Actually, if you read the article, they mention using two probes made of the same metal to eliminate any current generated by the differences in redox potential between them.
Oxidation of a metal is a much more likely source of electricity than a tree, so much so, that they need a much better experiment before I'll be convinced that the tree is making electricity.
For example, let it run for a long time and check for any evidence of corrosion.
Are there metals that don't corrode? Perhaps palladium?
They could also try a variety of different metals for the probes (both at the same time) and see if they get different voltage readings.