Dokument

Summary:
Species of the Salicaceae family, particularly Salix species and their hybrids, are typical invaders of river systems throughout the world with severe consequences for native ecosystems. Along Patagonian streams, riparian softwood forests structured by dominant invasive Salicaceae are increasing significantly in abundance, area and species diversity. The region of the Río Negro in northern Patagonia has been invaded by several woody plant species with a dramatical increase in the recent decades. The most dominant taxa are willows of the Salix alba L. - Salix fragilis L. complex, a hybrid of this complex and Salix babylonica L. and Populus spp. Additionally, Elaeagnus angustifolia Willd. and Tamarix spp. have been spreading aggressively in recent years. This thesis provides information on the possible consequences of these invasions for Salix humboldtiana Willd., the only native woody species along the Río Negro, due to interspecific competition and invasion development. Vegetative reproduction is especially for S. fragilis and its hybrids of great importance for the colonisation of new habitats and probably is the most important reason for their invasion success. In the first study (Chapter 2), a greenhouse experiment was conducted in order to assess the vegetative reproduction capacities of native and invasive Salicaceae (S. humboldtiana, S. × rubens Schrank, a S. babylonica hybrid and Populus spec.) under various soil composition and moisture gradients. The invasive willow hybrids showed better vegetative re-sprouting capacities and performed sinificantly better than S. humboldtiana and Populus spec. They developed more living cuttings as well as more and longer sprouts under all treatment combinations and they generated significantly higher above- and belowground biomass. After completely removing the shoot and root biomass of the cuttings re-sprouting capacities were evaluated in a second experiment. Again, the invasive willows had a superior re-sprouting ability and growth performance. These results demonstrate that invasive willows are able to establish more successfully by vegetative reproduction than the native willow and that they have a higher re-sprouting potential after disturbances. This ability in combination with the advantages of vegetative reproduction compared to generative reproduction seems to be the key factor for invasive success and, simultaneously, a potential threat for S. humboldtiana through out-competition. In a further approach (Chapter 3), age structure analyses were carried out which revealed that mixed adult forest stands are the results of joint establishment events with all Salicaceae taxa involved. The analysed 20 stands on islands had low mean ages <15 years. Investigation on growth performance (basal area, crown diameter, tree hight and crown base hight) of adult stands revealed that non-native willows and poplars are able to actively displace the native S. humboldtiana by competitive strength: A significant better growth performance with e.g. higher basal area and crown diameter was found for the non-native willows in adult life stage. Furthermore, adult S. humboldtiana was more frequent at the middle river stretch, while invasive willow hybrids showed an opposing pattern suggesting a downstream directed invasion process. In order to evaluate the competition potential of invasive alien woody plants (Salicaceae, Elaeagnus and Tamarix) for S. humboldtiana, habitat distribution models for different life stages (seedling, juvenile, adult) were developed (Chapter 4). The objective was to identify ecological niches and relationships between species occurrence and different abiotic factors. Data on species (presence/ absence) and explanatory environmental variables were gathered in the field on 167 plots using a grid-based, stratified-randomized sampling design. The environmental variables flood duration, the amount of gravel and the location (upper or middle river valley) proved to be the best explanatory variables to describe the occurrence of the species. For all life stages, a strong niche overlap could be observed for S. humboldtiana and invasive taxa, particularly Salicaceae, with no remaining exclusive habitat for the native willow. Additionally, investigations on vegetation change offer insights into invasion development (Chapter 5). Multispectral satellite images from the years 1986 and 2003 of the upper Río Negro were used to detect land cover changes. This study demonstrated that during this time period floodplain forests expanded considerably their distribution area by almost 150%, probably due to the explosive spread of non-native species. The results of this thesis can be summarized as follows: Invasive willows have significant better vegetative reproduction and re-sprouting capacities, better growth performances in the adult life stage and exhibit a strong niche overlap with the native willow species. These findings indicate a high competition potential of invasive Salicaceae with, consequently, a possible out-competition and suppression of S. humboldtiana.

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