First-entry prescribed burns occur when an area is burned without pre-treatment/thinning. Given the scale of our wildfire fuels problem in dry western forests, there are many areas which are too steep to thin, or where the economics of mechanical or hand-thinning make the work too expensive to justify. While there are many opportunities to use logging to thin forests while also making some money to pay for other management, we need to get better at using fire as the primary tool to thin forests.
We often think of prescribed burns as being low-intensity events, where the flames creep slowly across widely-spaced trees, remaining on the ground, and killing very few of the trees in the area. However, intentional fires can also be applied under more intense prescriptions to kill thickets of dense, small trees, while still leaving the larger trees, alive.
This video documents a late-summer prescribed burn in Nevada County, California, which included a portion of forest which had not been thinned before the area was lit afire.
Link in comments, below.