Particle Surface Created
In the image above the dark blue mesh on the left has a particle system system has particles overlapping. On the far right is a mesh with shape originating the dark blue and showing faces and other spacing vertices. The middle shape shape show the new mesh with no overlap of particles.
Create a second new grid surface only for particle trees new, widely spaced and bigger faces to accomodate a tree with extra space to suit your tastes. This is easy and fast. 8 minutes or less. If you use a more coarse and larger grid (second newly created) you can control the spacing with no change to visible terrain. This grid can use the [shrink wrap modifier] to lay near the visible terrain. Lastly if you duplicate the visible surface you can randomly via selection and manually remove faces to control face duplication location.