There have been tree tapping (the process of cutting a tree to expose its sap) threads before (see bibiliography at bottom) & tree tapping for a long time in human history, its a very basic art of stripping off the bark that yearns back to the practices of our earliest & closest ancestors deliberately looking for food between the trunk in the form of insects, requiring to eat the bark for fiber or being interested in the sap.
Fortress usageIn fortress mode, tree tapping is a skill carried out by herbalists generally in the field & the trees in question must be designated within a gathering zone (meaning they can be burrowed) to be selected when the command is given via a farmer's workshop.
> A metal tool called a spile is used in conjunction to a bucket, when the order is given the herbalist will go out and "milk" the tree of its sap and deposit the extract product in the bucket which can be bottled up into barrels undergoing the same process as milk (and then into vials/or used further ahead for certain things), the tree will regenerate sap over time.
Different propeties of tree sapsBased off real life counterparts, there might be many varieties of sap to tap, but only a few edible ones (which the selection could be restricted or left wide open, given the largely useless blood extracts) and even so with such things like rubberplants, not every materials has to be edible or even safe.
Only certain varieties of mushrooms have sap (
hence with 'underground orchard' and the stile tool you could collect underground tree sap to embark with despite being no fruits to collect) leaving the option open for modders. For example you could reason that towercaps have waxy rubbery sap, ideal for making cheap wax objects once it undergoes a stoking process to become a semi-solid globular material at cost of fuel/lavaheat by turning extract into glob using a kiln.
Further & deeper down, the goblincaps have toxic saps being more exotic & out of range, and this would be another kind of material that would turn up in world gen in places (alchemy plus wizardry) as well as good old cloak & dagger poisoning, a alternative substance for underground animal men & crafty adventuerers to use.
Having the saps be openly moddable & also varied (for instance, bloodthorn could literally bleed a blood extract & sagaros offer water for buckets to pour into ponds & use in hospitals) would be exceptionally useful.
The rest of the worldTree tapping is a common practice, and it is one unlikely source that elves also enjoy. Rather than hurt the plant they use a natural set of wooden grown tools & the plant "offers" its sap rather than the brutal method of harming the tree.
Offering tree sap implies that the tree was harmed (bleeding it out is distressing) so elves create their own 'natural' range of tree saps. In the hands of non-grown only entity site members, the wooden grown stile is useless, but elf entities extract their own range of ethic free-range tree sap amongst the many trees they hang around within their specific biome.
Though glumprong might have a sap, it is undoubtedly bitter & not good for your health in large quantities (therefore alcoholic) so goblins aren't interested, the extract like dwarven syrup (which is virtually the same goopy stuff) might go well with cooking however so there's still functionality for people settling in those regions. Mmm choking to death on death-sap.
Thanks for reading, i hope this thread kept you interested especially since i think it's a good step foward towards the eventual farming development roadmap goals & greater agricultural production.
Bibliography/Related Threads>
Expanding on wood and trees (author- AceSV / December 13, 2015 / Read 340 times) Notes - Very good & detailed thread, contributors also put forward lots of suggestions like certain dye/improvement extracts for lacquering/aroma being present within resins/saps
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Tapping trees and bushes for extracts (alfie275 / March 26, 2009 / Read 1031 times) Notes - popular old thread going over different things like the saguro water gathering i mentioned here on my own thread as a alternative way to gather water.
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Exotic Foods (Dwarmin / March 01, 2007/ Read 293 times) Notes - Another old thread from waaay way back I found quite charming, with fingers crossed for some oceanic big game whale hunting later, generally mentions tree tapping, had initially good feedback.