20 YEAR LICHEN GROWTH RATES ON SVALBARD

Lichenometry is commonly used to date geomorphic surfaces, however growth rates of lichen thalli are often poorly constrained. We present the results of detailed lichen measurements documenting lichen thalli growth rates in the high Arctic over an approximately 20 year interval. In 1984 and 1985, lichen growth stations were established on Svalbard (78°N 15°E) and dozens of lichen thalli belonging to the genera Rhizocarpon and Pseudophebe were measured and photographed. In 2002 and 2004, two of these growth stations were revisited and 35 lichens were relocated, re-measured, and re-photographed. Detailed measurements taken from the photographs and processed in image analysis software allowed us to determine precise growth rates. Growth rates were calculated based short and long axis diameters as well as total thallus area.

Traditional lichen growth rate curves show fast growth rates for small lichens and slower growth rates for larger, presumably older lichens. Our calculations reveal that over the past 20 years, Svalbard lichens grew at nearly linear rates largely independent of initial size. For example, published growth rate curves suggest Rhizocarpon of 10mm diameter on Svalbard grow about 2.0-2.5mm/yr as measured by the thallus short axis, however our measured Rhizocarpon lichens grew at much lower rates of 0.5-1.5mm/yr since the mid 1980s. In contrast, Rhizocarpon about 40mm in diameter grew at rates of 1.5-2.0mm/y rather than the slower 0.2-0.4mm/yr (short axis growth rates) predicted from published growth rate curves. Several possibilities may explain overall faster lichen growth rates (warmer climatic conditions since the 20th century, fertilization effect of atmospheric nitrogen pollutant) but it is not clear why these factors would slow growth rates of smaller lichen and enhance larger lichen growth rates. Our study continues but detailed computer-aided image analyses of lichen photographs provides the opportunity for reliable short-term measurements of lichen growth rates and improved lichenometry dating estimates.