Dryopteris expansa (Alpine Buckler Fern or Spreading Wood Fern) is a species of fern native to cool temperate and subarctic regions of the Northern Hemisphere, south at high altitudes in mountains to Spain and Greece in southern Europe, to Japan in eastern Asia, and to central California in North America. The species was first described from Germany. It prefers cool, moist mixed or evergreen forests and rock crevices on alpine slopes, often growing on rotting logs and tree stumps and rocky slopes. It is characteristically riparian in nature, and is especially associated with stream banks.
It has a stout, woody, creeping or ascending stock with large, green lacy fronds 10-60 cm (rarely 90 cm) long. The fronds are bipinnate at the base, pinnate toward the apex. The rhizome is erect or ascending, often producing offshoots. sori occur medially on the underside of the pinnae. Propagation is by spores and vegetatively by division of the rhizome.
It is easily confused with the related Dryopteris dilatata (Broad Buckler Fern), differing in the usually smaller fronds, and in the pale brown scales on the frond stem being more uniform in colour, rarely having a dark central stripe. It also differs in cytology in having 2n = 82 chromosomes (164 in D. dilatata).
The species name of this fern, expansa, is from the Latin expando, meaning "to spread out, spread apart, to expand". Other common names include Northern Wood Fern, Arching Wood Fern, Spiny Wood Fern and Crested Wood Fern.