The influence of forest floor properties on tree regeneration under changing environmental conditions

First-year seedlings, with nascent tissues and limited nutrient reserves, are much more sensitive to environmental stressors than mature trees. Seeds and seedlings rely on the forest floor (FF) as a source of water and nutrient supply while it harbours soil microbes that are both beneficial (e.g. mycorrhizae) and detrimental (pathogenic fungi) to early seedling establishment. Therefore, FF changes, along with and as a result of climate change, may have major impacts on tree regeneration success and composition, which have received little attention in forest research so far. Our greenhouse experiment investigates how changes in FF properties affect seedling establishment and growth of three Central European tree species differing in seed size: Fagus sylvatica (European beech), Acer pseudoplatanus (sycamore), and Picea abies (Norway spruce). We used FF material and mineral soil collected from three mixed European beech forest sites in Germany differing in soil P availability. Seeds were sown in December 2023, and seedling establishment and growth were monitored under different treatments (tree mixture; precipitation regime; application of fungicide; shredding of the FF) throughout one vegetation period before harvest in autumn 2024. Preliminary results indicate that shredded, fine-textured FF enhanced substantially the establishment rate of spruce in two out of three soil origins, while no consistent pattern was observed for beech and sycamore. This suggests that intact FF may act as a physical barrier for small-seeded species but is less mechanically impactful for species of larger seed size. The application of fungicides increased the establishment success of beech, indicating a significant impact of pathogenic fungi in FF. In terms of growth, broadleaved seedlings exhibited enhanced shoot length in phosphorus-rich soil, with sycamore showing a twofold increase and beech a 1.3-fold increase compared to phosphorus-poor condition. We also found that beech seedlings grown in monoculture achieved double the shoot length of those mixed with sycamore, indicating intense interspecific competition of sycamore under abundant light condition in the greenhouse. Interestingly, while fungicide application reduced length growth of both beech and sycamore in phosphorus-poor soil, it marginally improved growth in phosphorus-rich conditions, underscoring the importance of mycorrhizal associations in nutrient-limited environments. Further analyses are ongoing to assess the susceptibility of seeds and established seedlings to fungal infestations with changes in seasonal distribution of precipitation and to drought stress in dry periods. How soil nutrient availability mediates the growth responses to such factors will also be explored. These findings will deepen the understanding of the impact of forest floor properties on tree regeneration dynamics under changing environmental conditions.