New results are in...
Methods
Clarification: The front row is 6 trees with their origins roughly at ground-level. The rear row is 2 trees, duplicates of the rightmost 2 in the front row, but lowered origins.
Results
1)
The height of the origin does influence the highlight gradient (effect #2). I now assume that is more of a circular brightness gradient, and it is additive.
2)
The origin does not influence the shadow gradient (effect #1). In the first row, the two trees on the right with their extended geometry seem to have less of the shadow gradient simply because it starts a few meters below ground-level. The lower it starts, the less we see of the effect.
Conclusion
The ksTree shader has two separate effects that separate it from the alpha-only shaders.
(#1) a shadow gradient (subtracting brightness) starting at the bottom of the tree's geometry, extending up to the horizontal cut-line in the tree. This affect appears to be linear and following a 1-dimensional gradient. This effect is absent if the trees origin lies below its geometry.
(#2) a highlight gradient (adding brightness) starting at the origin of the object and extending outward in a somewhat radial fashion. This effect increases as the origin moves up towards the cut-line, and decreases as the origin moves below the ground.
So what I'm gonna do is probably to go a middle way between the two rightmost trees in the front row, i.e., a roughly 10m extension below ground with origins at ground level.
Appendix
This is what it looks like from the underworld
edit
improved phrasing clarity, updated conclusion