Plants of Southern California: Opuntia acanthocarpa var. coloradensis

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Introduction
Locations
    The Extreme Northeast San Diego County Voucher
Key and Photographs

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Introduction

Many botanists in southern California became greatly confused when the Jepson Manual did not have a main entry for the most common cholla by far on the extreme western side of the Sonoran Desert, Opuntia ganderi = O. acanthocarpa var. ganderi. Its only mention was an obscure note under O. parryi that claimed it was a hybrid between O. parryi and O. echinocarpa, without citing any supporting evidence.

When O. ganderi was deleted as an entry in the Jepson Manual, many botanists began labeling specimens of O. ganderi as the only variety of O. acanthocarpa given in the Jepson Manual, var. coloradensis. Such determinations of O. ganderi specimens are incorrect; these are two distinct taxa.

This page presents a distribution map for Opuntia acanthocarpa var. coloradensis, discusses the controversial voucher for this taxon from extreme northeast San Diego County, and gives pictures of true specimens of this taxon.

For an overview of chollas in southern California, see Plants of Southern California: Chollas.

For Opuntia ganderi, see Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Pictorial Identification Guide.

Locations

Lyman Benson recognized this taxon in 1969, and named it var. coloradensis since this taxon is confined to the area of the Lower Colorado River. In California, all undisputed specimens are within ~20 miles of the Colorado River. The same concentration to the area of the Colorado River is seen in Arizona, but there a number of specimens extend to ~70 miles east of the Colorado River, with one specimen ~150 miles east.

The following map gives California voucher locations with positions from the Consortium of California Herbaria, augmented with California locations from Benson (1969), and Arizona voucher locations from the Flora of Arizona (1999). It also shows a slightly-modified smoothed Distribution Map from Flora of North America:

The smooth blue contours delineate the range of all vouchers that are undisputed Opuntia acanthocarpa var. coloradensis.

There are two vouchers far to the west of that contour. The one on the west side of the Salton Sea is the voucher discussed below from extreme northeast San Diego County. I have not examined the other voucher from just west of Ridgecrest (UC1121720 from the year 1934), but specimens in the field there should be examined before that voucher is accepted as being Opuntia acanthocarpa var. coloradensis.

The Extreme Northeast San Diego County Voucher

In 1937, Frank Gander collected a cholla from extreme northeast San Diego County that has been the source of much interest and debate. This location is disjunct by some 70 miles west of the nearest known population in California, a total of 80 miles from the Colorado River.

Gander's voucher consists of only a single stem segment and one flower, mounted upside down, so confidence in its determination is not absolute. His voucher was probably originally determined simply as Opuntia acanthocarpa, since varieties ganderi and coloradensis were not defined until 1938 and 1969, respectively. The latest (1986) determination of his voucher is Opuntia acanthocarpa var. coloradensis (henceforth O.a.c.), from the reddish filaments of the then-49 year old flower and the characteristics of that single stem segment.

Because of uncertainty in the determination of this voucher, Jon Rebman has long been encouraging people to look for the plants that were vouchered by Gander. Bill Sullivan has searched three times for this taxon in 2005-2006 without success, finding only infrequent specimens of O. ganderi. Wayne Armstrong and I joined Bill to search for those plants in the field on 21 January 2008 and found only very clear O. ganderi in a single location.

Of course, extreme northeast San Diego County is a very large area, and it is therefore impossible to claim definitively that O.a.c. does not exist someplace here without extensive further searches. But there are three good reasons to believe that O.a.c. does not occur here:

• Since Frank Gander did not return with any samples from this area now determined as O. ganderi, the simplest interpretation of the known facts is that his cholla sample is actually from the O. ganderi population observed in several different locations here, and Gander's voucher was misdetermined as O.a.c..

  Gander spent only one whole day here and part of another here, collecting 79 taxa on one day and 3 cacti on the next. He could not have covered much more ground than was covered in the three searches. Of course, he could have covered different ground than has been searched so far. However, Gander most likely started from the same point as the recent searches, limiting the possible areas he could have searched.

• Despite expectations that this is a very dry area, this area is actually very similar in wetness to the area immediately west of the town of Borrego Springs. Thus this habitat turns out to be exactly that of O. ganderi, much wetter than the very dry habitat in which O.a.c. grows. Also, O.a.c. lives in areas that receive equal summer and winter rainfall, whereas extreme northeast San Diego County receives almost entirely winter rainfall.

  The expectation that this is a very dry area came from two sources. First, rainfall contour maps (Pryde 1992; Climatic Atlas of the United States 1968) give the annual rainfall here to be well below four inches per year and similar to that of Imperial County. However, those rainfall contours are unreliable since they are entirely extrapolations due to the lack of weather stations in this area. Such maps were removed by Pryde (2004) for precisely that reason. Second, this area is on the rainshadow side of the Santa Rosa Mountains, and might be expected to even drier than the area ten miles to the southwest on the other side of the Santa Rosa Mountains.

  The plants found on the 21 January 2008 survey clearly indicated such expectations are incorrect. That survey found abundant healthy annuals here, which during that time period were only found to the west of the town of Borrego Springs. Those annuals here germinated and were nurtured by rainfall spilling over from the crest of the Santa Rosa Mountains, just as the annuals west of Borrego Springs were nurtured by rainfall spilling over the crest of the San Ysidro Mountains. Furthermore, just ten miles to the south, the vegetation includes such indicators of a very dry area such as desert holly, Atriplex hymenelytra, and silver cholla, Opuntia echinocarpa, both of which are absent here.

• If O.a.c. actually existed at this location, it would be expected to be found in the many desert ranges between this location and the Colorado River, including Joshua Tree National Park, the Orocopia Mountains, the Chocolate Mountains, and the Chuckwalla Mountains. Those all have much more suitable O.a.c. habitat than the much wetter habitat in northeast San Diego County, and are much closer to the established range of O.a.c..

  Given the apparent absence of O.a.c. in those areas, it would seem extremely surprising for O.a.c. to skip over those areas and be present in a site with climatic conditions very different from those in the rest of its habitat.

Of course, plants sometimes disregard good reasons similar to the ones above, and further surveys in this area are planned. But I would not bet that any true O.a.c. will ever be found here.

Key and Photographs

Opuntia acanthocarpa var. coloradensis can be distinguished from O. ganderi using the following key:

1. Filaments red; inner tepals bright yellow; trees or sometimes shrubs, with few branches, usually at acute angles, without strongly ascending branches; stem segments 15-30 cm long; stem tubercles (1.5-)2-3 cm; spines 10-15 per areole .... O. acanthocarpa var. coloradensis

1'. Filaments greenish-white to yellow to green, sometimes suffused with bronze or rose, but not red; inner tepals greenish yellow, often with tips reddish abaxially; shrubs or sometimes trees, with many branches, with tips of major branches strict, ascending; stem segments usually 10-26 cm long; stem tubercles usually 1.3-2.6 cm; spines 15-25 per areole ... O. ganderi

Fortunately, since flowers are not frequently seen in the field, the other characteristics work well to separate these two taxa.

Photographs of specimens from Ogilby Road at the base of the Cargo Muchacho Mountains 20 miles west of the Colorado River are given below. The Cargo Muchacho Mountains are at the southeast tip of the Chocolate Mountains labeled in the above map.

The photographs were taken in harsh noon-time sunlight in summer, and hence the plants do not show up as well as photography taken under more optimum conditions.

Go to:

• Native and Introduced Plants of Southern California
• Voucher in northeast San Diego County
• Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Pictorial Identification Guide
• "Intergradation" between Opuntia parryi and O. ganderi
• Chollas
• References