Can't figure out what I need to use the Old Growth Forest tile. No tech I've researched seems to give me the required building. Is it disabled for now ?

In addition, I have a question regarding Caravans. They seem to work ok for the most part but, once I've a few going between my cities, I'll sometimes get a prompt to issue orders to one that just arrived in a city. Which leads me to wonder if there's a limit to the number of caravans that can interact with any given city. Also, is there a way to find out just how much a caravan is contributing to my my economy or, in the bigger picture, is there a way to get a view of my entire Caravan trade network ? I'm asking because once you have 6 or 7 cities going it's hard to keep track how many caravans from which city you sent where, and I'm always wondering if I've truly maxed out my internal trade potential.