The role of successional stage, vegetation type and soil disturbance in the invasion of the alien grass Cortaderia selloana

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Abstract
Question: This paper studies the establishment and performance of Cortaderia selloana (Pampas grass), an alien South American ornamental species that is invading many parts of the world. We asked whether (1) early successional stages were the most susceptible to C. selloana invasion; (2) soil microdisturbances increased invasion at any point of succession, and (3) C. selloana invasion of later successional stages was modulated by vegetation type Location: Delta del Llobregat (Catalonia, NE Spain). Methods: We monitored survival and growth of transplanted C. selloana seedlings in disturbed and non-disturbed plots throughout a successional gradient with an age range of < 1 to > 10 years in different vegetation types and within the area of influence of coexisting species with similar growth form. Results: Although seedling survival was extremely low in all treatments, our results revealed that early successional stages were not the most easily invaded since we found no significant differences in the percentage survival of C. selloana along the successional gradient. However, survival and seedling biomass were enhanced by soil disturbance at any seral stage. This result suggested that inhibition ruled C. selloana invasion. Invasibility neither depended on the invaded vegetation type nor on the co-existing species with similar growth form. Finally, C. selloana invasion was not enhanced by decreasing competition with Phragntites australis, a native coexisting species because survival rates after a year were not significantly different. However, Phragmites increased C. selloana leaf length probably due to shading. Conclusions: C. selloana recruitment appears to be positively affected by soil disturbance but it is independent of successional stage or vegetation type.