Here is a simple way to use altitudes (for color additions/variation, or opacity - of leaves or so) into a population. As the 'use Y' isn't exact further from 0/0/0 (as Y='flat'), there is an alternative method, which measures distance from the center of the planet.
For leaves on/off per tree/instance use a transform shader set to instance anchor point, and any mask will obliterate all leaves from one tree (instance) and keep them on the next.