I was struck by how much income a person could make if they found a way to undercut the suppliers of wood to the cities that were growing vast at this time. Certainly it would have to be handled with some care to not simply magic wood on the street to be sold, however I can see how a covenant could found and maintain a income resource that would last for many generations, by simply applying a few small magic spells or even items.
A easy enough level 10 Cr/He spell would create 10 cubic paces of wood. This wood would last for a moon duration so there would be no problem with it suddenly disappearing, especially in massive cities where tons of wood was burned every day. I went with this spell:
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Conjuring the Mightiest of Oak Boles Cr/He lvl 10
base 1 (it is simply a tree trunk 10 paces long 1 pace thick) R: Touch D: Moon T: Ind (+1 size)
The spell was developed to provide a steady source of winter fuel for a covenant in a wood poor region. This is the second attempt of Garrett ex Bonisagus to create wood by magic. His first spell created long, blemish free oak boles. Problems arose when covenant carpenters, ignorant of the spell's limits, attempted to use these logs in production, only to have a covenant wall crumble after the new moon. Garret then developed this spell creating a bole of oak twisted, knobbed, and useless for construction.
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I figure there are a number of methods to making a enchanted item that a servant could then use to make a endless supply of such trunks of wood. Depending on the greed of the covenant, one could have a item with a endless number of uses for what I can tell is two pawns of vis and a season of work. Do it with enough cunning and probably using a Agent and you could become a dominant source of wood for London, Rome, Paris, Constantinople, ect.
Granted it would lead to charges at Tribunal but that's what you get for being greedy. I would suggest you stay small, meager, and grow it slow enough to to shock the region. Could even simply provide ships which bring the wood in daily, taking care to not let people follow you. Needless to say, for a very low level spell, a season and a couple pawns of vis, one can provide a substantial income source.
This has probably been done and thought of before... I just was reading about how much wood Rome used during the height of it's power and thought, WOWZER!