…I sure got a big surprise.
There were odd clumps of white fluffy stuff everywhere – and no, it wasn’t snow.
Was it fur or hair?
Was it mold?
Some kind of fungus?
An undiscovered lichen?
Or just some cotton candy missing the red dye #5?
Nope.
They’re ice crystals.
They seemed to be growing mainly on alder shrubs, which I suspect contain a lot of moisture. Last night the temperature dropped to just below freezing. My hypothesis is that this created a vapour pressure gradient between the warm branches and the cold air: moisture on the outside of the branch froze, releasing heat and cooling the branch surface. This gradient drew more moisture from the inner to the outer surface of the branch, allowing for more freezing. The process continued and the crystals grew longer, like hair or fur.
In searching the literature for a similar process, I came across an article on ice flowers, which looked similar but weren’t exactly the same. The article mentioned that ice flowers form similarly to needle ice – a formation I’m familiar with from mountain geomorphology. Which means my hypothesis was correct, except for one small – but super interesting – detail. For the crystals to grow as long as they have, they require a constant source of water. For needle ice, that water comes from a groundwater source. In the case of the crystals I observed, it likely comes from continuous stemflow within the alder branches. How cool is that?
I’m interested to hear more from plant physiologists on the mechanism behind these crystal formation – and from other readers who might have seen something similar on their woodland wanders.
UPDATE: Thanks to a number of comments both here on the blog and on Twitter, I’ve discovered that these crystals are called ‘hair ice’ or ‘frost beard’, and are formed by a bacterial pathogen that inhabits many shrubs and trees. It’s called pseudomonas syringae, and seems to have an ice nucleation protein that raises the freezing temperature of intracellular plant tissues. There’s a bit about them in this 2012 article from Scientific American (by Jennifer Ouellette, aka @JenLucPiquant) – note her example picture is from just south of me, on Salt Spring Island! You can read more in this 2008 report by Gerhart Wagner and Christian Matzler (English translations sprinkled throughout).
Watershed Moments: Thoughts from the Hydrosphere 
It’s cool isn’t it!? We see it fairly often here in our woods in northern Sweden and I was told once (I believe by a teacher in forestry school) that it’s caused by a snow making bacteria, I’m guessing the “Pseudomonas syringae”. This bacteria creates ina proteins which makes water freeze at higher temperatures and that is also sometimes used for artificial snow making.
It’s really neat, and so much like needle ice! I’ll be looking up that bacteria – got a comment on Twitter about it as well so sounds like that might be the key. Apparently there was also a recent article about it in @MijnNatuurpunt (my Flemish/Dutch is rusty so I’ll be stumbling through that one…)
I’m happy to know I remember something from school, lol! 😉
Beautiful pictures! If you are interested, the September-October 2013 issue of American Scientist has a related article in it.
Hi Pappie – yes, very interested! First time I’ve seen this phenomena so looking for any info to help explain it. Do you think they have it online or would I need to look at your old copy?
Cool. It looks like the branches are dead, so this wouldn’t be a physiologically mediated process, but a physical process. Sort of like water movement through soil?
Hi Todd – I thought the same thing, but it’s actually happening at the base of still-living stems, too. I didn’t get any good shots of that, unfortunately. Seems like the bacteria angle is key – have had a few comments here & on Twitter about it. Going to look it up!
Very cool. Also reminded me that barkless alder can be alive. No wonder they can live anywhere and survive most any disturbance!
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Thanks for posting this – because this is how I found you. I had seen this phenomenon on a blog post not long ago, and today was thrilled to see it in the “flesh.” I’ll post a photo soon. I’m in the Seattle area and we were in the forest, in an area with alders. There were just a few pieces, but I was rewarded for scrutinizing the ground for more by finding bird’s nest fungi (Nidula niveotomentosa) just a few feet away – only the second or third time I saw those. You do a really good job of balancing content and photos, and not having so much text that the post is off-putting, but enough to give me something to think about.
I just saw more of it this morning – fascinating phenomenon! Glad you found my page, and thanks for the follow
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