A GROWTH RATE FOR THE LICHEN XANTHOPARMELIA SOMLOENSIS FROM CENTRAL VERMONT – A TOOL FOR DATING ROCK SURFACE EXPOSURE

Lichen growth rate has been used as a Quaternary dating technique for several decades (i.e., lichenometry). Certain species of lichen grow on rock surfaces at what is presumed to be a regular rate, and if this rate is known and presumed to be uninterrupted, then the length of time that a rock surface has been exposed may be determined by measuring the maximum lichen diameter. In central Vermont, lichenometric analysis of rock surfaces may yield a corroboration of the timing of deglaciation (ca. 14,000 calendar years based on varve chronology), and the age of landslide scarps or talus deposits, which are common in the steep terrain of the Green Mountains, might be determined.

Lichen dating has not been used in Vermont due to a lack of information on lichen growth rate and variables affecting growth. Lichen growth rate is affected by: physiology of the particular species; elevation; moisture availability; amount of direct solar radiation; annual mean temperature; and nutrient availability of the substrate. This study provides the first determination of a lichen growth rate in Vermont.

We used a statistically robust measurement of diameter for lichens on granite surfaces (pylons on the Norwich University campus) of known age in order to calculate the growth rate of the lichen Xanthoparmelia somloensis. In addition, by measuring the lichen abundance on various rock faces, and at different heights above ground, we find that a major variable in growth rate is the facing direction of the rock surface, presumably related to solar radiation. Therefore, we can now address the two critical variables necessary for using lichen growth as a dating tool in central Vermont. First, we provide a maximum rate of growth of 0.31 cm/yr and, second we have established that the maximum growth occurs on the northwest face of rock.