone pine forests with moderate participation of Siberian dwarf stone pine in the undergrowth. The natural hybrid are widespread everywhere. The ratio of fructifer- ous Siberian stone pine, Siberian dwarf stone pine and natural hybrid was found to be approximately 300:10:1. About 90% of the examined natural hybrids took an intermediate position between the two parental species by most features (structure of needles, shoots, and crown), i.e. representing putatively the first generation hybrids. Therefore, in contrast to the parental species they are subjected to the destructive effect of snowbreak (broken off or dislocated from part of the root system). Like the Siberian dwarf stone pine the natural hybrid has specific root sources forming from latent buds. Therefore, the hybrids are not subjected to ageing, as well as have no internal limitation of age and size. Siberian dwarf stone pine, Siberian stone pine and their natural hybrid grow together in the Upper Angara delta in the bog regions. In the most productive sites the ratio of fructiferous Siberian dwarf stone pine, Siberian stone pine and natural hybrid amounts approximately to 60:3:1. The ratio of fructi- ferous Siberian dwarf stone pine and natural hybrid reaches about 20:1 in the less productive sites where Siberian stone pine is sterile. Analysis of cone structure showed that the natural hybrid have substantially increased in comparison with the species' mortality and aplasia of reproductive structures at all stages of the generative cycle, from differentiation of the seed-bearing scales to differentiation of the embryo. The portion of the ovules, which develop into the valuable seed with differentiated embryo, amounted in Siberian dwarf stone pine to 69%, in Siberian stone pine to 44%, and in natural hybrid to 25%. Thus, the fertility of natural hybrid in the Upper Angara Delta substantially decreased in comparison with the pure species; however, it was demonstrated that natural hybridization between Siberian dwarf stone pine and Siberian stone pine species occurred.