Plants of Southern California: Opuntia bigelovii and O. fosbergii

These two species are usually easily distinguished in the field at a glance even from a car going 55 mph:

The pictures below were taken in Mason Valley, eastern San Diego County, on 3 and 8 January 2007.

The following pictures shows the clear average difference in height and plant coloration between the two species:

The labeled plants outlined in thin black lines in the above picture are representative of mature plants. Kate Shapiro kindly provided the scale in the above photo; she is 5.4 feet (1.64 m) tall. Both specimens, and Kate, are all at the same distance from the camera. The O. fosbergii is 4.8 feet (1.46 m) tall, one-third taller than the 3.5 foot (1.08 m) tall O. bigelovii, and has noticeably darker pinkish spines.

The following picture shows a field containing both species:

O. bigelovii are the shorter lighter-colored plants in the top half of the picture; some of these plants are surrounded by the smaller circle on the left. O. fosbergii are the taller darker plants in the lower half; some of these plants are surrounded by the larger circle on the right. The spatial separation of the species is typical of areas we have surveyed so far.

Closeups of the species are given below. The left picture was taken in normal reflected light; the right picture shows light shining through the spines:

In the pix on the left, the O. bigelovii is the young plant in the front, and is much shorter than typical plants of that species. James Dillane kindly provided the scale in this picture; he is ~6.0 feet tall. In the pix on the right, the O. bigelovii is the taller plant in the back. This picture shows a not-fully-mature specimen of O. fosbergii.

As seen in the first pix above, mature plants of O. bigelovii are usually significantly smaller than mature plants of O. fosbergii, with the difference apparent at a glance in fields where both are present.

The spines of O. bigelovii are whitish / silverish, giving the upper part of the plant a definite white appearance. In contrast, the spines of O. fosbergii, especially the young ones, are unmistakably pinkish.

Up close, the gradient in color from the young spines at the tip of the joints to the older spines below is quite noticeable for O. fosbergii:

The tubercles are roundish for O. bigelovii (pix on left below), whereas the tubercles are noticeably elongate for O. fosbergii (pix on right below):

One tubercle for each plant is surrounded by a black ellipse that closely follows the tubercle edges. Note again the difference in spine color throughout the plant.

Our field observations on 3 January 2007 found numerous cases where the species were found in distinct bands, appearing to follow different rock types. (We didn't have time to investigate whether the plants actually were following different rock types, but this hypothesis seemed consistent with what we saw in the tilt of the rock bedding in places.)

For example, the first place we examined the plants was on a south-facing hillside at the southeast base of Granite Mountain, where the species were found in separate diagonal bands. O. bigelovii was near the top of the hillside on the right; O. fosbergii appeared in a middle diagonal band; and O. bigelovii reappeared below in a lower diagonal band. This hillside is shown in the following panorama composed from four camera pictures:

Approximate boundaries of the species bands are shown with the black superimposed lines. The lower band of O. bigelovii is not seen very clearly in this low-resolution panorama, but is obvious in the higher-resolution version. The second picture on this page, on the left in the set of two above, was taken where the lower two bands meet at the base of that hillside.

It is not obvious to us how this diagonal banding could have been produced by vegetative propagation.

Tom returned to this spot on 23 January 2007 and examined more of this location from the east side. He found additional bands for a total of at least five bands, and it is probable that there are additional bands beyond the bands he could see.

The following pictures show the bands from first the south side of the hills (the same perspective as in the picture immediately above), and then from the east side, showing the bands in three dimensions:

The bands have arbitrarily been numbered with "1" being the lowest band on the west portion of the south-facing slopes. The pictures were taken from different distances; Kate Shapiro is visible in both pix to provide a scale, along with Wayne Armstrong in the first pix. The picture from the south side was taken from a position slightly to the west of where Kate is standing in the picture from the east side.

The boundary between the bands is approximately a two-dimensional plane, but the angle it makes with the slope depends on the angle of the slope itself, as well as the viewing angle. Hence it appears to have different angles in the two photographs.

More bands were visible by eye to the right of the location shown in the last photograph, but were too distant for the species to show up well in a photograph.

Norrie Robbins, a geologist, accompanied Tom on 23 January 2007 and immediately picked out numerous lineations in the rocks in this outcrop and surrounding ones with the same trend as the bands. The following photographs show such outcrops to the west:

The first pix above gives the larger scale view; the second pix zooms in on the lineations. Black lines have been added just above the most obvious lineations. The lines are not parallel since it is the intersection of the slope with the planes of the lineations, along with the amount of weathering at each spot, that determine the apparent slopes of the lines.

The Santa Ana Map Sheet of the Geologic Map of California shows that the Elsinore Fault defines the southern boundary of Granite Mountain here. These lineations are parallel to the trace of the Fault. The rock here is mapped as pre-Cretaceous metasedimentary rocks, and is given as the Julian Schist in the El Cajon map sheet. The Julian Schist is a sequence of metamorphosed submarine fan deposits and intercalated volcanic rocks. The species are clearly following different metasedimentary layers within this formation; it would be very interesting to find out if O. fosbergii is mostly confined to the volcanic series, or soils with a high concentration of volcanic rock.

Note on the species name given here: O. fosbergii is called O. Xfosbergii in the Jepson Manual. We do not use the X in the name since there is no definitive evidence that this species is a hybrid, only speculation. This species may of course be of hybrid origin, but many species without an X in their name also originated from past hybridization events. It is very misleading to place such widespread species with definite geographic ranges in the same category as infrequent F1 hybrids such as Quercus Xmorehaus. Q. Xmorehaus typically exists as only isolated specimens many miles distant from each other, with a total number of specimens existing that can be counted on one's fingers and toes.

We thank Jane Strong for her excellent comments we have incorporated in this page. The panorama photographs were put together by Matthew Brown's fantastic program Autostitch.

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Copyright © 2007 by Tom Chester and James Dillane
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Last update: 25 January 2007