Germination, Establishment and Distribution of Hardwood Floodplain Forest Species

Abstract

Hardwood floodplain forests are among the most structure- and species-rich ecosystems in Central Europe. Due to anthropogenic changes, they have been drastically reduced and belong nowadays to the most endangered ecosystems. Therefore, the restoration of floodplain forests is a central goal of alluvial restoration projects, but at the same time a difficult task due to the complex environmental conditions. Since flooding tolerance can be regarded as key factor for successful establishment of plant species in floodplains, this thesis deals with flooding tolerance of different hardwood floodplain forest species. Further, this thesis combines experimental studies under controlled hydrological conditions with field experiments. The latter where conducted in one of the largest alluvial restoration areas in Germany – the dike relocation area “Lenzen-Wustrow”. In general, the results of my thesis showed that an increasing flooding duration negatively affects plant performance. Further, after second flooding in the following year, similar flooding tolerance patterns were observed. However, most species were able to recover better after the second flooding compared to the first one, probably also because the individuals were one year older at this point. Nevertheless, a previous flooding experience, regardless of its duration, showed no changes in the flooding tolerance of the saplings, indicating that there is no flooding stress memory in the investigated species. Furthermore, I revealed that for the classification of flooding tolerance, it is highly important to include a recovery period. By this, misjudgments of flooding tolerance patterns of species can be avoided, as plants can either recover after flooding or suffer even more. In general, the species-specific differences in flooding tolerance could be explained by their ability to react to the resulting flooding stress by using morphological, physiological and metabolic adaptations. Species that are able to quickly generate different structures in a large number will cope better even with long periods of flooding. Although the assessment of the flooding tolerance of different species depending on flooding duration is useful in the context of restoration planning processes, it must be considered with caution as the incorporate variations in site conditions, hydrological parameters and ecological requirements are not included. The field study showed that processes in floodplains are highly dynamic and that long flooding as well as long drought periods must be considered as separate extreme events. Both are unfavorable during early establishment, even though, the mean annual number of flooding days was within the typical growth range of the hardwood floodplain forest zone. Further, under field conditions, additionally to the hydrological regime, many other factors can influence the establishment of trees, such as herbivory, which caused a high mortality in the study area. As comparable studies in restored floodplains do not exist until now and I could demonstrate how necessary long-term study periods are, it is important that more studies and assessments of establishment success are carried out in the field, covering longer study periods and also take into account the interaction of the various influencing factors more precisely. Only then, it will be possible to provide better predictions and possible solutions for future restoration measures.