Seed germination of the invasive shrub Mimosa pigra is negatively impacted by water level and grass shading

Background/Question/Methods

Invasive species are changing ecosystems worldwide and their effects are particularly evident on islands.  Tropical wetlands are often highly susceptible to invasion due to their long history of human-caused disturbance. Mimosa pigra is becoming a major invader of both wetlands and agricultural areas in Puerto Rico; but no previous work has focused on this species on the island.  The Laguna Cartagena National Wildlife Refuge in southwest Puerto Rico contains one of the most important freshwater wetlands on the island. M. pigra is rapidly invading the site so information on how environmental variables (flooding and shade from grasses) affect seed dynamics may have management implications.  We conducted a seed burial experiment in the greenhouse to determine the effects of water depth and shade on seed survival and germination. Seeds were collected from Laguna Cartagena in November 2012 and equal numbers of scarified and non-scarified seeds were sown into replicate trays. Each tray received one of three water treatments (moist soil, minimal flooding, complete flooding). Shade treatments (shade from grass litter vs. no shade) were applied to half of each tray.  A total of 2,400 number of seeds were sown and germination was measured every week for sixteen weeks.    

Results/Conclusions

Overall 18% of seeds germinated in the study. Our experimental results demonstrate that both shade and flooding reduced total seed germination over the course of the study. Seed germination under grass shade was nearly two times lower than unshaded (266 vs.161) while flooded conditions (either minimal or complete flooding) reduced germination by more 50% when compared to unflooded moist soil.  The effects of both shade and flooding were highly significant (p=0.003 and p<0.001, respectively), and there was no interaction among treatments.  Under unflooded conditions and no grass shade seed germination was faster and lasted longer (seeds germinate form week 3 till week 12). The relative elevation and the flood duration of the Lagoon may contribute to the distribution of the invasive species M. pigra in this area. A decrease in seed germination will be expected if the water levels for Cartagena Lagoon are maintained high. Efforts to reduce M. pigra regeneration should potentially target areas that experience extended periods of moist soil without inundation and where shading is minimal.