ForestThe Nowitna's vegetation forms part of the circumpolar northern coniferous (boreal) forest. On the refuge, forests dominate at elevations below treeline. Open stands of black spruce are common in low-relief terrain. White spruce, occasionally growing with paper birch and aspen, can be found in the better-drained and warmer sites.

Seven major vegetation classes were distinguished in a mapping process conducted on the refuge in 1985 using Landsat images. Forests are the most widespread vegetation type, covering 88% of the refuge. Of the five recognized subclasses, open needleleaf forest and broadleaf forest are the most extensive, comprising almost 1.5 million acres or 72% of the surface area of the refuge. The five forest subclasses are described below:

Closed needleleaf forest - This subclass has 60 to 100% cover, occurs on moist to well-drained sites from the lowlands to mountain slopes and is particularly well developed on alluvial sites along the Nowitna River and on some islands in the Yukon River. The dominant tree species is white spruce, which may grow in excess of 100 feet tall along the Nowitna River. White birch and balsam poplar are secondary species. This subclass comprises 2% of the refuge surface area.

Open needleleaf forest - This subclass has 25 to 60% tree cover and is found on moderately to poorly-drained soils. It is usually dominated by black spruce or larch. This subclass comprises 42% of the refuge surface area.

Needleleaf woodland - This subclass, which is sometimes called "muskeg", has 10 to 25% tree cover and is found on moderately to poorly drained soils. Black spruce is the most common tree, and dwarf shrubs such as Labrador tea, bog blueberry, lingonberry, and small cranberry are important in the understory. Sphagnum moss covers much of the ground, insulating the permafrost layer beneath. This subclass comprises 10% of the Refuge surface area.

Broadleaf - This subclass has 25 to 100% cover and occurs in well to imperfectly-drained sites. White birch, aspen, and balsam poplar dominate the overstory. Other types of broadleaf deciduous forests occur on hills where strips of birch forest line many hillside streams, and aspen is present on south-facing sandy hillsides. This subclass comprises 30% of the refuge surface area.

Mixed forest -This subclass has 25 to 100% cover. It consists of deciduous broadleaf and evergreen needleleaf trees distributed over large areas of moderately to well-drained soils on the lower mountains. It grows tallest in lowlands along rivers and on islands in the Yukon River. Principal species are white birch, aspen, and white spruce. This subclass comprises 4% of the refuge surface area.