The Gheranda Samhita describes vrikshasana as standing on one leg with the other foot set at the root of the opposite thigh, the practitioner upright and unmoving. Vriksha is the tree, and the image is a trunk rising from a single base.
It is one of the very few genuinely standing postures in the classical corpus. Counting across both surviving manuals there are three or four, and the tree is the only one that stands flat on a single foot. The Hatha Yoga Pradipika does not carry it.
The image has a long history in Indian religious narrative that the posture entry does not invoke. Standing motionless on one leg for years is the standard literary description of extreme ascetic practice, tapas, performed by sages and by demons alike in the epics and Puranas in order to compel a boon from a god. The manuals say nothing of this and the atlas asserts no connection, but the resonance is there for any reader of the wider literature, and the posture is the only one in the corpus whose shape matches a scene the tradition tells about constantly. What the Gheranda Samhita records is a balance held still, described in one verse, with no claim attached.
Trees themselves carry heavy religious weight across South Asia, from the fig under which the Buddha is said to have reached awakening to the village shrine tree and the temple tree tended as a presence in its own right. The Gheranda Samhita draws on none of this. Its interest is the silhouette, a single support with everything above it held still, and the atlas records the name as a resemblance because that is all the text offers.