The map (click on it to see a larger version) represents the estimated wildfire frequency for the period 1650-1850, according to research by Daniel Dey, Richard Guyette, and Michael Stambaugh.
Their description of the project:
Knowledge of historic fire frequency is important in guiding restoration of fire dependent ecosystems, but it is often missing or cannot be determined locally due to lack of fire-scar tree records. A Northern Research Station scientist and collaborators have developed a new model called PC2FM that predicts historic fire frequency for the continental United States. The model uses mean maximum temperature, precipitation, their interaction, and estimated reactant concentrations to estimate mean fire intervals. Having science-based estimates of historic fire frequencies for specific project areas is a major advancement in ecosystem restoration. Another important use of the model is in assessing potential changes in climate (temperature and moisture) on the likelihood of wildland fires. The PC2FM model can be used to map large-scale historic fire frequency and assess climate impact on landscape-scale fire regimes.
UPDATE June 5, 2015:
One of the comments from a reader what that they would like to have a poster-sized print of the image. If you go to Fine Art America you can order prints starting at $17, ranging in size from 8″ x 6″ up to 60″ x 44″. Frames are optional for an extra charge. The map can also be printed on cell phone cases, greeting cards, duvet covers, and throw pillows.