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The Santa Rosa Basalt Brodiaea | 
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Geographic Distribution and Basalt Endemism
Geographic Distribution
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Fig. 2. Geographic distribution of B. filifolia (+), B. santarosae (X), and B. orcuttii (O) in southern California, USA from vouchers, both ours and ones at RSA, SD and UCR verified by the authors. The minimum original extent of the Santa Rosa Basalt is outlined using the present position of the Basalt.
A. Map showing all populations.
B. Expanded-scale map showing remaining areas covered by Santa Rosa Basalt (black outline areas) and trails of the Santa Rosa Plateau Ecological Reserve in grassland (light gray).
Zoomable PDF versions of above maps: Fig. 2A, Fig. 2B.
Brodiaea santarosae is found only in a unique location in southern California, an area of low topographic relief that was flooded by the Santa Rosa Basalt 8-11 million years ago. Its range mostly, but not entirely, separates the populations of B. filifolia and B. orcuttii.
Fig. 2A shows the geographic range of these three species, along with the estimated minimum original extent of the basalt. Populations of B. orcuttii are found entirely to the south and southeast of the basalt. Populations of B. filifolia are found mostly to the north and west of the basalt, with just a few occurrences to the south (see also the note below), and only one population within the basalt area. That population, at the Mesa de Colorado, occurs at the biggest vernal pool, in heavily-leached dark vernal pool soils unlike the reddish basalt-derived soil elsewhere on the basalt; it is not accompanied there by B. santarosae.
The Santa Rosa Basalt has been dated as 8-11 million years old (Hull and Nicholson 1992; Hawkins 1970; D. Krummenacher, cited in Kennedy 1977). It formerly covered an area of at least 500 km2 (Kennedy 1977), from Elsinore Peak on the north, to the Murrieta Hogbacks and Oak Mountain on the east, to somewhere south of the Mesa de Colorado on the south, and to the area of Miller Mountain on the west. This is a minimum extent, estimated simply by taking the surviving patches of basalt and assuming basalt originally was continuous between them. Erosion has removed at least 97% of the original basalt area; the basalt now covers only disjunct areas of about 15 km2 (within a region of approximately 40 km2) plus a few very small patches such as the two patches at Elsinore Peak 11 km distant.
We include the "Basalt of Elsinore Peak", dated only as "Miocene" (Morton et al. 1999), as part of the Santa Rosa Basalt. Because these basalts most likely derived from the same or very similar source, and are from the same time period, they are highly likely to be substantially the same, and are treated as such here.
Fig. 2B shows the largest remaining areas of the basalt, along with all vouchered locations of B. santarosae and B. filifolia in that area. Voucher locations of most B. santarosae specimens are not precise; locations such as Mesa de Burro have been arbitrarily placed on the Mesa. Most of the extant basalt is on the mesas of the Santa Rosa Plateau, not all of which have yet been surveyed for these species. All Santa Rosa Basalt areas surveyed to date contain B. santarosae.
Perhaps the most remarkable feature of B. santarosae is that every population we surveyed grows in soils derived from basalt. Every surveyed population except one, and all vouchers we determined as B. santarosae, were from soils underlain by the Santa Rosa Basalt as mapped in Kennedy (1977). The exception was the population immediately west of the Mesa de Burro at Clay Hill. Although the basement rock there is mapped as metasedimentary, it turns out there are actually two previously-unknown areas of basalt to which this population is confined. Furthermore, the discovery of basalt at this location led to a geologic discovery of an ancient valley preserved in this basalt!
This distribution thus shows tremendous fidelity to the basalt areas, since both the populations and the major remaining basalt areas (not including the unsurveyed Oak Mountain) span a distance of 15 km by 15 km, yet both B. santarosae and the basalt only cover almost the identical ~7% (15 km2 / 225 km2) of that area.
This coincidence is not due to habitat considerations; there are extensive grasslands inhabited by B. terrestris Kellogg subsp. kernensis (Hoover) T. Niehaus outside the basalt areas. No other populations of B. santarosae, B. filifolia, or B. orcuttii have been documented even though those grasslands have been intensely studied. The first and third author surveyed the grassland trails of the Santa Rosa Plateau Ecological Reserve shown in Fig. 2B in 2001-2006; Lathrop and Thorne (1985) surveyed most of the area shown in Fig. 2B south of Murrieta; Boyd et al. (1995) surveyed the rest of the area shown in Fig. 2B; and there have been numerous surveys for residential development.
Thus, in Figure 2B, it is likely that there are no other populations of B. santarosae other than those shown, except for more populations in the basalt areas themselves. In particular, Redonda Mesa and Mesa de la Punta may contain additional populations. In addition, there are a few locations that have small remnants of the Santa Rosa Basalt, not shown on the map, such as the Murrieta Hogbacks and Oak Mountain. These areas should be searched for B. santarosae. It is also possible that small populations of B. santarosae might occur in drainages from basalt areas, carried there by runoff.
It is possible that B. santarosae might also be found in basalt soil derived from other geologic formations. Two different types of basalt rocks, in unknown proportions, form the basalt soil in the Clay Hill area. The ground surface contains basalt rocks from the ~145 million year old Santiago Peak Volcanics. The weathered Santiago Peak Volcanic surface of Clay Hill was covered by the Santa Rosa Basalt ~10 million years ago until erosion removed it in very recent geologic times.
If the soil at Clay Hill derives primarily from the Santiago Peak Volcanics, B. santarosae may be found on exposures of the Santiago Peak Volcanics elsewhere. There are extensive exposures of the Santiago Peak Volcanics to the west of the Santa Rosa Plateau that should be searched for B. santarosae. In addition, as at Clay Hill, the Santiago Peak Volcanics are often interbedded with the Bedford Canyon Formation, providing additional possible locations.
There are two locations that dramatically illustrate the confinement of B. santarosae to the basalt: the population near Clay Hill just west of the Mesa de Burro, and the population at Avenaloca Mesa.
Clay Hill. We found three populations near Clay Hill in 2006. At this time, we didn't even suspect that this species was confined to basalt since we did not think basalt occurred at Clay Hill. The first population was from a transect near the top of Clay Hill; two other populations were found from a survey along nearby roads. Only later did we discover these locations were in basalt areas.
In November 2007 we did a survey to delineate the basalt areas near Clay Hill. At that time, no trace of B. santarosae was visible. To our amazement, the only places where the basalt patches crossed the roads were the exact two locations of the populations of B. santarosae from the 2006 survey!
The following map shows the mapped basalt areas outlined in green, with the B. santarosae locations indicated:
Basalt Endemism
There is nothing different about the basalt areas other than the soil; the habitat is almost identical throughout the area of the map above. (See pictures of this area.)
See Ten Million Year Old Drainage At The Santa Rosa Plateau Reconstructed By Analysis for more information about the geologic discovery obtained from B. santarosae's remarkable ability to detect areas of basalt.
Avenaloca Mesa is a second dramatic place to witness the confinement of B. santarosae to the basalt, in two respects.
First, the following picture shows nearly the entire north-south distribution of B. santarosae:
Wayne Armstrong is standing on Avenaloca Mesa; Elsinore Peak is the highest peak in the distance, 7.5 miles north The entire area shown in the photograph was once covered with basalt. As can be seen in the photograph, the basalt has been completely eroded except for the foreground patch, where only boulders of basalt are left, and the small patch on top of Elsinore Peak. Those basalt areas are also the only occurrences of B. santarosae in the areas shown in the photograph.
The black line in the photograph marks the locations of the remaining basalt boulders in the photograph.
Second, B. santarosae at Avenaloca Mesa follows closely the areas that still contain basalt, and avoids areas where the basalt has eroded away. At the location of the picture above, the remaining basalt is primarily in three linear features marked by exposed basalt boulders.
Zach Principe mapped the locations of B. santarosae back when it was confused with B. filifolia or a hybrid of B. filifolia with B. orcuttii, and nothing was known or suspected about its confinement to the basalt. A portion of his map for the location seen in the previous picture is shown below:
The mapped locations precisely follow the chains of basalt boulders at this site. In the above photograph, the black line notes where the basalt boulders turn from a northwest/southeast orientation to the west-east orientation, exactly as shown in Zach's mapped locations of B. santarosae.
There is a third outcrop of boulders along the ridgeline to Wayne's right, which again corresponds exactly to locations of B. santarosae in Zach's map.
This coincidence is not due to any habitat preference of B. santarosae for boulders. In all other parts of its range surveyed where there is still contiguous basalt coverage, it is perfectly happy to live in areas without boulders.
Note added after the Madroño publication: Fred Roberts has kindly informed us that he knows of an additional about a dozen locations in Oceanside, Carlsbad, and Vista, all in northwest San Diego County, for B. filifolia. In Fig. 2A, this would form a chain along coastal San Diego County from the Orange County border to San Marcos. We will add those locations to the plot when vouchers are available for our examination.