Spreading Wood Fern (Dryopteris expansa)

There is something about ferns that is naturally soothing to the human eye. Long popular with houseplant enthusiasts, ferns seem to radiate tranquillity. Whether it is the form and texture of their delicate fronds, the nature of the settings in which they occur, or the fact that they predate flowering plants, ferns capture our imagination and calm our city-shattered nerves. Growing up in the nursery business, I always had a particular fondness for ferns. I associated ferns with exotic, tropical locales and enjoyed watching them develop seemingly spontaneously beneath our greenhouse benches and, quite literally, out of the woodwork. In my travels, I am always drawn to locations where ferns grow. As a youth (a noun I can no longer use to describe myself) I never imagined that so many different species of ferns grew so close to home.

The true ferns belong to the Division Pteridophyta. California’strue ferns belong to the Class Filicinae, which consists of four orders, nine families, 24 genera and 74 native species. In San Francisco, 12 native species in eight families can still be found and another two are recorded historically but have not been seen recently. (For more information on San Francisco’snative ferns, see the “Focus on Rarities” article “San Francisco Ferns”, December 1996.

In continuing my exploration of locally rare species (species known in only one or very few locations on the San Francisco Peninsula) there is one fern that especially merits mention. Dryopteris expansa (spreading wood fern) is a stout, woody, creeping or ascending fern with fronds up to one meter long. It is in the same family (Dryopteridaceae) as western sword fern (Polystichurn rn uniturn) and is typically found in similar habitats, namely redwood and mixed evergreen forests. It is, however, more characteristically riparian in nature, and is especially associated with stream banks. While it is rare in San Francisco, snreadino wood fern is recorded from San Mateo County to Del Norte County, and extends northward to Alaska. It is also known from Canada, the Rocky Mountains, New England to North Carolina, Greenland and Eurasia. The type locality for the species is Germany, where it was originally named Polystichum dilatata by Hoffmann in the early 1800s.

Spreading wood fern captured my attention for a couple of reasons. I wasn’t familiar with it when I first came across it. I was conducting surveys of the vegetation in Lobos Creek to assess damage from the infamous slope failure that caused a home to tumble into the creek during the heavy rains on December 11, 1995. Walking in the creek as I was, surrounded by dense stands of aquatic plants with old oaks lining the banks, I could scarcely believe that I was in San Francisco. And there was this rather large fern, growing in standing water in Lobos Creek, where it formed 4-foot tall clumps on decaying logs. I had to keep reminding myself that I was still in the city and not in a much more remote and exotic locale.

Co-occurring with spreading wood fern in Lobos Creek are punctate knotweed (Polygonurn punctaturn), small-fruited bulrush (Scirpus microcarpus), slough sedge (Carex obnupta), common large monkey-flower (Mimulus guttatus), American oenanthe (Oenanthe sarmentosa), duckweed (Lemna minuscula), common scouring rush (Equiseturn hyemale), stinging nettle (Urtica dioica) and wire rush (Juncus effusus,). Woody species found along Lobos Creek include arroyo willow (Salix lasiolepis), shining willow (Salix lucida ssp. lasiandra), and American dogwood (Cornus sericea ssp. sericea). Lobos Creek supports one of three remaining native populations in the Presidio of wax myrtle (Myrica californica), which is abundant on its banks.

Spreading wood fern is truly rare in San Francisco. To my knowledge, Lobos Creek is the only site where it can be found in the City. To the south, it is recorded from the San Francisco Watershed Reserve and Butano Creek; it is not listed in the flora of San Bruno Mountain. To the north, it is recorded from Marin County (Steep Ravine, Mount Tamalpais, Bear Valley and Point Reyes Peninsula); it is listed as rare in the floras of Sonoma and Mendocino counties. Although not officially recorded for the East Bay, it has been noted on East Bay Municipal Utility District lands.

Spreading wood fern is not considered a “special status species” by the U.S. Fish and Wildlife Service, California Department of Fish and Game, or the California Native Plant Society. As such, it has no status as a protected species under any existing laws or policies. And although it is widespread in North America and Europe, the tiny population in San Francisco most certainly represents a unique biological resource. Spreading wood fern reaches its southernmost limits on the San Francisco Peninsula, representing a relict of a cooler, moister epoch. If you’d like to take a trip back in time, visit the Lobos Creek restoration area. At the same time, you will have a glimpse into the future and be able to marvel at the tremendous efforts of the staff of the Golden Gate National Recreation Area and their volunteers to restore and preserve these natural wonders.