The quickest growing trees in mid-temperate latitudes are suitably chosen eucalypts. Certainly for the wood-pulp industry, in mid-Chile (200-600km south of Santiago, where wood is produced faster than anywhere else on the planet, and would be comparable to many parts of France south of the Loire), suitably chosen eucalyptus species produce a greater quantity of wood pulp per hectare per year than any other, and are ready for felling very quickly. Many species are also suitable for pollarding. Even in Britain, Raven Valley Nursery near Woking (http://www.ravenvalley.20m.com/index.html), which is a eucalypt specialist, accidentally produced a hybrid (they call it Eucalyptus x "Weirdo") which produced a trunk about 25cm diameter in just 4 years, and was about 8m high.

The other fast-growing tree of choice in such climates, at least for the wood-pulp industry, is Pinus radiata, or Monterrey Pine. This covers vast areas of New Zealand and mid-Chile. But it doesn't pollard, and slows down significantly if taken into cooler climates, eg Britain.

Whether you like these particular trees is another matter.