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More than 250 vascular plants, 74 moss and liverwort species, and a large number of fungi can be found within Fort Clatsop National Memorial. The park's habitat diversity, ranging from coastal rainforest to riparian and estuarine marsh, shrub, and swamp wetlands, is responsible for its high diversity of plant and fungi species.
The Sitka spruce vegetation zone forms a narrow band along the northwest coast from northern California to southeastern Alaska. The mild, wet maritime climate and rich soils create ideal growth conditions for conifers, resulting in climax spruce-hemlock forests of magnificent size and lush biodiversity. Sitka spruce (Picea sitchensis) is the dominant conifer in the park's forests. Western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), Douglas fir (Pseudotsuga menziesii), red alder (Alnus rubra), cascara (Rhamnus purshiana), red elderberry (Sambucus racemosa) and several willow species are the common subdominant trees. Salal (Gaultheria shallon), huckleberries (Vaccinium spp), seven fern species, and a large number of mosses, liverworts, and lichens share the forest understory. In November of 1805 explorer William Clark wrote of the coastal forests, "Spruc Pine grow here to an emense Size & hight maney of them 7 & 8 feet through and upwards of 200 feet high ... I observed in maney places pine of 3 or 4 feet through growing on the bodies of large trees which had fallen down, and covered with moss." Logging removed most of the site's trees between 1850 and 1950. Regenerating forests presently cover 50 acres in the park, 20 of younger and 30 of older trees. Several spruce are between five and six feet in diameter and over 100 years old. The park's older forests are slowly redeveloping characteristics of forest ecosystem health: multilayered canopies of diverse species and age classes, forest openings, standing dead snags and large decaying logs.
The Memorial protects valuable estuarine resources within the nationally significant Columbia River estuary. The 1979 National Wetlands Inventory identified 10 types of wetlands within Fort Clatsop: seven palustrine, two estuarine and one riverine. Wetlands comprise approximately half of the park's acreage and include the tidally-influenced Lewis and Clark River, low-gradient brackish sloughs, freshwater ponds and several freshwater streams and springs. In February of 1806 Meriwether Lewis described the wetlands near Fort Clatsop, writing, "The grasses of this neighbourhood are generally coarse harsh and sedge-like, growing in large tufts ... the salt marshes also produce a coarse grass, Bull rushes and the Cattail flagg." During the 19th and early 20th centuries the river shore was extensively diked and tidegated, converting former floodplains to agricultural lands. Removal of a section of dike within the park in 1961 restored former pastureland to a functional high tidal marsh populated by a diversity of estuarine plants including Lyngbye's sedge (Carex lyngbyei) and the uncommon narrowleaf cattail (Typha angustifolia). Two rare estuarine species, flowering-quillwort (Lilaea scilloides) and water-pimpernel (Samolus parviflorus), grow on the river's tidal mudflats and slough banks. The park also has several acres of willow shrub wetlands, a habitat type that has largely disappeared from the Columbia estuary during the last century. Species and hybrids of two beautiful native impatiens, jewelweed (Impatiens capensis) and spurless touchmenot (I. ecalcarata), occur in this habitat, the westernmost extent of a linear hybridization zone that extends for 30 miles along the lower Columbia River.
One third of the park's vascular plant species are nonnative, due to the site's extensive agricultural and residential use history. Invasive species control projects target a number of these exotics, including Scotch broom, English ivy, English holly, and yellow iris.
A permit is required in order to collect plants within the Memorial.
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