It seems that success at high latitudes (versus lower latitudes) may be due at least in part to historical effects if Salix radiated in high latitudes. In terms of success in general, I would guess that wind dispersal would be a big factor in sustaining and creating new local populations within meta populations (wind dispersal wouldn't be a good strategy in the tropics), whereas vegetative reproduction and the combination of wind and animal pollination may contribute greatly to success in the local environment.
It seems that an instructive discussion might also be: Why is Salix so successful relative to a closely related genus that is not as successful? Scott Chamberlain
