Ultramafic rocks can be either igneous or metamorphic. They are characterized by a very low silica content (<45%), high magnesium and iron content and low potassium. Serpentine soils, which are rather rare and localized in distribution, are derived from ultramafic rocks. These soils lack many essential nutrients (nitrogen, potassium and phosphorous), have high concentrations of heavy metals (nickel, iron, chromium and cobalt) and have low calcium. Much of the Klamath and Siskiyou Mountains in Northwestern California and Southwestern Oregon are composed of serpentine soils and ultramafic rocks.
Many unique plants have evolved to survive in these “less than hospitable” substrates. One fern that is adapted to serpentine soils and ultramafic rocks is pod fern (Aspidotis densa), although it is not restricted to these soils. This small fern is found growing in the crevices and mossy cracks of rocky areas from low to mid elevations in the Western United States, Quebec and Southeastern Canada.
Growing in small clumps or large colonies, pod ferns may have two different types of leaves. The fertile leaves have a triangular blade and long chestnut-brown petiole. The blade is composed of many leaflets divided into untoothed segments. The sori (clusters of spores) are continuous along the length of the leathery leaflet undersides. The edges of the leaflets curve over to partially conceal the sori forming a fake indusium (sori cover and protection) The leaflet tips are pointed. Unfertile leaves, if present, resemble parsley leaves.
Other common names for A densa are Indian’s dream, dense lace fern and cliffbrake. Synonyms for this plant are Cryptogramma densa and Onychium densum.
These pod ferns were photographed along Althouse Creek near Cave Junction in Southwestern Oregon.