Paper, in large quantities. Usually plural, "dead trees" is a term that probably came about during the environmental movement of the 70s and 80s to refer to paper that had been printed out unnecessarily, as in misformatted computer printouts, bad magazines, trash novels, junk mail, or computer manuals for advanced hardware but clearly written for a complete idiot.

Sample usage:

• "I just printed out six hundred pages of dead trees and forgot the page numbers in the corners."
• "Do you have any idea of how many dead trees they used for confetti in the Rose Parade last year?"