I think they both look good, but somewhat unrefined. For a tree that size, there's really not a lot of details. If you want to work on your pixelling technique, I would try to make them smaller, or give them more details, or work on making the lines smoother with AA. I personally would like to see some work on individual leaves, when you're working at this scale, but that's just my personal preference.

In terms of lighting, I notice that the trunk has more light on the left side, shadows on the right side, yet the green leaves on top have an equal amount of light on both sides.

On the snow-covered tree, I think the branches are a bit unrealistic, by being too thick, too spike-like and too densely packed together. I don't recall ever seeing a tree like that. If you wanted to make it alien and unusual, I guess it's not a problem.

The snow at the base of your tree is covering the top of the roots, where in reality you would expect the snow to be filling up the crack between the roots, due to wind, or even ice to gather in those cracks from periods of melting snow.

Only things I really have to say are (on the full tree, use some stippling on the light edges, it kind of looks posterized now. And on the barren, I'm not sure I like the short spiky look. I know you said it was stylistic, but maybe darken the palette so it fits, it looks a bit more like 'odd cactus tree' right now.