Hickories (Carya) are not native to Europe. They were introduced in the last few hundred years, but I am not aware of European hickory being used as commercial timber.
Hickories are quite a common in the Central and Southern USA, West from the Rockies, and they are native to the South-Eastern part of Canada too.
Traditionally, hickory was used for bows, lances, war clubs, later on for wagon hubs, axles, skis, hockey and lacrosse sticks, golf clubs and baseball bats, and of course for striking tool handles. Many of those appplications are now taken over by the cheaper ash wood, which is also somewhat lighter.
The American ash species are considered somewhat weaker than hickory for striking tool handle applications:
http://mdc.mo.gov/documents/forest/woods/ash.pdf
As for the European ash, according to
http://en.wikipedia.org/wiki/European_ash
"Because of its high flexibility, shock-resistance and resistance to splitting Ash wood is the traditional material for bows, tool handles, especially for hammers and axes, tennis rackets and snooker cues, although American hickory, from trees of the genus Carya arguably performs even better for these purposes."
Several true hickory species have higher shock resistance than any ash.
However, hickory wood is often sold as a mixture of several species, because the timber looks very similar. Sometimes even the pecan hickory group is mixed in, which is bad, since the pecans are inferior to the true hickories.
That, and also the way the wood is dried and/or stored, can lead to suboptimal mechanical properties in the handles made of such wood.
German axe-makers still use ash for axe handles: I have seen Ox-Head/Iltis and Helko Hunter axes offered with ash handles, especially for the lighter and shorter axes.
I think, if straight grained and vertical, both hickory and ash wood are good for axe handles.
You need to swing an axe all day long for a long time to feel any difference. None of us is a traditional logger, so for camp purposes, and especially for hatchets, I think even birch (see Roselli/Kellam and the Russian axes) and oak handles would be OK.
As for durability defined as decay resistance, neither ash, nor hickory are durable, but this should not be an issue with axe handles which are properly cared for. Hickory might be a little bit more sensitive to rapid or extreme changes in humidity than ash, but if the wood is properly sealed and the tool is not left in the damp or near the heater, you don't have to worry.
A popular myth derived from the time when old growth forests were abundant is that the darker heartwood of the hickories is inferior in strength compared to the paler sapwood.
http://www.fpl.fs.fed.us/documnts/usda/amwood/241hicko.pdf
"Over the years a prejudice has developed against the heartwood of hickory. Red hickory (heartwood) is often placed in a lower grade than white hickory (sapwood) simply because of its color. Tests by the Forest Products Laboratory have shown conclusively that red, white, and mixed red-and-white hickory have the same strength characteristics, regardless of color. The negative attitude toward red hickory developed during the days of virgin hickory stands. Under virgin-stand conditions the heartwood was often less dense and not as strong as the sapwood. In the second-growth stands of today this density difference does not exist, and specifications and utilization practices should be adjusted to take this fact into account."
On the other hand, pecan hickories exhibit greater contrast in color between the sapwood and heartwood, so some of the problems reported with mixed-color or dark color hickory handles might mean that they were made of species with a weaker wood to begin with.
Ash is so versatile and durable for its reslience/springines/toughness etc but its a shame it wont do for outdoor constructional work. and be more durable in that respect
