Hola Shafagh

Es and está are sometimes interchangeable when we want to use the passive form. When we use it with estar we are thinking more of a "result" of a process, and when we use ser, we are describing the actual process. 

In this case, we are using "está protegida por unos pinares" thinking more of the result. The trees have been put there, have grown, and as a result we have a protected beach. 

I hope this clarified it

Saludos