I've discovered a problem in my current build. In doing some research on sapling growth I've discovered that jungle trees have half the chance of producing saplings per leaf block as other trees (1 in 40 vs. 1 in 20). This means my mundane jungle tree farm isn't going to produce a viable number of saplings to maintain growth.
The solution seems to be using the giant form jungle trees, planting a 2x2 square area of saplings, to grow enough leaves to produce a viable farm. The problem now becomes finding someplace to plant such trees underground. My rules for this build say all farming / industry must be below the surface, so I'll need to rebuild the jungle section of my tree farm. Unfortunately there's not enough space to grow the current farm chamber up or down. Up will break through the surface, and down will run into my fishing pond and ranch area.
Right now I'm thinking I'll convert the current jungle farm to another tree type, and build a new jungle space at the far end of the current tree farm zone. A little poking with Minutor indicates there's enough vertical space there to support the build.
Sounds like it's time for some digging...