Subdividing by Province is useful to allow members to geographically group themselves for meets and other social events.
However, many are also interested in specific activities such as climbing impossible hills, crawling over truck sized rocks, swimming through bottomless mud holes or even just touring with a trailer or tent. The rock climbers and cross country campers have little in common other than probably a Jeep and a intimate knowledge of the location of every fuel stop.
So I suggest Provincial forums as well as 4 of 5 special interest areas.
Probably not a bad idea to also have a "How do I fix ...." forum since things do seem to break once and awhile!