Using some random numbers off of the Internet, it looks like you can get 100 books out of one tree.

I also have doubts about how environmentally safe ereaders are compared to printed media: books come from renewable resources, are recyclable, have a fairly long shelf life, are made from materials that are easier to refine, go through a simpler manufacturing process, etc..

So maybe it's best to say that you like using ereaders, and not bring up the trees bit.