I plan on using the firewood as the primary trading currency, and for this I need logs. Lots of them. But as the seasons progress, I have noticed that a Forester's lodge, which usually produces around 200-300 logs per season (with 4 workers) in the initial stage, reduces to less than 150 in some of the seasons (even though there are a lot of mature trees left in the lodge's range). Is this a bug or am I missing something?
There are many factors involved in how efficiently the workers actually perform, including their age, their condition (hungry, cold, etc) and their happiness. They also have to have the right tools, like using iron tools over stone, etc. If you don't have enough stone or iron, they will end up using their hands (or equally ineffective tools) and the production speed will plummet. Personally I've noticed that the tools is one of the biggest factors, although hunger may be a close second.
Make sure they have tools, (and the best ones you can make), and check to make sure your storage facilities have the room.