Note

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

The bug

The method net.minecraft.world.gen.feature.WorldGenBigTree.generate(World, Random, BlockPos) sets the given world as value for the net.minecraft.world.gen.feature.WorldGenBigTree.world field. The problem is that after generating or not generating the tree the field is not set to null again. Because each biome has a WorldGenBigTree constant and the biomes are created once on startup of the game, a different world (or the title screen) which does not generate big trees has still a reference to the previously loaded world and prevents the garbage collection from collecting it.