It looks to me like something that flows, probably from being warm, though there are epoxies that would flow just the right amount before setting.
The less-than-glossy finish looks almost like hot glue to me, and that's available in black. Hot glue is slightly soft at room temperature, and dents with a fingernail (though you probably can't try that test). It would be easy to test on scrap materials, which would be a good idea as it's not easy to sculpt with. Rewarming might have been used too, either with an oven or a hot air gun (or possibly even both, using the oven to soften it all and the hot air to melt it locally.
You can buy black epoxy, or black dye for clear epoxy. Most epoxies designed to be a visible finish are very glossy, runny, and set rather slowly. You probably want one with a setting time of a few minutes (but not the 1 minute epoxy glues, they're usually too runny). So this could be a black epoxy glue (example). Warmer ambient temperatures speed up setting but also make it flow more. When I've used epoxy to coat things with vertical surfaces, I've had to scoop some up at the bottom and move it to the top, early in the setting time. This makes it hard to the quantity right, and you have to stop while it's still runny enough to flow smoothly. Epoxy can't be melted to smooth, once set; any smoothing has to be mechanical and that's not going to work here. Again, test on scrap materials.
The coating is clearly quite a variable thickness, as we can see the shape of the twisted wires in the trunk, but see a thicker layer on the base. That could have been done in 2 layers. In this case it looks like a similar material has been used to even out the underside of the base. If you do this, be sure to use a jig to hold the tree steady as you cast the bottom.