We decided to remove the Timber Production model from InVEST because of how simple it was. As Stacie described, the actual calculations were trivial and are more easily calculated in a spreadsheet according to the specific parameters of a study.
Whenever we have a model in InVEST, we need the benefit of maintaining the program to outweigh the cost of maintaining it. In this case, the calculations were so simple and largely non-spatial that if someone really wanted to use the model, we believe that it would be more useful implemented in a spreadsheet than available as an InVEST model so we removed it from the suite. As Stacie mentioned, the model is still available in InVEST 3.3.1 (and might well still work for you!), but we can’t offer much, if any, support for it.
If you’re interested, the InVEST 3.3.1 User’s Guide chapter, available online, has the details of the model.
Hope you’re well,