It took me a while to spot your problem, but you mean the boxes the appear around the trees.
That is because you have used 'billboard' style (flat) trees.
I don't do a lot of renders but using 'physical' render settings should help.
I found this post that might help you out.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10