Tuesday, August 16, 2016

The Night's Alluring Advertising

Oenothera nuttallii.

I awoke in the middle of the night to a powerful fragrance emanating from the kitchen—a mix of jasmine and wintergreen and the benzyl benzoate beloved of veterinarians. I didn’t notice the abundant nitrogen-bearing compounds, for their disgusting stink was masked by perfumey scents and sharp animalic notes. But I should be clear—only later would I learn these details. At the time, I only recognized the thick sweet yeasty smell of Nuttall's evening primroses in bloom.

True to their name, evening primroses open at dusk. Nuttall’s evening primrose is well-suited to dim light, with a strong fragrance that lures pollinators to a large white target, where they’re rewarded with nutritious nectar. While sipping the nectar, the visitor inadvertently picks up pollen, and hopefully carries it to the receptive stigma of another evening primrose of the same species—leading to fertilization, seed production, and the perpetuation of life.
“A 5-spotted hawkmoth, Manduca quiquemaculata, drinks nectar from an opening Oenothera harringtonii flower. If you look closely [click on image], you will see that pollen has accumulated on the moth’s proboscis. Hawkmoths are important pollinators of many members of the evening primrose family, Onagraceae, and other night-flowering plant species.” (Krissa Skogen; used with permission)
The evening primroses in my kitchen were collected from a nearby abandoned railroad. I brought them home intending to make macro portraits, but I waited too long. The petals wilted and turned pink, signaling “all done.” Other flowers were waiting in the wings, however—folded and rolled up in their buds. They bloomed that night, filling my kitchen with their potent scent. After a few photos, I took them outside to the woodshed, leaving the door open for any pollinators that might be around.
This flower’s clearly finished, being pink. Pollinators, you might as well look elsewhere.
Midnight in the kitchen.
Morning in the woodshed.
The flowers remained open for several hours that morning. Inside the shed, they stayed still enough for the macro portraits I had hoped for.
The petals are highly reflective, and release aromatic compounds—terrific advertising.
There really is yummy nectar down in this hole!
Evening primrose pollen comes in strings, held together with viscin threads. Most online sources define “viscin” as a product of the mucilaginous sap of mistletoe, but there's more to it. In some plants, including orchids and evening primroses, pollen grains are held together with clear elastic viscin threads. More info here and here.
Viscin threads and pollen grains hang from anthers. Note extended 4-lobed green stigma.
Oenothera nuttallii  is thought to be an outcrosser. Keeping the stigma beyond the anthers discourages self-pollination.
Finally starting to wilt. But the stigmas are still out there waiting for pollen.

I had read that some evening primrose flowers open very quickly, so I collected stems with buds, hoping to catch them in the act. I put them in a vase in the kitchen. 
Drooping slender buds looked too young. Plump ascending buds looked more promising.
Surely these are ready to bloom!
The calyx of one of the buds had split, providing a glimpse of carefully packaged petals with green stigma lobes at the tip.

At 7:30 I turned off the lights and opened all the windows, bringing the cool dimness of dusk to the kitchen. I took up watch, reading by headlamp, but for an hour I noticed nothing. At 8:35 I let the dog in, and returned to that strong distinctive fragrance—a flower was fully open! These evening primroses are fast indeed. Soon others followed.
8:42:  Splits develop in the calyx, stigma lobes emerging.
8:42.5:  Calyx folded back, petals are starting to unfurl.
8:43:  Petals continue to spread and unfold.
At 8:44, I heard a faint but definite “pop”—a third flower had literally popped open!
This one wasted no time.
Petals spread and unfold. Fold lines remain visible until flowers close the next morning.
By 8:50, when I turned off the camera, four flowers had opened, and were filling the kitchen with their thick distinctive scent. I moved them outside and wished them the best.

Nuttall’s evening primrose is said to have a strong or even unpleasant fragrance. Eager to know more, I went searching. But Google and academic databases produced nothing specific for Oenothera nuttallii. I found a slender lead—a paper about volatile compounds in yellow-flowered evening primroses. Author Rob Raguso promptly responded to my inquiry:
“Specific to O. nuttallii, we have a large data set and a paper in prep on the phylogenetic distribution of floral scent in Oenothera section Anogra, including both lab and field data for O. nuttallii. It is a fascinating plant and there is much more to be learned about it! I hope to have the paper submitted by the end of the year, with my co-authors. Will be happy to share with you once it is.”
Floral scents are complex. Sometimes only a single compound is involved, but more often there are 20 to 60, or even 100 (Knudsen & Gershenzon 2006). Raguso and his colleagues have identified 16 volatile compounds in Nuttall’s evening primrose. The majority, about two-thirds of all emissions, are really foul-smelling nitrogen-bearing compounds. Yet to me, these evening primroses smell sickeningly sweet at worst, and only when they're confined to a small room.

The explanation lies in our differential reactions to specific compounds, as Raguso explained:
“… an important point is that human (and insect) perception of plant volatiles is not always dominated by the most abundant compounds. In the case of O. nuttallii, there is a relatively large and consistent presences of methyl salicylate, known to many as wintergreen oil! Also, there is a small amount of a very sweet, perfumey compound called cis-jasmone, which has a very penetrating effect to human subjects. So the blend is very dimensional, from sharp "animalic" notes (as described by the great chemist/perfumer Roman Kaiser) to distinctive wintergreen and jasmine scents, with a heavy, almost medicinal presence lent by the largest volatile compound in our sample, benzyl benzoate.”

Which pollinators find this blend irresistible? Moths. The Raguso lab is currently studying responses of hawkmoths to compounds released by Nuttall’s evening primrose. Krissa Skogen has shown that the hawkmoths that pollinate Harrington’s evening primrose will travel up to twenty miles in a night. Do equally wide-ranging hawkmoths pollinate Nuttall’s evening primrose? If so, the required outcrossing is almost certainly assured!
A receptive stigma extended well beyond the anthers, waiting for pollen from afar.

Sources (in addition to links in post)

Many thanks to Rob Raguso, Professor and Chair of the Dept. of Neurobiology and Behavior at Cornell University, for sharing his research results for Nuttall’s evening primrose.

Krissa Skogen of the Chicago Botanic Garden provided the great photo of the hawkmoth and evening primrose. For more about her research on hawkmoths and Harrington’s evening primrose, see Science Scents.

Knudsen, JT, and Gershenzon, J. 2006. The chemical diversity of floral scent, in Dudareva, N, and Pichersky, E, eds. Biology of floral scent. CRC Press.

Raguso, RA, et al. 2007. Floral biology of North American Oenothera Sect. Lavauxia (Onagraceae): advertisements, rewards, and extreme variation in floral depth. Annals of the Missouri Botanical Garden 94:236-257. http://www.jstor.org/stable/40035498

Tuesday, August 9, 2016

Trees on a Fold

“Forest” on Hutton Lake, next to pale sandstone outcrops. Pattern on lake is windblown waves.

Intent on following my serviceberry tree, neglected since March, I returned to the unexpected forest at Hutton Lake—trees growing where there "shouldn’t" be any. 
Off to see my serviceberry.
The Hutton Lake Forest is the patch of trees on the steep slope across the lake.
Hutton Lake lies in the southern Laramie Basin, in southeast Wyoming. To the east are the Laramie Mountains; the Medicine Bow Mountains lie to the west. The result is a double rain shadow, with the Laramies sucking moisture out of summer storms coming from the plains, and the Medicine Bows doing the same for winter storms from the west. The Basin receives only 11 inches (28 cm) of annual precipitation on average—not conducive to forests. Cottonwoods line the rivers, but the rest of the Basin is covered in grass and shrubs.

Yet on the south side of Hutton Lake stands a charming little forest, with two narrow leaf cottonwoods (Populus angustifolia), a patch of aspen (Populus tremuloides), and about twenty serviceberries (Amelanchier sp.), four of which I consider small trees, being taller than 65 in or 1.65 m (my height). All are growing in a narrow zone less than 100 m long on the northeast slope of a small ridge.
Narrow leaf cottonwood on a windy evening (windy is normal here).
Looking down into the aspen grove. The tilted rocks are a clue as to why these trees are here.
The aspen patch includes about a half dozen trees and many saplings. Aspen readily reproduce vegetatively, via root sprouts, so botanists are quick to conclude that stands are single “individuals” (clones). But there's more than one individual here, for example this tree:
This aspen and several smaller ones nearby are growing out of cracks in the sandstone.

The serviceberry tree that I photographed last March, when it was leafless, is now easy to identify, with its distinctive leaves and fruit.
In spite of the wind, some of these leaves ended up in focus. Click on image to see the distinctive oval leaves with obvious veins and toothed margins.
The berries look like little apples, not surprising given that apples and serviceberries are in the same subtribe of the Rose family (Malinae in the Rosaceae). None were ripe, but some were getting close.
Serviceberries have many names including juneberry, saskatoon, shadblow and others. I wish I could tell you the scientific name of mine, but unfortunately specimens from Hutton Lake have been called both Amelanchier alnifolia and A. utahensis. The two are difficult to separate:
“Serviceberries (Amelanchier spp.) intergrade and hybridize readily, making species identification difficult” (USDA Forest Service).
“Identification is best undertaken in the field, with visits during flowering and fruiting seasons, and observations of habitat, habit, presence of congeners [related species], and flowering time relative to sympatric congeners” (Flora of North America).
Sigh. Maybe I will look into this for a future post, but for now there are much more interesting things to ponder … like the ant hordes.
Little brown dots winding up the right side of the aspen trunk are ants (click on image to view).
There were ants everywhere!—on sandy ground, on sandstone rocks, on aspen trunks growing out of the rocks, and on my bare legs. Fortunately they didn’t bite, but they swarmed up my legs whenever I stopped to take photos and notes.

Why so many ants? Why trees? Maybe the reason is the same—a fold in the land.

When the Laramie and Medicine Bow Mountains were uplifted via massive folds and faults, about 60 million years ago, minor folds were created in the downwarped basin. Hutton Lake lies on the northeast side of such a fold—the Boulder Ridge anticline.
From Ver Ploeg et al., 2016. Added arrow points to exposures of steeply-tilted sandstones
Much of the Boulder Ridge anticline lies out of sight, buried under younger sediments. But erosion has exposed it in places:
“On the south side of Hutton Lake the two limbs of the Boulder Ridge anticline are shown [exposed], the beds on the west side dipping south of west at an angle of 15° and those on the north side dipping north of east at angles of 50° to 85°. The steeply dipping beds … outcrop in a prominent ridge along the south side of the lake.” —Nelson Horatio Darton, 1909
Steep northeast limb (side) of Boulder Ridge anticline. 
Steeply-tilted Muddy Sandstone (lower Cretaceous) behind narrow leaf cottonwood.
Front to back: golden currant, serviceberry, Muddy Sandstone, aspen.
It’s not unusual to find trees associated with rocks in an otherwise unforested landscape. Perhaps there are suitable microenvironments where seedlings were able to grow in the absence of competition from grasses and shrubs. Perhaps there’s more moisture here, from snow drifts, or rain running off tilted rocks. We known that fractures in bedrock serve as reservoirs, funneling and storing water that can be accessed by roots.

As for the ants … well … I don’t know why they were so abundant on the loose sandy soil. Maybe it makes for better burrows. In any case, it appears that they also benefit somehow from the folded rocks of the Boulder Ridge anticline.

Walking back to the car, through the din of complaining prairie dogs, I thought about the little forest and how it came to be. How did seeds manage to land in that little patch of hospitable habitat? How did they hit such a small target? Serviceberry, aspen and cottonwood must cast many seeds far and wide—so many and so far that a lucky few will land in just the right place, even if it’s tiny.

White-tailed prairie dogs are summer company at Hutton Lake. To warn their neighbors of invaders, they chatter and cry loudly … until the invader gets too close at which point they quickly disappear down their holes.

Monthly virtual gatherings of tree followers are kindly hosted by The Squirrel Basket. Find tree news from around the world here.


Darton, N.H., and Siebenthal, C. E., 1909, Geology and mineral resources of the Laramie Basin, Wyoming: U. S. Geological Survey Bulletin 364.

Knight, DH, et al. 2014. Mountains and plains; the ecology of Wyoming landscapes, 2nd ed. Yale University Press.

Ver Ploeg, A.J., Larsen, M.C., Taboga, K.G., 2016. Characterization of evaporite karst features in the southern Laramie Basin, Wyoming. Wyoming State Geological Survey Report of Investigations No. 70, 34 p.

Wednesday, August 3, 2016

Fecund Fireweed’s Far-flung Seeds

0.065-0.069, 9, 40, 81, 80,000 and 8,980,000—just a few of fireweed’s impressive numbers.

Last week I visited the railroad garden west of my house, and collected stems in flower and fruit to take home for portraits. Surprise! When I opened the plastic bag just twenty minutes later, I found that what had been this:
had become this (with white campion):
Fireweed capsules had split open and were releasing seeds. Apparently they were ready to send their offspring out into the world. They just needed a reason—in this case, being cutoff from moisture and nutrients.
Fireweed, Chamerion angustifolium. Long narrow structures below flowers are seed-filled capsules.
Fireweed capsule fully dehisced and empty of seeds.
From now well into fall, many thousands of fireweed seeds will be passing by, high overhead. When it’s windy, they fly. When it’s calm, they hang almost suspended. The feather-light seeds descend only when there’s no air movement whatsoever, slowly drifting down at about 0.065-0.069 m/hour. From 100 m up, it takes 25 minutes to reach the ground (exceptions include downdrafts and rainfall).

No wonder fireweed is so widespread. It’s circumboreal, native to much of the Northern Hemisphere, which explains why it has so many names. In Russia it’s called Ivan Chay (chai), in parts of Canada great willowherb, and in Britain rosebay willowherb. There was a time when Brits called it bombweed because it quickly colonized bomb craters during World War II. Somewhere in the world, fireweed is known as blooming sally—a name often mentioned but never explained (UPDATE: see PtP's Comment at end of post). In 1753, Karl Linnaeus gave it the scientific name Epilobium angustifolium, but fireweed is enough different from other willowherbs (Epilobium) that the Czech botanist Josef Holub moved it to the genus Chamerion in 1972. Chamerion means low Nerion (Nerion is oleander in the US; source). angustifolium means narrow leaf.

UPDATE: for additional information on nomenclature, common names, and tea, see Pat the Plant's very interesting Comment at the end of the post.
Epilobium angustifolium, today’s Chamerion angustifolium. From Flora von Deutschland, Österreich und der Schweiz; 1885; Otto Wilhelm Thomé (source).
Veins in fireweed leaves do not end at the leaf margin but rather join up again to form reticulate venation. Plants can be identified without flowers because of this distinctive pattern.
Fireweed grows profusely where vegetation has been removed, exposing bare unshaded soil. It’s best known as a fire follower but other disturbances will do, such as logging, bulldozing, gardens and volcanos. Often it’s the most common herbaceous species post-disturbance, being extremely good at reproduction, dispersal and establishment.
Fireweed is flourishing where railroad tracks were torn up near my house; more here.
Like many pioneering plants, fireweed is fecund. A single plant may produce 80,000 seeds per year. They’re tiny and light—only one mm long and almost paper thin, perfect for long distance travel. Their long silky hairs carry them away with the slightest breeze.
Seeds are hard to photograph. They move with the slightest disturbance, including sighs of frustration.
The adaptations of fireweed's seeds are highly effective, as has been shown many times. In Saskatchewan, Archibold and assistants placed seed traps (germination trays filled with potting soil) on a burned site in April, and retrieved them the next year. Of the seeds that had germinated, 63% were from fireweed. Extrapolating from their seed traps, they estimated there were 8.98 million fireweed seeds per ha (about 22 million per acre).

One year after Mount St. Helens erupted, Dale and assistants trapped wind-borne seeds on a debris flow; 81% were from fireweed. In northern Quebec, analysis of seed rain (seeds caught falling from the sky, usually with sticky traps) showed that fireweed contributes 40 seeds per sq m (3.7 seeds/sq ft; source).
Fireweed capsules in various stages of dehiscence.
Solbreck and Andersson took a different approach. They found a television tower with large suction traps (for flying insects) in a forest clearing with abundant fireweed. In September, they counted fireweed seeds trapped at different heights. There were thousands, even as high as 100 m (only a few of the seeds were from other plants). Given their near weightlessness, these high travelers “are likely to stay suspended in the air for long periods during sunny summer days with updrafts. These seeds will undoubtedly be carried long distances by the wind. … We suggest that seed dispersal distances of the order of 100-300 km are quite common …”

Most seeds lucky enough to fall on a suitable site germinate quickly—100% germination in ten days has been documented in some studies. Fireweed seeds are non-dormant and can germinate over a wide range of temperatures, though they do best when it’s warm, sunny and humid. Fireweed does not contribute to long-term seed banks; after 18 to 24 months, most seeds are no longer viable. This is a true opportunist!

But fireweed doesn’t reproduce just by seed. In fact, vegetative reproduction maybe be more common. Growth is especially profuse when disturbance cuts underground rhizomes, stimulating sprouting—as many as 9 sprouts per meter of rhizome.
Rhizomes of Chamerion angustifolium, by Rasbak.

What’s behind the recent appearance of fireweed plants on the old railroad bed near my house? Are they sprouts from rhizomes that laid dormant for years, and then responded to the bulldozer that tore up the railroad tracks and cleared out the thistles and tumbleweeds? Or did they grow from seed? Without digging, I’ll never know, but I don’t want to disturb them. In fact, I hope they continue to flourish and spread!
Maybe I'll sow these myself.


Archibold, OW. 1980. Seed input into a postfire forest site in northern Saskatchewan. Canadian Journal of Forest Research 10:129-134.

Dale, VH. 1989. Wind dispersed seeds and plant recovery on the Mount St. Helens debris avalanche. Canadian Journal of Botany 67:1434-1441.

Pavek, DS. Chamerion angustifolium. In: Fire Effects Information System [Online]. USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Accessed 2016, July 31. http://www.fs.fed.us/database/feis/plants/forb/chaang/all.html 

Romme, WH, Bohland, L, Persichetty, C, and Caruso, T. 1995. Germination ecology of some common forest herbs in Yellowstone National Park, Wyoming, USA. Arctic and Alpine Research 27:407-412. PDF

Solbreck, C, and Andersson, D. 1987. Vertical distribution of fireweed, Epilobium augustifolium, seeds in the air. Canadian Journal of Botany 65: 2177-2178.