Call it an exercise in curiosity.

It is somewhat difficult to find information about Dudleya densiflora online or in field guides (at least I have struggled). It is visually distinct where it grows and it is stunning - it is near-impossible to confuse with anything else in the mountain range - and there is a population easily accessible from a public park along a well-maintained trail. But despite this, information on its ecology seems to remain somewhat elusive.

I wondered: why does it grow here, in these few canyons, and nowhere else? Does it actually grow nowhere else? Why not on other steep granitic cliffs in the San Gabriels? How is it pollinated? How do its seeds disperse? Why does it grow, not only in just a few canyons, but only on select rock faces and talus slopes within those canyons? For which organisms is it a food source? How did it evolve? The other Dudleya in the San Gabriels are variable and hybridize; Dudleya densiflora does not hybridize with them. Did it diverge from their populations further back in its lineage? Is it related to other Dudleya with similar habit? How long does a given individual live? How many are left in the wild, really?

Because I am so drawn to them, and because the act of observing and searching is enjoyable even when it does not turn up answers, I have returned to them time and time again and searched for them in other parts of the mountains. In doing so I have given myself even more questions to ponder and an excuse to try new trails and new adventures. I have also found some leads, if not answers, to questions already posed.

Insect and Arachnid Interactions

Because of the short flowering period and obstacles like a 9-6 job I have had only limited opportunities to observe the plants in flower. Nevertheless, I tried to take some time to watch for pollinators or other insect/arachnid interactions whenever I had the chance.

Very scientifical descriptions of behavior, I know. My main notes are that I have (on multiple occasions) found the flowers crawling with beetles, as in https://www.inaturalist.org/observations/195597956 . I believe I have seen beetles more often than bee or fly pollinators. I am not sure what the beetles are doing, but they don't appear to have a way to carry pollen, nor did they appear to be laying eggs. I have not seen hummingbirds or butterflies visiting these flowers. I am honestly not sure what percent of flowers even get pollinated at all - in almost all photos I have of the species flowering, no pollinators are visible. I do not believe I have records of aphids being present on D. dens, but I could be mistaken.

Habitat

This has been one of the more baffling questions to address.

D. densiflora appears - appears - to prefer the pale, granitic, steep cliffs of the San Gabriel River canyon. It is most accessibly abundant on the river's east bank where the trail runs; through binoculars or a telephoto lens it does not appear to inhabit the west bank much. Even on the east bank it grows abundantly and then not at all. At first, it seemed as if the lines of abundance were drawn along the shadows of the high summer sun - Dudleya densifloras grew largely in the shade and were absent in sunnier areas. This proves inconsistent further down the trail. The question of aspect is raised - a preference for north- or west-facing banks? - but the sharp meanders of the San Gabriel River have plants facing every cardinal direction.

Geology, kind of

The lightness or darkness or composition of the rocks on which they grow was also something I wondered about. Maybe reflectivity, or heat absorption, or porosity, or chemical composition play a role? Even the level of fracture or drainage in the rock? It seems like the plant prefers very bright white granitic rock - until it doesn't. See https://www.inaturalist.org/observations/164577824 for an example of what I would have previously considered a "typical" substrate color (on top) with darker substrate on the bottom, still inhabited. I had also suspected that maybe there was a variation in mineral composition between the yellower-toned rocks on which it is mostly absent along the riverbank vs. the bluer-toned ones, but @asimow provided swift evidence against this with a small swing of a hammer. The yellower rock is white-gray on the inside; the surface is weathered. Perhaps the weathering is suggestive of different exposure conditions which the plant disprefers? I am not sure.

It also seems that the plant prefers very steep cliffs, until it doesn't. Further down the trail, past the end of the maintained portions, one can find it more-or-less happily inhabiting talus slopes and the remnants of landslides. (I ought to link specific observations here.)

It also surprised me to see it on the west bank of the river in with my telephoto when I could have sworn I'd looked across the river before. In that area, it appears to grow below the road and not above it - I have pulled over at many parking areas and turnoffs to hunt the cliffs with my telephoto - but clearly I'm capable of missing it time and time again.

With every misconception I develop and then discard, I'm not sure I can say much at all about its preferences.

Locations within the mountains

I have encountered the plants in four places in the San Gabriels: the main population accessible from the park, a very small population (est. <20 plants) on the east bank further downstream of the park, blocked by chain link fence but visible from the bike trail (https://www.inaturalist.org/observations/164577824); a small population further upstream along the 39, on the west bank, perhaps a hundred individuals but only spanning a section of roadcut I measured at around 25-30 meters (https://www.inaturalist.org/observations/165851923); and in a canyon between Fish and San Gabriel accessible through a housing development (no observation). I have looked for, and not found them, in two smaller canyons between Fish and San Gabriel, along several stretches of Highway 39, in a couple spur canyons immediately adjacent to the roadcut on which they are visible, and along the West Fork of the San Gabriel River. Their absence from the West Fork confuses me greatly. It seems like similar habitat to the main branch just a few miles further south, but I have never spotted one despite visiting many times. Many other Dudleyas inhabit those canyon walls. Why not densiflora?

Plant community associations

I have often found these alongside Plummer's mariposa lily, scarlet larkspur, Bloomeria crocea, Dudleya lanceolata, and various Clarkias. They often grow in clumps of moss or dirt anchored by grasses or other plants. At first I didn't think they affiliated with more xeric plants like prickly pears, fountain grass, or buckwheat/sagebrush, but they have proven me wrong time and time again.

Total population

The population is often given as <2,000 individuals remaining in the wild but this seems patently false. There are many outcroppings where over a hundred individuals can be seen - I've gone and counted in photos to be sure. https://www.inaturalist.org/observations/170638702 is one such example.

Flowering and fruiting

I have limited commentary here besides the description in Jepson.

I will say that the budding inflorescence often "hangs its head", then unfurls as the flowers open. They appear to typically open starting closer to the stem and progressing away from the plant. The flowering may span enough time that the flowers closest to the stem are beginning to fruit before the ones furthest from the stem have opened. Not all mature plants flower every year; I've seen individuals which have remained flowerless multiple years in a row as well as individuals which have flowered multiple years in a row (e.g. https://www.inaturalist.org/observations/121357337 and https://www.inaturalist.org/observations/170513248).

I have seen swollen/inflated fruits occasionally - https://www.inaturalist.org/observations/140598900 - I don't know what's different about them.

Other thoughts

I sat down to write this half-envisioning that I'd have more information to contribute, but I feel like most of what I've landed on is uncertainty. I've had many theories on why it grows here and not there, but those theories have been disproven whenever I try to feel about their edges or test the margins. That's part of what keeps drawing me back to them: the mystery. I plan to update this post with further questions and thoughts as I keep exploring.

Thanks

to @toyonito, @pleistocene, @g_heaton, @slarm, @elizabeth_lockhart, @nathantay, and @gianni20033 for identifications of my observations and bonus thanks to @asimow for indulging my geological curiosity and hiking with me (especially despite my lack of basic geological info retained since college).

Posted on January 2, 2024 09:58 PM by velodrome