title: Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?
abstract: OBJECTIVE
  The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions.
  
  
  METHODS
  Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified.
  
  
  RESULTS
  For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics.
  
  
  CONCLUSIONS
  The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.
authors.affiliation: Department of Biological Sciences, Plant Biology, Aarhus University, DK-8000 Århus C, Denmark. tenna.riis@biology.au.dk
authors.forename: Tenna
authors.initials: T
authors.lastname: Riis
authors.forename: Carla
authors.initials: C
authors.lastname: Lambertini
authors.forename: Birgit
authors.initials: B
authors.lastname: Olesen
authors.forename: John S
authors.initials: JS
authors.lastname: Clayton
authors.forename: Hans
authors.initials: H
authors.lastname: Brix
authors.forename: Brian K
authors.initials: BK
authors.lastname: Sorrell
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics / Q000502:physiology; D000704:Analysis of Variance; D014644:Genetic Variation / Q000235:genetics; D031321:Hydrocharitaceae / Q000145:classification / Q000235:genetics / Q000502:physiology; D019684:Magnoliopsida / Q000145:classification / Q000235:genetics* / Q000502:physiology*; D009520:New Zealand
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