It could be rot.
nut
A nut comes from a tree and so does sap.
Sap
ElmOak
There isn't one that begins with an 'S' A three letter word for an evergreen tree is fir.
...
Yew (which is a kind of tree).
Sap is the word your looking for.
· Gorilla · Green Tree Python
You "TRIM" your Christmas Tree.....
The cure for malaria is from quinine (not sure of spelling), which I believe comes from the bark of a tree by the same name.
Oak