One metric for plant health is simply how "hydrated" the xylem is. This can be an indication of how much water the plant has been drinking and could change if it is living in a stressful environment. 

This sensor would be very similar to capacitance sensors used to measure volumetric water content (VWC) in the soil. An open sourced implementation of that is the Gravity Capacitance Soil Moisture Probe which is from DFRobot. It uses the TLC555I pulse generator to generate a signal which passes through a transmission line (on a pcb) that is immersed in the soil. Depending on the VWC of the water, the properties of that embedded transmission line changes.

 Image of the Gravity SM Probe. Notice the transmission line going around the edge of the sensor. A thinner version could be implanted in a tree to determine "hydration".

Image of the Gravity SM Probe. Notice the transmission line going around the edge of the sensor. A thinner version could be implanted in a tree to determine "hydration".

For the tree implementation, the transmission line would go along a probe similar to the one used for the thermal sap flow sensor.