If you used the randomize command at the top of your code, that means that the rnd() that you use will always return the same value for the same randomize value.
However, if you made a tree map, ie an image which has a coloured dot representing the location of a tree, that would be a quickish way to position objects in the same location time after time. Youd just need to read the pixel position from the image, and then position an object at the relevant 3d location. This would also stop trees from loading inside each other.