Invasive populations of Spiraea tomentosa (Rosaceae) are genetically diverse but decline during succession in forest habitats

Abstract

Population genetic and ecological data may help to control invasive plants, which are considered to be a major threat to natural habitats. In contrast to expected bottleneck events, genetic diversity of such invasive populations may be high due to extensive propagule pressure or admixture. The ecological impact of invasive species has been broadly evaluated in the field, however, long term-studies on the fate of invasive plants are scarce. We analysed genetic diversity and structure in invasive Spiraea tomentosa populations in eastern Germany and western Poland by using an Amplified Fragment Length Polymorphism approach. Potential hybridisation between co-occurring diploid Sp.tomentosa and tetraploid Sp. douglasii was investigated using Flow Cytometry. The genetic analyses were complemented by data from a thirteen-year vegetation study in an area invaded by both Spiraea species. We found no evidence for spontaneous hybridisation between Spiraea species. In populations of Sp.tomentosa genetic diversity (He=0.26) and genetic structure (ΦPT=0.27) were high and comparable to other outcrossing woody plants. Low levels of clonality, the presence of seedlings and of new patches in sites that had been colonised over the last 13 years imply that populations also increase and spread via sexual reproduction. In all habitat types, species diversity declined following Sp.tomentosa invasion. However, detailed aerial mapping of a forest reserve with ongoing succession revealed that Spiraea spp. populations declined over a ten year period following invasion. Despite its potential dispersal ability and its negative effects on plant communities, invasive Spiraea populations may be controlled by increasing canopy cover in forest habitats.