Rarest Species in the Borrego Desert Flora
Table of Contents
Introduction
Rarest Species Seen In Our Surveys
Rarest Species From Vouchers without Taxonomic difficulties
Rarest Species From Vouchers with Taxonomic difficulties
Species Classified as Rare-In-California that are Not Rare In This List
Introduction The lists on this page were made for the first time on 9 February 2014, and it will take some time, and lots of review, before they can be considered mature, especially since it is very difficult to compile a list of the rarest species anyplace. If you know of other species that should be added to the lists, or know that we have missed other populations are of any of the rare species, or have any other comments on this page, please let me know. For now, I'm concentrating on trying to make the lists complete; I'll work on the abundance numbers, and species comments, later.
I consider only plants native to San Diego County in all of the following lists. This list concentrates on species of the Borrego Desert, defined roughly as the northern half of Anza-Borrego Desert State Park. It includes a few species we have seen in surveys in the southern part of the Park, or know about from other means.
For this page, I consider a species to be rare if it is rare in the Borrego Desert area, and is found in a small number of locations in all of San Diego County, typically six or seven, with fewer than 1,000 plants known from all the populations combined. This is very different from the criteria used by CNPS and others to define rare species in California as a whole. Many times, species rare overall in California can be very abundant in the few places where they live. Such rare-in-California species would not make this rare list, simply because there are many other species that are rarer when one considers just the Borrego Desert.
To be clear, none of the lists contain species that are rare in the Borrego Desert, but are common elsewhere in San Diego County. There are many species that are rare in the Borrego Desert simply because they are coastal / montane species that barely make it into the Borrego Desert, and it would be silly to include them in a list of truly-rare plants in the Borrego Desert.
The list of rare species on this page includes plants that may be common outside of San Diego County. Thus these species listed here are by no means all rare species statewide, and should not be confused with those species.
Compiling a list of rare plant species in a given area is a very difficult task for a number of reasons:
- Many species may appear to be rare only because no one has surveyed the areas which have the greatest numbers of plants of a given species. As James Dillane is fond of saying, there is a difference between species that are truly rare, and species that are rarely seen. In particular, species that live in very inaccessible areas, such as on crumbly vertical slopes, are extremely difficult to get good abundance information for.
Since we have no information on plants not seen, all we can do is give abundances from actual sightings.
- Some species appear only in certain years, and if no one surveys the right areas in the right years, they will appear to be rare. This is especially true for monsoonal species.
- Species with taxonomic difficulties are prone to being present on the list, from misdeterminations of a small number of vouchers of a widespread species.
- The rarest plants, ones not yet seen, are missing from the list.
Since I and colleagues have done ~400 quantitative surveys of many different areas in the Borrego Desert, we have an excellent knowledge of the abundance of species we have seen, and have pretty good evidence that the species we have not seen are uncommon. In fact, five of the species on this rare plant list were ones discovered in the course of our surveys. Our surveys also allow us to ascertain the actual abundance here of species that are classified as rare or uncommon overall in the state, which often results in them not being present in the list of rare species given here.
Vouchers take advantage of over one hundred years of many people scouring the Borrego Desert for plants, and thus give names of species we haven't seen. But locations of older vouchers are often vague, and sometimes wrong, and determinations of vouchers are sometimes suspect.
As a result, I present the rarest species in four different tables. Table 1 gives species that are rare in our surveys, defined as occurring in roughly six or fewer locations, with fewer than 1,000 plants found in our surveys and from our estimate of the number of plants from additional vouchered locations. We have excellent abundance information for those species from the areas we have surveyed.
Table 2 gives vouchered species that are probably correctly determined, with good locations, that we haven't seen, and which appear to be rare in San Diego County as a whole. Rare here is defined as having fewer than eight locations, and which are likely to have fewer than 1000 plants (that is very much a judgment call, since vouchers rarely give abundances). We will target these species in future surveys to see if we can verify the determinations and locations, and get good estimates of their abundances. Not all the species in Table 2 have been studied in detail, so some of them might be removed to Table 3 if it becomes apparent that they suffer from taxonomic difficulties.
Table 3 gives vouchered species that are rare, but which have taxonomic problems that raise a question about the correct determination of the vouchers. We will look at these vouchers to try to verify the determinations, as well as target their locations for field study. Often the determination of a single voucher can be problematic, but when looks at the population in the field the determination becomes clear. I.e., the species in Table 3 might not be present at all in the Borrego Desert.
Table 4 gives species that are classified as rare or uncommon statewide, but which are not rare in the Borrego Desert using the criteria given here.
Rarest Species Seen In Our Surveys The column header #Pls gives the number of plants of each species we have seen in our surveys, augmented by my estimate from voucher locations. Estimates from voucher locations are of course extremely uncertain except in the very few cases where the voucher label reported how many plants were seen.
Since a population of 20 or fewer plants is very likely to be extirpated by normal population fluctuations, it is highly likely that species with fewer numbers in Table 1 are more abundant than has been seen so far in our surveys and in vouchers. Most of those species have never had targeted surveys done for them, which might have discovered areas in which they are more abundant. These are all species that need targeted surveys done for them!
No abundance estimates are given for some species, and for many others the estimates are just wild guesses. I'll attempt to refine the numbers in the future. The first priority was just to get the names on this list. Some of the numbers for observed species were from my memory; I'll look up the actual numbers from our survey data in the future.
The column header #Areas gives the number of areas includes locations seen from our surveys, as well as other locations from vouchers in San Diego County. The number of locations is generally from the clustering algorithm of the Consortium, obtained by scaling the map so that San Diego County fills the screen.
Species discovered for the first time in San Diego County in our surveys are marked with ^ after the species name.
Some species have links to Borrego Desert pages for them.
Table 1. Rare Species Seen In Our Surveys
#Pls #Areas Species(^) 1 1 Phacelia rotundifolia^ 2 1 Horsfordia alata 2 2 Astragalus pachypus var. pachypus 3? 1 Boechera not-xylopoda^ 3? 2 Astragalus pachypus var. jaegeri 8 1 Abutilon abutiloides 10? 2 Stanleya pinnata var. pinnata 10 2 Andropogon glomeratus var. scabriglumis 10? 3 Glycyrrhiza lepidota 10 4 Petalonyx linearis 17 2 Prenanthella exigua 20? 3 Chamaesyce pediculifera 20? 4 Penstemon thurberi 30? 1 Echinocactus polycephalus 30 1 Eriogonum pusillum^ 34 1 Thymophylla pentachaeta var. belenidium 50? 3 Lycium parishii 50? 6 Crossosoma bigelovii 50? 7 Salvia eremostachya 94 3 Nemacladus twisselmannii var. botanywomaniae^ 100 1 Chamaesyce revoluta 100? 3 Cryptantha barbigera var. fergusoniae 100? 1 Atriplex elegans var. fasciculata 100? 5 Quercus palmeri 110 2 Trianthema portulacastrum^ 143 11 Matelea parvifolia 200? 3 Tiquilia canescens var. canescens 200? 4 Chaenactis carphoclinia var. peirsonii 200? 5 Boerhavia coulteri var. palmeri 200? 5 Chamaesyce arizonica 200 5 Lessingia glandulifera var. tomentosa 200? 7 Chamaesyce abramsiana 300 1 Herissantia crispa 300 6 Scutellaria mexicana 800 3 Calliandra eriophylla 1000? 6 Cylindropuntia wolfii 2 Camissonia campestris ssp. campestris 2 Cryptantha holoptera 3 Hoffmannseggia microphylla 3 Plagiobothrys jonesii 4 Astragalus aridus 4 Sphaeralcea angustifolia 5 Allenrolfea occidentalis 5 Boerhavia wrightii 5 Helianthus niveus ssp. canescens 5 Linanthus jonesii 5 Malperia tenuis 5 Monolepis nuttalliana 5 Nemacladus tenuis 5 Opuntia chlorotica 6 Cryptantha ganderi 6 Linanthus bigelovii 6 Nemacladus sigmoideus 6 Stylocline micropoides 7 Cryptantha costata ^ Species discovered for the first time in San Diego County during our surveys.
Rarest Species From Vouchers without Taxonomic difficulties Table 2. Rare Species From Vouchers without Taxonomic difficulties
#Areas Species 1 Astragalus magdalenae var. peirsonii 1 Astragalus sabulonum 1 Eriogonum deserticola 1 Eucnide rupestris 1 Gilia mexicana 1 Gutierrezia microcephala 1 Lepidium flavum var. felipense 1 Opuntia polyacantha var. erinacea 1 Phacelia ivesiana 1 Spermolepis echinata 1 Wislizenia refracta ssp. palmeri 2 Helianthus niveus ssp. tephrodes 2 Nama hispidum var. spathulatum 2 Phacelia neglecta 2 Salvia pachyphylla 2 Spermolepis infernensis 3 Abutilon palmeri 3 Anemone tuberosa 3 Aristida californica 3 Eriophyllum multicaule 3 Leptosiphon floribundus subsp. hallii 3 Mimulus rubellus 4 Astragalus insularis var. harwoodii 4 Astragalus nuttallianus var. imperfectus 4 Cuscuta denticulata 4 Lupinus shockleyi 4 Mentzelia desertorum 5 Stylocline intertexta 6 Eriogonum saxatile 6 Mimulus parishii 7 Gilia scopulorum 7 Poa bigelovii
Rarest Species From Vouchers with Taxonomic difficulties Table 3. Rare Species From Vouchers with Taxonomic difficulties
Species Baileya pleniradiata Boechera pulchra Eriogonum angulosum Camissonia boothii ssp. decorticans Chylismia brevipes Chylismia claviformis ssp. aurantiaca Chylismia claviformis ssp. claviformis Ephedra trifurca Eriogonum ordii Phacelia longipes Phacelia tanacetifolia Phacelia umbrosa Comments on some of these species:
- The only voucher location of Baileya pleniradiata is exactly where there are many vouchers of B. multiradiata. These two species can easily be confused.
- Specimens of our Chylismia claviformis ssp. peirsonii are distinguished by having spreading hairs on the lower stem, and can have either yellow or white flowers. Specimens that have less-obvious spreading hairs on the lower stem are often misdetermined as Chylismia claviformis ssp. aurantiaca and ssp. claviformis, depending on the color of the flowers. We have yet to find a population of Chylismia claviformis in the Borrego Desert in which most individuals do not have spreading hairs.
- Eriogonum angulosum is almost surely not in San Diego County. One voucher of it has three dupes determined as E. maculatum, and the other voucher has another voucher by the same collector with locality near Tehachapi, San Diego County. Tehachapi, of course, is in Kern County, where this species actually lives.
- Phacelia tanacetifolia is almost certainly not present in San Diego County except as an escape from cultivate plants, or a waif. This species only occurs in the wild north of the San Gabriel and San Bernardino Mountains. It is easily confused with P. distans unless you know how to reliably distinguish it. In particular, specimens of P. distans with exserted stamens are often taken to be P. tanacetifolia. See Phacelia distans, common phacelia, and P. tanacetifolia, tansy-leafed phacelia.
- Phacelia umbrosa may not exist as a species separate from P. distans. The description of this species makes it appear that it is just a depauperate or smaller version of P. distans. However, a number of true species also have similar descriptions (of being a smaller version of another species), and they are indeed separate species.
Species Classified as Rare-In-California that are Not Rare In This List My use of rare in this section is in the same sense as given above, which differs from the use of the same word in the Jepson Manual First Edition (JM1) and in the CNPS Lists.
CNPS considers a species to be rare in California, at rank 1 or 2, if there are fewer than 50 populations of it. Rank 4 is for species with 50 to 150 populations. Since there have to be fewer than eight populations to make my list here, many species correctly determined as being rare by CNPS might not make my rare list.
My placement of the species in this section therefore does not necessarily imply that any of these species should be removed from the rare category used by others. In fact, most of the species are on this list simply since there are more than six or seven locations in San Diego County, although some of these are very abundant species (Ferocactus cylindraceus and Selaginella eremophila). I need to collect abundances from our surveys for most of them to see whether some of them should be placed in Table 1 based on the number of plants.
I'll add our observed abundances of these species in the future.
Table 4. Species Classified as Rare-In-California that are Not Rare In This List
Species Rarity Designation in JM1 Acmispon haydonii RARE Astragalus crotalariae UNCOMMON Astragalus lentiginosus var. borreganus UNCOMMON Ayenia compacta RARE in CA Bursera microphylla RARE Carlowrightia arizonica RARE in CA Caulanthus simulans RARE Cylindropuntia fosbergii Uncommon Delphinium parishii ssp. subglobosum UNCOMMON Ferocactus cylindraceus UNCOMMON; threatened by collecting Galium angustifolium ssp. borregoense RARE Lupinus excubitus var. medius RARE Lyrocarpa coulteri UNCOMMON Mirabilis tenuiloba UNCOMMON Penstemon clevelandii var. connatus UNCOMMON Pilostyles thurberi UNCOMMON Selaginella eremophila UNCOMMON Senna covesii RARE in CA Streptanthus campestris RARE Xylorhiza orcuttii RARE
I thank James Dillane for a number of additions to the initial list, and Adrienne Ballwey for noticing that Herissantia crispa was missing from the list.
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Updated 5 June 2020.