Depending on how precise you're hoping to be, and how complex the shape of the boundary is, the easiest/quickest approach would be to use a mask with a feathered edge on the primary forest clip, which would let you fade/blend to a background image.
You don't mention if the lake & forest clip has motion or not, and it's possible that you'd need to make a custom mask shape and also use keyframes to track the contents in the main clip. It may also be possible to use a color swap to create a green/yellow screen area where you want the new content to go, and then chroma key that out to add the new image.
Only the first suggestion is basic editing, though, and more advanced techniques might be needed to really address the jaggedness and difficulty in avoiding artifacts. They will also require significantly more editing time, not to mention the time needed to learn how to use them effectively, so on some level, you'll have to balance "good enough" with how much time and effort you're willing to expend to get it nearly perfect.
A screenshot of the section of video you're looking to transform would be very helpful for forum members to make the best suggestions. Save the sacreenshot as an image and then upload it using this button below the forum's text box: