。
The mountains were dyed in a brocade-like tapestry of trees.
Literal
Brocade weave-[classical-あやなす] trees-[plural] [with-で] mountains [subject-が] were-being-dyed-[continuous-ていた].
Poetic autumn description. 錦 ('brocade, rich multi-colored cloth') is used metaphorically for autumn foliage — the changing leaves are a classic Japanese 'brocade of the mountains.' あやなす is a literary/classical verb meaning 'to make patterns, to weave, to variegate' — preserved in poetic register. 染まる ('to be dyed') is used figuratively for scenes being colored. This is classic 季語 (seasonal-word)-rich prose.