Also, the reason different species of wood trees show up different with thermal is different tree species are more vascular than others and transpire hundreds if not thousands of gallons of water per day, ie they are big water pumps. The water holds the heat real well and adds to the emissivity of the wood.
In fact, water molecules transfer heat so efficiently that they are the definition of British Thermal Units (BTU) as 1 cc of water changed 1 degree Centrigrade takes 1 BTU of thermal energy to happen.
Same things go for different types of rocks giving off different thermal emissivities at night, I don't know how many times I have stalked out a rock outcropping at night, but it has been frequent.
Here is a pic of some hogs I shot at a waterhole, you can see their thermal emissivity reflection in the water itself.
The one with the crosshairs on his head dumped alot of hot blood into the cool water that you could see too....