The answer is: the area in Alaska once had a warmer climate.
A fern and other large-leafed plants were adapted to live in a warmer climate. The large leaves prove this. It is known that the largest leaves are characteristic of the warmer climate for many reasons, such as increased of transpiration surface, or increased of photosynthetic increase, etc. If fossils of such plants are found in some colder area, such in Alaska, it suggests that this area once was warmer than today.
