[Agema]

I think...

A province without a fort only allows you to use 50% of it's resources (e.g. a 82 resource province tells you it has 41). Put a fort on, you get the whole 100%.

If you put forts next to the province, I think they first suck out the "inaccessible" 50%, and then once over 50% you'll start seeing a resource decrease.