I thought I'd try to get my head around this deck by looking at themes, patterns and recurring images. There are six cards that feature trees, so let's have a look at them.
The Emperor and Empress
These two cards swap black and white -- Emperor is black on white, Empress is white on black. The Emperor tree is a longer, pointier shape, which could perhaps be seen as phallic, while the Empress tree is a rounder shape. The Emperor tree has lots of spaces between branches, while the Empress tree is more lush. The Emperor tree stands out in stark black on white, and the sun is blotted out by a total eclipse, though the sky remains colorless, neither blue nor dark, but white. The Empress on the other hand is beneath a waning crescent moon. The sky is black. The tree is tinged with red, though, and this strikes me both as a warm colour, and also a reference to fecundity, the richness and the darkness of the womb.
The Tower and the Moon
The Tower and the Moon cards both feature evergreen trees in white against a black background. In the Moon card, we again see a waning crescent. I don't know if this has significance, or if it's just because the artist likes the moon drawn in the shape of a letter C. I am just not sure how much purposeful meaning went into these cards, beyond design, and how much we are putting into the after the fact. So-called 'intuitive' readers can load imagery with all sorts of meaning--can take anything, a photo from a magazine, an advertisement, a beermat--and see symbolic significance there. That said, the two trees in the Moon card are probably acknowledging the two towers in the RWS Moon.
The Tower card shows a tree being struck by lightning, which seems to be a very straightforward Tower depiction.
The 6 of Cups and Ace of Pentacles
People talk a lot about how 'natural' these cards are, and they are, but actually there is manmade stuff all over the imagery in Wild Unknown. We have cups, swords, domesticated flowers (roses), domesticated animals (horses, goats, cats), we have crosscuts of tree trunks, such as you get from using a saw, there are ribbons wound around swords and twigs -- there is evidence of man in many of the cards. For me, this does not detract from the deck, but rather adds to it. Most of my favourite decks feature people very heavily, and a way in to the cards for me is the human element present there.
So in the 6 of Cups we have a tree that is growing upward against a sky that has horizontal lines. These lines make me think of measuring the height of the tree over time, as it grows. The view of underground shows that the roots of the tree are deeper and wider than the tree itself, which of course is true of trees, but we also have the suggestion that this is also true of ourselves. Our roots, our past experiences, our lessons learned, the things we have been through, loom deeper and larger and wider than our present manifestation. Without them, our present manifestation could not exist. The card shows it to the point that all the actual colour is in the roots. That is a strong message!
Finally the Ace of Pentacles shows a cross-section of a tree trunk. In the heart of it, a glowing orange core with five points radiating outward, like a cross-cut of an apple. The tree trunk itself seems to glow outward--not only do we grow upwards, but we also expand. Height suggests aspirations to the spiritual, whereas outward growth suggests an expansiveness that encompasses the earthly plane, and all its sensations and experiences.
Tarot cards are about people. These Wild Unknown cards are about people, too. This deck explores the human experience and spirit through images of nature, which in some ways could be seen as adding a layer to get through to get to the heart of the meaning. The tree cards aren't about trees. The cat cards aren't about cats. They're about us. Figuring out how a tree is like a mother is one degree of separation from an image of a human mother. In this way, for some at least, the Wild Unknown Tarot can have the same barriers presented by any 'themed' deck, such those based around mythology, or fairy tales, or Jane Austen.