A cyclic space-time concept in which one cycle corresponds to one year, can be used as a basis for an integral theory of a mythic world-and-time tree. According to this theory, mythic oaks of the runo songs are symbols both of the time/year and world/nature: growth of an oak symbolises growth of a year-nature (1) reaching its summit or balance at midsummer (representatives of the 'good oak' in runo song poetry: the beer oak, the gold wheel oak, the love oak, the sun oak), (2) leading, due to its overgrowth, into the loss of balance reaching its bottom at midwinter when the sun disappears (the 'bad oak' in the runo song poetry: the great oak overshadows the sun), when (3) a saviour ends the old year-nature cycle, starting, at the same time, the new one (by chopping down the great oak, he liberates the sun). For the world-and-time tree theory speak (1) the semantic connections iso tammi 'great oak' - isotammi 'January' - tammi 'axis, pole', (2) those variants of the Great Oak Song according to which the great oak grows on the location meaning the centre of the world, (3) the double nature of the oak-chopper expressing his role of a mediator between the old and the new cycle. The oak-chopper's double nature, his function as a bringer of a turn of the year, denomination ukko(nen) 'old man, thunder-god, thunder', fire-blazing axe, bell, beard, and hair strongly suggest that the runo song singers of the northern runo song areas interpreted him as a thundergod. The closest cross-cultural counterparts to the great oak of the runo song poetry are the great apple and the great birch of the Mordvinian folk songs. It is very probable that the Runo Song of the Great Oak and the Mordvinian Song of the Great Apple/Birch have both been formed from one and the same proto-song which was very similar to them. Regardless of the unclear status of the oak-chopper in the proto-song, in Viena Karelia the thunderstorm myth or its transformations have most strongly influenced the Runo Song of the Great Oak.