Generating more energy from a tree
Apparently voltage from trees is actually a thing. Who'd have thought. 

This study suggest you need to use copper electrodes for the highest voltage. Power is not specified here.
Series connection of trees may not be possible because one of the electrodes is a ground rod.

To actually use this voltage you'd need an energy harvesting circuit.  Of which there are some kits available. Silabs has one.



I'm unsure if I'm allowed to share the pictures of the research site here, see for yourself:

Hao Z, Wang G, Li W, Zhang J, Kan J (2015) Effects of Electrode Material on the Voltage of a Tree-Based Energy Generator. PLoS ONE 10(8): e0136639. https://doi.org/10.1371/journal.pone.0136639