The tundra is a sub-arctic zone characterized by long, cold winters and short, warm summers.
(The tundra; is; a sub-arctic zone characterized by long, cold winters and short, warm summers) (a sub-arctic zone; characterized; by long, cold winters and short, warm summers)
The tundra a sub-arctic zone
is characterized
a sub-arctic zone characterized by long, cold winters and short, warm summers
by long, cold winters and short, warm summers