How does a lake turn into a forest?

Answer 1

A lake can become part of a forest through gradual primary succession.

The process goes that waves and storms wear away the surrounding shoreline filing the bottom with mud. The Lake is also filled with low lying vegetation (mostly leaves) that normally hangs over the water. As the lake shallows plants take root, with their stem in the water and leaves poking through the surface, the speed of its fill with vegetation slowly increases. Eventually it gets so shallow tall grasses grow in the pond-like surface. Eventually trees take root in the soil (normally Pine like trees at first).

After a period of time new trees take root, one that will ensure a ecosystem climax.

The process take thousands of years but is different from lake to lake.