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Université de Bielefeld (2005)

Schlüsselfaktoren der Invasivität von Acacia longifolia : experimentelle Untersuchungen im Vergleich zu heimischen Arten mediterraner Sandökosysteme

Peperkorn, Ralf

Titre : Schlüsselfaktoren der Invasivität von Acacia longifolia : experimentelle Untersuchungen im Vergleich zu heimischen Arten mediterraner Sandökosysteme

Key factors of invasiveness of Acacia longifolia : experimental comparison with native species from Mediterranean sand ecosystems

Auteur : Peperkorn, Ralf

Université de soutenance : Université de Bielefeld (Allemagne)

Grade : Doktorgrades der Universität Bielefeld 2005

Résumé
During the past decades invasion of alien species became an important research focus in ecology and population biology. Exotic species cause serious changes in natural ecosystems, having as a consequence enormous ecological and economical impacts. Even knowing that no key factors for the invasive success of alien species could be identified up to now, some common traits have been characterised : e.g. higher phenotypic plasticity to abiotic environmental factors, higher nutrient uptake rates and higher reproductive rates. In the middle of the 20th century 13 Acacia species including Acacia longifolia were introduced into the sand dune ecosystems of the Portuguese Atlantic coast for dune stabilisation and ornamental purposes. In our days it became clear that Acacia longifolia became invasive and shows a clear dominance in the habitats. Particularly at sites that have been disturbed by fire and/or man action through tourism pressure. Along the North-South gradient of the Portuguese coast, which is associated with a clear gradient in climate (precipitation, temperature), edaphic and anthropogenic factors (e.g. disturbance, fire frequency), a decreasing invasive pressure of A. longifolia was observed. This fact the questions how plastic are the responses of A. longifolia to biotic (competition) and abiotic factors (resource availability) compared to native species (Halimium halimifolium and Pinus pinea) and what are the possible consequences for the invasive power of A. longifolia. The present work examines these questions with the following approaches : (1) A seedling experiment to compare the plasticity of the invasive and the indigenous species along a nutrient gradient and at different light treatments. (2) A drought experiment to compare the adaptation and plasticity to successive water stress through carbon isotopes discrimination. (3) A competition experiment to evaluate the specific competition ability both short and longterm using 15N tracing and nitrogen uptake rates. One major result is the similar plasticity, quantified according to the phenotypic plasticity index (Valladares 2000), of both, the invasive and native species, which was independent of the investigated parameters. Contrasting to numerous investigations on invasive species, this result excluded phenotypic plasticity as one main cause for the invasiveness of A. longifolia in sand dunes. It could further be shown, that the growth rate (above- and belowground) of A. longifolia was significantly higher compared to native species. The advantage of high growth rate was consistent over all treatments and was enhanced (aboveground) under increasing nutrient availability. Since this trait was independent of growth conditions, it can be expected to be important for the invasive success of A. longifolia. The results from 15N-tracer experiment indicated, that A. longifolia is able to take up nutrients more efficiently because of a higher root biomass and a higher N uptake rate of ammonia and nitrate, as a nitrogen source. Additionally, A. longifolia was able to use ammonia and nitrate equally efficient. Together with its capability for N2-fixation, which enables its high biomass production and turnover, A. longifolia seems to be well adapted to the nitrogen limited conditions of the Mediterranean coastal dunes. The nitrogen status of the ecosystem can be enriched by the high amount of nitrogen-rich litter of A. longifolia (e.g. Witkowski 1991). This accumulation in turn promotes Acacia more than the native flora and supports establishment of the invasive species. Results from the experiments indicate established A. longifolia plants as being tolerant against competition and as good competitors towards less competitive species. Consequently this leads to a stabilisation of the Acacia population and helps to replace native species. On the other hand, seedlings during establishment exhibited a less high competitiveness. However, after disturbance and/or "nursery plants" the competitive characteristics would help A. longifolia to dominate the ecosystem. A. longifolia exhibited a lower tolerance to water deficits as compared to the native species. Dry environmental conditions force it to reduce photosynthesis and, at very negative water potentials, to reduce a high quantity of its photosynthetic area by leaf shedding. The native species were more efficient to minimise water loss and keep assimilation rate on a relatively high level. Under suboptimal conditions, A. longifolia once introduced into an ecosystem, can persist at the location but is not able to dominate the more tolerant native species.

Mots clés : Acacia longifolia, Invasiv, Isotope, Mediterran, Konkurrenz, Plants (Botany)

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Page publiée le 24 mars 2009, mise à jour le 6 janvier 2019