- The second cavern hasn't had any life in it for quite some time. The cavern is covered in mud and doesn't sport any trees (which is not surprising, since my world gen parameters are set to reduced water in caverns). An FB has been rampaging in the cavern for 3 or so years killing everything encountered before I managed to divert efforts to kill it off. I then saw no life in the cavern for a fair while, mapped it, without seeing any life, got my gremlin trap going and caught a gremlin, but haven't seen anything since, despite having restored the trap since half a year or so. Can a cavern layer be completely depopulated in that short a time? The world gen parameters have been set up stop split caverns from appearing, and DFHack "reveal" has shown no separate piece exists.
A rampaging forgotten beast can annihilate your wildlife rather quickly. You can use DFHack's "region-pops" command to check your site's wildlife populations (and bump them back up if you like).
- The surface of my embark had two tiles of scarcely wooded sand desert (the rest being completely devoid of vegetation). I cut down the 3 trees, but haven't seen a sapling anywhere (on the surface). Shouldn't there be at least some tree regrowth? This is the first time I've had trees on embark without any regrowth at all. Sure, the ground cover is sparse, but given the amount that does exist, at least a few saplings should have appeared and matured by now.
Did you dig out the layer underneath the surface? Aboveground trees need an undisturbed layer below the surface to grow.
I would have considered depopulation if all caverns were affected, but while the FB slaughtered a fair number of jabberers, I've seen some in the 3:rd cavern since. As far as I understand, region-pops doesn't discriminate between caverns.
I haven't dug out the layer below the top one where there is surface vegetation (except a single tile wide entrance tunnel, which wasn't dug until a couple of years into the fortress), so inability for roots to grow is not an issue.
Unless you modded it or something, clay loam is basically the same thing as silty loam so far as the game goes - it is too "impure" to count as clay, so it is treated just the same as any other type of non-clay, non-sand soil. If it's being treated as a boulder, then it's behaving as a stone, not as clay. Again, unless modded, it's probably a bug that it's being imported at all.
The FB shouldn't be able to kill off a whole underground area, unless you did something in worldgen to also kill off most of the population. I know LoudWhispers was still getting wildlife with a half-dozen undead FBs slaughtering everything even 50 years into his game. The populations are generally in the thousands, and only half a dozen come onto the map every few months. It may just be that there is still a flag for some sort of underground wildlife from another layer or something blocking the appearance of the next season's wildlife.
Also, at least in the last few versions, underground wildlife appearance rates are tied to fortress wealth. Generate more wealth (by making masterwork roasts, for example,) and you generate more underground wildlife diversity.
The wiki disagrees, and claims "Five different soil types are classified as clay - clay, clay loam, sandy clay, silty clay, and fire clay. Each of these can be gathered for use in Ceramics.". That also matches my experience. In addition there are a few junk soil types whose names indicate they ought to be clay, sand, or both, but support neither.
I had a 0.40.24 fortress in a reanimating biome with hundreds of undead critters in one cavern. It was a pain to get rid of them, especially the campers... I known campers and creatures captured in a cavern but released in confinement while wild will block further appearance of critters from that cavern. It even seems to happen with silk farm GCS' that revert to wild, but I've caught a single critter in that cavern, and that's a trained gremlin, and that capture happened half a year after that cavern had been found to be empty (I did have some hope that gremlin was blocking other groups from entering, but nothing has showed up for a couple of years, and the gremlin trap has been restored to operation). Hm, a thought: is it possible for a gremlin to have gotten itself stuck somewhere, despite there being no trees? I know I've seen critters get stuck on ramps and then not move, and if it's a gremlin it would be invisible...
I'm making lavish meals of all non brewable food I get, and the meat off of a few FBs plus a couple of rutherers killed by one FB in the 3:rd cavern produced a fair amount of lavish meal source material for my single cook, so wealth should not be an issue. I have to say, though, that I haven't seen any correlation between wealth and cavern populations. One group comes in, walks around for a while, leaves, and is replaced by the next one. Occasionally two groups are present at the same time (I almost always embark in savage biomes, and both biomes in my current embark are joyous wilds).