Once again I utilize the topo to surface conversions which I've shown you before.
1. Draw a closed area with model lines. (Must be higher than the topo)
2. Select the model lines in Dynamo
3. Select the topo in Dynamo
4. Set the number slider according to your taste for tree quantities..
The definition looks like this:
Inside Revit after placement:
And a little screencast video as usual: Screencast