Flora of Sweeney Pass and Volcanic Hills Area, Anza-Borrego Desert State Park
Introduction
Botanical Highlights
Procedure For Compiling The Checklist
Floral Regions and Coverage Maps
Species Checklist
Fig. 1. The Sweeney Pass / Volcanic Hills Area. Sweeney Pass itself is inside the red ellipse, where S2 switchbacks up from the Carrizo Creek lowlands to the tableland of the Volcanic Hills Area. The Volcanic Hills are capped by lava flows resulting from the opening of the Salton Basin 15 to 22 million years ago.
Introduction Sweeney Pass and the Volcanic Hills (Fig. 1) are in the southernmost part of Anza-Borrego Desert State Park. Sweeney Pass itself is a sweeping omega-shaped switchback of highway S2 in the area dividing the Jacumba Mountains from the Coyote Mountains, roughly 14 miles north of I-8. Even in dry years, one typically picks up some blooms along the highway from a good stand of Lupinus arizonicus, Arizona lupine, in the pass, along with blooms of one notable plant of Asclepias albicans, white-stemmed milkweed, just before one emerges from the top of the pass. In good years, the bloom is stupendous!
Driving southeast, one switchbacks up from about 700 feet elevation at the Carrizo Creek crossing to 1200 feet elevation in the flattish alluvial plain leading to the Border Patrol checkpoint at the San Diego / Imperial County border, and then the town of Ocotillo (not to be confused with the off-road vehicle area of Ocotillo Wells 28 miles to the north). (See a one minute panoramic video from the top of the Pass.)
300 feet of that relief is seen on the north side of the top of Sweeney Pass at the Carrizo Badlands overview, looking down in Canyon Sin Nombre. Less visible are the ~200 feet deep canyons of Sweeney Creek to the south, which are an impressive place to botanize; see Fig. 2.
Past Jojoba Wash, S2 enters the Volcanic Hills and even passes through a water gap eroded into the lava flow. The Volcanic Hills are capped by lava flows resulting from the opening of the Salton Basin 15 to 22 million years ago. Very little of the lava itself is left except for thin caps on the top of the mesas and scattered remnant rocks below the mesas.
But the influence of the lava is still seen in the flora. Despite the barren-looking landscape in places, the diversity of the flora is high, and this area contains plant species not found elsewhere in ABDSP, such as Linanthus filiformis and Leptosiphon aureus ssp. decorus. On 14 April 2020, Fred Melgert and Carla Hoegen found 135 species in bloom on a 7.5 mile loop in the Volcanic Hills, which is their all-time record for the number of species in bloom on a single hike. They wrote: "No way! We just broke our all time bloom record by 3 plants and in the most unlikely place".
The ~200 foot high east bank of Sweeney Canyon that is just below the top of Sweeney Pass, but is unseen by motorists on S2. A short stretch of the S2 guardrail is labeled at left. One can walk to an unmarked, unsigned overlook area (labeled in the photograph) by parking along the roadside just after the last curve at the top of Sweeney Pass. Photo taken on 16 December 2012. Click on the picture for a larger version.
Botanical Highlights There are a number of botanical highlights here, some of which have been mentioned in the Introduction. One additional major highlight is that Sweeney Canyon has the largest population of Carlowrightia arizonica, Arizona carlowrightia, in California. This species was discovered and vouchered here on 12 February 2011 by Jon P. Rebman, Mike Rood & Samantha Rebman-Rood. This was an amazing find, extending the range of this species in California by 15 miles to the south. They reported that the species was "rare with about 8 plants seen".
Tom Chester and Kate Harper specifically went to their location on 16 December 2012 to see this population, and were amazed to find a large population of at least 50 plants (we counted them!) that were unbrowsed. Nearly every other plant of this species that we've seen from five of the six other locations in California has been severely browsed. Subsequently, further surveys by Walt have found over 99 plants of this species here.
Procedure For Compiling The Checklist The Checklist was compiled from extensive fieldwork by the authors, mostly by the first author Walt Fidler, augmented by voucher records, iNat observations, and Calflora records.
Fieldwork
- 16 December 2012 by Tom Chester and Kate Harper, covering just 0.75 miles of Sweeney Canyon from where it is closest to S2.
- 24 November 2014 by Carl Hoegen and Fred Melgert.
- 4 March 2020 by Walt Fidler.
- 8 March 2020, a short survey of Sweeney Canyon by Tom Chester, Nancy Accola, and Don Rideout.
- 14 March 2020, a survey of Jojoba Wash and Volcanic Hills by Tom Chester, Walt Fidler, Don Rideout, Joe Woods, Steve Rogers, Gaylee Rogers, Jim Roberts, and Abbyann Sisk.
- 26 and 27 March 2020, surveys of the Volcanic Hills by Walt Fidler.
- 28 March 2020, a survey of several areas in this vicinity by Tom Chester.
- 31 March 2020, a survey of the Sweeney Pass area by Walt Fidler.
- 1 April 2020, a survey of the Volcanic Hills by Walt Fidler.
- 11 April 2020, a survey of the Carrizo Creek area near Sweeney Pass by Tom Chester.
- 11, 12, 13, 14, 15, and 19 April 2020, surveys of the Volcanic Hills by Walt Fidler.
- 14 April 2020 by Carl Hoegen and Fred Melgert, their bloom-record-setting trip.
- 20 April 2020, survey of Sweeney Pass area by Walt Fidler.
- 1 and 2 May 2020, surveys of Sweeney Pass area by Walt Fidler.
- 13, 14 and 15 January 2021, Cylindropuntia ramosissima and other surveys by Walt Fidler.
- 21 and 22 January 2021, Cylindropuntia ramosissima and other surveys by Walt Fidler.
- 27 and 28 January 2021, Cylindropuntia ramosissima and other surveys by Walt Fidler.
Vouchers
Voucher records were obtained from three separate searches of the Consortium of California Herbaria on 14 March 2020 by Tom Chester:
- a search using coordinates for the Sweeney Pass Flora area, for the rectangular box from 32.80 to 32.848° north latitude and -116.20 to -116.16° east longitude;
- a search using coordinates for the Volcanic Hills area, for the rectangular box from 32.77 to 32.83° north latitude and -116.16 to -116.10° east longitude; and
- vouchers with Sweeney in their localities.
Duplicate vouchers were removed, and the localities were reviewed manually to remove ones not in the target area. That left 235 vouchers for the Sweeney Pass Floral Area and 316 vouchers for the Volcanic Hills Floral Area.
The top collector in this area by far was Joe Barth, with 190 vouchers. Larry Hendrickson was next, with 85 vouchers, followed by Jon Rebman with 60 vouchers. No other collector had more than 30 vouchers.
iNat observations
There were a whopping 2,425 iNat observations from this area. The top two observers are authors of this flora: the team of Fred Melgert and Carla Hoegen contributed 1,292 observations, and Don Rideout contributed 304 observations. Birgit Knorr contributed 154 observations, and "ebergman" contributed 143 observations. No other observer contributed more than 55 observations.
Calflora records
Calflora records contributed no additional species.
Floral Regions and Coverage Maps The checklist contains a total of 245 taxa, with separate columns for the Sweeney Pass area flora and for the Volcanic Hills Flora. The floral boundaries are given in Fig. 3, which also shows all GPS points for species in this area.
Fig. 4 shows separately the GPS locations where species were recorded by Walt Fidler (6,332 species GPS points) and/or Tom Chester (857 species GPS points), and the 2,425 species GPS points from iNat.
An additional 1,721 GPS points were taken by Walt Fidler to map the Cylindropuntia ramosissima locations in this area. See Figs. 7 and 8 in Cylindropuntia ramosissima Geographic and Elevational Distribution, and Habitat for a discussion and maps of those points.
Fig. 3. The Floral Boundaries are shown for the two sections of this flora, along with all species GPS points recorded in this area by the authors and by iNat observers (map not yet updated with the iNat points).
Fig. 4. Left: GPS points recorded by Walt Fidler and/or Tom Chester. Right: GPS points recorded by iNat observers, mostly by Fred Melgert and Carla Hoegen. Click on the maps for larger versions.
Checklist for Sweeney Pass and Volcanic Hills Area For more information on this checklist, see:
- Notes on the Scientific Names Used At This Site and
- Information about the order in which the species are presented, and the links from the Scientific Name and Common Name.
The family name is abbreviated to the first six characters in order to save space in the table rows.
An asterisk before the Common Name indicates a non-native taxon.
The column with label BW links to the photo gallery page for each species at Fred Melgert and Carla Hoegen's BorregoWildflowers site.
The last two columns give a minimum estimate of the number of plants from the field surveys, up to a maximum of 99 plants, for the Volcanic Hills area (column label VH) and the Sweeney Pass area (column label SP). The main intent of these estimates is to indicate the species for which we found very few plants. If the columns contain V, it means we have not seen the species but it is vouchered from that area. If the columns contain iN, it means we haven't seen the species; the species hasn't been vouchered; but there is an iNaturalist observation. If the columns contain FC, it is found only in the Fred Melgert / Carla Hoegen plant list for this area.
Version for printing, without other text on this page: html (7 pages) or pdf Clickbook booklet (2 double-sided pages). (See printing instructions for an explanation of these options)
Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/)
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Copyright © 2012-2021 by Walt Fidler, Tom Chester, Carla Hoegen, Fred Melgert, Don Rideout, Nancy Accola, Gaylee Rogers, Steve Rogers, Joe Woods, Jim Roberts, Abbyann Sisk, and Kate Harper
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
http://tchester.org/sd/plants/floras/sweeney_pass_area.html
Comments and feedback: Tom Chester
Last update: 30 January 2021