Are you saying you want to change a bush to a tree, or is that a bug that you are trying to fix?
If it is the first one, you would have multiple groups of sprites (or tilesets), and have an enum in your class stating which tileset you are currently using.
Then, you use DateTime.UtcNow to store the last change-tileset time, and when ten seconds have elapsed, you change your tileset.
Cheers,
ge-force.
P.S. If you are doing a platformer, check out the Platformer Starter Kit from the App Hub (
http://create.msdn.com/en-US/education/catalog/sample/platformer[
^])
It has lots of good information on tile set animation.