I couldn't find anything about it being held under a tree... But I also couldn't find anything about where it was held, other than being in Niagara (then Newark), on 17 September 1792. What sources said it was under a tree?

Edit: I found some sources, but they hardly seem reliable.. hmm...

Edit2: This 1907 book says the following:

As Miss Carnochan, the indefatigable historical enthusiast of Niagara, writing at that time .said: "The much vexed question as to the first parliament house may yet be settled, but so far, it is wise not to assert too confidently, unce no less than five places have been mentioned as deserving the honor , 'Navy hall,' 'The Indian council house,' 'The Parliament Oak,' 'Fort Niagara,' and 'Government house near the present court house.' Since parliament met here during five years, it is quite likely that more than one of these can claim the honor."

Edit3: This 1996 book writes that the parliament may have been held in the local Masonic lodge that served as the Indian council house.

Seems like it's unclear! The most reliable sources say that we will never know, but it was like inside a building one way or another.