Plant Species of the Borrego Desert:
Species Ranked by their Maximum Number of Plants per Mile

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Table of Contents

Introduction
Ranking by Maximum Number of Plants per Mile
Comments on Individual Species, and Additional Density Measurements

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Introduction

The vast majority of the Borrego Desert plant species are not distributed uniformly through the region. Most are found in only a small fraction of the total area here. Even those that are widespread throughout the area usually vary in plant density between areas.

In our Floras and Plant Trail Guides, we almost always report the abundance of each species, but only up to a maximum of 99 plants. Those abundance numbers were primarily intended to note which species were present in only low numbers in a given area. A different abundance measurement is needed in order to report the actual abundance of the more common species.

In late 2020, the authors began a program to measure that abundance by counting the total number of plants of a given species in a measured distance as we hike, giving the number of plants per mile. We are first concentrating on shrubs, trees, and cactus. Annuals are another beast entirely, since their numbers are usually much higher, and they are only present on the desert floor in good years.

We count only a single species at a time, and we usually, but not always, pick one of the most abundant species at our location.

We aim to count species over a distance of about one mile, but sometimes cannot do that if the habitat along our hike suddenly changes and that species disappears. It would give a misleading maximum density if we included a large distance where no plants were present, if there are other areas where the species is present over a larger distance. Note that counts made over distances much smaller than a mile require considerable extrapolation to report the number of plants per mile, which may produce numbers larger than any actual count in a mile.

We standardize that density measurement by only counting plants seen within a distance of 100 feet on either side of our hiking route. We begin the count on a hike by stepping off a distance of 100 feet on either side of our hiking route, to calibrate our eyes for that species to that distance.

Counting plants within this 100 foot distance works well for large plants, such as creosote and ocotillos, but smaller plants, such as Mammillaria, can not be completely counted out to 100 feet as part of a normal hike. Nonetheless, we report our derived number of plants per mile for those species, recognizing that a correction factor may need to be applied in order to accurately compare actual two-dimensional densities between species. Our reported number is still of interest to botanists in giving the likely maximum number of plants of those species that could be seen on a hike.

The intent of this page is to eventually report the numbers from where the species is most abundant. However, at first, and for some time, this page will report the measurements we have taken so far, which are unlikely to be where each species is most abundant. Nonetheless, the numbers are still of interest to give an idea of the densities we've measured for each species, and to serve as the "target to beat" for each species.

In our past surveys, when we have found an area with a much larger number of plants of a given species than we usually see, we have sometimes noted such areas as the "Happy Home" of that species. We will eventually try to return to those areas for each species to count them.

It is easy to convert our counts to a density per hectare by simply dividing by ten. 1000 plants per mile in our count is 1000 plants in a rectangle one mile long and 200 feet wide. This equals 100 plants per hectare (41 plants per acre).

Ranking by Maximum Number of Plants per Mile

Table 1. Maximum Number of Plants per Mile in Descending Order

#/Mile	Scientific Name	Survey Length (mile)	Location
7000	Cylindropuntia bigelovii	--	Density per mile derived from typical counts of 200 plants in a circle with radius of 100 feet
1820	Cylindropuntia ganderi	0.48	Lowermost Mine Wash Road
1700	Justicia californica	0.13	Wash in Pinyon Canyon near upper end of Pinyon Wash Road
1650	Larrea tridentata	0.33	Calcite Mine Parking Area
1600	Stephanomeria pauciflora	0.03	Lowermost Mine Wash Road
1350	Agave deserti var. deserti	0.15	Mescal Bajada between Mine Wash and Chuckwalla Wash
1300	Senna armata	0.30	Lowermost Mine Wash Road
970	Hyptis emoryi	0.26	Wash in Pinyon Canyon near upper end of Pinyon Wash Road
950	Fouquieria splendens ssp. splendens	0.21	Mescal Bajada between Mine Wash and Chuckwalla Wash
825	Psorothamnus spinosus	0.31	Mine Wash Road mile 2
700	Mammillaria dioica	0.11	Mescal Bajada between Mine Wash and Chuckwalla Wash
585	Ericameria paniculata	1.51	Lowermost 1.5 miles of Pinyon Wash above SR78
500	Xylorhiza orcuttii	1.00	Fish Creek (from Fred Melgert)
440	Hesperocallis undulata	0.27	Carrizo Creek older non-active wash below S2
377	Cylindropuntia ramosissima	8.80	Sweeney Pass / Volcanic Hills Area
47	Echinocereus engelmannii	0.58	Lowermost Mine Wash Road
32	Opuntia basilaris var. basilaris	0.22	Mine Wash Road mile 1

Comments on Individual Species, and Additional Density Measurements

Agave deserti. Plants of this species are typically found in clumps of 3 to ~15 rosettes that are probably genetically identical, and possibly physically connected. We counted recognizable clumps, since it would be difficult to count individual rosettes from 100 feet away. A count of individual rosettes would probably yield a density something like five times higher.

Cylindropuntia ramosissima. See other density measurements, many with maps.

Ericameria paniculata. There were 294 plants per mile in 0.96 miles of Nolina Wash just above its junction with Pinyon Wash, and 22 plants per mile in 1.02 miles of upper Nolina Wash. There were 41 plants per mile in 1.75 miles of the Vallecito Wash east of Vallecito County Park.

Fouquieria splendens. There were 740 plants per mile in a distance of 2.4 miles in the Coyote Creek Road area from the Alcoholic Pass Trailhead to Ocotillo Flat.

Hesperocallis undulata. This species is widespread at fairly low densities, but has small very dense patches in widely-scattered areas. The densest patch we know of consists of 163 plants in about a half acre in Don's yard in Borrego Springs. That density of ~320 plants per acre translates to a whopping 7800 plants per mile if the density continued for a mile! But we've never seen anyplace where such very dense patches continue over areas larger than a half acre, and most dense patches are even smaller in area.

The number in the Table comes from an area in Carrizo Wash that has a small dense patch, along with about an equal stretch with less dense plants. Even then, the density in the table comes from just 0.27 miles. A count over a larger distance might produce a lower density.

Larrea tridentata. There were 1550 plants / mile in a distance of 0.12 miles in the Mescal Bajada between Mine Wash and Chuckwalla Wash.

There were 775 plants / mile along 0.95 miles of the Coyote Creek Road from Second Crossing to First Crossing.

Our maximum density so far of 1650 plants per mile is roughly six times less than the density of 959 plants per hectare measured on Mojave Desert sites in Rock Valley, Nevada, which is 9500 plants per mile in our units.

Stephanomeria pauciflora. Our survey ended abruptly when the habitat changed, so the number of plants per mile is highly uncertain due to the very short survey distance.

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