Threats to the park include overuse, urban traffic, encroaching development, invasive flora, and lack of maintenance money.

[4] About eight separate Grande Ronde Basalt flows have been mapped in the Tualatin Mountains (West Hills), where they underlie the steepest slopes of Forest Park and form the columned rocks visible along Balch Creek Canyon and Northwest Cornell Road.

It also includes smaller numbers of grand fir, black cottonwood, red alder, bigleaf maple, madrone, and western yew.

[11] In the mid-1990s, about 1% of the total vegetation in the park consisted of grasses, bracken, thistle, and fireweed in sections of the forest cleared two to five years earlier.

Forest areas 10 to 30 years old that contained tall alder and maple trees and smaller conifers accounted for about 20% of the park.

[11] The last forest stage, old growth, is reached after 250 years and includes many snags, downed and dead trees, and fallen logs.

[11] The largest tree in Forest Park is a Douglas-fir near the Stone House, the remains of a former public restroom near Balch Creek.

[11] Invasive species include English ivy, European holly, clematis, morning glory, and Himalayan blackberry.

[14] and The Forest Park Conservancy engage in projects to remove ivy, maintain trails, and plant native species.

[13] Sixty-two mammal species, including the northern flying squirrel, black-tailed deer, creeping vole, bobcat, coyote, Mazama pocket gopher, little brown bat, Roosevelt elk, and Pacific jumping mouse frequent Forest Park.

Blue grouse, great horned owl, hairy woodpecker, Bewick's wren, orange-crowned warbler, osprey, northern pygmy-owl, and hermit thrush are among the more than 112 species of birds that have been observed in the park.

[18] Amphibian species frequenting the Audubon Society pond include rough-skinned newts, Pacific tree frogs, and salamanders.

[13] Pressure from habitat loss, pollution, hunting, and urban development has reduced or eliminated the presence of wolves, bears, and wild cats and has led to increased numbers of weasels, raccoons, and other small predators.

Invasive plant species such as English ivy have made the habitat simpler and less supportive of native insects and the salamanders and other amphibians that feed on them.

[22] Near the east end of the park, the free-flowing reaches of Balch Creek support a population of resident cutthroat trout.

[13] Near the west end, furthest from the city center, Miller Creek retains much of its historic nature and supports a greater diversity of aquatic organisms than other Forest Park streams.

[22] Biological field surveys of Miller Creek in 1990 noted sea-run cutthroat trout, coho salmon, and short-head cottid, as well as abundant macroinvertebrate species including stoneflies, mayflies, caddisflies, water striders, and crayfish.

After logging, the steep slopes and unstable silt loosened by heavy rains caused landslides that defeated construction plans, and claims were defaulted or donated to the city.

[7] Civic leaders beginning with the Reverend Thomas Lamb Eliot, a minister who moved to Portland in 1867, sought to create a natural preserve in the woods that eventually became Forest Park.

[23] The Olmsted Report, received in December, emphasized creation of a system of parks and linking parkways that would take advantage of natural scenery.

In 1897, Donald Macleay, a Portland merchant and real-estate developer, deeded a 108-acre (44 ha) tract of land along Balch Creek to the city to provide an outdoor space for patients from nearby hospitals.

In 1928, the City Council's Delinquent Tax Committee transferred land to the Parks Bureau for a wildflower garden along Balch Creek.

Multnomah County Sheriff's deputies in 2007 seized 114 mature marijuana plants found growing in the park on a hillside near Portland's Linnton neighborhood.

[53] Further southeast, Wildwood Trail, while still in Forest Park, passes Pittock Mansion and its panoramic views of Portland and five volcanic peaks: Mounts Rainier, Adams, St. Helens, Hood, and Jefferson.

[9] Beyond the mansion, the trail connects to adjoining Washington Park and attractions such as the Oregon Zoo[54] via the Barbara Walker Crossing, a pedestrian bridge over Burnside Street.

[55] From here and from more remote Forest Park trailheads near the St. Johns Bridge, other components of the 40-Mile Loop system of trails encircle the city.

[54] This trail network links more than 30 separate parks that offer diverse recreational opportunities, such as horse-back riding, in-line skating, canoeing, and viewing of wetland wildlife, in addition to hiking and biking.

Metro, the regional government, plans to link the 40-Mile Loop to trails along the Willamette River to Wilsonville, south of Lake Oswego.

[6] It is also the longest section of the 40-Mile Loop, a trail network of roughly 150 miles (240 km) reaching many parts of the Portland metropolitan area.

[6] In 2019, the City of Portland constructed Barbara Walker Crossing to allow Wildwood Trail users to safely pass over West Burnside Street.