To examine whether there was any physical or thermal interaction between trees and lava when a lava flow inundates a forest we studied the Kīlauea's July 1974 lava flow. We mapped the location of ∼600 lava-trees and the lava type (pāhoehoe versus 'a'ā), and sampled an additional ten lava-trees for chemical and textural analysis to infer flow viscosity and dynamics. The emplacement event lasted 3.5 hours and markers on the outer surface of the lava-trees allowed us to define initial high effusion rate and velocity (∼400 m 3 /s and 5-10 m/s) that then declined to 9 m 3 /s and 4 m/s during a waning phase. We find that lava passing through the forest underwent an initial cooling rate of 4 °C/km which increased to 10 °C/km late in the eruption. This is no different to cooling rates recorded at Kīlauea for tree-free cases. There thus appears to be no effect on cooling for this case. The lava-trees did, though, form a network of vertical cylinders obstacles and evidence for local diversion of flow lines are noticed. However, this varies with lava type, as almost no lava-trees form in ʻaʻā. We find a relation between the percentage of ʻaʻā and the number of lava-trees per hectare. The pāhoehoe-ʻaʻā transition for this flow occurs at a viscosity of 10 3 Pa s and this appears to be a threshold below which lava-trees can form so as to behave as a network of obstacles, and above which they cannot.