It is native to the west coast of North America from British Columbia to California and east to the Rocky Mountains in Idaho, Montana, and Wyoming; there is a disjunct population on serpentine soils in Quebec.

Where the petiole joins the leaf, the stem color grades to green and acquires a groove on its adaxial (top) surface.

[2] A. densa is native largely to the west coast of North America from British Columbia to California and east to the Rocky Mountains in Idaho, Montana, and Wyoming.

There is a disjunct population on serpentine soil in Frontenac National Park in Quebec, and closer probably disjunct populations in the Wasatch Mountains of Utah and in the Cuyamaca Mountains near San Diego California, neither apparently associated with serpentine.

[7][8] Relationships between vascular plants with an emphasis on the ferns as summarized from morphological and molecular phylogenetic analyses.