Near two native ferns, their hybrid is discovered, along with possible backcrosses
The Jasper Ridge herbarium team has monitored plants on the Preserve for 25 years carrying on the fieldwork tradition of Herb Dengler, John Thomas and many prominent botanists, spanning 150 years. Because the local flora is rich, the rewards of fieldwork are reaped year after year as plants and docents come and go.
In October, Diane Renshaw and Rebecca Reynolds were collecting Hooker's evening primrose and California rose, respectively, along San Francisquito Creek when an unusual patch of ferns on the south creekbank demanded our attention.
Jasper Ridge has a strong representation of the ferns found in the Santa Cruz Mountains, and as with all other plants on our plant list, the ferns are represented by vouchers (sheets of pressed plants) in the Oakmead Herbarium. One fern not on the list (though it had once been)1 is the California shield fern (Polystichum californicum), a purported sterile hybrid whose ancestors in our region are the locally common Sword fern (P. munitum) and the less common Dudley's shield fern (P. dudleyi)2. Close examination of the intriguing clump revealed all three species growing within a few feet. The California shield fern fronds are intermediate in gross morphology between those of its two ancestors.3 Also, our California shield fern appears from its sporangia (spore sacs) to have experienced chromosome doubling, restoring fertility, and also allowing backcrossing. Further plant hunting revealed one additional California shield fern, along Bear Creek, and two possible backcrosses with P. munitum (a speculation for further investigation).
And heads up, Fragile fern (Cystopteris fragilis) will soon be leafing out at the edge of the Herb Dengler Trail west end of the redwood grove; watch your step.
1A frond with apparently normal sporangia had been vouchered in 1961 as P. californica, but reexamination of the sheet revealed it to be P. dudleyi, which made it the first record of Dudley's shield fern for the Preserve but left us without a voucher or any other record for California shield fern.
2 The complex taxonomic story of Polystichum is summarized in the Flora of North America. The ancestor (with P. dudleyi) of P. californicum in the northern part of its range from Northern California to British Columbia is thought to be P. imbricans, based on morphologic and ecological data. In other words, the taxon P. californicum is an amalgam of interfertile species with polyphyletic origins. Polystichum: from the Greek polys, "many," and stichos, "row," referring to the rows of sori on the type species; munitum: armed, fortified; dudleyi: named for William Russel Dudley (1849-1911), professor of botany and head of the Botany Department at Stanford University. (Charters, M. California Plant Names)
3 There is a handy key to JRBP ferns by affiliate Dr. Katherine Preston.
John Rawlings, JRBP Herbarium team. John recently completed the Cyperaceae (Sedge family) treatment for the revised Flora of San Francisco County (edited by Tom Daniel, Curator of Botany, Emeritus, California Academy of Sciences), and is in the midst of an ongoing survey with naturalist Ken Hickman of Locally Rare plants on Midpeninsula Regional Open Space District preserves in the Santa Cruz Mountains.