I drove next to this recently, and from what I can tell, is that there is some story in the same place that there is the "Cepelīšu priede", it might be the place where Latvian partisans where shot, but I didn't really look in to the sign. The sign can be seen in this photo:

https://www.mapillary.com/app/?lat=56.8811762&lng=24.544578499999943&z=19.033644149710096&pKey=799637331440215&focus=photo

On the right, it is a pine tree. With a small sign on the bottom.

The tree is this one: https://www.openstreetmap.org/node/11140624628