Konkurrenz und Begünstigung in kalkalpinen Rasengesellschaften

Der Versuch Muster in Pflanzengesellschaften zu erklären war lange Zeit vom Faktor Konkurrenz geprägt. Die, Ende des letzten Jahrhunderts entwickelte, Stressgradientenhypothese (SGH) berücksichtigt zur Erklärung dieser Muster zusätzlich die positiven Interaktionen zwischen Pflanzenindividuen. Sie sagt aus, dass mit zunehmendem abiotitschen und/oder biotischen Stress die positiven Interaktionen gegenüber den negativen zunehmen und an Bedeutung gewinnen. In dieser Arbeit wurde getestet, ob diese Vorhersagen auch dann zutreffen, wenn die Identität der untersuchen Arten und die Eigenschaften der Nachbarschaft entlang eines Stressgradienten konstant gehalten werden. Zusätzlich wurde überprüft, ob die Reaktionen unabhängig von den verschiedenen Lebensphasen sind. Dazu wurde ein „Neighbour-Removal-Experiment“ in zwei kalkalpinen Rasengesellschaften in den Nord-Ost Alpen durchgeführt. Der Stressgradient zwischen den beiden Standorten ergibt sich vor allem durch den vorhandenen bzw. fehlenden Schneeschutz während der Wintermonate. Adulte Individuen und Samen der drei untersuchten Arten wurden in Plots transplantiert bzw. gesät. Die oberirdische Biomasse an diesen Plots wurde entfernt. Um die Präsenz von Nachbarn zu simulieren, wurde die Hälfte aller Individuen bzw. Samenflächen von einer künstlichen Struktur umgeben, welche standardisierte Nachbareigenschaften garantieren sollte. Die laut SGH zu erwartenden Trends konnten im Allgemeinen weder anhand der Überlebensrate noch anhand der sechs erhobenen Wachstumsvariablen bestätigt werden. Zusätzlich waren die Keimungsraten für zwei der drei untersuchten Arten, auf den ersten Blick, sogar entgegengesetzt zu den Erwartungen. Die Interpretation dieses Ergebnisses ist allerdings schwierig, weil die Interaktionen verschiedener Stressgradienten das Resultat eventuell in unerwarteter Weise beeinflusst haben. Das Experiment hat jedoch deutlich gemacht, dass verschiedene Lebensphasen einer Art am selben Standort unterschiedlich auf das Entfernen von Nachbarn reagieren können. Vor allem konnte gezeigt werden, dass die sich ändernde Identität der Nachbarn, eine wichtige Rolle bei möglichen Verschiebungen der Bilanz von positiven und negativen Interaktionen entlang von Stressgradienten spielen kann.The importance of positive interactions as a structuring force of plant communities has already been demonstrated in several studies and has given rise to the stress gradient hypothesis. This hypothesis suggests that competitive effects are more important, or at least more intense, in relatively benign, low stress environments, whereas facilitative effects prevail under abiotically severe conditions. In this study we evaluated whether these predictions hold true even if the identity of the investigated species and the properties of the neighbourhood along a stress gradient are kept constant. Additionally, we assessed if different life stages show consistent interactive responses along a given stress gradient. We performed a neighbour removal experiment in two alpine plant communities of Northeastern Calcareous Alps of Austria. The stress gradient between the two locations arises mainly from the differential build-up of a protective snow cover during the winter months. Adult individuals and seeds of three different target species were transplanted or sown into plots, where the surrounding natural vegetation has been removed. To mimic the presence of neighbours, half of the individuals and seed areas were surrounded by an artificial structure which should guarantee identical neighbour properties. The expected trends were generally not confirmed for target plant survival and growth. In addition, germination rates were, at first glance, contrary to expectations for two of the three species. However, the interpretation of this finding was difficult because the interactions of different stress gradients could have influenced the outcome in unexpected ways. Our experiment has given clear evidence that different life stages of a species may react different on the removal of neighbours, even at the same location. Above all, our results have shown that the changing identity of the neighbours may play an important role for the possible shifts in the balance of positive and negative interactions along stress gradients. This suggests that severity affects the net interaction outcomes in a more indirect way, namely via selection of varied species pools

Pathways to polyploidy : indications of a female triploid bridge in the alpine species Ranunculus kuepferi (Ranunculaceae)

Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (B-III hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen > 27 mu m was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis

Niche dynamics of alien species do not differ among sexual and apomictic flowering plants

Biological invasions can be associated with shifts of the species’ climatic niches but the incidence of such shifts is under debate. The reproductive system might be a key factor controlling such shifts because it influences a species’ evolutionary flexibility. However, the link between reproductive systems and niche dynamics in plant invasions has been little studied so far. We compiled global occurrence data sets of 13 congeneric sexual and apomictic species pairs, and used principal components analysis (PCA) and kernel smoothers to compare changes in climatic niche optima, breadths and unfilling/expansion between native and alien ranges. Niche change metrics were compared between sexual and apomictic species. All 26 species showed changes in niche optima and/or breadth and 14 species significantly expanded their climatic niches. However, we found no effect of the reproductive system on niche dynamics. Instead, species with narrower native niches showed higher rates of niche expansion in the alien ranges. Our results suggest that niche shifts are frequent in plant invasions but evolutionary potential may not be of major importance for such shifts. Niche dynamics rather appear to be driven by changes of the realized niche without adaptive change of the fundamental climatic niche