bake shadows on only on small part of the plane ? is it possible ?

i have a huge plane in a scene with some trees here and there.

i want to bake shadows only in area where shadow of those trees fall

because if i bake shadow of whole plane it takes too much memory and i dont want to generate shadows of mountains, and if i try to save some memory shadows of those trees look too pixelated.

i want something like a reflectionprobe, where i can define an area like bake crisp shadows inside this area,

crisp shadows of trees, but pixelated shadows of mountains ?