I've been thinking about a herbam specialist and how you could create a living tower from the hollowed out trunk of a vast tree.
I've put together the following and would appreciate your learned criticism.
Conjuring the Tower from the humble seed
Cr(Re, Mu)He40 (Ritual)
R: Touch, D: Mom, T: Ind
The caster conjures a mighty tree that writhes and thickens to tremendous size. The trunk swells to thirty feet across and the canopy is some 100 feet above the ground. Its thick roots dig into the soil deeply and broadly anchoring it in place. The interior of the vast trunk is hollow and shaped into rooms, complete with spaces for windows, doorways and stairs. The tree can hold about six floors with 10 foot high ceilings within the trunk.
(Base 1, +1 touch, +3 elaborate design, + 4 size, +2 reqs)
My thoughts on why it looks like this
Base 1, we are conjuring a plant
+3 elaborate design, lifted directly from CtMT. If it applies to a hollowed out block of stone I see no reason it should not apply to a hollowed out tree
+4 size. Herbam and terram have the same base size, plant of block of stone one pace square. I can see that this might need to be higher for herbam given that a plant one place across is far less material than a stone block one pace across.
+2 reqs. Muto to make it really big, rego to form the hollowed out rooms.
a. Can you create a plant this big with creo alone? Wouldn't you be creating a plant that doesn't exist in nature?
b. If you need muto, what happens at the end of the ritual? Does it stay big? If not, how can I get it that big?
c. +4 size? I've worked it out as everything between +4 and +7.
Obviously this tree tower has issues that a stone tower doesn't but any herbam mage capable of casting this ritual isn't going to have a problem sorting them out.
Great spell. My 2c, to take or leave. I'm often a little generous on spells though.
I don't think you need both Muto and Rego. It seems a little heavy handed. I'd argue that the spell might need one or the other, but for a reason I can't put into words I think Rego is the only one needed. Perhaps tha is because it will assist in shaping the tree (?).
size +4 is fine if you're matching to size, perhaps a little extra space/size is needed so that the tree can maintain it's internal structure too. Terram is more efficient for building as its non-living.
I don't think you need to stress about it being so dramatically bigger than a normal tree as long as you've already paid for the size increases. This is a mythic setting, so its fair to allow things which are, err Mythic! The local non-wizards might think its very strange or unnatural though. For comparison another spell might create a magical beast with CrAn which is far larger than normal and probably pay for it with the size increases in mags alone. If a permanently conjured magical dog can be huge, then why not a tree? (YSMV .....) For more "realistic" games I respect the view that it's not allowed.
a) The largest real world tree by volume is General Sherman, a sequoia. It is 275 ft / 83m tall and 36 ft / 11m wide at base. I think this makes a good case for a tree the size you describe as probably naturally achievable. (General Sherman is a New World tree, not a European one, but I don't think that should be a bar.)
The volume of trees is a bit sketchy because of all the branches. I'm now having painful flashbacks to Computer Modeling 393 in undergrad for thinking that I could just treat it like a cylinder and use V=PiR^2h and having it totally wrong and having to integrate the area function over a vectorized line or something like that.
Only in Ars Magica can a discussion about creating a home base almost immediately spiral into a debate about whether trees exist.
Anyway, the spell looks fine to me, and I agree that Creo Herbam with a Rego requisite is plenty without the Muto or ritual, as long as you start with a valid seed for a large enough tree. Perhaps hunt down a mythically large tree for the project; more stories are always a plus. I'm curious of the results if you did it with a seed from a tree with Might, too. Oh, how regularly Ars Magica excites me with the unknown.
Tower of Living Oak CrHe30, Touch/Mom/Ind - Ritual
Base 3, +1 Touch, +3 Size, +3 Elaborate)
Creates a living tree with space for living inside, 25 paces high, 5 pace diameter inside (6,5 outside), 5 floors appr. 18 sq paces per floor
IMHO Rego or Muto req is not needed. The example from MoH has Te req due to the inclusion of earthen hearts. My version doesn't, I kept it simple and purely Herbam.You can just build a hearth yourself using rocks, bricks, clay or something. Or - wild suggestion - use magic for heating the place? If you can manage this spell you should be able to conjure up some heat.
As for the Californian trees: IMHO they do not exist because the American continent is unknown in ME and hence such large trees are unknown as naturally occuring plants. Supernatural plants may be as big (or oddly coloured) as need be. Go nuts.
Then, in you saga, they don't. In my saga, there are also no California sequoias. I was making no claim that California, or any other, sequoias exist.
A - Trees exist in the real world and in Mythic Europe.
B - In Ars Magica in general, for purposes of general discussion, things that do exist in the real world make examples of things that could exist in Mythic Europe.
C - For example: Tree X Really Exists, Therefore A Tree Of Similar Size Could Be Justified In The World Of Mythic Europe, at least through proper application of magic.
There may indeed be no trees of this species, giant sequoia, and their existence is not required to grow a tree to that size.
Putting this a little differently: In the real world, wood plant objects, called trees, can naturally grow to gigantic sizes, showing that wood plant objects can physically support such sizes. In Mythic Europe, magic may therefore call upon the natural physical structural quality of wood to grow a tree to a size that is impressive, but not supernatural.
My Mythic Europe presumes an objective reality that exists without the observation or opinion of mortals.
The largest plane tree in Europe (on the fringes of Europe, in Georgia, but planes are found all over Europe) is about 30 feet in diameter at the base (more precisely, at breast height), and about 180 feet tall. The largest chestnut tree in Sicily is over 20 feet in diameter at the base (again, at breast height), and approximately 60 feet tall. So trees of the size requested by the OP not only can, but do exist in Europe.
I'd grow such a tree from a seed (perhaps the "best" seed in a lot, as determined by an Intellego Herbam spell) with a Creo(Rego) Herbam non-ritual effect of 7th magnitude: Base 15 (bring a plant to maturity over the course of a single day or night), +1 Touch, +2 Sun, +1 Rego requisite (to shape the tree as it grows, as an experienced gardener would).
I think T:Individual without size modifiers would be sufficient, since the spell is initially cast on a small seed, but if that is not the case T:Circle could be substituted without a change in magnitude.
EDIT: Now that I think of it, it's not 100% clear what "maturity" means for a tree. Unlike animals, trees just keep growing. The chestnut tree above is 1000 years old, the plane tree 2000. I'd probably just ignore the issue and assume that the aforementioned spell gets the tree to the size the OP wanted
I would even propose a "group" variant of Fafnir spell, which would allow you to combine several trees growing together and waving them together to make smaller, yet functional housing.
Considering that the base spell is level 35, you have several magnitudes t o play with if complexity or area of effect need to be increased. Gwidion of Verditius, in Magi of Hermes is an Herbam specialist with a lot of relevant spells for tree-house building.