The influence of forest floor changes on the success and composition of tree regeneration

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Climate change will greatly impact tree species composition and hence biodiversity and ecosystem functioning of forests. So far indirect effects such as changes in regeneration success as a result of altered substrate conditions for seed germination, modifications in nutrient availability or seed predation and subsequent shifts in the competition among tree species have received very little attention, although they may be crucial for future forest resilience and adaptive capacity. In central Europe, drivers such as increasing temperatures, increasing proportions of deciduous species and ongoing eutrophication may lead to a reduction in forest floor (FF) mass and other FF properties with unknown consequences for the regeneration of tree species. Most research on the influence of FF on tree regeneration dates back to the 1960-1970s, long before widespread P deficiency and climate change were important considerations. Our project aims to close this knowledge gap by studying the influence of changes in FF properties on the regeneration success of important tree species in temperate mixed European beech forests. Specifically, we will address the combined influence of climate change and FF changes on the regeneration success of three target species, Fagus sylvatica, Picea abies, Acer pseudoplatanus. In addition to the direct influence of these two factors, we will analyse indirect influences through changes in the availability of nutrients and water or mycorrhization, changes in biotic factors such as fungal pathogens and seed predation, and changes in the competition among tree species as well as between seedlings and other understorey vegetation. For that purpose, we will assess the germination rates as well as the mortality, growth (e. g., density, height, biomass of roots and shoots), the competitive status as well as mycorrhization and nutrition of seedlings in an experimental approach. In mesocosm experiment with soil columns from sites differing in soil P status and hence FF mass, we focus on the germination and initial establishment process. Here we will test whether decreasing FF thickness influences the susceptibility of seeds and germinants to 1) fungal infestation with changes in seasonal distribution of precipitation (wetter winters) and 2) to desiccation in dry periods. We further test whether 3) the establishment success depends on FF structure. In a field experiment, where seeds of the three species are sown at six sites contrasting in P availability mean annual temperature, we test whether decreasing FF thickness promotes 4) competition for tree seedlings from understorey and 5) increases competition for P and other nutrients between mature trees and seedlings. If successful, our project will provide an important contribution to understanding the regeneration dynamics in forest ecosystems under changing environmental conditions.