Plants of Southern California:
Arctostaphylos parryana ssp. desertica, and comparison to A. glandulosa ssp. adamsii

Table of Contents

How To Recognize Arctostaphylos parryana ssp. desertica
Geographic Distribution


This page was just a shell to hold the maps of the geographic distribution of Arctostaphylos parryana ssp. desertica and A. glandulosa, with the rest of the page incomplete. I began adding some comments on how to recognize this species on 4 November 2023.

This page is primarily on Arctostaphylos parryana ssp. desertica. However, it is so similar to what I call A. glandulosa ssp. adamsii that they need to be discussed together. In fact, prior to the 1997 recognition of A. parryana ssp. desertica, plants of that species were called A. glandulosa ssp. adamsii. Munz, in his paper describing A.g. adamsii, cites a specimen of what is now A.p. desertica, from the Santa Rosa Mountain Road, as "another collection" of A.g. adamsii. One dupe of this collection still has the original determination of A.g. adamsii, whereas another dupe of this collection is determined as A.g. ssp. glandulosa!.

How To Recognize Arctostaphylos parryana ssp. desertica

The main difficulty in recognizing A. parryana ssp. desertica is distinguishing it from A. glandulosa ssp. adamsii.

Both of those two taxa live mostly, and perhaps entirely, in the eastern Peninsular Range (PR). In that area, they are the only manzanita species with a burl. (A. rainbowensis lives only in the western Peninsular Range, in the Fallbrook / Rainbow area.) So if you can establish that the plant you are studying in the eastern PR has a burl, it has to be A. glandulosa or A. parryana desertica.

A. parryana desertica lives only on the desert edge of the PR, so if your plant is well away from the desert edge, it has to be A. glandulosa. See the geographic plots below.

A. glandulosa has a number of defined subspecies. The only plants of A. glandulosa that can be confused with A.p. desertica are plants that only have short-white hairs on their twigs and nascent inflorescence, and no glandular hairs on either. I will call those plants A.g. adamsii in this page, whether or not the leaves are "strong white-glaucous" or not.

A.g. adamsii has had several quite different circumscriptions in the last few decades. Wells included plants with glandular hairs in that taxon. Keeley included only plants with non-glandular hairs in his circumscription, but retained this taxon as having "strongly white-glaucous" leaves.

I have extensively studied plants of A. glandulosa in the eastern PR that have only non-glandular hairs, and their leaves range from white-glaucous to green. Very few plants are "intensely white-glaucous", including plants at the type locality of A.g. adamsii. In fact, in the original description of A.g. adamsii by Munz, he says it has "light or yellow-green leaves".

The distinguishing characteristics of these two taxa that I know about are given in Table 1, with some shown in Fig. 1.

Table 1. Characteristics that distinguish A. glandulosa ssp. adamsii and A. parryana ssp. desertica

CharacteristicA.g. adamsiiA.p. desertica
BurlHemispheric (flat-topped)~Spheric (not as flat-topped)
OvaryDensely white-hairyGlabrous to sparsely white-hairy
Fruit hairssparsely white-hairyGlabrous to sparsely white-hairy
Fruit Width / HeightGen ~1.3, but can be oblong~1.1
Fruit Weight251 mg^329 ± 17 g
Stone mean # segments3.2 (Individual plants range from 1.7 to 4.3)#1.3 ± 0.1

^Value is for the entire A. glandulosa species, and might be different for A.g. adamsii.

# See Arctostaphylos glandulosa Subspecies Number of Stone Segments

The nascent inflorescence does not appear to distinguish these two taxa, which is very surprising. See pix of the nascent inflorescences for A.p. desertica, and compare them to the ones in these iNat obs of A.g. adamsii.

Ovary hairs. Fig. 1 shows the difference in the ovary / young fruit hairs.

A.g. adamsii

A.p. desertica

Corolla manually removed
SnJt Cedar Spring Trail

Corolla manually removed
SnJt PCT north of Cedar Spring Trail

Developing very young fruit
Corolla naturally has fallen off
SnJt east of Goff Flat

Developing very young fruit
Corolla naturally has fallen off
SnJt east of Goff Flat
Fig. 1. Photographs of ovaries / very young fruit. Click on the pictures for larger versions.

Fruit width / height. Young fruit of both species start out essentially spherical, and the fruit changes shape as it develops. Although the mature fruit of A.g. adamsii is supposed to be a bit more flattened than that of A.p. desertica, immature fruit can be more flattened in A.p. desertica; see Fig. 2.

A.g. adamsii

A.p. desertica

These young fruit of A.g. adamsii have the opposite shape of what A. glandulosa is supposed to have!
SnJt Cedar Spring Trail

These young fruit are somewhat depressed spheric, consistent with the ratio of ~1.1 given by Keeley et al.
SnJt PCT north of Cedar Spring Trail

iNat obs by Derek Stakelum
Cuyamaca Ranch State Park

SnJt PCT North of SR74

Cherry Canyon above Ranchita, San Diego County

SnJt Andreas Saddle North
Fig. 2. Photographs of developing fruit and mature fruit, with the width to length (= width to height) ratio shown for some fruit that were oriented to display the full length. A. glandulosa "generally" has a ratio of ~1.3 for mature fruit, and A.p. desertica has a ratio of ~1.1. It is clear that there is quite a wide variation in the ratio.

There is no significant difference between these two taxa in the hairs on developing and mature fruit.

Click on the pictures for larger versions which show more fruit.

Stone number of segments. The name manzanita means "little apple" in Spanish. When ripe, the fruit is red, and does look somewhat like a little apple; see the bottom of Fig. 2 above.

The fruit of manzanitas is called a drupe by botanists, which is a fleshy fruit with thin skin and a central stone containing the seed or seeds. See a photograph of a half-opened A.p. desertica fruit. The pulp surrounding the stone is ± mealy, and can be solid, as in the previously-linked photo, or can turn almost to dust.

The stones contain roughly six to seven individual seeds, that can each grow into a genetically distinct plant. Some stones can separate into as many as six segments, whereas others remain intact.

Whether a stone remains intact, or separates into a number of different segments, can sometimes distinguish manzanita species, and is important for distinguishing A.g. adamsii, with "generally separable" stones, and A.p. desertica, with mostly intact, or fused, stones.

I was confused for years about exactly how one determined the number of segments that a stone split into. For example, I didn't know if one used a hammer to split the seeds, or not. I finally learned from Tom Parker that just opening the fruit with one's fingernails was the proper procedure, and that separable stones would fall into segments in the process of opening the fruit.

It is a bit misleading that the floras say that the stones of A.p. desertica are "fused". The mean number of stone segments for A.p. desertica reported by Keeley et al is 1.3, not 1.0.

The floras say the stones of A. glandulosa are "generally free", and that seems to be accurate. That does not mean that it has no fused stones! The data collected by Chester et al for A. glandulosa show an enormous variation in the mean number of stone segments ranging from 1.7 to 4.3, depending on the individual plant. The plant with a ratio of 1.7 had 5 of its 10 stones fused.

Sampling a single fruit and finding the stone fused is thus not a guarantee that the plant is A.p. desertica. Eight out of ten A. glandulosa plants had at least one fruit, out of ten, that remained solid.

Similarly, sampling a single fruit and finding it splits into two segments does not guarantee the plant is A.g. adamsii. Eight out of 23 fruit samples for A.p. desertica split into two segments.

However, sampling a single fruit, and finding it splits into three or more segments is a good indication that plant is A. glandulosa.

Sampling ten fruit per plant usually gives a pretty solid discriminant between these two taxa.

Photographs of the stones of both species are given in Fig. 3.

A.g. adamsii

A.p. desertica

PCT east of Fages Monument, San Diego County
Photo by Don Rideout

SnJt Andreas Canyon Saddle
(Need to find a pix of two segments for A.g. adamsii)

PCT east of Fages Monument, San Diego County
Photo by Don Rideout
(Intentionally blank, since A.p. desertica doesn't split into five segments)
Fig. 3. Photographs of the stones for each species, including examples of stones that fell into different numbers of segments when the fruit was split open. Click on the pictures for larger versions.

Geographic Distribution

The geographic distribution of these two species, from my GPS points, cleaned-up vouchers and verified iNat observations, is shown in Fig. 4.

Fig. 4. Geographic distribution of A. glandulosa adamsii (left) and A. parryana ssp. desertica (right), from my GPS points, cleaned-up vouchers, and verified iNat observations. See also expanded version of these plots for the Garner Valley area for A. glandulosa and A. parryana desertica. The blue horizontal line separates Riverside County on the north from San Diego County on the south.

Keeley, Boykin, and Massihi chose the subspecies name of desertica to recognize "the consistent tendency of this taxon to be restricted to the desert edge of chaparral" (Keeley, Boykin, and Massihi 1997).

Fig. 5 shows that A.p. desertica in the San Jacinto and Santa Rosa Mountains is found only on the desert side of those mountains, up to and including the Desert Divide.

Fig. 5. Google Earth view looking south/southeast from an elevation of 27,000 feet showing known locations of A. parryana desertica (red dots) and the Desert Divide, the mountain crest separating the desert side of San Jacinto Mountain from the coast side. Click on the pix for a view of a slightly-larger area.

Most of the desert side of San Jacinto Mountain is unexplored, due to very steep treacherous slopes, so there may be many more locations of A.p. desertica not shown in this map. On the other hand, A.p. desertica may prefer not to live on steep slopes. Fig. 6 shows where Mark Reese and I surveyed for A.p. desertica on 18 May 2022, and found it only on ridgetops and flattish areas.

Fig. 6. Google Earth view looking west showing A.p. desertica locations (red dots) on the PCT and the access road to the PCT from Goff Flat. The blue line shows the survey track. Note that plants of A.p. desertica are preferentially found on flattish ridge tops (top left) and flattish ridges on the desert side of the Desert Divide. Click on the picture for a slightly-larger version.

Even on the Desert Divide ridge itself, there are significant stretches where there are no or few plants of A.p. desertica. Fig. 7 shows one such stretch.

Fig. 7. Google Earth view looking southwest showing A.p. desertica locations (red dots) on the PCT in an area that has been surveyed for it. Note the large gap with no A.p. desertica locations at all. There are only small numbers of plants of that species at the locations of the three red dots on the right of this picture. Click on the picture for a version showing a larger stretch of the Desert Divide.

The plants in San Diego County inhabit a somewhat-similar desert side / mountain crest location, although some of the plants are found in a more coastal side location, such as the ones below Eagles Nest and in the Ranchita area, and they do not prefer ridgetop locations in general; see Fig. 8.

Fig. 8. Google Earth view looking almost due north from an elevation of 27,000 feet showing known locations of A. parryana desertica (red dots) in San Diego area. Click on the pix for a view of a slightly-larger area that shows part of the Santa Rosa Mountain population as well.

As shown in Fig. 4, there is a considerable distance gap between the population in the San Jacinto / Santa Rosa Mountains, and the population in San Diego County. Fig. 9 shows that plants in those two areas largely have significantly-different elevations, although many of the San Diego County plants have the same elevations as the Santa Rosa Mountain plants.

Fig. 9. Latitude vs. Elevation for specimens of A.p. desertica.

Fig. 10 gives the latitude vs. elevation plot for both species.

Fig. 10. Latitude vs. Elevation for specimens of A. glandulosa and A.p. desertica. The symbols for A.p. desertica were made slightly larger so they could be seen amidst some of the A. glandulosa symbols.

Page not revised past this point.

Considerable clean-up of the vouchers was needed to produce the maps shown in Fig. 2:

I thank Don Rideout for considerable assistance with field surveys for these two species; Mark Reese for assistance with the field survey on 18 May 2022; and Fred Melgert and Carla Hoegen for their survey work in the Ranchita Area and their iNat posts of those plants.

Voucher data provided by the participants of the Consortium of California Herbaria.

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Copyright © 2021-2023 by Tom Chester
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Last update: 18 November 2023