Future global change scenarios predict a dramatic loss of biodiversity for many regions in the world, potentially reducing the resistance and resilience of ecosystem functions. Once before, during Plio-Pleistocene glaciations, harsher climatic conditions in Europe as compared to North America led to a more depauperate tree flora. Here we hypothesize that this climate driven species loss has also reduced functional diversity in Europe as compared to North America. We used variation in 26 traits for 154 North American and 66 European tree species and grid-based co-occurrences derived from distribution maps to compare functional diversity patterns of the two continents. First, we identified similar regions with respect to contemporary climate in the temperate zone of North America and Europe. Second, we compared the functional diversity of both continents and for the climatically similar subregions using the functional dispersion-index (FDis) and the functional richness index (FRic). Third, we accounted in these comparisons for grid-scale differences in species richness, and, fourth, investigated the associated trait spaces using dimensionality reduction. For gymnosperms we find similar functional diversity on both continents, whereas for angiosperms functional diversity is significantly greater in Europe than in North America. These results are consistent across different scales, for climatically similar regions and considering species richness patterns. We decomposed these differences in trait space occupation into differences in functional diversity vs. differences in functional identity. We show that climate-driven species loss on a continental scale might be decoupled from or at least not linearly related to changes in functional diversity. This might be important when analyzing the effects of climate-driven biodiversity change on ecosystem functioning.

Obwohl der Bedeutung von hyporheischen Zonen als Übergangsbereiche zwischen Flüssen und angrenzenden alluvialen Aquiferen eine wachsende Anerkennung zuteilwird, sind dynamische hyporheische Reaktionen auf instationäre hydrologische Bedingungen weiterhin signifikant untererforscht. Um diese Lücke zu schließen, liegt der Fokus dieser Doktorarbeit insbesondere auf den Effekten transienter Abflussverhalten und Temperaturschwankungen in Flüssen auf die raumzeitliche Variabilität von hyporheischen Austauschprozessen. Unter Beachtung dieser Ziele wird ein neues physikalisch basiertes numerisches Modell vorgeschlagen und schließlich angewandt, um systematisch die hyporheischen, durch Sedimentoberflächenstrukturen ausgelösten Reaktionen auf eine Reihe von künstlichen und natürlichen Abflussregimen abzuschätzen. Parameter wie das räumliche Ausmaß der hyporheischen Zone, hyporheische Austauschrate, mittlere Aufenthaltszeit, Temperatur des hyporheischen Flusses sowie das Denitrifikationspotenzial werden definiert, um den Einfluss der Antriebskräfte und Regulatoren auf dynamische hyporheische Reaktionen zu quantifizieren. Die Ergebnisse zeigen, dass mit zunehmendem Abfluss generell das räumliche Ausmaß der hyporheischen Zone vergrößert wird; jedoch bestimmen geomorphologische Bedingungen und Grundwasserflüsse erheblich das Ausdehnen und Zusammenziehen hyporheischer Zonen zusammen mit Strömungen, Wärme- und Stoffaustausch zwischen Fluss und Grundwasser. Temperaturvariabilität, ein wichtiger Faktor, welcher oft in hydrodynamischen Studien vernachlässigt wird, zeigt direkte kontrollierende Effekte beim Bestimmen hyporheischer Austauschraten und mittlerer Aufenthaltszeiten. Weiterhin spielt die Dynamik von Grundwasserständen eine entscheidende Rolle bei hyporheischen Austauschprozessen. Das Optimieren der Terminierung von Grundwasserförderung ist ausschlaggebend für die Regulierung von Wasserqualität, Nährstoffkreisläufen und der Entstehung thermischer hyporheischer Refugien. / Although there is a growing recognition of the importance of hyporheic zones as transitional areas connecting rivers and adjacent alluvial aquifers, the dynamic hyporheic responses to unsteady hydrological conditions are still significantly understudied. To bridge this gap, the present PhD thesis primarily focuses on the effects of transient river discharge and temperature fluctuations on the spatiotemporal variability of hyporheic exchange processes. With these objectives in mind, a novel physically based numerical model is proposed and then applied to systematically evaluate bedform-induced hyporheic responses to a series of synthetic and natural hydrological regimes. Metrics including spatial hyporheic extent, hyporheic exchange rate, mean residence time, temperature of hyporheic flux, and denitrification potential are defined to quantify the impact of drivers and modulators of dynamic hyporheic responses. Results indicate that increasing river discharge generally enlarges the spatial hyporheic extent; however, geomorphological settings and groundwater fluxes substantially modulate the expansion and contraction of hyporheic zones along with flow, heat and solute exchange between river and groundwater. Temperature variability, an important factor which is often neglected in hydrodynamic studies, displays direct controlling effects in determining hyporheic exchange rates and mean residence times. Groundwater table dynamics also play a critical role in hyporheic exchange processes. Optimizing the timing of aquifer pumping is crucial for regulation of water quality, nutrient cycling, and the formation of thermal hyporheic refugia. The findings largely advanced our mechanistic understandings of dynamic hyporheic responses to varying transient flow and temperature conditions, and therefore shed lights on improving river management and restoration strategies.

Future global change scenarios predict a dramatic loss of biodiversity for many regions in the world, potentially reducing the resistance and resilience of ecosystem functions. Once before, during Plio-Pleistocene glaciations, harsher climatic conditions in Europe as compared to North America led to a more depauperate tree flora. Here we hypothesize that this climate driven species loss has also reduced functional diversity in Europe as compared to North America. We used variation in 26 traits for 154 North American and 66 European tree species and grid-based co-occurrences derived from distribution maps to compare functional diversity patterns of the two continents. First, we identified similar regions with respect to contemporary climate in the temperate zone of North America and Europe. Second, we compared the functional diversity of both continents and for the climatically similar subregions using the functional dispersion-index (FDis) and the functional richness index (FRic). Third, we accounted in these comparisons for grid-scale differences in species richness, and, fourth, investigated the associated trait spaces using dimensionality reduction. For gymnosperms we find similar functional diversity on both continents, whereas for angiosperms functional diversity is significantly greater in Europe than in North America. These results are consistent across different scales, for climatically similar regions and considering species richness patterns. We decomposed these differences in trait space occupation into differences in functional diversity vs. differences in functional identity. We show that climate-driven species loss on a continental scale might be decoupled from or at least not linearly related to changes in functional diversity. This might be important when analyzing the effects of climate-driven biodiversity change on ecosystem functioning.