Carbon limitation and aluminium toxicity prevents dominance of Crassula helmsii on weakly buffered soils

Invasions of the aquatic weed Crassula helmsii pose a serious threat to native vegetation of soft water lakes, which increases the need for understanding the factors that regulate the success or failure of C. helmsii. Creating favorable conditions for native species seems promising to control C. helmsii, as previous research indicated that C. helmsii is a poor (root) competitor. We studied the development of C. helmsii growing together with the native Littorella uniflora (biomass, cover, tissue nutrient composition) at two different calcareous poor soil types (organic/acid, mineral/buffered) and under two different water tables (not inundated, submerged) in a greenhouse experiment. We found that when growing under submerged conditions, C. helmsii coexisted with L. uniflora and other native species without becoming dominant due to carbon limitation in the water layer. In contrast, we found that C. helmsii can easily become dominant over L. uniflora when growing on desiccated buffered fen soils with moderate nutrient availability. On the acidic/organic soils, C. helmsii development was poor and die-off was observed under both water level treatments, probably induced due to aluminum toxicity under a low acidity. These results indicate that creating oligotrophic and carbon-poor conditions are required for controlling C. helmsii. Restoration measures can preferably be taken before the onset of the rainy season and/or be followed by maintenance of a temporal artificial high water table to stimulate carbon limitation. Liming more or less organic shores could alleviate aluminum toxicity under acidic conditions which can potentially stimulate the development of C. helmsii.