Conditional crown fire [Conditional surface fire]
n. A hypothetical type of fire in which the conditions required for sustained active crown fire are met but conditions required for crown fire initiation are not.
A conditional crown fire is not directly observable. Depending on the initial type of fire, one would expect to observe either a surface fire or an active crown fire. If the fire begins as a surface fire, then it is expected to remain so because conditions required for crown fire initiation are not met. If a crown fire has already initiated (in an adjacent stand, for example), then it may be able to spread as an active crown fire.
Conditional crown fires are possible in fire environments conducive to active crown fire – due to high canopy bulk density – but not conducive to crown fire initiation – due to high canopy base height or low surface fuel load, for example. Even-aged stands in the stem-exclusion phase may exhibit such characteristics.
Conditional surface fire is synonymous with conditional crown fire, which is preferred because it avoids the potential for significant underprediction of fire behavior.