Invasion patterns of European larch and Swiss stone pine in subalpine pastures in the French Alps

In some French mountain areas, the land-use changes which started around 1850 have resulted in a complete abandonment of any agro-pastoral activity by 1950. Thus, the landscape underwent important modifications of structure and functioning. Tree invasion did not take place homogeneously in space or time. Using aerial photographs I mapped the recolonization areas at timberline and analyzed the spatial pattern and age structure of sample plots established in stands in the recolonization zones above 2000 m a.s.l. I identified four main establishment periods, with the most important forest edge dynamics happening during the most recent periods: (1) immediately after abandonment of the pastures 50 years ago, European larch (Larix decidua Miller) formed very dense colonization patches close to the ancient chalets. The invasion was limited to narrow fringes related to colluvial soil conditions alongside the forest edges. Now Swiss stone pine (Pinus cembra L.) is regenerating in these young larch woods. (2) During the last two decades, stone pine has regenerated massively at tree line, forming dense seedling patches, mostly on convex relief forms. The spatial restriction of the young forests, be it stone pine or larch stands, enhances the importance of relief and soil conditions for vegetation dynamics at timberline. The landscape structure and the land-use history are determinant for the assessment of forest edge dynamics and consequently for the study and prediction of timberline fluctuations. (C) 2001 Elsevier Science B.V. All rights reserved.