There have been 230 genera and over 700 species of vascular plants recorded in Nahanni National Park Reserve. The park has a richer diversity of vegetation than any other area of comparable size in the continental Northwest Territories.
Reasons for the extremely rich flora are complex. The presence of highly specialized habitats such as wet calcareous substrates, mist zones of waterfalls, hot and cold mineral springs, and unglaciated terrain are important. Areas of discontinuous permafrost and periglacial habitats also have influence. In addition, the length of the park intersects major differences in local climate and weather systems.
Nahanni's vegetation is predominantly boreal forest, with a transition from lowland wet areas to alpine tundra. The main tree species are white and black spruce, lodgepole pine, jackpine, subalpine fir, larch, balsam poplar, trembling aspen and white birch. White spruce and trembling aspens characterize the valley bottoms, while pine and spruce mixtures dominate the montane and sub-alpine zones.
Lodgepole pine, found predominantly in the western portion of the park, is at its northern limit of distribution. Naturally occurring wildfires ensure that the complex mosaic of vegetation is maintained.
The park contains the only known occurrence of the Nahanni aster (Aster nahanniensis). Bryophyte and lichen flora in the park are extensive compared to other northern areas. Other significant features of vegetation distribution are two large tracts of alpine tundra on the Tlogotsho Plateaus and unvegetated areas, some with permanent snow and ice cover in the west end of the park.