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The role of tree diversity for individual tree growth, crown architecture and branch demography
(2012)
In the light of the concurrent loss of biodiversity, biodiversity and ecosystem functioning (BEF) research attracted a great deal of attention and emerged as one of the important fields of research in ecology. Since important ecological interactions such as competition occur between individuals, the understanding of individual tree growth was considered to be fundamental for forest related BEF research. Individual tree growth is determined by the above- and belowground interactions of a tree individual with its local neighbourhood. To obtain a deeper understanding of BEF relationships, I broadened the focus from individual tree growth (usually measured as diameter or biomass increment) to the arrangement and dynamics of the above-ground modules of trees in dependence of their local neighbourhood. More precisely, the main objective of the present thesis was to analyse the impact of tree diversity on individual tree growth, crown architectural and branch demographic variables. Thereby I considered crown architectural variables as important indicators of the competition for light. In addition, crown architectural variables impacted ecosystem services such as erosion control. Furthermore, the results of the present thesis contributed to the current discussion on species coexistence theories, which may be differentiated by two opposing views: one that relies on neutral processes and one that implicates a role for meaningful differences in the ecological strategy (niche) of co-occurring species. The studied forest ecosystems were the subtropical broad-leaved evergreen forests of southeast China, which have been under high human pressure due to a long history of intensive land-use. The area is of particular interest for BEF research due to the high species richness of woody plants, including many, yet poorly studied species, and due to the rough terrain with steep slopes, which cause severe soil erosion. The present thesis combines three observational with two experimental studies, applying the local neighbourhood approach along an age gradient from tree saplings to mature trees. In the Gutianshan National Nature Reserve (GNNR), I conducted two observational studies on permanent plots which were chosen according to a space-for-time substitution design. The aim of the first study was to reveal the effects of diversity (species richness, functional diversity) together with other biotic and abiotic variables on morphological growth parameters (crown area, crown displacement and stem inclination) of target trees of four tree species (Castanea henryi, Castanopsis eyrei, Quercus serrata and Schima superba). In the second study, the same target trees together with their neighbours were used to analyse the relation between stand related functional diversity and the horizontal and vertical structure of the canopy. The third study was conducted in a young secondary broad-leaved evergreen forest. Using two target species (Castanopsis fargesii and Quercus fabri), the role of diversity, intra- vs. inter-specific competition and the mode of competition (symmetric vs. asymmetric) on the target individuals was tested by analysing five-year radial growth increments. The two other studies were carried out in an experimentally established plantation, using saplings of four tree species (C. henryi, Elaeocarpus decipiens, Q. serrata and S. superba), which were planted in monoculture, twoand four-species combinations and in three densities. The fourth study focused on mechanisms of coexistence and the role of species richness, species composition, species identity and density on sapling growth. The fifth study tested the effect of sapling density and identity on the througfall kinetic energy, which represents a measure for the erosive power of rain. It was found that functional diversity does affect crown architectural and canopy related parameters of forests in the GNNR. However, no effects of species richness on radial-growth were detected in the younger forest. Since I also did not find strong effects of species richness on saplings in the experimental plantation, diversity effects may evolve at a later age stage. The importance of the diversity effect may be related reversely to that of species identity in an age gradient of forest stands. The findings suggest that different mechanisms of coexistence operate simultaneously but that their relative importance may shift through the life stages of trees. During the sapling stage, species-specific differences in growth and architectural traits support niche theory. In older forest stands, no species-specific differences in growth parameters could be detected. However, I did find effects of functional diversity on horizontal canopy structure. I conclude that mechanisms of coexistence may not only change with forest stand age, but may also differ for distinct traits. The present thesis, being the first to apply the local neighbourhood approach with regard to crown architecture and branch demography within the BEF field of research, stresses the importance of this individual based approach. Although the observed forest systems are very complex, crown architectural and canopy structural variables were found to be affected by diversity. The finding that the degree of erosive power of rain could be elucidated by crown architectural variables, encourages further studies to reveal possible relations between biodiversity and other ecosystem functions or services, which might be mediated by crown architectural and canopy structural variables.
Air quality models are important tools which are utilized for a large field of application. When combined with data from observations, models can be employed to create a comprehensive estimation of the past and current distribution of pollutants in the atmosphere. Moreover, projections of future concentration changes due to changing emissions serve as an important decision basis for policymakers. For the determination of atmospheric concentrations of air pollutants by means of numerical modelling it is essential to possess a model which is able to create anthropogenic and biogenic emissions with a temporally and spatially high resolution. The emission data is needed as input for a chemistry transport model which calculates transport, deposition, and degradation of air pollutants. To evaluate the impact of changing emissions on the environment a flexible emission model with the capability to create diverse emission scenarios is needed. Further, it is important to always take into account a variety of different species to properly represent the major chemical reactions in the atmosphere (e.g. ozone chemistry, aerosol formation). Currently there are only a few high resolution emission datasets available for Europe. The amount of substances included in these datasets, however, is limited. Moreover, they can not be used as basis for the creation of new emission scenarios. To enable the creation of emission scenarios in the course of this doctoral thesis the American emission model SMOKE was adopted and modified. On the basis of a multitude of different georeferenced datasets, official statistics, and further model results the newly created emission model “SMOKE for Europe” is capable of creating hourly emission data for the European continent with a spatial resolution of up to 5x5km2. In order to demonstrate the universal applicability of the emission model the carcinogenic species benzo[a]pyrene (BaP) was exemplarily implemented into the model. BaP belongs to the group of polycyclic aromatic hydrocarbons. Because of its high toxicity the European Union introduced an annual target value of 1 ng/m3 in January 2010. SMOKE for Europe was used to create a variety of emission scenarios for the years 1980, 2000, and 2020. These emission scenarios were then used to determine the impact of emission changes on atmospheric concentrations of BaP and to identify regions which exceed the European target value. Additionally the impact of different legislation and fuel use scenarios on the projected atmospheric concentrations in 2020 was investigated. Furthermore, additional use cases for a flexible emission model are pointed out. The SMOKE for Europe model was used to simulate the transport of volcanic ash after the eruption of the Icelandic volcano Eyjafjallajokull in March 2010. By comparison of modelled concentrations for different emission scenarios with observations from remote sensing and air plane flights distribution and concentration of the volcanic ash over Europe was estimated. The results of this thesis have been presented in four scientific papers published in international peerreviewed journals. The papers are reprinted at the end of this thesis.
The world wide population growth and the increasing water scarcity endanger more and more the human society. Water saving measures alone will not be sufficient to solve all associated problems. Therefore, people in arid countries might come back to any kind of water available. In this context the way people regard wastewater must change in terms that it has to be recognized as a water resource. The reuse of wastewater, treated and untreated, for irrigation purposes in agriculture is already established in some semi-arid and arid countries. Countries with absolute water scarcity like Israel might not only be forced to reduce their water consumption, but even to transfer reused water to other sectors. Concerns of authorities and the general public about potential health risks are completely understandable. The health risks of wastewater are mainly originating from pathogens which are negatively correlated with its treatment. Therefore, the quality of a wastewater effluent derived from mechanical-biological treatment can be further improved by additional treatment steps like soil aquifer treatment (SAT). This process is adopted at the Israeli Shafdan facility in the south of Tel Aviv. Conventionally treated wastewater is applied on surface basins from where it percolates into the coastal plain aquifer which supplies approximately one quarter of Israel ́s drinking water. After a certain residence time in the subsurface the water is recovered by wells surrounding the recharge area. Although the pumping regime creates a hydraulic barrier to the pristine groundwater, concerns exist that a contamination of the surrounding drinking water wells could occur. So far, little is known about the removal of organic trace pollutants during the SAT process in general and for the Shafdan site in particular. Consequently, the need arose to study the purification power of the SAT process in terms of the removal of organic trace pollutants. For this purpose reliable wastewater tracers are essential to be able to differentiate between degradation and sorption processes on the one hand and dilution with pristine groundwater on the other hand. Based on their chemical properties, their worldwide usage in a variety of foodstuffs and beverages, and first data about the fate and occurrence of sucralose, artificial sweeteners came into the focus as promising tracer candidates.
Thus, in the present work an analytical method for the simultaneous determination of seven commonly used artificial sweeteners in different water matrices, like surface water and wastewater, was developed (see chapter 2). The method is based on the solid phase extraction (SPE) of the analytes by a styrene-divinylbenzene (SDB) copolymer material, and the analysis by liquid chromatography-electrospray ionization tandem mass-spectrometry (LC-ESI- MS/MS). The sensitivity in negative ionization mode was considerably enhanced by postcolumn addition of the alkaline modifier tris(hydroxymethyl) aminomethane. In potable water, except for aspartame and neohesperidine dihydrochalchone, absolute recoveries >75 % were obtained for all analytes under investigation, but were considerably reduced due to matrix effects in treated wastewater. The widespread distribution of the artificial sweeteners acesulfame, saccharin, cyclamate, and sucralose in the aquatic environment was proven. Concentrations in two German wastewater treatment plant (WWTP) influents ranged up to 190 μg/L for cyclamate, several tens of μg/L for acesulfame and saccharin, and about 1 μg/L for sucralose. For saccharin and cyclamate removal rates >90 % during wastewater treatment were observed, whereas acesulfame and sucralose turned out to be very persistent. As a result of high influent concentrations and low removal rates in WWTPs, acesulfame was the dominant sweetener in German surface waters with concentrations up to 2.7 μg/L. The detection of acesulfame and sucralose in recovery wells in the Shafdan SAT site in Israel in the μg/L range was a promising sign for their possible use as anthropogenic markers. As acesulfame and sucralose showed a pronounced stability in WWTPs and were detected in recovery wells of the SAT site in Israel it became worthwhile to assess their tracer suitability compared to other organic trace pollutants suggested as anthropogenic markers in the past (see chapter 3). Therefore, the prediction power of the two sweeteners was evaluated in comparison with the antiepileptic drug carbamazepine (CBZ), the X-ray contrast medium diatrizoic acid (DTA) and two benzotriazoles (1H-benzotriazole (BTZ) and its 4-methyl analogue (4TTri)). The concentrations of these compounds and their ratios were tracked from WWTPs with different treatment technologies, to recipient waters and further to river bank filtration (RBF) wells. Additionally, acesulfame and sucralose were compared with CBZ during advanced wastewater treatment by SAT in Israel. Only the persistent compounds acesulfame, sucralose, and CBZ showed stable ratios when comparing influent and effluent
concentrations of four German WWTPs with conventional wastewater treatment. However, by the additional application of powdered activated carbon in a fifth WWTP CBZ, BTZ, and 4-TTri were selectively removed resulting in a pronounced shift of the concentration ratios towards the nearly unaffected sweeteners. Results of a seven months monitoring program along the rivers Rhine and Main showed an excellent correlation between CBZ and acesulfame concentrations (r2 = 0.94), and still good values when correlating the concentrations with both benzotriazoles (r2 = 0.66 - 0.82). In RBF wells acesulfame and CBZ were again the compounds with the best concentration correlation (r2 = 0.85).
The dissertation deals with the impact of nitrogen deposition on the functioning of heathland ecosystems. Special interests were the displacement of heather (Calluna vulgaris) by the purple moor-grass (Molinia caerulea) as well as the fate of nitrogen loads in dry heathland ecosystems. The results of the studies undertaken in the field and in the greenhouse are presented as five individual journal articles. The nature of nutrient limitation was studied by means of fertilisation experiments with nitrogen (N) and phosphorus for heather and purple moor-grass (Articles I and II). The impact of nitrogen deposition on the outcome of competition between these two species was analysed during a competition experiment in the greenhouse (Article III). The aim of a 15N tracer experiment was to determine the fate of nitrogen deposition as well as allocation patterns (Article IV). In addition, the response of purple moor-grass to the combined effects of nitrogen deposition and summer droughts was investigated in a second greenhouse experiment (Article V). The fertilisation experiments showed that the growth of heather as well as of purple moor-grass is predominantly limited by N (Articles I and II). However, the results of the competition experiment demonstrated that only purple moor-grass has the ability to benefit from additional N loads, which in turn gives the grass the opportunity to displace heather (Article III). Drought treatment resulted in strikingly reduced biomass production of purple moor-grass in N-fertilised pots, mainly as a result of dying aboveground biomass during dry periods (Article V). This striking susceptibility of purple moor-grass to the combination of nitrogen deposition and drought must be taken into account, when predicting future developments of dry heathlands. The results of the 15N tracer experiment showed that the investigated heath is still in an early stage of N saturation, as indicated by a high immobilisation capacity and negligible leaching losses of 15N (Article IV). The findings of the dissertation contribute to a better understanding of the processes underlying the encroachment of purple moor-grass in dry heathlands and can enhance heathland management. The results can also be used to to evaluate the current and future status of this ecosystem particularly with regard to the various stages of N saturation as well as in the determination of “Critical Loads”.