These days, when I close my eyes at night, I see images of circular, yellow-green lichen with black edges. As I walk across the Kangerlussuaq landscape, I check every rock surface for Rhizocarpon, the lichen in my dreams.
My preoccupation with these ubiquitous green circles comes from the fact that I have measured hundreds of them during the past week, employing the dating technique called lichenometry. Lichenometry hinges on the fact that Rhizocarpon lichens grow steadily in a circular fashion. By measuring the diameter of lichens growing on a rock, and relating it to a known rate of growth, you can determine an approximate age of a surface – the amount of time that a rock has been exposed. Using lichenometry in the field is quite simple (although time consuming and, I must admit, a bit tedious at times). All you need are calipers, a notebook, and, for me, a long measuring tape.
Why have I been measuring so many lichens? Here in Kangerlussuaq, I’m interested a common landscape feature – patches of ground where soil has been eroded by the wind, leaving a rocky surface behind. Once these rocks are exposed, lichen can start colonizing the surface. By measuring the lichen on rocks within the exposed patches, I can start to estimate when the soil erosion occurred. Knowing when something occurred opens the door to many possibilities – I can see if the timing relates to any known climate fluctuations in the area, I can estimate the rate of erosion, and I can see if all the patches formed at the same time.
But I shouldn’t get ahead of myself. I haven’t even entered all of my data (and there are a lot of numbers to enter), let alone crunched the numbers. However, just by observing the lichen on the patches, I have definitely seen a consistent pattern. Lichen diameters near the edge of the exposed patches are very small – only a few millimeters – while lichen diameters near the center of the patches are much larger. Outside of the patches, on bedrock and large boulders, the lichen can reach gigantic sizes – 5 or 6 centimeters in diameter! This consistent pattern leads me to think that many of the patches are actively being eroded, since the rocks right at the edge are host to such young lichens.
Although I’m glad to have a break from lichen measurements, I’m looking forward to figuring out what all the numbers mean. For now, I encourage you all to take a look at the lichen around you – each one has a story to tell!