Plant Species of the San Jacinto Mountains
Arctostaphylos glauca, Bigberry Manzanita
Fig. 1. An arborescent specimen of Arctostaphylos glauca on the Old Control Road 0.3 miles above Fisherman Point at an elevation of 4130 feet. Left: View of most of the entire plant. Top Right: The nascent inflorescence, twigs, and leaves, all glabrous except for the pedicels. Bottom Right: The "bigberry" fruit, about 15 mm in width. Photographs by Don Rideout. Click on the pictures to go to Don's iNaturalist post, where you can click on the photos twice to successively enlarge them. Fig. 2. One of the few photographs of A. glauca in bloom from the San Jacinto Mountains, by Bruce Watts in the Kenworthy area of Garner Valley on 16 February 2018. Click on the photograph for a version showing more of the inflorescence.
Out of 109 iNaturalist observations of A. glauca from the San Jacinto Mountains, only two show flowers! They are by Neil Frakes, 16 January 2017 near the South Fork Trail; and Morgan Stickrod, 29 January 2020 on SR74 below Pinyon Pines.
Apparently A. glauca usually blooms in January and/or February here, when most iNaturalist observers are taking observations in the desert.
Arctostaphylos glauca is one of our easiest manzanita species to recognize. Wells 2000 wrote that this species is "a very strikingly distinct species that is remarkably uniform over its extensive range" in the coastal ranges from San Francisco to northwestern Baja California.
The best and easiest distinction in our area for this species is that it is the only manzanita species with glabrous (smooth; no hairs or glands) twigs. That's all you need to conclusively determine a San Jacinto Mountains manzanita plant as being this species.
There are two other characteristics of note. The first is berry size. True to its common name, this has berries that can be larger than those of our other manzanitas, 10 to 15 mm wide. However, there is some overlap in berry size with the 6 to 12 mm wide berries of A. pringlei. Our other species have a maximum berry width of 10 mm. In addition, the berries of A. glauca are sparsely glandular-sticky; only A. pringlei and some A. glandulosa have similarly glandular-sticky berries.
The second is the color of the leaves. True to its scientific name, the leaves in age are white-glaucous (more accurately, green tinged with a whitish tint). Interestingly, young leaves are straight-up green, and only become glaucous in age. As far as we know, none of our other manzanitas have leaves that change color with age like this. A. glandulosa can also have white-glaucous leaves, and A. pringlei has gray-glaucous leaves. Our other manzanita species have green leaves.
A. glauca is found in chaparral, both on the coastal side of the San Jacinto Mountains at 1800 to 5000 feet elevation, and on the desert side at 2900 to 5000 feet elevation, with some specimens on the Skyline Trail apparently up to 5800 feet. A. glauca does not grow in the pine belt proper here.
On the coastal side, specimens are generally erect, often with a single stem, as the plants try to grow above the surrounding chaparral; see Fig. 1. In areas that haven't burned for a century or more, it can grow into a 20 foot tall tree-like plant.
A. glauca is an obligate postfire seeder. It is killed by most fires, since it doesn't have a burl, but fire is needed to break the dormancy of its seed. It germinates abundantly after a fire, and if they get enough rain, plants can reach three to six feet tall in just three years.
One serious danger to this species on the coastal side is the increased fire frequency caused by humans. Repeated short-interval fires eliminate it entirely from such areas, such as in some areas of the San Dimas Experimental Forest that burned at 15 year intervals (Riggan et al, quoted in Howard 1993). We may already have lost some populations in the San Jacinto Mountains in the last few decades of repeated fires in many places.
On the desert side, specimens are generally shrubby, staying low to the ground. Like most manzanitas, stems that rest on the ground surface can root and eventually become a separate plant. Since fire has historically been infrequent to non-existent on the desert side, plants can sometimes form an expanding circle of clones from stem-rooting. Jepson named these plants as A. glauca var. eremicola in 1922, but Wells reduced it to A. glauca forma eremicola noting that this plant habit is only a tendency.
For more information on this species, see Howard 1993: Arctostaphylos glauca In Fire Effects Information System: .
Distribution in the San Jacinto Mountains
The following plots show the distribution of A. glauca in the San Jacinto Mountains and nearby areas. The locations derive from:
- our observations, 50 GPS points, of which 43 come from a detailed survey on the PCT south of SR74 on 20 November 20. We have very few other locations since our surveys have mostly been at higher elevation where A. glauca is not found.
- 107 vouchers with coordinates. Two additional vouchers with coordinates were tossed, since one had just a vague location of "San Jacinto Mtns." and the other had an erroneous locality in conflict with the reported elevation.
- 109 iNaturalist observations with accurate locations; we tossed observations with obscured coordinates, and with position uncertainties larger than 100 m. An additional 12 iNat observations turned out to be misdetermined A. pringlei; another one was a misdetermined A. glandulosa; and another one was a misdetermined A. pungens.
We only reviewed the determination of iNat observations, and the voucher localities, from areas where we thought the presence of A. glauca was unlikely, which were mostly ones above an elevation of 5000 feet. It turned out that all of those observations above about 5000 feet were indeed misdetermined (iNat) or had inaccurate locations (vouchers), except for some iNat observations of plants on the Skyline Trail at up to 5800 feet.
It is possible there are other misdetermined iNat observations, or vouchers with inaccurate locations, in the plots below, but if so, removing them is unlikely to change the plots, since there is general agreement between vouchers and iNat observations in those areas.
The first three plots below show the geographic distribution from the above sources. The first plot shows all the locations in the San Jacinto Mountains. The next two plots zoom in to show the higher elevation area of the San Jacinto Mountains.
The following graph plots the elevation vs. longitude for all the accepted locations. This plot identifies the source for each location. An annotated version of that graph is also shown.
It is striking that there appears to be a fairly strict upper limit of ~5000 feet elevation for A. glauca on the west and south side of the San Jacinto Mountains, with only a few locations extending up to 5800 feet elevation on the very steep east side, which is a very different environment.
It is also striking that there are vouchers, but no iNat observations, of A. glauca of the westernmost part of the San Jacinto Mountains where the elevation is below 4000 feet, including along the lower part of SR243 and SR74. Many of those vouchers are from the Red Mountain Road, with some from the Soboba Indian Reservation, places where there are likely to be few iNaturalist observers. Some vouchers are along SR243 and SR74. It is possible that A. glauca has been extirpated in some of those areas by too-frequent fires. It would be interesting to resurvey voucher locations in some of those areas to find out.
Distribution in Southern California
Copyright © 2020 by Tom Chester and Dave Stith.
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Updated 24 November 2020.