The Great Maine Spirit Forest is one of many spirit forests which appeared after the barrier to the
Spirit World was broken, when
nature spirits inhabited the trees, plants, and animals of this forest causing the trees and plants to grow at a greater rate than normal and possessed the animals evolving them to be more resilient. Spirit forests are thus fairly hard to permanently clear without the use of
Magic which can prevent the spirit's expansion of the forest, or more aggressively have a water mage exorcise them to the Spirit World using magic.
Geography
The Forest travels along with the Appalachian mountains starting in Northern Vermont ending in Northern Maine and Eastern Quebec. The Middle of the Forest tends to be either mountains or hills, and as one leaves the forest it becomes flatter. Within the Northern Half are hundreds of lakes which are the source of the Penobscot, Kennebec and Androscoggin Rivers(listed north to south). In total It occupies about twenty-eight thousand sq miles(73,000 sq km) within Vermont, New Hampshire, Maine, and Quebec.

The Great Maine Spirit Forest by Mike1027
Flora & Fauna
The region is home to a variety of animal and plant life which can be found elsewhere in North Eastern America. Unlike a normal forest, Nature Spirits play a major role in the ecosystem. These spirits bond with plants and animal changing in various ways usually making survival easier for it. In particular, the region is home to many cougars, known locally as mountain lions, which are bonded with nature spirits allowing their population to make a come back. Nature spirits have increased the population of prey animals, which have also helped the Lynx and Wold Populations to rebound from their near extinction levels due to centuries of hunting. In plants, the spirits cause them to grow at an accelerated rate which helps support the vegetarian animals such as Elk, Moose, deer, rabbits, and other rodents.
Natural Resources
The various business which operates within the forest most pay homage to the local Yokai, homage varying from currency to vegetables depending on the Yokai in question.
The forest two main exports are wood and tourism, although there are limited mining attempts. Lumber companies will use often use automated drones instead of augmented workers allowing them to do the logging with a few human overseers. This is usually because its more efficient, but spirits can rarely turn violent when their host is harmed and while not particularly strong, the worker can't see them at first, this means dodging is impossible, making them extremely dangerous. Loggers will usually hire a mage to watch for spirts, and intervene when one becomes violent, as well as having them protect the camp and machines from being overgrown with plant life.
Dispite it being fairly difficult to cross without an airplane, the forest is a major tourist attraction with several walled settlements to provide opportunities for Camping, Hunting and Sight Seeing in a secure location. For the Most part, the amount of hunting is not regulated because populations tend to be fairly strong, but in order to hunt in the forest, park authorities require one to possess a degree of skill and knowledge which enable one to survive within the forest. Sight Seers are guided on a set path which ensures the tourist is safe, but also allowing them to see dangerous predators from a distance and beautiful views.
This is an interesting take on what I assume to be a real life location of some sort? I like the information on how the cougars bonding with nature spirits helped bring their population back up. I'd like to see this fleshed out a little more, but you've got a good start!
Yes, it is indeed based on the real life, New England Quebec area, so unless you are familiar with US geography it may seem confusing. I haven't fleshed it out much, mainly cause I didn't want just explain real-life plants and animals, I'll have to figure out how the forest differs from the real-life location more though.