Forest of 2D trees of which rotation is related to camera's rotation

I have a few pngs of trees. I want to import them and use them as a particle system to create a forest with dozens of trees. How can i set their rotation to be dependent on the camera’s rotation, so a plane meshes with trees rotate always to the front to the camera to hide their flatness?

Use the locked track constraint:

use that copy constraints operator to duplicate it to multiple objects

I managed to do it, but without particle system, just copying the same object again and again.

I want to use particle system, because the ground i plan to set the trees on is hilly. By copying hundred of tree-planes (when not using particles) i would need to set every tree on the ground level, so it doesn’t levitate. It would be very time-consuming. Unless there’s more automatic way (?)

When adding a planes to a group and then making a particle system with them - i can’t calibrate it properly:

As you can see:
  1. I try every combination in the constraint and can’t find the good one (i think it’s because the main plane must be lying, so the “hairs” could be vertical, in result the whole reliance plane-camera is ruined)
  2. Even if above would be fixed it also wouldn’t be good, because hair-planes just repeat moves of the main-plane from the group, ignoring their real placement in regard to the camera (Here’s particle settings of the ground plane):

I have definitely run into those issues with particle systems, so I avoid them whenever possible. You can snap multiple objects to a surface at once:

Here is a group of 4000 tree planes all floating above the ground plane

If you go to top view, enable snapping to faces and projection snapping, then just press g to start moving the trees and immediately hit enter to accept. All your trees should snap to the ground:

Much easier than working with particle systems, imo. Especially if you are doing a few dozen trees. There are some optimizations with particle instances, so if you wanted to do tens of thousands of trees, it might be worth wrestling with the wierd orientation issues.

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Thanks, everything is clear now.