SPITFIRE vs incoming Scott & Reinhardt crown fire model calculated crown mortality #1373
Description
Describe the changes
In current SPITFIRE model, there is crown scorching and crown damage subroutine for calculating fire-induced crown fraction burned, thus post-fire crown mortality. I suppose this is a way to account for potential crown fire behavior and resulting impacts while SPITFIRE itself is a surface fire model. Since we are working on a actual crown fire model and there is a way to calculate crown fraction burned when a passive or active crown fire happens, keeping both SPITFIRE and crown fire model derived crown fraction burned thus is redundant and can result in mismatch between the two calculated crown fraction burned and any other variables that are based on it.
I'm suggesting to remove the SPITFIRE way of calculating crown fraction burned (CFB) and only account for post-fire crown mortality when there is a crown fire by using the crown fire model derived CFB. This will impact post-fire mortality for any calibrations with current FATES configuration as for surface fire there will only be cambial kill left, which is determined by fire heating duration that is purly based on fuel moisture content. This then concerns me as there is no good observations to tune the relevant parameters, thus make surface fire mortality tunning tricky. Related, SPITFIRE calculates crown mortality as a function of fire intensity, and I have thought about changing cambial kill to be a function of fire intensity as well a while ago and did a test branch) using EQ. 18 in Mercer & Weber 2001. This approach has less parameters even I don't think it is easier to obtain observations. However, it can make prediction of fire mortality more consistent when both fire intensity (used for calculating fire heating duration in surface fire) and rate of spread (used for calculating CFB in crown fire) are fire intensity metrics, reducing the dimension of uncertainty in modeling fire mortality. To avoid further disturbance for current calibrations, we can include this as another option for calculating cambial kill instead of completely removing the original approach.
Another potential is to keep the SPITFIRE way of calculating crown mortality but only apply that to corhots under certain DBH size since crowns of small trees can definitely get torched and burned even during a surface fire, but I don't see how that can happen for big trees that are often out of the flame zone of a surface fire.
Discussions are welcome and appreciated!
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