Trees as batteries

A topic slightly off topic in this blog, but I think it is really cool.
Do you remember the experiment we probably all did in school where you stuck two metal-rods into a lemon to create electricity.
At first sight this sounds fairly similar is based on completely different principle however.

Voltree has developed a method utilizing the pH difference between the inside of the tree and the soil to create electricity. They use this to power remote sensor nodes which form a wirless mesh network in order to monitor remote forests.

[Link]