Ok, this is a typical problem of the billboards when used on animations: If you play the animation from a top view, you can see how the billboards are rotating to face the camera, but sometimes the planes intersect between them because the high density of objects. This creates a flickering effect when rendering.
There are different solutions to this problem:
a) Enable Collisions. To get a similar density of trees from the original scene, select a "Dense" distribution map and adjust the "Density->Units" parameters.
b) Use a Two/Three/Four planes geometry for the trees and disable the "Camera->Trees Facing the camera" parameter. Using this configuration, you will need to adjust some illumination parameters. Suggested values: Material->Self-Illum factor = 100%, and set the RGB output value of the textures to 0.5 to darken them.
c) Use 3D trees. You will get the best result for animations, although render times will be higher. You can find some free 3D HQPlant trees in our website.
I also noticed some flickering effects in this scene caused by self-shadowing. To avoid it, disable the "Shadows->Raytrace->Every tree over itself" parameter. I also suggest to set "Camera->Limit to Visibility" parameter to off, or increase the "Expand" value to fix the popping effect of some trees.