Plant Species of the Borrego Desert:
Distribution of Cylindropuntia bigelovii and its Recent Death Rate

Table of Contents

Geographic and Elevational Distribution
Death Rates

Geographic and Elevational Distribution

This page shows the geographic and elevational distribution of Cylindropuntia bigelovii in the Borrego Desert, and also reports the results of five surveys of the percentage of dead plants in some areas. I plan on doing more such surveys in the future, guided by the plots here.

As of 24 December 2020, I've digitized 162 GPS points of C. bigelovii from surveys I have done with colleagues, and there are 846 iNat observations with location uncertainties less than 100 m. Terry Hunefeld has contributed the most iNat observations, 196; Fred Melgert and Carla Hoegen have contributed 94; and Camden Bruner has contributed 74 observations, with 258 other observers contributing 49 or fewer observations.

One additional iNat observation was rejected due to being a misdetermination.

There are only 43 vouchers from this area, many with uncertain locations, so they were not used in these plots.

The geographic location of 1008 GPS locations (162 from the GPS points of my surveys with colleages and 846 from iNat) are shown in Fig. 1, both individually and also using the clustering algorithm of the Consortium of California Herbaria.

See also the iNat zoomable map linked to all observations of C. bigelovii, which also includes observations posted after 24 December 2020, and Fred Melgert and Carla Hoegen's species page on C. bigelovii containing much more information on this species and a zoomable map that includes Calflora and voucher locations.

Fig. 1. Maps of the distribution of C. bigelovii using 1008 locations. The blue lines separate Riverside, Imperial and San Diego counties, as well as the U.S. and Mexico.

Left: Map showing individual points.

Right: Map using the clustering algorithm from the Consortium of California Herbaria, showing how many individual points are in a small area. Having a lot of points in a small area does not necessarily imply the density of plants is greater there; it just means someone has taken more GPS points or observations in that area.

Click on the pictures for larger versions.

Fig. 2 shows a plot of elevation vs. longitude for these points.

Fig. 2. Plot of elevation vs. longitude for 1008 locations of C. bigelovii. The GPS points from my surveys with colleagues and the iNat points are shown separately. The quasi-linear features are typically from a number of GPS points taken along a single wash or drainage, as seen in Fig. 1..

Most C. bigelovii locations are between 500 and 2500 feet elevation, with a smaller number of locations ranging up to 3500 feet elevation.

Death Rates

Someone (I can't quickly find the source) wrote that "The desert is always dying", since many (not all) shrubs of the desert generally only recruit new individuals from seed in rare wetter periods of a year or more, and therefore only lose individuals in the rest of the years.

The years since the year 2000 have been claimed to be the worst long-term drought in the U.S. southwest in 1200 years. Early in that 20 year period, massive die-offs of desert shrubs and perennials were recorded in the severe drought year of 2002 in several places in the southern California deserts.

C. bigelovii, however, is an interesting exception to the generalization that the desert is always dying, since it is always recruiting new members from its unique (among our cholla species) vegetative propagation of shedding joints that form new individuals. In fact, this is the only way it reproduces, since it does not produce viable seeds since it mainly a triploid (2n=33) and therefore cannot undergo normal meiosis.

However, because it takes much longer than 20 years to achieve a steady-state population (Ebert 2019), the populations of C. bigelovii will have been reduced by the recent drought years, with little replacement from shedded joints.

In late 2020, I did five surveys to find the percent of dead plants of C. bigelovii in different areas, usually counting the plants inside a circle of radius ~100 feet (some of the earlier counts had varying radii, and some counts were of half-circles). Three of those surveys were in close proximity in Cactus Valley, finding 15, 40 and 40% of the plants dead; one was near the beginning of the Buttes Pass Road at Borrego Mountain, finding an amazing 97% of the plants dead, and another survey near that found 57% mortality.

The area with 97% dead plants is the farthest east location of C. bigelovii in the southern Borrego Valley area, and is also the lowest elevation location for it in that area. It is perhaps not surprising that a warming, drier climate, coupled with a record period of severe drought, would extirpate plants at that location first, since that is also probably one of the driest locations in the Borrego Valley area.

On the other hand, in early 2021, Don Rideout and I did two surveys in the Mescal Bajada, one with 85% mortality and another with 48% mortality. So there can be a significant variation in mortality even between nearby locations.

As of 13 February 2020, I have done surveys in 11 areas. Fig. 3 shows the survey locations labeled by the % dead.

Fig. 3. Plot of the locations where I have done surveys for the death rate of C. bigelovii. Each location is labeled by its death rate..

See also the survey locations on on a plot of elevation vs longitude, and a plot of the percent dead plants vs. longitude.

Bobich, Wallace and Sartori (2014, Madrono 61:126) measured the mortality of C. bigelovii at two sites in Deep Canyon on the north side of the Santa Rosa Mountains. They found a mortality rate of 56% in the flood plain of Deep Canyon at 210 to 290 m (690 to 950 feet), compared to 26% at Agave Hill at 820 m (2690 feet), and concluded that the difference was due to "milder temperatures and greater precipitation" at the higher elevation site.

Fig. 4 shows the death rate plotted as a function of elevation, along with a best-fit line to the data. The linear fit explains only 14% of the variance in the data.

Fig. 4. C. bigelovii death rate plotted as a function of elevation, along with a best-fit line to the data.

Precipitation is not a simple function of elevation in this area, although the lowest elevations are usually the driest areas. In Fig. 4, the two lowest elevation points are in areas with C. echinocarpa; all other points are in areas with C. ganderi, which occupies higher-precipitation areas (which are also generally higher-elevation areas) than does C. echinocarpa. Yet those two points in dry C. echinocarpa areas at elevations of 700 to 800 feet, with mortalities of 97% and 57%, are not terribly different from the two points at 1200 to 1400 feet elevation, with mortalities of 85% and 48%.

So although elevation and precipitation certainly are factors in explaining the death rate, other factors play a larger role. Other factors might include density and size of the C. bigelovii population (resulting in competition for water), habitat composition (boulder, gravelly, or sandy areas), ground slope, and slope orientation. Keith Haworth noticed that the maximum death rate areas are clustered, and suggested a pathological disorder, disease or insect infestation might also be a factor.

Anne Kelly supplied these thoughts:

I think Anne really nailed it with the patchiness of rainfall, which destroys any simplistic correlation of the death rate with elevation, etc.

The surveys done to date are listed below in declining order of mortality rate. Many of the surveys have panoramic pictures posted at iNat and linked from the text here:

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Comments and feedback: Tom Chester
Updated 24 January 2023