Zentner, as discouraging as our climate is for this hobby, keeping many native and Northern European trees is possible with a little winter protection. When I first started messing with bonsai, I got told that it was a hobby that only made sense for rich old men in temperate climates. I have found native spruce, hardy elms, junipers, pines and honeysuckle have all done well if collected or purchased locally. Prepping trees in situ as SKBonsaiGuy suggested certainly works well, but there are other ways to go about it.