Located within the Klamath–Siskiyou region, known for its high biodiversity, the monument supports 391 vascular plant species as well as many species of bryophyte, lichen, and macrofungi.
Trees in the monument include Douglas fir, oak, white fir, and alder.
Among the oldest trees is Big Tree, thought to be the widest Douglas fir in the state.
It is 41 feet (12 m) in circumference near the base.
Its age is estimated at 600 to 800 years, and it was described in the 1930s as 14 feet (4.3 m) in diameter.
The monument contains no plants with special conservation status.

Species lists for the park include about 50 mammals, 86 birds, 11 reptiles and amphibians, 8 bats, more than 200 arthropods, 8 snails and slugs, 75 butterflies, more than 55 moths, and 8 aquatic macroinvertebrates.
Of these species, 160 are found inside the cave.
Outside the cave, the black-tailed deer, Steller's jay, the common raven, and Townsend's chipmunk are among animals often seen in the park.
Less commonly sighted are the black bear, cougar, northern flying squirrel, and Pacific giant salamander.
Springs and other wet places support flatworms, frogs, and snails.

Animal species in the park with special conservation status are the northern spotted owl, California mountain kingsnake, tailed frog, Del Norte salamander, northern goshawk, olive-sided flycatcher, little willow flycatcher, Siskiyou gazelle beetle, and Pacific fisher.
Five at-risk bat species are found in the cave: Townsend's big-eared bat, and the long-eared, fringed, long-legged, and Yuma myotis.
Located within the Klamath–Siskiyou region, known for its high biodiversity, the monument supports 391 vascular plant species as well as many species of bryophyte, lichen, and macrofungi. Trees in the monument include Douglas fir, oak, white fir, and alder. Among the oldest trees is Big Tree, thought to be the widest Douglas fir in the state. It is 41 feet (12 m) in circumference near the base. Its age is estimated at 600 to 800 years, and it was described in the 1930s as 14 feet (4.3 m) in diameter. The monument contains no plants with special conservation status.
Species lists for the park include about 50 mammals, 86 birds, 11 reptiles and amphibians, 8 bats, more than 200 arthropods, 8 snails and slugs, 75 butterflies, more than 55 moths, and 8 aquatic macroinvertebrates. Of these species, 160 are found inside the cave. Outside the cave, the black-tailed deer, Steller's jay, the common raven, and Townsend's chipmunk are among animals often seen in the park. Less commonly sighted are the black bear, cougar, northern flying squirrel, and Pacific giant salamander. Springs and other wet places support flatworms, frogs, and snails.
Animal species in the park with special conservation status are the northern spotted owl, California mountain kingsnake, tailed frog, Del Norte salamander, northern goshawk, olive-sided flycatcher, little willow flycatcher, Siskiyou gazelle beetle, and Pacific fisher. Five at-risk bat species are found in the cave: Townsend's big-eared bat, and the long-eared, fringed, long-legged, and Yuma myotis.
Oregon Caves National Monument and Preserve