Plants of Southern California:
The Malacothamnus Species of San Diego County
The Malacothamnus Species of San Diego County
Keir Morse and Tom Chester
| M. fasciculatus | M. densiflorus | M. enigmaticus | |||
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Whitish calyx; short calyx bracts | 
Greenish calyx; long calyx bracts | 
Whitish calyx; long calyx bracts Fig. 1. Left column: Malacothamnus fasciculatus. Middle column: M. densiflorus. Right column: M. enigmaticus. For common names, see below.
| Top Row: The corollas (petals) do not distinguish any of our three Malacothamnus species. The differences in the centers of the flowers are due to the stage of maturity of the stamens. Bottom Row: In contrast, the calyx, and calyx bracts, readily distinguish our three species. Some of the three calyx bracts are marked by the white arrows in each photo.
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Table of Contents
Introduction
How To Distinguish the Three Malacothamnus Species in San Diego County
Geographic Distribution Map
Photographs and Locations of Our Three Species
Appendix 1: Variation in Calyx Appearance
Appendix 2: Variation in Stem Appearance
Appendix 3: Distinguishing the Three Malacothamnus Species Using Only Morphologic Measurements
Introduction
We now solidly have three Malacothamnus species in San Diego County. We have always had M. fasciculatus and M. densiflorus in the county, but whether a third species exists here has been up in the air since 1963 until now.
Malacothamnus enigmaticus is a new species recognized in 2019 (Morse and Chester 2019; see also free pdf posted in February 2021). It is endemic to the desert edge of the Peninsular Range in San Diego County, from the San Ysidro Mountains in the north to the Laguna Mountains in the south. It is the only Malacothamnus species in its geographic range, which contains a unique habitat in San Diego County where the eastern edge of montane elevations transition rapidly to the low desert. To the south of its range, montane elevations disappear. To the north of its range, the montane regions are not in as close proximity to low desert elevations and are oriented differently with respect to the low desert.
Plants of M. enigmaticus were incorrectly called "M. aboriginum" in the 1993 First Edition Jepson Manual and the 2015 Flora of North America treatments by Bates. But any reference to that name for the San Diego County plants disappeared in the 2012 Second Edition Jepson Manual! That disappearance was one of the motivations for us to work on the Malacothamnus species of San Diego County.
Our analysis work clearly showed that the San Diego County plants were distinct in a large number of ways from the true M. aboriginum in northern California, and also distinct from the other two species in San Diego County, M. fasciculatus and M. densiflorus.
The purpose of this page is to show the reader how to distinguish our three Malacothamnus species in San Diego County, and provide a detailed geographic distribution map for the three species.
The common names for two of our three species are unfortunately potentially confusing and misleading, with one common name even being applied to both species! Only the common name for M. enigmaticus is unique, since we suggested its common name, "enigmatic bushmallow", in our paper.
The common names in use for M. fasciculatus are:
- "chaparral mallow", contained in 25% of the webpages that contain "Malacothamnus fasciculatus", and used by the San Diego County Checklist, the Jepson eflora, and Calflora;
- "chaparral bushmallow", contained in 8% of such webpages;
- "coastal bushmallow", used by just 3 webpages, and Lightner in his San Diego County Native Plants Third Edition.
The common names in use for M. densiflorus are:
- "yellow stem bush mallow", contained in 24% of the webpages that contain "Malacothamnus densiflorus";
- "many flower mallow" or "many flower bushmallow", contained in just 6 webpages, and used by the San Diego County Checklist and Calflora; and
- "chaparral bushmallow" used by Lightner in his San Diego County Native Plants Third Edition.
The name "many flower" is misleading since all three of our Malacothamnus species have many flowers per node.
For those curious about how we got involved in this project, and how much effort it required, see Tom's Malacothamnus enigmaticus Project Timeline.
How To Distinguish the Three Malacothamnus Species in San Diego County
The corolla (petals) of nearly all Malacothamnus species are indistinguishable; see the top row in Fig. 1. However, many species can be readily identified simply by looking at the calyx and its three calyx bracts; see the bottom row in Fig. 1.
The calyx is the part of the flower just below the petals The calyx serves to protect the petals before the bud opens up, and may also help to stabilize the petals and form the shape of the flower. The surface of the calyx itself is usually greenish, but may appear whitish due to the hairs covering it.
The calyx bracts are immediately outside the calyx, attached at the base of the calyx. They are a set of three usually-identical narrow structures with coloration similar to the calyx.
In San Diego County, our three species are distinguished as follows:
- M. fasciculatus is usually easily distinguished by its white or yellow stems and shorter calyx bracts.
- M. densiflorus is easily distinguished from the other two species by its green stems and calyces. In the other species, the underlying green color of their stems and calyces are obscured by their denser hairs. M. densiflorus is also distinguished from M. fasciculatus by its longer calyx bracts. It also frequently has longer calyx hairs than the other two species.
- M. enigmaticus is usually easily distinguished by its white stems and longer calyx bracts.
See a gallery showing the calyx variation in Appendix 1.
See the stems of the three species photographed under the same lighting conditions, and a gallery showing the stem color variation in Appendix 2.
There is a slight overlap in the lengths of the calyx bracts for M. fasciculatus and M. enigmaticus, which is why we used the word "usually" above for those two species. If there is any confusion for a specimen between M. fasciculatus and M. enigmaticus, there are three options to pursue:
- examine other calyx bracts on the same plant, and nearby plants, to see if there is a trend one way or other that places the plants in one of these two species; or
- use their strong geographic separation to make the determination; or
- examine the inflorescence more closely and make detailed measurements that will clearly separate the two species; see Appendix 3.
The geographic separation is definitive and very easy to use in nearly all cases, since the two species are separated by more than 25 miles except for one small area. With the exception of that one small area, M. fasciculatus is a coastal area species in San Diego County, found only within 25 miles of the coast, whereas M. enigmaticus is found only farther than 50 miles of the coast. The exception consists of just one or two plants in lowermost Coyote Creek that are washed down from the M. fasciculatus population in the Santa Rosa Mountains of Riverside County. See Fig. 2 for the geographic distribution of the species.
Geographic Distribution Map
Fig. 2 shows the distribution of specimens that we have determined from inspection, as well as the distribution of specimens that we have measured in detail and appear in the plots in our paper.
Fig. 2. The full geographic range of M. enigmaticus (filled squares), along with nearly the full geographic range of M. densiflorus (open circles), and the nearby portion of the range of M. fasciculatus (asterisks). Measured and analyzed specimens are shown with larger symbols. Malacothamnus enigmaticus likely exists at or near the single location marked with a question mark since we have observed specimens washed down from that area. The Malacothamnus specimens in northeast Baja California (open stars) appear to differ from each of these three species, but too few specimens are known to generate reliable conclusions.
Photographs and Locations of Our Three Species
Photographs of M. enigmaticus can be found at Calphotos, 52 pix. Photographs and some locations for M. enigmaticus can be found at Calflora. Photographs and locations for M. enigmaticus can be found at iNaturalist, 64 observations as of 8 December 2019.
Photographs and locations of M. densiflorus are similarly found at Calphotos, Calflora and iNat.
Photographs and locations of M. fasciculatus are at Calphotos, Calflora and iNat.
iNat is probably the best source if you want to find out where it has been observed recently, and whether the plants or in bloom or not, by filtering it by date. This is particularly important for M. densiflorus and M. enigmaticus, which are strong fire-followers and mainly present only in the first few years after a fire. Known locations that haven't burned in the last five years may have no extant plants of these species.
Keir curates the iNaturalist observations for all Malacothamnus species, and has supplied his determination for essentially all iNat observations of that genus. He also has reviewed all the Calphotos pix for the genus.
Appendix 1: Variation in Calyx Appearance
Fig. 3 shows some of the variation in the appearance of the calyx and calyx bracts for the three species.
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| Fig. 3. Variation in calyx and calyx bract appearances for Left: M. fasciculatus; Middle: M. densiflorus; Right: M. enigmaticus.
Click on the photographs for larger versions. | ||
Appendix 2: Variation in Stem Appearance
Fig. 4 shows some of the variation in stem appearance for our three species. The photographs of M. densiflorus are placed in the middle column to show its greener appearance compared to the other two species.
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| M. fasciculatus | M. densiflorus var. viscidus | (intentionally blank) |
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| Fig. 4. Variation in stem appearance for Left: M. fasciculatus; Middle: M. densiflorus; Right: M. enigmaticus. The stems of M. fasciculatus and M. densiflorus are roughly ordered from whitish to yellowish. The last three rows show M. densiflorus var. viscidus. Note that even in the M. densiflorus stem with the highest hair density, in Row 6, you can still see the green stem in-between the hairs, which never happens with the other two species. | ||
Appendix 3: Distinguishing the Three Malacothamnus Species Using Only Morphologic Measurements
Warning! This section is only for hard-core botanists who have a specimen with calyx bracts in the small overlap length range for M. fasciculatus and M. enigmaticus, and who don't want to rely on the much-easier geographic separation to distinguish the two species.
First, you have to learn the terminology of the different parts of the inflorescence glomerule, which is given in detail in our paper. The inflorescence of Malacothamnus is, frankly, a mess of confusing parts until you become quite familiar with the parts. Each glomerule of flowers (the dense collections of flowers with reduced branches at each node in the inflorescence) typically has 5 to 20 flowers in clusters of 1 to 5, with each flower cluster subtended by 1 to 5 stipular bracts, and each flower subtended by 3 calyx bracts (shown in Fig. 3).
Worse, the parts are small, and the glomerule is very hard to dissect in the field.
But if you can dissect the glomerule, reliably identify the parts, and have a ruler handy, all three species in San Diego County, as well as Orange and Riverside counties, separate perfectly using the following key:
1. Stem hairs 26 to 75 in a 2 mm x 2 mm square, green stem generally visible to the naked eye between hairs . . . . . . . . . . . M. densiflorus1'. Stem hairs 164 to 235 in a 2 mm x 2 mm square, stem yellow or white due to densely overlapping hairs, green stem rarely visible distally . . . . . . 2
2. Longest calyx bracts 2.0 to 6.0 (8) mm; widest stipular bracts 0.5 to 2(4) mm wide . M. fasciculatus2' Longest calyx bracts 5.5 to 13 mm; widest stipular bracts 1 to 8 mm wide . . . . M. enigmaticus
Note that if the stipular bracts are lobed, the width is measured at the widest point, which may be the distance between the lobe tips for a lobed bract with widely-spreading lobes.
This key was able to distinguish all measured specimens of the three species; see plot of the longest calyx bracts vs. the stem hair density and plot of the longest calyx bracts vs. the widest stipular bract. In both plots, points marked with additional symbols represent samples from the same population or plant, showing the variation within a species. The key was also verified on a number of additional specimens from all three counties.
Tip 1: The stipular bracts are often much easier to see when the inflorescence is immature and less crowded, especially near the base of the glomerule.
Tip 2: the easiest part to recognize are the three narrow calyx bracts that are directly underneath the calyx, as shown in Fig. 3. Once you can find them, the vast majority of the specimens will separate simply by measuring the length of the longest ones you can find in an inflorescence.
A small minority of specimens will have calyx bracts in the narrow overlap region, but they should separate using the width of the stipular bracts.
Tip 3: Malacothamnus species are wildly variable with some specimens of some species very similar in appearance to some specimens of other species. Much of that variation is intrinsic to each species, but some of it is due to currently-unrecognized taxa within some species. We presented evidence in our paper that some previously named varieties of M. densiflorus and M. fasciculatus that are currently unrecognized possibly should be recognized. Keir is currently working on resolving the taxonomy of the entire genus which will clarify if there should be more taxa recognized in San Diego County as well.
Tip 4: The only fairly-clear hybrids between M. densiflorus and M. enigmaticus that we have come across are specimens in the San Felipe Valley. Those hybrids are only in the narrow zone where those two species are found in close proximity. We have seen possible intermediates between M. densiflorus and M. fasciculatus in the narrow overlap zone between those two species, particularly near Ramona.
If you are interested in seeing how the four Malacothamnus species in Los Angeles, San Bernardino, and Kern counties are different, see the link at the bottom of Keir's Malacothamnus page.
All photographs by Keir Morse, except the one of M. enigmaticus in the top row of Fig. 1 by Fred Melgert.