Right now, wood is (almost) the most valuable resource for an enterprizing fortress lord. So here are a couple of suggestions to make wood more common.
1: Make tower-caps grow in 1 year or less. This doesn't increase the wood you can harvest per year (unless older saplings prevent young saplings from sprouting) but it does make wood available sooner. Considering that tower-caps are gigantic mushrooms, it would make some sense that they grow quickly.
2(a): Make trees produce more logs. If each tree that gets cut produces multiple logs, then wood would be less scarce. If certain tree types produced more logs than others, this would be even better.
2(b): Make trees produce a "felled tree" item. This item could be hauled to a lumber yard and processed into multiple. Very similar effect to suggestion 2(a), but it could be set so it requires about the same effort to get X logs under this system as it currently takes to get X logs. The advantage is that more logs are available on a given map. Again, if different tree types produced different log quantities, that would be a good thing.
3: As often suggested, humans could bring logs in their caravans. In addition, maybe as a reward for honoring their tree quota, the elves would bring logs that they shaped out of a tree. (The story dealing elf who turned evil suggests similar things.) This would make living under the tree quota more bearable.