Donno Skunker, it would have to be a bush suspended (or traveling through) midair as I don't see any stalk, trunk or even the shadow touching the ground at any point.
Looks like mud/oil or perhaps even the residue of two vehicle explosions which happened long enough ago for the models to fade and disappear?
EDIT: the splodge on the left is more faded than the right hand one. which is why I initially thought of footprints (drying over time), but that would also be seen as battle debris fades and dissapears over time, no?