Your harvesting level is equal to your adventuring level, unless your crafting level is higher in which case it is equal to your crafting level. There is no other way to affect your harvesting level.
When I say level, I mean the number which is used to determine your skill cap values, not your actual skill in harvesting. A foreman who has 10 levels in adventuring, but has never harvested will not be able to harvest anywhere near as well one that has harvested and raised his skills. Both are considered 10th level Foreman characters for various in game purposes, but the difference between these two is huge.
They will not have experience. They will have a skill potential that increases with your adventuring level OR crafting level, whichever is highest. As you unlock the potential for more skill, you will gain more skill when performing Harvesting. So that means you can be an Uber harvester if you are high level in adventuring or crafting, but you are not able to be a Uber harvester if you are a low level adventurer or crafter. - 6 September 2005