Year of publication
- 2009 (3) (remove)
- English (3) (remove)
- Carbon dynamics under natural and manipulated meteorological boundary conditions in a forest and a fen ecosystem (2009)
- Current climate models predict changes in the amount, intensity, frequency and type of precipitation within this century. These changes are likely to result in an increasing frequency of severe drought periods in summer, causing irregular and extreme drought stress in well-drained soils or a lowering of the water table in water-logged soils. Due to rising temperatures precipitation is more likely to occur as rain rather than snow, resulting in reduced snowpacks in winter. In some regions, this can lead to an increasing frequency of soil frost. In summary, changes in the global water cycle can significantly affect boundary conditions within soils. This thesis investigated the impact of extreme meteorological boundary conditions on CO2 fluxes in two ecosystems in South-eastern Germany. Using a combination of field site manipulation and laboratory experiments we investigated the effects of prolonged summer drought and soil frost on soil C dynamics in a Norway spruce forest. In a minerotrophic fen located nearby, the effect of water table lowering (as a result of summer drought) on ecosystem C dynamics was quantified. Additionally, soil C dynamics at both sites were modeled under current meteorological conditions. For the Norway spruce forest, modeling indicated that soil C turnover predominantly occurred within the organic horizons. During the last decades, the soil has acted as a small sink. The possibility of altered C dynamics at the site due to undocumented liming has to be considered when comparing results presented here to results from other sites. For the fen, modeling revealed that soil C turnover was dominated by processes occurring within the uppermost 15 cm of the peat and that root biomass was a very important soil C stock. Most important, modeling indicated that the fen was turned into a net C source during the last decades, presumably because of disturbance of the hydrological conditions. Results from this fen cannot be regarded as representative for undisturbed peatlands. Soil frost was induced at the forest site by removing the snowpack in the winter of 2005/2006. On the snow removal plots, soil frost occurred down to a depth of at least 15 cm and for several weeks, in contrast to the snow-covered control plots where no soil frost occurred. Soil C losses were significantly reduced not only during the soil frost period but also in the summer of 2006. This phenomenon could be explained by changes in the composition of the microbial community due to soil frost, primarily a reduction of fungal biomass. To investigate the effect of drought on soil C dynamics we experimentally induced prolonged drought at the forest-site by excluding throughfall with a transparent roof during the summers of 2006-2008. Additionally, undisturbed soil columns from the site were subjected to drought in the laboratory. In both experiments, drought reduced total soil C losses in comparison to C losses from a control. This reduction was mainly owed to decreased soil respiration rates during the actual drought period, but water repellency also hindered rewetting of the dry soil, thus further prolonging the period of reduced soil respiration rates. In the past, mobilization of stabilized C due to drying-wetting has been repeatedly discussed as a possibility to actually enhance soil C losses. In the studies presented here, no evidence for this assumption was found. Soil C mineralization rates were reduced during drought and recovery was slow, possibly delayed by water repellency and preferential flow. At the fen site we used two approaches: (i) Experimental lowering of water tables to measure resulting C fluxes in comparison to C fluxes under natural conditions (i.e. control plots), and (ii) repeated measurements under varying natural conditions to be able to later statistically identify the main drivers of CO2 fluxes. We included measurements of C uptake and respiration by aboveground vegetation, thus being able to study ecosystem rather than soil C dynamics at the fen site. In summary, the impact of the water table on CO2 fluxes in and out of the fen ecosystem was minimal. Soil respiration was not affected at all by the manipulative lowering of the water table from ca. 15 cm down to more than 60 cm, most likely due to low substrate quality in deeper peat. Measurements of the natural C dynamics indicate that water table could have an impact on soil respiration within the uppermost 0-15 cm of the soil, but predominantly low water tables during summer under current boundary conditions make it unlikely that further lowered water tables due to climate change will markedly affect soil respiration rates at this site. In summary, CO2 fluxes at the site are presumably very resilient towards an increasing frequency of summer drought resulting in lowering of the water table.
- Vegetation ecology of springs: ecological, spatial and temporal patterns (2009)
- Acidification is a phenomenon, which affected the forested catchments of the northern hemisphere severely over recent decades. Acidic depositions depleted the buffering capacities of soil and groundwater, what lead to an impairment of forests, headwaters, and lakes. Even though the depositions were reduced considerably since the early 1990s, the recovery of catchments was found to occur time-delayed. The grade of recovery was found to vary significantly between regions. Biomonitoring is an appropriate tool to detect spatial and temporal patterns of ecosystem alterations, such as acidification and recovery. However, to know the interrelationships between organisms and their environment is an indispensable precondition for the identification of indicator species. The complexity of ecosystems and ecological processes hampers this quest oftentimes. Springs provide a natural setting that minimises such constraints. Compared to other habitat types, external factors are less relevant, which makes it easier to relate changes in species abundances to changes in their environment. Studying this species-environment relationship, here the response of plant species to the acidification of the spring waters was of particular interest. In a survey of five regions in Central Europe - taking spatial, hydrophysical as well as hydrochemical parameters of the springs into account - it was clearly shown that the species composition of springs is essentially determined by the spring water chemistry, and more precisely by the gradient of acidity and nutrient availability. This connection was reflected by spatial patterns within and between the regions. These patterns provide useful ecological information about spring water quality and in return about the acidity status of their forested catchments. Including catchment traits - like bedrock, climatic parameters, and forest vegetation - in the analyses, these emerged to be relevant for the species composition of springs, but less than the spring water chemistry. A path analysis showed that the catchments affect the vegetation of springs not directly, but indirectly via the determination of spring water quality. Hence, the catchments are a part of the functional chain, which is driven by the atmospheric depositions. The pH-value was found to represent the gradient of acidity and nutrient availability best. It can serve as a proxy measure that can be related to species occurrence and to species dynamics respectively, aiming to identify indicator species for assessing the status and alterations of spring water quality. With the aim to delineate niche optima and amplitudes, which in return can serve as indicator values, the realised niches of spring-inhabiting species were modelled with respect to pH. The niche attributes were found to be a matter of sampling scale. Larger plot sizes (grain) weakened the species-environment relationship, what consequently resulted in broader niche amplitudes. In contrast, the grain did not influence the species’ pH optima. Monitoring approaches that target to assess processes in time, such as acidification and recovery, are dependent on the response time of indicator species to changes in their environment. Investigating an interval of four consecutive years, inter-annual variability of the species composition could not be attributed to changes in the acidity of the spring waters. Looking at single species, bryophytes did not show a higher sensitivity to the inter-annual variability of the environment than vascular plants. Actually, only a minority of all species featured abundance changes which were significantly correlated to variations in spring water acidity. Our results suggest that the species inertia retards the vegetation dynamics of forest springs. A delayed or long-term integrating response of potential indicator species must be considered when evaluating their indicator suitability. In conclusion, the biomonitoring of spring water acidification or recovery is expedient only for longer time intervals. In a nutshell, the vegetation of springs is closely related to the hydrochemical traits of the spring waters, in particular to a gradient of acidity and nutrient availability. Individual species as well as whole plant communities are suitable indicators which allow for the monitoring of the acidity status of forested catchments. The results of this study contribute to a better understanding of the species-environment-relationships, and in return to an improvement of indicator systems.
- Analysis of flow patterns and flow mechanisms in soils (2009)
- Matrix flow and preferential flow can occur concurrently in the same soil. Both flow regimes produce typical flow patterns that can be visualised in dye tracer experiments. To extract quantitative information from dye tracer studies a vast variability of approaches exists. One of them is to describe dye patterns by the so called dye coverage function, i.e. the percentage of stained area per soil depth. Based on extreme value statistics the dye coverage function can be reinterpreted as a probability function to find the tracer in a certain depth. Therefore, the two-parametric probability distribution 1 – H, H being the generalised Pareto distribution, can be fitted to the dye coverage function. The form parameter of this distribution serves as a risk index for vertical solute propagation. We did tracer experiments with Brilliant Blue FCF at three study sites: in a Norway spruce forest in southeast Germany, in a tropical mountain rainforest in southern Ecuador and on an agricultural field in southern France. We tested the ability of the risk index to summarise main information obtained in dye tracer studies and characterise flow patterns in different soils. Our results suggest that the risk index is to some degree invariant to changing experimental conditions (such as irrigation rate). The initial soil moisture, however, seems to have a large influence on the risk index. It is difficult to adjust the parameters of the generalised Pareto distribution when the dye coverage function fluctuates or does not decrease monotonically. This might be due to tortuosity of paths, varying flow mechanism or changing soil physical properties. Thus, we restricted the analysis to the lowest part of the profile. Since the theory of the risk index is based on extreme values of vertical solute propagation it is the lowest part of the profile that is the most interesting. We propose to combine the two parameters of the generalized Pareto distribution and to use the complete distribution 1 - H to estimate the risk of vertical solute propagation in soils. Despite a certain resistance to changes of experimental conditions, the risk index is not an intrinsic soil parameter. Since the flow regime in the same soil can be dominated either by preferential flow or by uniform matrix flow, the risk of vertical solute propagation will change. The adjusted parameters of the generalised Pareto distribution will capture the dominant flow regime as reflected by tracer flow patterns. Bearing in mind the boundary conditions of the tracer experiment like irrigation rate, the tracer employed, soil initial moisture or type of vegetation (permanent or seasonal, deep rooted or shallow rooted) it is possible to compare different study sites or to consider the same site at different boundary conditions and to access the risk of vertical solute propagation. Pattern analysis based on the risk index for vertical solute propagation revealed the occurrence of preferential flow at the German study site. To gain insight in flow mechanisms and possible impacts on soil chemistry we analysed soil texture, fine root density, soil bulk density, exchangeable cations, pH and total C and N contents in preferential flow paths and soil matrix. Results from linear mixed-effects models suggested that at this study site roots constituted main preferential flow paths and induced macropore flow, especially in the topsoil. In the subsoil root density decreased and inhomogeneous infiltration from preferential flow paths into the soil matrix caused non-uniform flow. There were no textural differences between the flow domains, but smaller bulk densities in preferential flow paths. This is probably due to a higher soil organic matter content in preferential flow paths. We found smaller pH values, more Ca, more Mg, more C and more N in preferential flow paths. Compared to the adjacent soil matrix, more Al and more Fe (but small absolute amounts) were found in the subsoil where macropore flow along root channels decreases and heterogeneous matrix flow dominates. These distinct chemical properties can be explained by root activity and translocation of solutes and DOC (dissolved organic carbon) via preferential flow paths. During transport along preferential flow paths contact time between DOC and soil is reduced so that DOC is transported to greater depth where it potentially forms organo-mineral associations. If this holds true, preferential flow is a mechanism that promotes C sequestration in subsoil and does not only influence its immediate environment around paths, but also underlying subsoil horizons.