Tree damage can provide insights into internal dynamic pressure changes of pyroclastic density currents (PDC). On 18 May 1980, Mount St. Helens erupted a laterally directed PDC that decimated ~600km2 of forest, referred to as the blowdown zone. The head of the current contained the peak dynamic pressure, which uprooted or broke off most trees and stripped them of vegetation; however, some partially stripped tree trunks were left standing. Tree damage was assessed using aerial photography taken one month after the eruption. The flow direction of the PDC was mapped from shadows of root balls of toppled trees and directions of fallen trees. Along given flow paths, the density of standing trees was measured by the number of shadows within 200m2 areas. Towards the northwest, the average tree density increased from 0.01 to 0.58 (± 0.19) trees/m2 with distance. Additionally, analysis identified 95 clusters of trees still standing in the blowdown zone, situated on the lee sides of hills or plateaus. Blurry, cylindrical shadows versus well-defined, cylindrical shadows distinguished standing trees with foliage in clusters from those without. Five variables were used to determine the heights of trees: ground slope and aspect, bearing and length of shadows, and the sun angle above the horizon. Trees stripped of foliage in patches have average heights of 16 ± 7m and occur where the PDC reached 66 ± 24% of its runout. Foliage patches have average heights of 12 ± 7m and occur where the PDC reached 91 ± 9% of its runout. Tree heights in the patches indicate a localized height the peak dynamic pressure must jump as it travels over hills and away from its source. Patches with foliage imply that the peak dynamic pressure has risen above the tops of the trees, whereas patches without foliage suggest that the peak dynamic pressure was still low enough to damage trees even though the current had jumped over topography. Outside of the patches, increasing tree density suggests that dynamic pressure waned with distance.