…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?
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).