I note the description says "each 10 metres from the next"; it does not say "10 metres between centres". And each tree has a radius of "at least 20 centimetres". Which taken together would suggest an interpretation where fatter trees have greater distances between their centres.
It does not say what happens to trees that would intersect the circular boundary. I suggest that any tree that is wholy or partly inside the boundary should conform to the planting lattice, even if centred outside the boundary.
I suggest interpreting "see out of the forest" as simply having a clear line of sight to any point outside the boundary, even if it's the surface of a tree that's partly inside the boundary.
I would then posit a configuration of 4 trees, each 490 metres in diameter, planted on the compass points of the border. Even though the like-sized trees planted on the lattice outside the forest protrude into the forest, there's still outside visibility of around 1°20’ of arc at each of the compass bearings 30°, 60°, 120°, 150°, 210°, 240°, 300° & 330°.
The requirement for a regular lattice suggests there could be two sizes of tree, but the fatter trees would still have to be small enough be 10 metres apart on the diagonals (since otherwise that would be the nearest tree).
Enlarging the compass-point trees to a diameter of 690 metres means that the the "small" trees on the alternate lattice points are only 290 m in diameter, and lie entirely outside the forest, visible at the compass bearings of 45°, 135°, 225°, & 315°.
Conversely, reducing the compass point trees to 290m means that the 10 m gaps to the nearest trees are now nearer to square-on to the observer at the centre, leaving wider views beyond the forest.