N. S. Gill writes the following here:
A pine tree was made to represent the dead Attis for the day of the entrance of the tree.
The reference given is “The Cannophori and the March Festival of Magna Mater,” by Duncan Fishwick. Transactions and Proceedings of the American Philological Association, Vol. 97. (1966), pp. 193-202. This states:
Certainly the entry of the pine tree decked out to represent the dead Attis and the startling rites that followed two days later are ceremonies of a funerary festival that ended with the washing of Cybele’s image in the Almo; cf. Arrian, Tactica 33.4 (ed. A. G. Roos): ἡ Φρυγία τιμᾶται ἐκ Πεσσινοῦντος ἐλθοῦσα, καὶ τὸ πένθος τὸ ἀμφὶ τῷ Ἄττῃ Φρύγιον ὄν ἐν Ῥώμῃ πενθεῖται, καὶ τὸ λουτρὸν δ’ ἡ Ῥέα, ἐφ̕ οὗ τοῦ πένθους λήγει, τῶν Φρυγῶν νόμῳ λοῦται.
I admit that I had not heard of this work of Arrian, which never seems to have been translated. It was edited by Roos as part of a 2 volume collected works. The Greek seems to say:
“The Phrygian [goddess] from Pessinus is honoured, and the mourning for the sake of Attis the Phrygian is bewailed in Rome, and” … something about washing? What I don’t see is anything about how the pine tree represents Attis.