Fir trees were part of the environment nurtured by Jurassic Park staff, since they were found back in the Mesozoic, they made a nice addition to the variety of flora found on Isla Nublar. Firs were important enough to the park to be mentioned in the tour, by the computer monitors narrated by Richard Kiley, inside of the Toyota Land Cruisers
“…The world of the dinosaur included more modern plants, such as pine and fir trees, and swamp cypresses. You will see these as well.” Quote found in the Third Iteration: The Tour (page 135 paperback edition) of the Jurassic Park novel.
Ellie Sattler being a paleobotanist, had knowledge of this tree as well:
“When a Douglas fir tree is attacked by beetles, it produced an anti-feedant chemical-and so did other Douglas firs in distant parts of the forest. It happened in response to a warning alleo-chemical secreted by the trees that were under attack.” The Jurassic Park Novel Third Iteration: Jurassic Park (page 86 paperback edition)
Fir is a genus of 48–55 species of evergreen coniferous in the family Pinacaea. It is found through much of North and Central America, Europe, Asia, and North Africa, occurring in mountains over most of the range. Firs are most closely related to the genus Cedrus (cedar); Douglas Firs are not true firs, being of the genus Pseudotsuga.