Steps to Reproduce:

1. Use the world seed -770728766
2. /tp to the following coordinates:

   -91 63 57
   -83 63 67
   -1481 65 -2067
   -6050 63 -5463
   -4960 62 -4815
   

Observed Results:
The trees are growing on the wrong blocks such as sand, gravel, and clay.

Expected Results:
Trees should only generate on dirt or grass.

Original Description:
Upon loading a world in the new beta, I have found naturally generated Trees growing on Gravel, Sand, and Clay Blocks. 

Seed: 513337564 /with Cave & Cliffs experimental features switched on.

Upon first loading turn around 180 degrees, & walk 42 blocks. Then turn 90 degrees right, & walk 80 blocks. Trees are there.

The cords are 171, 63, -37.

However, the trees from this seed & cords are relocated or do not exist as a world generation bug fix changed the landscape.

I have still found trees naturally generated growing on both/either gravel & sand in the latest betas, though they are uncommon.