Sixty-six million years ago, a giant asteroid struck Earth, triggering the mass extinction that killed 75% of Earth’s species. The impact caused a brief heat pulse, potentially global wildfires, and a “nuclear winter” in which soot and dust blocked sunlight for months to years. Our study uses microscopic fossil leaf cells to show that some plants grew in this deep shade, and forest canopies temporarily collapsed in the aftermath. By examining leaf cell shape and fossil canopy density (leaf area index) at seven sites in the Western Interior USA, we reveal that forests in the north were more dense than those farther south. These results provide the highest-resolution picture yet of how forests responded to the K–Pg impact, showing both the initial collapse and the pattern of recovery across the ancient landscape.