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

气候变化将极大地影响树种组成，从而影响森林的生物多样性和生态系统功能。迄今为止，诸如由于种子萌发的基质条件改变、养分有效性或种子捕食的改变以及随后树种之间竞争的变化而导致的更新成功的变化等间接影响很少受到关注，尽管它们可能对未来森林的恢复力和适应能力至关重要。在中欧，温度升高、落叶物种比例增加和持续的富营养化等驱动因素可能导致森林地面（FF）质量和其他FF特性的减少，对树种更新的影响未知。大多数关于FF对树木更新影响的研究可以追溯到20世纪60- 70年代，当时广泛的缺磷和气候变化还不是重要的考虑因素。我们的项目旨在通过研究FF特性变化对温带欧洲山毛榉混交林中重要树种更新成功的影响来缩小这一知识差距。具体而言，我们将研究气候变化和FF变化对Fagus sylvatica、Picea abies和pseudoplatanus 3种目标树种更新成功的综合影响。除了这两个因素的直接影响外，我们还将分析间接影响，包括养分和水分或菌根化的变化，真菌病原体和种子捕食等生物因素的变化，树种之间以及幼苗与其他下层植被之间竞争的变化。为此，我们将以实验方法评估种子的发芽率、死亡率、生长（如密度、高度、根和芽的生物量）、竞争状况以及菌根化和幼苗营养。在不同土壤磷含量和FF质量的土壤柱的中观实验中，我们重点研究了萌发和初始建立过程。在这里，我们将测试减少FF厚度是否会影响种子和发芽物对真菌侵染的敏感性（1)随降水季节分布的变化（冬季较湿润）和2）在干旱时期对干燥的敏感性。我们进一步检验3)FF结构是否决定了企业的成功与否。在田间试验中，我们将三种树种的种子播种在6个不同磷有效度平均年温度的地点，测试减少FF厚度是否会促进4)林下对树木幼苗的竞争，5)增加成熟树木和幼苗对P和其他营养物质的竞争。如果成功，我们的项目将为了解变化环境条件下森林生态系统的更新动态提供重要贡献。

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.