Though it’s been known for some time that there was a small amount of electrical potential between many plants and the soil (the effect is described in an article at PLoS One) only recently has anyone put that trickle of electricity to use. The company is Voltree Power and their “bioenergy converter” is intended to be used (at least initially, and appropriately enough) as part of an Early Wildfire Alert Network.
As described in Source of Sustained Voltage Difference between the Xylem of a Potted Ficus benjamina Tree and Its Soil, electrodes connected between various parts of plants and the soil can routinely generate potentials from 50 to 200mV, through a mechanism that has only recently been studied in detail. Voltree Power has developed a device the size of a pack of gum that taps the metabolic energy of trees without harming them.
By eliminating the need for batteries that require periodic replacement, the "bioenergy converter" makes possible "Voltree's 'Early Wildfire Alert Network' (EWAN) [that] consists of thousands of tiny humidity and temperature sensor nodes distributed over remote forestland. Each node contains a wireless transceiver that enables EWAN to instantly detect the onset of wildfires as well as continuously monitor forest conditions for accurate, ultra-high resolution, day-to-day, fire hazard prediction."
An example of bio-utilization, the EWAN combines a "free energy" source and widely-distributed low-cost agents (sensors) using a self-managing mesh network. Beyond warning of fires, these sensors could be used for fine-grained, 'ground-level' monitoring of forest ecosystems.