Root system structure and functions across coastal saltmarsh flooding gradients

Salt marshes are ecosystems with extreme environmental conditions such as regular tidal inundation, mechanical disturbances, salinity, waterlogged and anoxic soils, and potentially toxic compounds in the soil. This adverse abiotic environment requires a high degree of adaptation of inhabiting plants and animals and results in a highly specialized community in this ecosystem. The gradual elevation from sea level from the lowermost pioneer zone, where the establishment of salt marshes begins, to the upper salt marsh is characterized by a gradient of these abiotic factors leading to a distinct zonation of salt marsh communities. The presented PhD thesis is part of the joint research project BEFmate (Biodiversity and Ecosystem Functioning across marine and terrestrial ecosystems) with the aim to study BEF relations during the succession from marine (tidal flat) to semi-terrestrial (salt marsh) ecosystems. As part of this project, the thesis investigated different root system functions and their relation to the abiotic conditions across the salt marsh flooding gradient. It thereby contributes to the understanding of ecological processes involved in the development of salt marsh zonation as well as their sensitivity to environmental change. To fulfill these objectives, the study included in situ investigations at two natural salt marsh sites on the German North Sea coast, investigations of communities transplanted to experimental islands, as well as an additional greenhouse experiment. Under the controlled conditions of a greenhouse experiment, we investigated effects of waterlogged sediments on seedlings of three saltmarsh species characteristic for the three zones in NW European salt marshes (Salicornia europaea from the pioneer zone, Atriplex portulacoides from the lower salt marsh, Elytrigia atherica from the upper salt marsh). Seedlings of these species were grown under different sediment waterlogging treatments within mesocosms simulating tidal inundation and subsequently subjected to an erosion treatment. The upper marsh species E. atherica showed fastest root and shoot growth under drained conditions but suffered strongest in the waterlogged sediment compared to A. portulacoides and S. europaea from the lower salt marsh and pioneer zone, respectively. Resistance against erosion decreased in all three species from drained to completely waterlogged soil conditions with the strongest negative impact of waterlogging on the otherwise strongly competitive species E. atherica. We found that resistance towards erosion was strongly influenced by root growth of the seedlings. This indicates that species-specific responses of root growth of seedlings under waterlogged soil conditions may be a first determinant of the distribution of species across the saltmarsh elevational and flooding gradient. In our second study we conducted an inventory of fine root mass across the saltmarsh gradient and across two geomorphologically different sites. Fine root biomass was dependent on soil texture and plant-available nutrient concentrations in soil: we found higher fine root biomass at the sandy (and nutrient poorer) site on the back-barrier island of Spiekeroog compared to the clayey mainland salt marsh near Westerhever. According to the optimal resource partitioning theory, fewer nutrients in the soil require increased investments in fine root growth for nutrient uptake. Furthermore, the more erosion-prone sandy sediment may require increased root biomass for plant stability under mechanical disturbances. However, the most important predictor variable for fine root biomass was species diversity. At both sites, fine root mass was greatest in communities with highest plant species richness (in the intermediate lower salt marsh), potentially caused by root space partitioning in this community. With this study we could demonstrate that salt marsh plants have adapted their below-ground organs to the harsh environmental conditions which allows them to successfully develop extensive root systems and ensures survival. Furthermore, their high below-ground biomass is an important contribution to the carbon and nitrogen pools in salt marshes. In our third study we used stable isotope measurements (δ18O) to investigate plant water use patterns (i.e. the use of saline vs. freshwater) across the saltmarsh flooding gradient. We found a marked gradient in plant water use from the pioneer zone (79-98 % seawater uptake by Spartina anglica) to the lower marsh (61-95 % in A. portulacoides) and the upper marsh (25-39 % in E. atherica) reflecting the predominant inundation regime at the three elevational levels. Only minor seasonal differences in water use patterns were detected, likely due to the absence of longer dry periods during summer in these temperate salt marshes. A. portulacoides significantly increased its uptake of seawater following transplantation to lower elevations with higher inundation frequencies on experimental islands which were set up within the BEFmate project. This indicates that certain species are able to show a flexible water use strategy under changing inundation frequencies which may enable long-term adaptation to rising sea levels. Integrating the data on vertical fine root distribution and the soil depth of water uptake, we found that fine root mass distribution correlated positively with water uptake across the soil profile. However, contradicting our expectations, the physiological activity of fine roots (determined by the water uptake/root mass ratio) did not decrease with soil depth demonstrating a good functionality of roots in deep and anoxic sediment layers in this habitat. In conclusion, this thesis demonstrates that saltmarsh plants have developed various species-specific adaptations of their below-ground organs enabling their survival under the harsh environmental conditions prevailing in salt marshes. Interspecific differences particularly corresponded to the gradient of inundation frequency and associated salinity, waterlogging and mechanical disturbance in all three studies. Supporting previous studies, it was underlined that the initial establishment as well as further ecological processes in salt marshes are driven by the gradient of abiotic conditions