Plant Species of the Borrego Desert:
Loasaceae: Eucnide rupestris, rock nettle
Loasaceae: Eucnide rupestris, rock nettle
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| 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
Pronunciation, Origin and Meaning of Name
Defenses Against Predation
Other Interesting Characteristics
Distinguishing Characteristics and Similar Species
Two Month Repeat Photography
Habitat, Distribution and Abundance
Introduction
Rarity in Space and Time
Eucnide rupestris, rock nettle, is one of the rarest species in California, known from only four locations: Torote Canyon / Indian Gorge in southeast San Diego County; Fossil / Shell Canyon and Painted Gorge in the Coyote Mountains in southwest Imperial County; and at Split Mountain. There are only nine 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 / Shell Canyon;
- on 18 March 1984 and 24 March 1998 from Indian Gorge;
- on 30 April 1989 from a "small side canyon of Split Mountain, north of anticline"; and
- on 22 January 1977 from a "dry wash north of Signal Mountain" that is probably a wash-down waif.
We had previously thought that there are probably fewer than 100 plants at most in Torote Canyon / Indian Gorge in most good years for this annual species, and possibly similar numbers in Painted Gorge and Fossil Canyon, 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.
However, in the exceptionally-good summer / fall rainfall year of 2022, where it rained at least someplace in the montane / desert transition area every month from June to November 2022, Jim Roberts, Fred Melgert, and Tom Chester counted 139 plants in Torote Canyon / Indian Gorge on 14 and 15 November 2022, with two hard to access locations not censused. Walt Fidler then surveyed those hard to access locations in December 2022 and January 2023, and found a total of 1,483 plants, showing what a difference it makes if surveys are only confined to easier-to-reach areas. Such unusually-good summer rainfall probably happens only once every 20 years or so.
The rarity of Eucnide 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 locations are the northernmost part of its population.
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| 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 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, and just 18 plants total in Torote Canyon / Indian Gorge despite two days devoted to surveying for this plant. Even in the very good year of 2022, when we found 139 plants in Indian Gorge and Torote Canyon, we only found three plants in Indian Gorge (although Walt Fidler found 26 plants total by surveying difficult-to-access areas).
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 a facultative annual (see below) here that apparently only germinates in response to good monsoonal rain, which does not occur where it lives here 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.
Pronunciation, Origin and Meaning of Name
Michael Charters gives the pronunciation of the name as yook-NIGH-dee roo-PES-tris.
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 six families which have some members with stinging hairs, and those six families are not close at all in evolution. Hence stinging hairs have independently evolved at least six times in separate lineages.
The six families with stinging hairs are Boraginaceae, Euphorbaceae, Fabaceae, Loasaceae, Solanaceae, 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 main defense is undoubtedly its habitat, growing on cliffs and steep unstable slopes that are hard to reach and hard to traverse.
The plant characteristics that deter predation are the stinging hairs and the very glandular herbage. Those stalked glands, and/or perhaps the secretion that produces the shiny leaves, probably produce the unpleasant odor of the leaves. The unusual flowers are very well protected by both stinging and glandular hairs; see Fig. 1 and this photo. As if these weren't enough, the Arizona flora reports that the plant also has barbed hairs!
Other Interesting Characteristics
Perhaps the most interesting other characteristic of this plant is that it plants its own seeds near the base of the mother plant! The fruiting pedicel elongates, curves and becomes reflexed, bending away from light, so that the fruit capsule is pointed toward the cliff when it is ready to release its seeds. Some of its sister species have such greatly-elongating pedicels that the entire fruit capsule is forced into the rock crevice in which the plant grows!
This species is also very unusual in its delay between germination and seed production. Most of our monsoonal annuals come into flower a month after germination, and are dead three months after germination. This species flowers three to six months after germination!
This unusual delay between germination and flowering may be because this species is a short-lived perennial in most of its range, and is a facultative annual in our area, probably due to the unreliability of our monsoonal rainfall. Perennials don't have the strong urge to flower in their first year that annuals have. A number of our perennial and shrub species germinate in response to monsoonal rain, but almost none of the other such species flower in their first year.
Distinguishing Characteristics and Similar Species
In Fall 2022, along with the greatly-increased numbers of Eucnide in Torote / Indian Canyons, there was an explosion of Perityle emoryi plants. This made it quite difficult to pick out the small numbers of Eucnide plants amidst the overwhelming number of similar-looking Perityle plants. For example, see a typical view of a hillside that has both species in it, where Jim Roberts was able to spot the Eucnide with binocs, and went up there to confirm it (note the strong color variation in the Perityle plants that made it even more difficult to spot the Eucnide).
If E. rupestris is flowering, it is easy to confirm since there is no other species even remotely similar. Its flowers are so unusual in our area that all it takes is a quick glance at the flowers to identify this species; see Fig. 1.
However, if it is not flowering, it is very hard to distinguish from non-flowering plants of Perityle emoryi until you have seen both species together a few times and picked up the gestalt differences between the two.
Fig. 3 shows some examples of both species growing together.
The main differences are:
- the leaves of Eucnide are shiny, and the leaves of Perityle are dull. This is sometimes hard to see under some lighting conditions. Fred says that the leaves of Eucnide look like they are wet from recent rain.
- the leaves of Eucnide usually have many prominent bumps on the upper leaf surface, with each bump having a stinging hair projecting from it. The upper leaf surfaces of Perityle have about the same number of hairs that are similar in color and shape to stinging hairs, but there are no bumps at their base.
There is often a difference in the leaf lobes, with Eucnide have less-deeply-lobed leaves, but each species is quite variable in its leaf lobes. Sometimes the leaf of Perityle is more deeply lobed, but sometimes not.
Surprisingly, the hairs of both species look very similar with a hand lens in the field. Perityle has long translucent hairs that look very similar to the stinging hairs of Eucnide, and they both have short-stalked glandular hairs. See a comparison of the leaf hairs.
If you are willing to rub the leaves, there is a difference in the texture of the leaves, and a difference in their odor. Fred says that the Eucnide leaves have a rougher feel, probably from the bumps on the leaves.
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| Fig. 3. Top: Photographs of two different areas where Eucnide and Perityle grow together. In these two areas, the leaves of Eucnide are a bit larger, more shiny, and a somewhat-darker green.
Bottom: photograph of two young plants growing side by side. The Eucnide leaves are shinier, with apparent bumps, and much less deeply lobed, but in this case are a lighter green than the leaves of Perityle. See a comparison of the leaf hairs for these two plants. Click on the pictures for larger versions. | |
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. 4 shows photographs of the same plants taken two months apart, on 17 January 2018 and 18 March 2018.
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| Fig. 4. 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.
We found roughly equal numbers of plants on north-facing slopes as on south-facing slopes.
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 in summer 2017, 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 1984 voucher from this area reported "ca. 20 plants on steep south-facing granitic talus & some in wash d.g." from "near mouth of Indian Gorge". Since we had to go to three locations to get 18 plants in 2018, this implies it is at least slightly more abundant in better monsoonal rain years.
We indeed found that this was the case in the exceptionally-good summer / fall rainfall year of 2022, when Jim Roberts, Fred Melgert, and Tom Chester counted 139 plants in Torote Canyon / Indian Gorge on 14 and 15 November 2022, with two hard-to-access locations not censused. The locations found in that survey and in the 17 January 2018 survey, along with the number of plants from the highest count at each location in either survey, are shown in Fig. 5.
Fig. 5. Map of Eucnide rupestris locations (red dots) found in the 2018 and 2022 surveys, with the number of plants from the highest count at each location in either survey. The total number of known plants from these surveys was 151. Additional plants are known to present in areas not surveyed in 2022 due to difficult terrain to survey. The surveys were restricted to the vicinity of the Indian Gorge / Torote Canyon floors, except for where Fred Melgert climbed a very steep talus drainage to map the distribution there. Click on the map for a slightly-larger version.
The number of six plants in that location came from the 2018 survey. We only saw four plants in that location in 2022.
The number of ten plants in that location also came from the 2018 survey. We did not attempt to climb the talus slope to get to those plants in 2022.
As in the 2018 survey, in the 2022 survey we found plants in the two canonical locations for this species, cliffs and bouldery slopes.
The location with the 67 plants is a very interesting one, possibly unique in Torote Canyon. It consists of a steep "drainage" made entirely from large boulders, which shows up as a unique feature from satellite views; see the images in Fig. 6. What is also unusual about this location is that there is abundant sand / gravel below many of the boulders, which allows Eucnide to grow roots in.
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| Fig. 6. Google Earth images of the narrow bouldery "drainage" which contains plants of Eucnide (red dots) throughout its length. Left: View looking to the north. Right: View looking down Torote Canyon to the east. Although there are other white areas on the slopes of Torote Canyon, those areas are mostly cliff faces.
Click on the images for larger versions. | |
The more typical bouldery slope location for Eucnide is at the top of rocky fans at the base of cliffs. Two locations are shown in Fig. 7, which are the two locations we didn't survey in 2022, since significant mountain goat ability is needed to reach those locations.
Since Eucnide only grows amidst the boulders at the top of those fans, it must be the case that only that upper part has enough sand / gravel to grow in.
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| Fig. 7. Top: Google Earth image of four Eucnide locations (red dots). The middle two locations are at the top of bouldery fans. A photo of the second location from the left is shown at bottom left, and of the third location from the left at bottom right. Note the person in each photo for scale.
Click on the pictures for larger versions. | |
Many of the other Eucnide locations are in cliffs, typically ones that have ledges and cracks. The pix in the top of Fig. 3 show one such location. Fig. 8 shows a location that is unusually rich in Eucnide, the one with 29 plants shown in Fig. 5.
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| Fig. 8. Photos of a cliff location that is especially rich in Eucnide plants.
Click on the images for versions showing a larger area. | |
In Fall / Winter 2022 to 2023 Walt Fidler devoted himself to surveying monsoonal plants in the southern part of ABDSP. He surveyed Indian Gorge / Torote Canyon in December 2022 and January 2023, and took on the very challenging task of climbing the steep unstable cliffs in order to get a much better count on the Eucnide population, surveying areas where mere mortals do not dare go.
In the area in the middle of Fig. 5, where the previous surveys had found a total of 67 plants, he found a total of 222 plants that could be confidently identified as Eucnide! In the Indian Gorge population, at the right edge of Fig. 5, where previous surveys had found a total of 6 plants, he found 26 plants. Most amazingly, in the area of the westernmost five points in Fig. 5, where previous surveys had found a total of 54 plants, with the caveat that we didn't attempt to count plants where "significant mountain goat ability was need to reach those locations", Walt found 1,211 plants!
Walt thereby increased the total number of known Eucnide plants here from 151 to 1,483, almost a factor of ten increase!
Walt found that the Eucnide population was actually continuous between those five points in Fig. 5. He surveyed the plants at the eastern end of the population, hiked upslope to the bottom of the cliffs, then did a western traversal at the bottom of the cliffs. He collected a waypoint at every plant that he could reach. This procedure resulted in 686 gps points, with several hundred more plants carpeting the slopes above. He used binoculars to count at least 525 addtional plants that he couldn't reach. Hence the minimum abundance of the westernmost 5 points in Fig. 5 is 1211.
Well above 50% of the plants consist of a flat basal rosette with a single flower. Hence more than half of the plants in the population are uncountable unless you are within a few feet.
Furthermore, Walt found that the time of day of the survey was extremely important. The immensity of the population is only obvious very early in the day when the slope is still illuminated by sunlight. For most of the day, the plants are quite inconspicuous in heavy shade when viewed from the wash. After 8 a.m. the population disappears in the shade.
Fig. 9 gives an updated map adding the new Eucnide locations reported by Walt.
Fig. 9. Map of Eucnide rupestris locations (magenta dots) found in the 2018, 2022, and 2023 surveys. The vast majority of the locations are from Walt Fidler.
Fig. 10 shows the locations of the westernmost points, which require special climbing skills to reach. Walt noted that the density of plants in the western rock nettle population is almost uniform, in both the cliffs and the steep sandy slopes between rocks. Every large gap in the gps point plot is simply terrain that he said he didn't have the temerity to attempt access. Throughout the traversal it was impossible to descend while vertical. Walt said that every upslope excursion seen in the plots ended with a controlled downslope slide on his rear end!
Fig. 10. Google Earth view looking south showing Eucnide rupestris locations (magenta dots) in very hard to reach areas, with nearly all the locations from Walt Fidler. Compare with the ground-level photographs in Fig. 7. In both Fig. 7 and 10, the top of the alluvial fan is visible, with the extremely steep, unstable cliff area above. See also a slightly-larger map of the very westernmost population.
Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).