Does Tree Age Predict the Occurrence and Abundance of Usnea Longissima in Multi-Aged Submontane Picea Abies Stands?

Abstract

In boreal Scandinavia Usnea longissima is a rare epiphytic lichen associated with old spruce (Picea abies) forests. To test whether tree age was an important determinant of the occurrence and abundance of U. longissima at the scale of a forest stand we estimated thallus numbers on 1008 trees within three submontane spruce stands in south-central Norway. The stands were of all ages and multi-layered and had been selectively cut previous to the 1940s. There was no relationship between number of U. longissima thalli and tree age among trees with dbh of ≥ 10 cm. The most consistent pattern was a positive relationship between occurrence and number of thalli and stem diameter. After controlling the covariance of stem diameter, there was a tendency for a negative relationship between number of thalli and tree age. The results suggest that in old-growth resembling stands, with already established U. longissima populations, tree size (here measured as stem Diameter) is a better predictor of thallus number than tree age. This does not contradict the possibility that other environmental characteristics, associated with stands of older age, may be important for the establishment and growth of U. longissima populations.