How do I add groups of trees to my generated worldmap

So I'm generating a worldmap that consists of land and water. If the tiles randomly assigned variable is above a certain value, they are land, else they are water. Now, I would like to decorate it with trees, bushes and mountain tops etc, but only where there's large groups of land tiles. Anyone have experience with decorating generated landmasses?

Here's an example of a landmass I'd like to populate with trees

I'm wondering if I should calculate how many tiles in each direction each land tile has until it hits water? then only spawn a tree on those sometimes?