The trees of North Ithilien are ripe with valuable sap which Thurindol believes can be made into a sustaining tonic for the soldiers.
'${PLAYER}, I have another valuable task for you.
'The versatile flora is not all this region has to offer the willing herbalist. I noticed this morning that some trees have begun to bear a rather curious sap. Harvest this sap for me, and I may be able to formulate potent sustaining tonics for the impending march. There are many fine sap-bearing trees wreathing the Fields of Cormallen.'
Sap-bearing Trees can be found near the Field of Cormallen.
Harvest the sap from North Ithilien trees.
Thurindol can be found in Henneth Annûn.
You have gathered a good quantity of tree-sap and should now return to Thurindol.