Mortality rates in FATES

[mpaiao]

I have been checking how mortality rate is computed and applied to changes in demographic density in FATES, and I am somewhat confused. In the typical ED formulation, mortality rate would be the continuous mortality rate, sensu equation 5 of Sheil and May (1996) (sometimes called hazard rate). But when I check the code, it seems in FATES the mortality is being applied as the mortality probability (i.e., equation 2 of Sheil and May).

The main practical differences between the two are that (1) hazard rates are additive (i.e., we can add the rates due to carbon starvation, hydraulic failure to get the total mortality), whereas mortality probabilities are not additive; (2) complete extinction can only occur when hazard rates go to infinity, which effectively prevents demographic densities (n) to go negative, as hazard rates are linearly related to ln(n), not n.

From reading the code (and I may be misreading it), it seems that both are being used interchangeably, i.e., as hazard rates in EDPhysiologyMod.F90 but as probability in EDMainMod.F90. From the practical point of view, this may not make much difference as long as the rates are small or time steps are short, but it may be adding some biases to the model predictions that could be avoided by simply using the hazard rates everywhere.