Jargon File
A saying from the "Jargon File" is that "You can't grep dead trees", from the Unix command which searches the contents of text files. This means that there is an advantage to keeping documents in digital form, rather than on paper, so that they can be more easily searched for specific contents. Another usage of this imagery, cited in the "Jargon File" is "tree-killer", which may refer either to a printer or a person who wastes paper.