Try using a power fractal in the blend by input to mask the trees. Disable the object populations. Add the power fractal to the end of the Shaders tab. Enable the test color.
Now check Apply low colour. Slide the Colour contrast to 1, and reduce the Colour offset to -0.8.
Now set the Scale tab inputs. Try Feature scale 250, Lead-in scale 8100, and Smallest scale to 10. These settings will need to be tweaked.
Disconnect the power fractal from the end of the Shaders nodes. Now add the to the Blend by shader input of your tree population or the distribution shader of the tree population.
By adjusting the Object spacing in a,b to can get the trees to grow in clumps and groups. These will have wide spaces between them.