You use trace of nature instead of forest (you can also exchange any type of trace to trace of nature through the black spirit). As far as I understood, as a rule: all alchemy recipes substitute whatever trace/fruit/essence they used to require with the one called "of nature".

yup! caithy is correct. we are in the middle of the item simplification, so if you do not have any trace of nature yet, you can exchange any trace that you do have with the black spirit, your comma menu, using the exchange button.