Plant Species of the Borrego Desert:
Loasaceae: Eucnide rupestris, rock nettle


Fig. 1. The unusual flower of Eucnide rupestris, with sparse stinging hairs and dense stalked-glandular hairs throughout, yellowish-green sepals, a cylindric yellowish corolla tube, with erect green corolla lobes. The anthers are visible reaching halfway up the corolla lobes; the stigma is below the anthers.
Click on the picture for a larger version.
See also Fred Melgert's photographs.

Table of Contents

Introduction
Origin and Meaning of Name
Defenses Against Predation
Other Interesting Characteristics
Distinguishing Characteristics and Similar Species
Two Month Repeat Photography
Habitat, Distribution and Abundance
References


Introduction

Rarity in Space and Time

Eucnide rupestris, rock nettle, is one of the rarest species in California, known from only three locations: Torote Canyon / Indian Gorge in southeast San Diego County; Fossil Canyon and Painted Gorge in the Coyote Mountains in southwest Imperial County. There are only six collection events for it, on 30 April 1930, 17 May 1938, and 29 January 1956 from Painted Gorge; on 6 April 1978 and 10 May 1978 from Fossil Canyon; and on 18 March 1984 from Indian Gorge.

There are probably fewer than 100 plants at most in Torote Canyon / Indian Gorge in good years for this annual species, and probably similar numbers in the other two locations, for a total of fewer than 300 plants of this species in California. In most years there are zero plants of this species in California.

Its rarity in California is because the main population of this species is in Mexico along the edges of the Gulf of California, and it just barely sneaks across the border into our area; see Fig. 2. Our three locations are the northernmost part of its population.

Fig. 2. SEINet voucher distribution map for Eucnide rupestris as of 19 March 2018. Click on the map for an interactive version from SEINet which might be updated from this version.

Even in our three locations this species is rare both spatially and temporally. For example, in Indian Gorge in 1984 about 20 plants were seen. In Indian Gorge in 2018, we found only six plants total in Indian Gorge, and just 18 plants total in Torote Canyon / Indian Gorge despite two days devoted to surveying for this plant.

This spatial rarity is due to its habitat of growing on steep cliffs, and steep unstable "talus with sandy soil" slopes, at least in Torote / Indian Canyon, with occasional plants found at the base of such slopes. This habitat is discontinuous and rare in Torote / Indian Canyon. The last monograph on Eucnide states for the entire genus that they grow only on cliffs and rocky slopes ... and as a result of their restriction to these habitats, the populations of Eucnide are relatively small and discontinuous.

The temporal rarity is because this species is an annual here that apparently only germinates in response to monsoonal rain, which does not occur where it lives in most years. For example, Karyn Sauber reports that she saw this species in Indian Canyon in 2005, a year with good monsoonal rain, and not again until 2018. Karyn specifically checked for this species there in 2017, which was a good rainfall year in the desert for winter rain, but it was not present. She also checked for it there in all or most of the years of 2010 through 2016, without seeing it.

Other reports of this species in Torote / Indian Canyons are from 1995 and 2002. The report from 1995 is from a friend of Karyn's who hikes those canyons regularly, and reported that he saw it just twice in 20 years, in 1995 and 2005, although he didn't specifically look for it there every year during that period. Dieter Wilken photographed it in Torote Canyon in February 2002 after it was found there by Kevin Downing.

Tom and others searched for this species in Fossil Canyon on 23 December 2010 and 13 February 2011, and did not see this species there that desert season.

Origin and Meaning of Name

The name Eucnide is from the Greek eu, "good or true," and knide, "stinging nettle"; see the entry for both eu and Eucnide at Michael Charters' page on the meanings and derivations of scientific names. The Jepson Manual description says the name Eucnide means "strongly nettle-like".

This puzzled us since Eucnide is in the Loasaceae family, not the Urticaceae family.

It turns out there are at least four families which have some members with stinging hairs, and those four families are not close at all in evolution. Hence stinging hairs have independently evolved at least four times in separate lineages.

The four families with stinging hairs are Boraginaceae, Euphorbaceae, Loasaceae, and Urticaceae.

Defenses Against Predation

Eucnide rupestris has an amazing number of defenses against predation, which it needs since it must often be the only green healthy fresh plant during most of its lifetime. It germinates from monsoonal rain in the late summer, and takes something like five months to produce seeds. Hence during the fall and early winter, it could be the only attractive plant to eat for critters while they are waiting for winter rains to grow other food plants.

Its first defense is its habitat, growing on cliffs and steep unstable slopes that are hard to reach and hard to traverse.

Other Interesting Characteristics

Distinguishing Characteristics and Similar Species


Two Month Repeat Photography

On 18 March 2018, Tom Chester, Aaron Schusteff, Karyn Sauber and John Randall revisited the locations of eight plants in Torote / Indian Canyon that were photographed on 17 January 2018. Amazingly, the first plant we checked looked quite similar from a distance to how it appeared two months earlier! However, the two smaller plants next to it were no longer present.

Of the eight plants we checked, only those two plants were missing. Four plants were green and looked good. One of those four was in good bloom and fruit; two of those four were all in fruit; and one wasn't checked up close. Two plants were brown and dead, with lots of fruit.

Fig. 3 shows photographs of the same plants taken two months apart, on 17 January 2018 and 18 March 2018.

Fig. 3. Left: Photographs taken on 17 January 2018. Right: Photographs taken on 18 March 2018.
Click on the pictures for larger versions.

It was quickly apparent from the repeat photographs that even the plants that were green and looked good on 18 March were significantly aged from their appearance on 17 January, appearing more yellowish and more ragged. A close examination of the photos found that in all cases some of the rocks around them were different, indicating that the rocks in their unstable environment had moved. One plant had been smashed in part by large rocks falling on it that weren't covering the plant on 17 January. This strongly suggests that the two missing plants were knocked off by rockslides, perhaps caused by bighorn sheep whose scat was abundant near these plants.

Habitat, Distribution and Abundance

As mentioned in the Introduction, the last monograph on Eucnide states for the entire genus that they grow only on cliffs and rocky slopes ... and as a result of their restriction to these habitats, the populations of Eucnide are relatively small and discontinuous.

Jeff Field checked the habitat of the plants we saw at each location on 17 January 2018, and reported that in all cases there was sandy soil along with the rock chunks. One plant appeared to be growing out of bedrock, but it was immediately next to a talus slope with sandy soil.

This habitat is what allowed the plants to look so good four months after the rain on 10 September 2017. The plants are mostly alone in their habitat, so they have the stored water in the sand all to themselves. Their habitat ensured that none of the rain ran off, and concentrated the water to the spaces between the rocks.

This concentration of the rainfall was important, since this area did not appear to have received much monsoonal rain. In our 17 January 2018 survey, we only found ~10 widely-scattered plants of Pectis papposa, 2 plants of Euphorbia eriantha, and no Amaranthus fimbriatus, all monsoonal species. It appears that the 10 September 2017 rainfall here was just barely enough to germinate some of the monsoonal annuals. Better monsoonal rain might result in more Eucnide plants.

The only voucher from this area was from 1984, from "near mouth of Indian Gorge", which reported "ca. 20 plants on steep south-facing granitic talus & some in wash d.g.". Since we had to go to three locations to get 18 plants, this implies it is at least slightly more abundant in better monsoonal rain years.

We found roughly equal numbers of plants on north-facing slopes as on south-facing slopes.

References


Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).


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Copyright © 2018 by Tom Chester, Carla Hoegen, Fred Melgert, Walt Fidler, Nancy Accola, Jeff Field, Jim Roberts, James Dillane, and Michael Charters.
Commercial rights reserved. Permission is granted to reproduce any or all of this page for individual or non-profit institutional internal use as long as credit is given to me at this source:
http://tchester.org/bd/species/loasaceae/eucnide_rupestris.html
Comments and feedback: Tom Chester
Updated 19 March 2018.