Here's an (extremely long) set of ideas for how growing trees might work:
Maybe you could have a system where the longer a tree grows, the higher the quality of it's wood, BUT, the maximum tree growth is determined by how much space, and time, the tree has to grow, along with the forest soil-quality (which might be determined by how long the soil has remained un-farmed, and the overall quality of any mosses and fungi growing on it.).
You might be able to increase forest soil-quality, perhaps even more-or-less permanently, by adding (a lot of) charcoal to it, which would be realistic, and which would increase the "cost" of growing trees, helping balance, and also making use of wood scraps--forest fires could do this naturally.
Tree maturity could be divided up into several levels, for instance: oak sprout, oak sapling, young oak, mature oak, giant oak, venerable oak, and great oak. Each level could start at q1 (q1 mature oak lumber, acorns, etc), and grow over time into q100.
Each level could require a certain amount of space to reach, though, along with a minimum quality level of sprout. In a given space, you might be able to plant up to 24 oak sprouts, which could grow into oak saplings. The same space would hold 12 young oaks which would be able to grow into mature oaks, 6 mature oaks, which would be able to reach old age, 4 old oaks, which could grow into giants, 2 giants, which could grow venerable, or a single venerable tree, which could become a great oak. (minimum space needed would depend on the species of tree--a mature redwood might require the space of a dozen mature lime trees).
Trees that don't have enough room to grow would simply stop growing, in both size and quality, but would have the same chance, or greater, to contract diseases, so it would be valuable for a player to cull trees, to fit the species space-requirements.
Reaching each new level of growth would always carry with it a small, but increasing, percentage chance that the tree would begin to die from disease. The percentage for a 100q sprout to become diseased might be 1%, until it reaches mature status. Past mature, the disease chance would become 5%. Disease chance could be calculated at, say, every 25 quality-points for that level of growth.
If the tree contacted a disease before it reached maturity, it would immediately die. You'd still be able to chop the tree down, and get the lumber from it, but if left alone, the diseased tree would rot fairly quickly.
If the disease were contracted at mature level, or later, it would survive for a while, although it would stop growing beyond it's current level--although, depending on the disease, it might perhaps still keep increasing in quality. The nice thing about this is that there are several tree diseases which actually have value. You might be able to harvest oak galls only from diseased oaks, for instance, or get heavily-knotted lumber from diseased trees which have been allowed to "mature". You could also get different types of mushrooms, from diseased trees, as well, so you wouldn't just lose your whole investment.
The quality of the sprout could/should affect surviveability--and it should always be a good idea to grow the highest quality sprouts you can find--but maybe you could have a system where the highest quality sprouts only come from the largest, and highest-quality, trees. If you want a 100q sprout, you've got to harvest it from a 100q great tree, which is a reasonable reward, given the risks and time/space commitment involved.
Logs taken only from the biggest trees might be needed for certain building projects, like ship's masts for instance, and logs from great trees (of any size) would be the best sources of material for things like high-end musical instruments, furniture, religious icons/totem poles, and any project requiring the straightest, strongest, most durable, and/or most beautiful wood. Great trees might even give the highest quality fruit, sap, or olive-oil.
Once a tree reached "greatness", it might still contract diseases--with maybe a 5% chance of contracting a disease, now calculated for every single quality point it gains, but if it reached 100q, it might then grow into an "immortal" level, where it will sit at 100q "great", but, unless chopped down, it will just keep living, basically forever, free of disease (since we still have trees today, that have survived since the middle-ages, and much longer).
"immortal" trees could still be harvested for sprouts, potentially forever, (as well as fruit, etc.), but the quality of saplings at that growth-level would become completely random--and it might not be possible to harvest 100q sprouts from any source other than a great tree, so you'd have a fairly narrow 100q sapling-harvesting window, which would make them rarer and more special.
Such a system would encourage, and reward, growing 100q saplings, without making having one 100q sapling a game-breaker.
It would also reward "sitting on" trees for as long as possible, and forcing players to balance immediate needs, with eventual gains.
I realize that this isn't a completely realistic system, but I think it retains enough realism to suspend belief, while meeting many game-balancing concerns, and still being simplistic enough to grasp, with a minimum of study and practice.