The 0 issue has been dealt with; you now get a warning. EDIT: spacing can be zero for equal distances, but population size at 0 was a problem before.
Nice first render, we hope you're here to stay. Some populations would be good indeed. As the island is rather far away, you'd only need to populate the front part, not the distance. Did you locate the cener of the population into the center of the island. You can see the lines of the square population, t5hat should be just big enough to cover that island. Bigger has no use. Then measure the water height (lake Y) and add a distribution shader with minimum height at that level, softened a few meters, as masking shader. Perhaps use a power fractal as mask for the distribution shader for some dispersal/patchiness of trees. If you can't figure out what goes wrong just post your tgd and we'll have a look.
