I use an eReader for the convenience and instant gratification when new books are released.
I do not think they have a siginifigant effect on the ecology as yet. They are as environmentally unfreindly as any electronics to produce. The main use of eReaders are novels which are normally kept any way (who throws away a book?). This may change as more people start reading magazines and newspapers electronically but today they are just another gadget.
You evidently do not know what a forest is today. Most areas where people lived in the US had been denuded of forest by the mid 1800's -- including the all of New England. The forest we see today and the plants and animals they support are entirely due to replanting. Much of it by paper manufacturers. While there are woodlots grown for the single purpose of providing pulp, much of the replanting is done to encourage natural growth. All types of trees are planted, hardwoods and softwoods of all varieties. In some places the ratio is 7 new trees planted for each tree harvested. I have learned this from 30 years in the printing industry where I have no vested interest in the source of the paper.