In the Cities: Skylines - Industries DLC, I have zoned a couple of forestry areas and built one of each type of building that is initially available; the main forestry building, a small plantation, a sawmill, and a storage yard. The industrial area itself covers a large area of the forestry natural resource. I long ago met the production criteria for levelling up (something like 5000/500) but I am stuck on the number of workers at roughly 90/150.
I assumed that in an industrial area zoned as a particular industry type, even the factories that get built by the AI would be related to that industry (but they don't seem to be) and the number of workers in the zone as a whole would be counted, but that does not seem to be the case. Is the only way to increase the number of workers to build more forestry specific buildings? Given that I am still at level 1 and only have one of each type of building, does it mean I have to build multiple plantations and sawmills?