National Drug Threat Assessment 2005: Marijuana
This rather dry report from the U.S Department of Justice is a good source of figures about the amount of marijuana seized in raids on public land.
A 2003 map of cultivation locations
This map from a 2003 US-DOJ report shows the locations where the largest amount of marijuana is being farmed on public lands.
FIFTY MARIJUANA GARDENS RESTORED
This article on a National Park Service site talks about the extent of the problem, and the clean-up work in Sequoia National Park.
NORML Report on U.S. Domestic Marijuana Production
The National Organization for the Reform of Marijuana Laws (NORML) published this 1998 report on the volume and value of Marijuana production in the U.S.