The process of recolonization after disturbance is crucial for the persistence and dynamics of patch-tracking metapopulations. We developed a model to compare the spatial distribution and spatial genetic structure of the epiphytic lichen Lobaria pulmonaria within the perimeter of two reconstructed 19th century disturbances with a nearby reference area without stand-level disturbance. Population genetic data suggested that after stand-replacing disturbance, each plot was colonized by one or a few genotypes only, which subsequently spread clonally within a local neighborhood. The model (cellular automaton) aimed at testing the validity of this interpretation and at assessing the relative importance of local dispersal of clonal propagules vs. long-distance dispersal of clonal and/or sexual diaspores. A reasonable model fit was reached for the empirical data on host tree distribution, lichen distribution, and tree-and plot-level genotype diversity of the lichen in the reference area. Although model calibration suggested a predominance of local dispersal of clonal propagules, a substantial contribution of immigration of non-local genotypes by long-distance dispersal was needed to reach the observed levels of genotype diversity. The model could not fully explain the high degree of clonality after stand-replacing disturbance, suggesting that the dispersal process itself may not be stationary but depend on the ecological conditions related to disturbance.

Bugmann, H. 2007. The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps. Silva Fennica 41(1): 55-70.Recent changes of climate and land-use are often regarded to affect the European Alpine region substantially and to trigger an increase in the elevation of the upper treeline. The patterns of tree invasion on a subalpine meadow at 1900 m a.s.l. in Sedrun, Canton Grisons, Switzerland, were studied in order 1) to reconstruct the process of tree establishment and tree-growth dynamics in space and time, and 2) to evaluate the influence of site properties, land-use change and climate on these processes. Dendroecological analysis of 105 Norway spruce combined with an assessment of 48 vegetation plots and 17 soil profiles revealed that the trees were established in one main period (1965-1980s), starting 15 years after the abandonment of the agricultural use of the meadow, and that there is a pronounced environmental gradient along the forest-meadow ecotone. Tree establishment and height growth were favoured close to the former forest edge, but all saplings irrespective of their distance to the forest edge and their age showed increased radial growth since 1990, coinciding with a period of higher summer temperatures in the region. Therefore, we conclude that the observed tree-line dynamics in Sedrun are the result of both land-use and climate change: Tree establishment was triggered by the abandonment of the agricultural use of the meadow, and strongly favoured by particularly good growing conditions in a warm decade, which illustrates the sensitivity of conifers near the alpine tree-line to temperature fluctuations.

Bolli J.C., Wagner H.H., Kalwij J.M., Werth S., Cherubini P., Scheidegger C. and Rigling A. 2008. Growth dynamics after historic disturbance in a montane forest and its implications for an endangered epiphytic lichen. Bot. Helv. 118: 111 -127.Endangered forest species are often negatively affected by disturbances, which may have long-lasting effects on the distribution, abundance and genetic diversity of such species. To understand the effects of historic disturbances, detailed knowledge of the conditions for survival and recolonisation is needed, and this requires precise information on the perimeter and severity of historic disturbance events. We reconstructed a major historic disturbance (intensive logging followed by windthrow and fire in 1871) in the Swiss Jura mountains to analyse its effect on the disturbancesensitive epiphytic lichen Lobaria pulmonaria. Tree-ring analysis of old and young Norway spruce trees (Picea abies L.), sampled systematically on a 100 m grid, revealed that (1) the disturbance was of intermediate severity, (2) a large, well-defined area of disturbance was created, and (3) an undisturbed zone remained in the centre of the disturbed area. A comparison with lichen genetic data from a previous survey revealed that genetic diversity was particularly high in the remnant zone. These results suggest that the lichen survived there, and that it re-colonised the disturbed area both from the edge and from the remnant undisturbed zone. This illustrates that a detailed reconstruction of historic disturbances, as achieved with dendroecology, is very important for understanding the recolonisation process and thus, the conditions for the long-term persistence of disturbance-sensitive species in a dynamic landscape.

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