Invasibility of boreal wetland plant communities

Question: How does germination and establishment of non-resident plant species differ among major types of wetland ecosystems in boreal forest landscapes? Location: A 250-km(2) large boreal forest landscape in northern Sweden. Methods: We performed a germination and establishment experiment to test for differences between three major wetland types: riparian zones, open wetlands and forested wetlands. These wetland types differ ecologically, hydrologically and in their distribution in the landscape. Six species of vascular plant native to the region but absent or rare in one or more of the three wetland types were sown separately in plots with two different levels of disturbance (complete vegetation removal and control) and were monitored for 3 yr. For two species, seed-sowing density was varied to test for effects of propagule pressure. Results: All six species were able to germinate and survive in all habitats and disturbance levels, suggesting that all three wetland types are invasible. There were positive correlations between germination or survival and species richness in resident vegetation for four out of six species, i.e. species-rich sites were more invasible. The germination frequency did not vary with seed-sowing density, indicating that density-dependent effects were small. All species had higher survival in their resident habitat, while the effect of disturbance was small. Conclusions: The results suggest that the low levels of plant invasion observed in boreal wetlands are better explained by low propagule pressure than high resistance to invasion. However, the habitat dependency of survival implies that population establishment is only expected in habitats to which the species are adapted. Nevertheless, levels of invasion might increase in the future, given that more species are being introduced as a result of increases in transport and trade.