Plant Species of San Jacinto Mountain: Arabis johnstonii, Johnston's rock cress

Arabis johnstonii and A. parishii are an interesting species pair. They are very similar, but distinct, taxa, and each species is endemic to a very small area, with those two areas separated by just 40 miles. Arabis johnstonii is found only at the south end of the San Jacinto Mountains between 4450 and 6800 feet, and A. parishii is found only at 6500-9800 feet in the San Bernardino Mountains.

Presumably these two species are very closely related. One possible scenario is that they differentiated from a common ancestor after populations of that ancestor became reproductively isolated. The San Bernardino Mountains and San Jacinto Mountains formed only geologically recently as part of the San Jacinto / San Andreas Fault System. Prior to ~five million years ago, both those areas were part of the same flattish contiguous area, without significant elevation ranges and with no barrier like the Banning Pass separating them.

Online vouchers of A. johnstonii are only from a triangular area of about three square miles, the Quinn Flat / Kenworthy / Pyramid Peak area at the southeastern end of Garner Valley.

There is a third recently-discovered species which is also probably closely related to these two species. A. hirshbergiae, found only near Lake Cuyamaca in the Cuyamaca / Laguna Mountains of San Diego County at 4600-4700 feet, was described by Steve Boyd in 1998 (Aliso 17:203-205). Boyd wrote:

All three species are associated with pebble plain habitats in montane areas characterized by relatively gentle relief compared with surrounding canyons and ridge systems.

The current habitat of these three species is consistent with an origin from an ancestor that flourished in the flattish pre-San Andreas Fault southern California.

Name changes. Many of the California species formerly assigned to Arabis in California have been reassigned to Boechera, since these California plants are genetically distinct from the Arabis of the old world (After all, Arabis was named for Arabia!). These three species were renamed Boechera johnstonii, B. parishii, and B. hirshbergiae in 2003 (Novon 13(4): 386-388).

Furthermore, Windham and Al-Shehbaz (Harvard Papers in Botany 11:61-88, 2006) consider A. hirshbergiae to be the same species as A. johnstonii, so those two species may be combined in the upcoming revision of the Jepson Manual. However, it is unlikely that A. hirshbergiae and A. johnstonii are currently exchanging genes, so these may be populations on their way to becoming morphologically-distinct species.

Keying. The plants shown in the photographs below were seen on the Pacific Crest Trail between the Cedar Springs Trail and Pyramid Peak (the location is intentionally made vague since this is a rare species). From my photographs alone, they key well to A. johnstonii, using the Jepson Manual:

1'  Seed ~1.8 mm wide; fruit ~2.5 mm wide
13' Basal leaves linear / oblong
22  Leaves grayish white with hairs dense
23' Pedicel ascending
28  Style ~1.5 mm; basal leaves linear / oblong
29  Style ~1.5 mm; seed wing wide; fruit ~4.9 cm ...A. johnstonii

The measurements are all approximate since they were made from the following photographs.


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Copyright © 2008 by Tom Chester.
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Updated 7 November 2008.