Large oak trees seem to have gone back to not spawning in forest biomes after being re-implemented in a recent snapshot in response to a previous issue report. Large oak trees were not found in forest biomes across multiple test worlds. They also could not be found in extreme hills biomes. (This is less reproducible due to there being less trees in this biome to begin with.)
Perhaps a consequence of the recent fix for certain biomes not spawning, or an intended removal. No evidence of this problem in 16w04a was found in any previous issue report on this topic.
Steps to reproduce:
Create a new world in 16w04a.
Find a forest, foresthills, or extreme hills biome.
Find a lack of large oak trees?
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I used the seed, "tree" in both 15w44b and 16w04a. Turn to the immediate left from spawn and you should see a large oak tree in 15w44b. However, in 16w04a, there is a smaller tree in it's place. It's worth noting that this tree does have a single "branch" coming off it it, making it larger than the surrounding trees. However, it is not a large oak tree as can be seen in the screenshots added. There are also other instances of large oak trees on the spawn island being replaced with this smaller type of tree.
There have been some tweaks to world generation since 15w44b. This has affected the specific trees that generate as large oaks, and it has also affected the configurations of large oaks that generate in the same spot across snapshots. (I've even seen some variability for the way in which a specific large oak generates in 16w04a; sometimes the same tree of the same seed will generate smaller or bigger, with different amounts of wood.)
In any case, I cannot reproduce a lack of large oaks. In each of my test worlds for 16w04a, large oaks are part of the world generation.
FYI, Kumasasa, tree size is not always seed-based; the same seed can generate different trees, as per MC-55596
I've tried generating the same seed a number of times. In this case, I used the seed, "1" which spawns in a larger forest biome. In each case, the largest tree I am able to find is the type described in my last comment. That is, a 5-block tall tree with an extra bit of leaves on one side of the foliage. Every "large" oak tree I find of this type generates exactly the same way, just with the extra leaves on different sides of the tree. I am still unable to find any large oak trees that resemble that in the 15w44b screenshot.
For example, the tree located at 175, 246 near spawn in the seed, "1" generates in the same size, shape, and orientation for me every time I generate that seed.
Nathan, are the large oaks you get in your test worlds the same type/size as the one in the 15w44b screenshot? If so, do you have an example seed I could use to test this?
After switching to a different version of Minecraft and then back to 16w04a, large oak trees now generate in abundance. They take the place of the 5-block tall trees I observed previously on the seeds, "tree" and "1" After numerous times generating these seeds again, I cannot reproduce the previously found smaller trees that were where the large oaks should be. Not sure what caused them to generate, but it seems it was not a bug.
Just for completeness, I checked out the forest at spawn for seed "1", and found large oaks in abundance (though not one at x=175, z=246, interestingly). In any case, sounds like it's all fixed for you, good to hear.
Just to clear things up for good, I believe the issue I was having was the effects of MC-11208, as I continued to intermittently experience a lack of large oak trees spawning after this thread was marked solved. This issue has now been sorted out as of 18w07a.
MC-29844 was fixed for 15w44a.
Can you provide a seed and coordinates where big oaks are generated in 15w44a but not in 16w04a ?