They hung up trees with ropes about 10 kilometers from the Russian border to hide the road from airplanes.
It looks like it's not enough to cover the road but, when the road was viewed from an angle (not directly above) and a high altitude, the trees concealed enough of the road to make it difficult to identify as a road.
Source for original picture is The Finnish Wartime Photograph Archive
Okay, so the trees provide enough coverage despite the spacing, but did they have to change the trees every couple weeks when the needles turned brown and started to fall? Wouldn't the Russians notice a straight line of dead trees?
Those are evergreens
They're dead evergreens though.
They are looking a little grey now that you mention it.