See https://github.com/Glitchfiend/BiomesOPlenty/blob/master/src/minecraft/biomesoplenty/worldgen/WorldGenWillow.java#L54 - the willow tree generator will overwrite air and Blocks.leavesColorized, but not Blocks.willow (the vine-like blocks that hang from willow trees) so overlapping willow trees can't be grown.

Also, it looks like the height limit is hard-coded as 128, but I'm not familiar with world-gen code so that may be intentional.

Willow trees no longer generate the willow vines, so I'll just go ahead and close this.