What your 7 year forest map looks like compared to someone else's 4 year savannah map is completely irrelevant to this discussion. The issue at hand is that after a few years a map of a tree-sparse biome will have become very tree-dense. Vattic has already provided evidence that this phenomena occurs; I have not seen any evidence suggesting the contrary.
Since the trees that are placed originally themselves bear no resemblence to what the trees ought to look like having had hundreds of years to grow, it is pretty much irrelavant that the total number of trees that we end up with on a savanna is greater than we started off with, since the trees are also a lot bigger than they started as well. Fact is that the original trees are not grown using the model that is used after embark, they are placed using a relatively crude script.
Since the total number of trees that existed on embark is *not* the number of trees that ought to be there given the mechanics actually in play in the game, it is far harder to prove there is a problem that simply pointing a finger at the number of trees we end up with at any given biome and pointing out that there were more that we started out with. In order to prove there is a problem we either have to prove there is actually no limit at all on tree growth or that the limit is exactly the same on all biomes.
Why not? The devs are human and capable of making mistakes, and Dwarf Fortress isn't even complete yet. Who are you to say that this isn't something Toady either overlooked or knows about and intends to fix later? You have zero proof of the dev's intent.
Neither do you. Nor do I need any proof, since the conclusion follows from reasonable premises; it is reasonable to assume that the devs do not want uncontrolled tree growth on all biomes, hence if they are developing a model for tree growth they will have implemented some limitations on said tree growth. You can of course prove me wrong by actually demonstrating that this is not the case, but the devs are stupid is not a reasonable premise to draw conclusions from.
Except we already have solid evidence.
Edit: Decided to let a fort run with the fps uncapped to get some quick comparison shots. The embark is a "sparse" savannah.
We have solid evidence that the number of trees we end up with is greater than the number of trees we start off with; not nobody is disagreeing with that. Comparison between my screenshot and Vattic's screenshot shows that the trees are overall far scarcer in his 'final state' than my 'final state'. There are very clearly observed clusters on the savanna map, areas where trees simply refuse to grow, the clusters on the forested map however are a lot smaller and more disputable. Since the comparison reveals that there is a quite distinct difference between the overall tree coverage in both biomes, it does not follow that the tree growth model is indeed growing a smaller number of trees in savannas than in forests. The drawbacks are that we do not know what a savanna looks like after 7 years but only 3.
Firstly, saplings do not pop up at once, the appear gradually over time (and probably completely independent of whether there are trees or not, except, of course, on the tree tiles themselves), so they should mature gradually as well. Secondly, the number of L shapes have gradually increased over the years, with the later ones appearing significantly after the original tree and the pairing tree, and I haven't cut down any trees since the original race, except to set up no grow zones around my fortress and the map edge, and the trees cut down at that time were those actually in those zones.
I've got one of these as well
TTT
T
.
T where the missing tile contained a herb that's since died, but no sapling yet.
Edit: The embark is not a jungle, but tropical savanna or shrubland, based on the animals present.
Saplings do pop up virtually at once. This is because as you said earlier they take 3 years to mature and Vattic's screenshot is only 3 years after embark with few or no saplings already present, meaning all those saplings popped up pretty much instantly upon embark; the amount of time it takes saplings to appear is basically negligable.
Animals often wander out of their home biome a few squares away; also you might have clipped the edge of a jungle when you placed your embark which would mean you would see jungle creatures on your map despite it mostly being savanna. I would also like to see a screenshot of your embark and a clarification of how many years have passed since embark as well.