In my headcannon, elves increase their population by growing from a Home Tree as fruit that is unpicked, which would also explain their proclivity to eat other sentients but also have fanatical respect for trees. To an elf, all other races are basically just mature fruit that has grown into a more mobile form. (Plump helmet men anybody?). While elves (in my headcannon), have no trouble consuming the fruits of The Hometree, they refrain from eating mature versions of those fruits (ie: other elves), preferring the plants that don't move away. However, if an elf does die, they become immobile again and thus fair game for others to consume at their leisure, so long as they aren't producing fruit. This slots into the various wars in the history screen where such and such dwarf was slain and eaten by the elves. It wasn't malice: they just stopped moving, and dead bodies don't produce fruit. Likewise this works for the elves being animal-friends: "The animals are moving? Don't eat them. Animals don't move anymore? Are they producing fruit? Don't eat them, eat the fruit. No movement or fruit? Eat them."
So it does make one wonder with this logic if non-elf slaves and animals producing babies are considered by elves as a way of these mobile beings to produce "fruits"? Are elves actually savoring baby-flesh as the "fruits" of these beings, at times allowing them to grow into a mobile version in order to keep up the populations? Horrific concept, I know, but the logic is sound.
What would be interesting is if the creation of workshops created a Workshop sapling that would mature into a fully mature Workshop Tree in a given number of seasons. Similar to placing a well or a paved road, each Workshop Tree would need certain reagents to be "planted", which would always include an "Elf seed". (Produced by the Home Tree as seeds in the fruit that both sustains and when unpicked increases the population of the elves.) Thus to elves, they would require the fully matured elves to eat an elf-fruit and then take the seed that fruit produces and plant it with the other required reagents to grow that workshop. (Perhaps the workshop matures like trees do in Vanilla, becoming first a sapling, then a full tree, then as the next step in its growth becoming the Workshop. Elf-fruit seeds might even be combined with a bunch of other rare ingredients to grow a new Home-tree for elves seeking to create new Retreats.) I'd imagine that elves require clean water the same as Dwarves require booze, so there would be a new workshop that would function as a water-producer if there wasn't a spring or other source of drinking water nearby. (Maybe a long thin spire that sweats water-mist, with a base that holds water like a 1x1 pool?)
With elves being immortal, having a Home-tree able to produce a certain amount of elf-fruit seasonally would become self-inhibiting as that fruit matured into mobile elves that begin consuming the elf-fruits. The carrying capacity of a single Home Tree would probably be able to completely support about a dozen elves, and it would be up to the elves to start eating other things than elf-fruit if they wish to increase their population from there. Growth of Workshops would of a necessity come at the expense of new population and require that the elves either find that Workshop's reagents themselves, or obtain them via trade or raids.
Another thought would be that elf-fruit grows into elves that at a certain age root into the soil to become a Workshop tree. Each Workshop would then be a caste of elf whose main goal in life would be to gather those reagents needed to fulfill its purpose. (The elven version of a "fey mood"). The elf would be given a designated "nest" area where they would "root" once all of their needs were met, but be otherwise mobile. In this version, the Workshop Tree would be planted at the expense of an already mature elf, whose corpse upon death would be the final reagent used to produce a "Workshop" sapling.
As mentioned before, the Workshops would grow their items as fruits, but only a few at a time per season. A Carpentry Tree or Craft Tree might produce a couple grown-wood chairs or grown-wood swords per season, so whole groves of Workshop-trees might need to be planted before any items would be produced en-mass. A large and varied Retreat would be a major investment in dedicated life to the elves, each tree also a friend and family member. If the Workshop Trees were still able to speak and be intelligent, that could be an even more interesting game, since the trees themselves would also still count as "citizens" of that site, and their personalities could be influenced by friendships and rivalries, even though they no longer move around.
Could be a fun to see the how an Elven Retreat shapes up based on this kind of organization.