The word viṣṭara is irregularly formed in the sense of 'tree' and 'seat'.,
This word is formed by adding the affix अप् to the root स्तृ preceded by the preposition वि ॥ Thus विष्टरो वृक्षः ॥ विष्टरमासनम् ॥
Why do we say when meaning a 'tree' or a 'seat'? Observe औलपिवाक्यस्य विस्तरः ॥ See sutra 3.3.33. by which अप् is added here instead of धञ् ॥,