Ecosystems provide important functioning and services, like biomass for bioenergy production or storage of atmospheric carbon. Two examples of such ecosystems are temperate grasslands and tropical forests. Both vegetation are rich of various species, whereby each of the respective ecosystem benefits from its species-richness concerning their functioning, i.e. productivity. In this thesis both vegetation are in the focus of the investigations. In the first chapter, a review of existing grassland and vegetation models provides an overview of important aspects, which have to be considered for modelling temperate grasslands in the context of biomass production. Based on the review, new conceptual modelling approaches for temperate grasslands are proposed. In the third chapter, derived from the suggested concept, the process-oriented and individual-based grassland model Grassmind is presented. In the fourth chapter, the model Grassmind is used in order to parameterize and simulate the annual dynamics of a typical Central European grass species. Grassmind is able to reproduce the structure and dynamics of a temperate grass species. With reference to the parameterized grass species, a simulation study using defined species groups is performed in order to investigate on the effect of the richness of species groups on aboveground productivity. We do not observe a significant positive effect of species group richness on productivity, which is explained by limitations of using the parameterized grass species as a reference. In the fifth chapter, comprehensive investigations are carried out on the example of stem size distributions in forests concerning their statistical analyses, i.e. by using maximum likelihood estimation. The effects of uncertainties, i.e. binning of measured stem sizes or random measurement errors, are examined in detail. Uncertainties bias the analyses of maximum likelihood estimations. It is shown, that the use of modified likelihood functions, which include either binning or measurement errors, reduce these biases to a large extent. For both studies, i.e. modelling of temperate grasslands and analysing stem size distributions of forests, the presented investigations are discussed and possible examinations are suggested for future research in the last chapter.

Pannes are rare intradunal wetlands. Though small, they are known to exhibit extremely diverse and sensitive vegetation and are home to a number of reptile and amphibian species. In the United States, pannes are known to occur only around the Great Lakes Basin and Cape Cod. At Indiana Dunes National Lakeshore, the fifteen known pannes have an unusually large variety of plant species for such a small geographic area and provide habitat for plant species found nowhere else in Indiana. However, these sensitive ecosystems have been exposed to over a century of atmospheric pollutants from the surrounding steel and coal industries. Since 1986, the native vegetation of the area is slowly being replaced by invasive species like Phragmites australis and Typha spp. This study attempts to explain the shift in vegetation. Pannes in two other locations, at a distance from the industrial complex, were used as control sites as they were not expected to be exposed to the same levels of heavy metal concentrations. Four of the fifteen pannes at the Indiana Dunes National Lakeshore, two of the four pannes at Sleeping Bear Dunes National Lakeshore, Michigan, and two of the three pannes at Warren Dunes State Park, Michigan, were studied, resulting in a total of eight pannes. The pannnes were stratified and sampled by hydroperiod. Surface soil samples and sediments at depth, were recovered from each of the pannes considered in this study and analyzed for heavy metal, phosphorus, carbon and nitrogen content. Results show that high levels of organic matter coupled with high nutrients and high metals, in the soil, are a combination that may be considered a risk factor for future invasion of pannes by invasive species. It appears to be difficult for the native vegetation to deal with the high metals and high nutrients which are deleterious to the native vegetation and facilitate establishment of invasive vegetation which is more tolerant to the altered geochemical conditions.

Forested watersheds in western North America are subject to significant change from natural and anthropogenic disturbance, including wildfire. Forest canopy changes have subsequent impacts on sub-canopy snow processes. A simple, process-based point energy balance model was developed to quantify differences in energy balance characteristics between a burned and a healthy forest stand. Potential model uncertainties were identified using sensitivity analyses. Simulated snowmelt accurately recreated measured snowmelt, providing confidence in the model’s ability to simulate energy balance processes in subcanopy environments where wind redistribution and sublimation are not major drivers of the local snowmelt energy balance. In the burned stand, sub-canopy snow accumulation was greater but melted more rapidly than in the healthy stand. The removal of forest canopy resulted in more energy available for snowmelt, including higher short-wave and lower long-wave radiation, and increased turbulent fluxes. Burned stands should be considered a separate land cover type in larger scale watershed models. / xii, 129 leaves : ill,, map ; 29 cm