Here's an overview that has a timeline and some jumping off points if you're interested in learning more. What happened as the ice retreated varied by area and some of the valleys in particular have been studied to see what species arrived, in what order, and to investigate delays by some.

https://www.nps.gov/parkhistory/online_books/olym/schalk/cha...

> In comparing the various palynological sources for Northwestern Washington and surrounding regions, it is clear that the vegetation history varies at least in its details from area to area. For example, Heusser (1978:1576) notes that treeless conditions persisted longer after glacial retreat in the Hoh Valley than in the northwestern corner of the Olympic Peninsula.

> ...the arrival of coniferous trees in this area was apparently delayed by aridity until sometime between 11,000 and 9,600 yr B.P.