Huge square of biome/house missing, replaced by different biome

A large 15x15 block chunk of landscape where I saved my game reloaded as a different biome-everything I built in the section has vanished.

What I expected to happen was...:

That I load the game and find myself in the structure where I saved my game. 

What actually happened was...:

The area of sequoia/spruce biome was gone, replaced by a section of oak forest biome.  House and other structures completely missing.

Steps to Reproduce:

1. It loaded in like this.
2. ...
3. ...