Normally, the Mesa Plateau F biome generates trees. It seems to not be doing this, but it still generates dirt and grass. The seed I used to find the Mesa biome was "Mesa?" Similar (possibly related) issues are occurring in other biomes as well.
Confirmed Cases:
Oak Trees in Mesa Plateau F
Spruce Trees in Extreme Hills+
Swamp Oak Trees in Swampland
Giant Mushrooms in Roofed Forest
Unconfirmed Cases:
Pine-Shaped Spruce Trees in Taiga
Oak Trees in Forest (too short?)
Giant Mushrooms in Mushroom Islands
Oak and Spruce Trees in Ice Plains
Linked issues
is duplicated by 1
Comments 2
From the release notes for 14w30c: "Some black spots will now occur. The black spots bug-fix was responsible for MC-62993, which was causing trees in certain biomes to not generate."
I can confirm that giant mushrooms don't get generated in roofed forest biome in 14w30b. Mesa plateau is also without trees.