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Kreyling, J; Stahlmann, R; Beierkuhnlein, C (2012). Spatial variation in leaf damage of forest trees and the regeneration after the extreme spring frost event in May 2011. ALLGEMEINE FORST UND JAGDZEITUNG, 183(2-Jan), 15-22.

Spatial variation in leaf damage of forest trees and the regeneration after the extreme spring frost event in May 2011. Late frost events in spring are climatic extremes with high ecological and evolutionary importance. Their significance is projected to remain constant with ongoing climate change. The late frost event in May 2011 reached local minimum temperatures of at least -10 degrees C over Germany. It happened after leaf development of all major forest trees. This event caused strong leaf damage (Figure 1) und reset the spring development of forest greening by 7-9 weeks (Figure 2, difference in 16 days maxima of MODIS NDVI; terra 16-Day L3 Global 250m; before and after the frost event; grey: difference >0 to black: difference <-250; white: no forest). Yet, clear spatial patterns in frost damage occurred, which can be explained either by meteorological differences (realized minimum temperature) or by biotic factors (phenological stages or species compositions). Studies developing fine scale models of minimum temperatures coupled with information on species compositions are proposed in order to understand the relative importance of these factors. A field survey at 91 forest stands in northeastern Bavaria indicated furthermore that frequency of frost damage varied strongly at the local scale (Table 2). In particular altitude (negative correlation), slope position (decreased frequency of damage at tops) and position relative to forest edges (increased frequency of damage at edges) were found (Figure 3). For juvenile trees, canopy cover by the adult trees played a decisive role with no frost damage in closed stands (cover >75%). Beech (Fagus sylvatica), ash (Fraxinus excelsior) and oak (Quercus petraea) showed very high frequencies of damage (Table 1). Maple species (Acer spec.), birch (Betula pendula) and conifers (with the exception of fir, Abies alba) were considerably more tolerant against late frost. Direct effects of the late frost event will probably be reflected in reduced annual growth increments. Yet, all observed individuals showed regeneration of the foliage within several weeks, even if completely defoliated days after the event (mainly ash). Indirect effects such as altered competitive balance or the predisposition against other stressors, e.g. herbivory, pathogenic attack, or drought stress over subsequent years may be more important. We conclude that frost and winter events need to be considered in forestry even in a warmer and drier (at least for European summers) world, in particular when deciding on future species compositions.



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