[gdw]
The adjective only appears to be attested in the plant name branche ursine. The medieval association with branche stems from an incorrect translation of the Latin name for the plant: branca ursina (literally ‘bear’s claw’).
The phrase branc de ursine (uniquely attested in BL, Sloane 282, cf. Plant Names 54), where ursine appears to be used as a substantive, is unetymological (and is possibly a transmission error of branche ursine).