Questions: How have the historical frequency and severity of natural disturbances in primary Picea abies forests varied at the forest stand and landscape level during recent centuries? Is there a relationship between physiographic attributes and historical patterns of disturbance severity in this system? Location: Primary P. abies forests of the Eastern Carpathian Mountains, Romania; a region thought to hold the largest concentration of primary P. abies forests in Europe's temperate zone.Methods: We used dendrochronological methods applied to many plots over a large area (132 plots representing six stands in two landscapes), thereby providing information at both stand and landscape levels. Evidence of past canopy disturbance was derived from two patterns of radial growth: (1) abrupt, sustained increases in growth (releases) and (2) rapid early growth rates (gap recruitment). These methods were augmented with non-metric multidimensional scaling to facilitate the interpretation of factors influencing past disturbance.Results: Of the two growth pattern criteria used to assess past disturbance, gap recruitment was the most common, representing 80% of disturbance evidence overall. Disturbance severities varied over the landscape, including stand-replacing events, as well as low-and intermediate-severity disturbances. More than half of the study plots experienced extreme-severity disturbances at the plot level, although they were not always synchronized across stands and landscapes. Plots indicating high-severity disturbances were often spatially clustered (indicating disturbances up to 20 ha), while this tendency was less clear for lowand moderate-severity disturbances. Physiographic attributes such as altitude and land form were only weakly correlated with disturbance severity. Historical documents suggest windstorms as the primary disturbance agent, while the role of bark beetles (Ips typographus) remains unclear.
Conclusions:The historical disturbance regime revealed in this multi-scale study is characterized by considerable spatial and temporal heterogeneity, which could be seen among plots within stands, among stands within landscapes and between the two landscapes. When the disturbance regime was evaluated at these larger scales, the entire range of disturbance severity was revealed within this landscape.
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The Greenland Ice Sheet (GrIS) is a key element of the global climate system and thus knowledge about its melting in the past is desirable. However, GrIS-melt records only date back until 1979 and climate data based reconstructions covering the 20th century differ with respect to absolute values. To extend our knowledge about the Greenland Ice Sheet we examined the potential of shrub ring-widths and wood-anatomy as proxies for GrIS-melt. We found significant correlations between shrub cell-wall thickness and regional melt derived from passive microwave satellite brightness for 7% of the total GrIS area. A respective transfer function calibrated over the period 1979 to 2007 successfully passed model calibration-verification tests and explained 42% of GrIS-melt variability. Consequently, the first GrIS-melt reconstruction based on shrub wood-anatomy covering the period 1909 to 2012 is presented and compared against two temperature-based reconstructions. For the period prior to 1930 the new reconstruction contrasts with existing literature but generally confirms that most recent record melt rates are amongst the highest since the early 20th century. We discuss the sensitivity of shrubs to several influencing factors besides summer temperature as possible reason for the observed differences and highlight the potential of using shrubs as multiparameter proxies within a network to increase our knowledge about 20th century Greenland Ice Sheet dynamics.
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