I’ve built an underground forest chamber in my main fort as a convenient lumber supply. It has a dirt floor and plenty of torches, but the trees I planted there haven’t sprouted yet. I have heard that wheat will grow under torchlight (I believe X’s Adventures in Minecraft mentioned this). Do trees also share this property?
Trees can definitely be grown underground with sufficient light. According to this thread on the Minecraft Forums, you need 7 blocks of space above the sapling. (Perhaps this condition isn’t being satisfied.)