Getting fire into the crowns is not the same as keeping it there. Van Wagner's second criterion asks whether the crown fire, once started, can carry itself: it compares the crown fire's rate of spread against a critical spread rate set by canopy bulk density, the mass of available fine foliage per cubic metre of stand. A common form is \( R_0 = \frac{3.0}{\rho_c} \) metres per minute for a bulk density \( \rho_c \) in kilograms per cubic metre, which puts the threshold near 0.1 kg per cubic metre for many conifer stands. This produces three named fire types: surface fire, passive crown fire — torching, where individual trees flare but the crown fire cannot sustain itself and the surface fire still drives spread — and active crown fire, where the crowns and surface burn as a single moving front. The confusion this resolves is why a thinned stand can still torch violently and yet never run: initiation depends on the vertical gap, continuation depends on horizontal crown density, and they are separate thresholds. After this Concept you can classify a modelled fire as surface, passive or active crown fire.
Crown fire is a wildfire behavior category in which a surface fire transitions into the forest canopy, occurring in three progressively more intense forms — passive (individual tree torching, not mov…