Our synthesis aims at estimation of the relative importance of each factor. This kind of knowledge is theoretically important, particularly because the marginal populations have not been studied intensively yet. The application and utilization of the results to gene conservation of minor species is straight forward. In addition the genetic improvement and production of adapted and variable planting stock will benefit from the results. Willows are apparently derived from aspen-like forms not later than in Creatceous more than 100 million years ago.
Willows are common shrubs and small trees in the northern boreal and subarctic areas.
The number of willow species there is surprisingly high as compared with other woody plant species. In Finland there are 24 native willow species, in Europe more than 50 species and in the entire northern temperate and boreal zone more than 150 species. Willows are well adapted to severe climate of northern latitudes, and dramatic environmental changes such as glaciations seem to favor the speciation and adaptive radiation of these effectively colonizing plants. In turn, chemical diveristy of willows apparently explains the exceptionally high number of plant-feeding insects associated with these plants. E.g. sawflies which form galls on the shoots and leaves are an insect group strongly restricted on willow plants only.