a1 Norwegian Institute for Nature Research, Gaustadalléen 21, NO-0349 Oslo, Norway. Email: firstname.lastname@example.org
a2 Norwegian Institute for Nature Research, P.O. Box 5685 Sluppen, NO-7485 Trondheim, Norway
Over the last decades, levels of sulphur deposition from air pollution have been substantially reduced in Norway, whereas levels of nitrogen deposition have been stable or somewhat increased. In parallel, a clear trend of increasing annual temperatures, as well as precipitation, is evident. Human impact on natural ecosystems is predicted to reduce biodiversity at regional scales through facilitating a few generalist species at the expense of species with narrow habitat requirements. In this study, we investigate changes in community composition and the abundance of dominant lichen species on birch in subalpine forests over a 15-year period. The study is based on repeated measurements in five monitoring sites in Norway, representing regional gradients in temperature and precipitation as well as in deposition of nitrogen and sulphur compounds. Two dominant species are studied in particular; the generalist Hypogymnia physodes and the subalpine birch specialist Melanelia olivacea. The largest change in species composition was found in the site with the biggest reduction in sulphur deposition during the 15-year period, whereas the site with low precipitation and low pollution loads had small changes in species composition. The abundance of the generalist H. physodes increased in all sites and the specialist M. olivacea decreased in abundance, especially in sites with both high precipitation and heavy nitrogen deposition. Our study thus suggests that a warmer, humid climate is beneficial for the generalist H. physodes.
(Accepted July 02 2012)