Horkelia

Horkelia clevelandii, copyright John Rusk.

Belongs within: Potentilla.

The perils of exposure
Published 7 May 2024

When many people think of conservation, they picture dense woodlands or rainforests. But, of course, more open habitats provide homes to their own characteristic biota that may face their own challenges to survival. This may be demonstrated by certain species of Horkelia.

Horkelia hendersonii, copyright Dee Shea Himes.

Horkelia is commonly recognised as a genus of about twenty species of perennial herbs found in western North America, the majority of them in California. I say ‘commonly recognised’ because phylogenetic studies in recent decades have nested Horkelia and other related genera within the diverse genus Potentilla, the cinquefoils. Competing suggestions have been made on how to deal with Potentilla’s resulting paraphyly, whether to sink Horkelia and others within a broad Potentilla (e.g. Eriksson et al. 2022) or maintain the segregate genera by carving Potentilla apart (e.g. Kechaykin & Shmakov 2016). At present, it may be too early to say which of these approaches will eventually win out.

Horkelia tridentata, copyright Mary E. Muchowski.

Whatever their nomenclatural status, there is broad agreement that the species of Horkelia form a distinct group. Most species are glandular with a distinct resinous smell. The leaves are pinnate usually flat, and are mostly clustered towards the base of the plant. Sparser, smaller leaves are borne on the more or less erect stem, which usually grows about a foot in height though at least one species H. californica may stand more than a metre. Flowers are typically white and borne in a cyme that may be open or composed of dense clusters. The shape of the flowers is characteristic with a broad, flat-bottomed hypanthium, five sepals and five petals with the sepals often reflexed, and ten stamens with flattened filaments that often form a tube around the style. Fruits are an achene similar to those produced by their close relative, strawberries, though Horkelia lack the swollen receptacle that strawberries have (Ertter 1993; Hickman 1993).

Barton flats horkelia Horkelia wilderae, copyright James Bailey.

As alluded to above, Horkelia species are typically associated with more or less open habitats. These may include the edges of forests and streams, meadows, chaparral and other scrub, or coastal dunes. The preferred micro-habitat of particular species may be sparsely distributed so species distributions may be patchy. Open habitats are also often vulnerable to disturbance, offering tempting locales for urban development, roads and trails, or establishment of campsites. As a result, certain species of Horkelia may be considered threatened. Horkelia bolanderi is known from only a few locations near vernal lakes in northern California and is threatened by development. Horkelia marinensis is restricted to beaches and other sandy coastal habitats between Fort Bragg and Santa Cruz, in the vicinity of San Francisco. Horkelia wilderae is known only from the vicinity of Barton Flats east of Los Angeles; it grows at the boundary between montane chaparral and pine forest, and is considered critically threatened by the effects of logging. Horkelia species are often not large plants; they may all too easily be ground under a foot or crushed beneath a tire.

Systematics of Horkelia
<==Horkelia
|--H. bolanderiH93
|--H. californicaH93
| |--H. c. ssp. californicaH93
| |--H. c. ssp. dissita [incl. H. elata]H93
| `--H. c. ssp. frondosaH93
|--H. clevelandiiH93
|--H. congestaH93
| |--H. c. ssp. congestaH93
| `--H. c. ssp. nemorosaH93
|--H. cuneataH93
| |--H. c. ssp. cuneataH93
| |--H. c. ssp. puberulaH93
| `--H. c. ssp. sericeaH93
|--H. daucifolia [incl. Potentilla daucifolia var. indicta, H. daucifolia ssp. latior]H93
|--H. fuscaEH03
| |--H. f. ssp. fuscaH93
| |--H. f. ssp. capitataH93
| |--H. f. ssp. parviflora [incl. H. fusca ssp. pseudocapitata]H93
| `--H. f. ssp. tenellaH93
|--H. hendersoniiH93
|--H. hispidulaH93
|--H. marinensisH93
|--H. parryiH93
|--H. rydbergii [incl. H. bolanderi ssp. parryi non H. parryi]H93
|--H. sericata [incl. Potentilla howellii]H93
|--H. tenuilobaH93
|--H. tridentataH93
| |--H. t. ssp. tridentataH93
| `--H. t. ssp. flavescensH93
|--H. truncataH93
|--H. tularensisH93
|--H. wilderaeH93
`--H. yadoniiH93

*Type species of generic name indicated

References

[EH03] Eriksson, T., M. S. Hibbs, A. D. Yoder, C. F. Delwiche & M. J. Donoghue. 2003. The phylogeny of Rosoideae (Rosaceae) based on sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA and the trnL/F region of chloroplast DNA. International Journal of Plant Science 164 (2): 197–211.

Eriksson, T., N. L. Persson & J. E. E. Smedmark. 2022. What is Potentilla? A phylogeny-based taxonomy for Potentillinae (Rosaceae). Taxon 71 (3): 493–505.

Ertter, B. 1993. A re-evaluation of the Horkelia bolanderi (Rosaceae) complex, with the new species Horkelia yadonii. Systematic Botany 18 (1): 137–144.

[H93] Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley (California).

Kechaykin, A. A., & A. I. Shmakov. 2016. A system of subtribe Potentillinae J. Presl (Rosaceae Juss.). Turczaninowia 19 (4): 114–128.

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