You’re right – the flexibility of having the vectors to move them around and edit is nice, and of course they are more crisp than rasterized objects.
I have two ways that I typically approach rendering master plan trees. One method is to use a high-resolution plan tree image that I don’t anticipate needing to change and marquee tool it + alt to copy it as nca suggested above. This puts all the tree images on one layer and I can apply a drop shadow to the layer. Starting with one high res and copying it on the same layer takes up much less space than copying that layer hundreds of times.
The other is if I’m looking for a less-photo-realistic tree and more sketchy: I will create one tree layer and put a color overlay/texture/drop shadow on that layer. Then I draw my circles with a white brush on that layer to whatever diameter is appropriate. This allows me to easily edit the colors, opacities etc. of that layer after the fact. And it’s easy with this method to just erase an area and re-space the trees by creating more white circles. Then add depth and dimension on new layers if desired.
Good luck!
Obviously a different project type but tree-wise, similar enough example. This is a tree symbol I use frequently – it is high enough resolution that when I zoom in, it’s still quite crisp. They are all on one layer and copied around the page, shadow added to entire layer. You could easily create 3-4 different similar layers if you wanted to vary the size of the trees/types while keeping the psd file size down.