So I created a forest scene using hair particles system for trees and bushes. These particles stay stationary and only the camera and my character moves in the animation. So to better the render time, i read that i can turn off physics. But when i try to do that, my particles just disappear. That kind of makes sense and if there is no physics/gravity, they will just float, but disappear?
Physics off: 
Physics on: 
Hair particles are in fact trajectories of regular particles ; the regular particle need to have a motion, so it needs physics. Of course this is not a real motion, as it don't vary in time, but a path, a trajectory.
To reduce render time, you can turn off Hair Dynamics, which is the motion applied to every hair particle. It is off by default.
It's kinda weird to use hair particles to render actual objects. A regular particle system would work fine and is much easier to configure.
