There are a few things we know about lichens. They grow to agreat age and in some habitats can have rapid early growth as they pioneer an area that is undergoing primary succession, like a newly formed moraine from glacial retreat. It is not entirely clear if these lichens exhibit the rapid early growth soon after they arrive on a stone in these cool climates. It is my understanding that the jury is still out on that.
Anyway we can still get a pretty good understanding of the lichen age by measurements, weather/climate data and collaborating with a lichenologist (yes, they exist. They even have their own professional journal!). What Al has found is that these lichens are in the neighborhood of 200 years old which tell us that the glacier (linnebreen) was at this point of the upper valley at this time and created the moraine that the lichens have colonized. This was the nearing the end of the little ice age. So, the glacier has retreated over 1 kilometer since the last death throes of the little ice age. Significant retreat in this part of the world.
Lichens come in lots of shapes, colors and sizes (fruiticose = leafy; foliose= hairy and crustose = held tight against the rock). Their are two really neat things about lichens... 1) they are a symbiotic mix of algae and fungi, meaning that the organism is mutually dependent on these too components, and 2) they often grow very slowly and therefore can be used to age date the deposit on which they are growing - assuming you know how fast they are growing! To get a handle on lichen growth rates you can measure lichens on deposits of known age (tombstones, rock pile graves, monuments, mining tailings etc.) or you can directly measure them at one point in time and then come back some years later and re-measure them and determine how much they have changed. I measured lichens in the early 80s and recently Steve Roof re-measured the exact lichen thalli 20 years later. Because the growth rates are so slow the way we measure them is to photograph them with a scale (precisely machined square of metal) and then compare the two photos. Steve has done this using GIS techniques (normally used to analyze large geographic data sets) to determine long and short axis dimensions, circumference and area. His results suggest that lichens have been growing significantly faster during the past 20 years than they did prior to that time. The cause of this increased growth rate could be recent climate change or it could be due to the deposition of nutrients (e.g. nitrogen) associated with atmospheric pollution - yet one more indicator that the Arctic is not as isolated or robust as we originally thought!