Impact of wind-induced microsites and disturbance severity on tree regeneration patterns: Results from the first post-storm decade

In two hemiboreal mixed spruce hardwood forests in north-east Estonia, we studied (1) which factors affect tree regeneration survival and development during the first post-storm decade and (2) how these effects change in time. Regeneration height and mortality of the tree species black alder (Alnus glutinosa (L.) J. Gaertn.), birch (Betula pendula Roth., Betula pubescens Ehrh.), Norway spruce (Picea abies (L.) Karst.) and European rowan (Sorbus aucuparia L.) were analysed in moderately and heavily damaged stands, in two types of windstorm-created microsites, i.e. root-plate pits and mounds of uprooted trees, and on intact soil at different stages since disturbance. Regeneration was significantly taller in heavily damaged areas and species traits regarding tree height only became noteworthy at later stages since disturbance. Mortality probability was initially indifferent to microsite type and increased later for regeneration on intact soil compared to regeneration on the storm-induced microsites. Mortality increased with storm severity for A. glutinosa and Betula, whereas mortality of P. abies was initially low and became higher with time since disturbance in areas with increased levels of coarse woody debris. Eventually, height and height increment in previous years were clearly negatively related to mortality probability and competition levels in previous years increased chance of death. The relatively high spatial heterogeneity and trends in dominance of post-storm microsites by different tree species increase disturbance-emulating management options. In conclusion, regeneration mortality and species composition are initially directed by exogenous factors linked to storm severity and microsite heterogeneity, generating a degree of spatial partitioning within a microsite, whereas gradually species' life-history traits and competition take over. (C) 2015 Elsevier B.V. All rights reserved.