I think your "Lead plants" are evidence of adaptation but not yet speciation. Nothing needs to change in the genetic material.
It's similar with people living in high altitude currently, and are adapted but genetic is not concerned. I we are able to observe this for a long time AND the gene flow will be firmly cancelled between high mountain and "downstair people", you might see the first signs of a speciation uprising in a few hundred thousand years...
Speciation is long for human but short for the geologic time-scale.
Denesha