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. 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. 

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.

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