Tree seedling mortality in wind-induced microsites in spruce-hardwood mixed forests, Estonia

Background/Question/Methods

Storm events increase heterogeneity in the range of potential tree establishment sites. This heterogeneity does not, however, guarantee survival. Many studies on post-disturbance stand development are limited to only the first few years after disturbance. In this investigation, we analyse regeneration height and mortality 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 to determine which factors are most important for survival and performance after the first post-storm decade. We expect that growth and mortality are initially directed by endogenous factors, generating a degree of spatial partitioning within a microsite, whereas gradually species’ life-history traits and competition take over. Comparison of logistic mixed effect models identified the most important factors for mortality probability 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.) at different stages since disturbance.

Results/Conclusions

Regeneration was significantly taller in heavily damaged areas and species traits only became significant at later stages since disturbance. Mortality probability is indifferent to microsite type and increases with storm severity for A. glutinosa and Betula, whereas P. abies initially benefits from increased levels of coarse woody debris. Generally, pits from P. abies as the uprooted tree species enhance survival probability of all regenerating species. Subsequently, height and height increment in previous years are more clearly negatively related to mortality probability and competition levels in previous years increase chance of death. Although regeneration height, density and mortality levels were not considerably different between and within microsites, each site was dominated by a different species community. This study indicates that it is important to distinguish disturbance severities and timing of a study when evaluating post-storm impact on forest succession.