Long-term implications of plant invasions: the significance of the soil seed bank

1. The Invasive Species Ireland Forum 2009 Long-term implications of plant invasions: the significance of the soil seed bank Margherita Gioria & Bruce Osborne Margherita.gioria@ucd.ie School of Biology and Environmental Science University College Dublin

2. Predicting invasions by IAS • A number of generalizations have been proposed to predict the factors that are responsible for successful invasions • Only a few have provided consistent results • Stochastic factors • Disturbance • Propagule pressure • Residence time • Changes in land use • Impacts at community level – standing vegetation • The impact of IAS on the soil seed bank (SSB) has been largely neglected

3. Soil seed banks • Determining plant community dynamics • Source of diversity and genetic variability • Survival of a species at a locality • Mitigating the effects of unfavourable seasons • Colonization of new habitats • Dispersal in space and in time - ‘memory’ • Thompson et al. (1997) classified SSBs : • Transient - Short-term persistent - Long-term persistent

4. SSBs and IAS • Alterations in the seed bank of resident species • The formation of a large SSB •  seed input, germination, viability •  species recruitment from the seed bank •  additional effects on the vegetation • Understanding the potential long-term implications of plant invasions •  changes in SSB must be examined

5. Mechanisms Species displacement from the vegetation  Reduce seed input Formation of a large above- and below-ground biomass Limitation mechanisms Changes in conditions for germination Reproductive strategies of resident species  Saturation Gioria 2007

6. Objectives • 3 large herbaceous plant invaders: Fallopia japonica var. japonica FJ Gunnera tinctoria GT Heracleum mantegazzianum HM • Seed bank of GT and HM • Effects on the structure (diversity, composition, and abundance) of resident SSB communities • Comparative assessment of the effects of these invaders

7. Characteristics • Large stature, biomass, and litter Reproduction: • FJ: exclusively by vegetative means • GT: sexual and asexual • HM: exclusively by seeds Reproductive potential: • GT: 700,000 seeds per plant (Osborne et al. 1991) • HM: 10,000-20,000 fruits per plant (Pyšek et al. 2007) Residence time: • FJ: 3-5 years • GT: 30-50 years • HM: 30-40 years

8. Methods • Multi-site comparative approach • 3 sites for each invader • Comparable invaded and uninvaded areas • 4 – 4m2 plots • 5 soil cores • 3 depths (0-5, 5-10, 10-15 cm) • May and October • Seedling emergence approach (Thomspon & Grime 1979) • Unheated greenhouses • 240 samples per site

9. Seed bank of GT and HM Gunnera tinctoria • 32,120 ± 31,837 SD seedlings m−2 in May • 28,308 ± 16,176 SD seedlings m−2 in October • 20% seedlings (5-10 cm) • 10% seedlings (10-15 cm) • Asynchronous Persistent seed bank (sensu Thompson et al. 1997) Heracleum mantegazzianum • 9,762 ± 390 SD seedlings m−2 in October • 0-5 cm • Synchronous germination • Requirement for chilling period Transient seed bank

10. Impacts of GT May October Fig. 1. nMDS configurations representing SSB communities invaded by GT at three sites

11. Impacts of HM May October Fig. 2. nMDS configurations representing SSB communities invaded by HM at three sites

12. Impacts of FJ October May Fig. 3. nMDS configurations representing SSB communities invaded by FJ at three sites

13. Dominance GT May October Fig. 4. Dominance-diversity curves based on SSB data collected in May and October at three sites

14. Dominance HM October May Fig. 5. Dominance-diversity curves based on SSB data collected in May and October at three sites

15. Dominance FJ October May Fig. 6. Dominance-diversity curves based on SSB data collected in May and October at three sites

16. SSB invaded by GT May Figure 7. Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by GT (Bray-Curtis, 4rt root tramsformed data)

17. SSB invaded by HM Figure 8. Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by HM (Bray-Curtis, 4rt root tramsformed data)

18. SSB invaded by FJ May Figure 8. Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by FJ (Bray-Curtis, 4rt root tramsformed data)

19. Effect of invasive species identity Table 1. Results of PERMANOVA analyses testing the effect of ‘invasive species identity’ (Sp) on soil seed banks

20. Invaded seed bank May Fig. 9. nMDS plots displaying multivariate patterns in invaded seed bank communities for the three invaders, at each study site and within each plot October

21. Conclusion 1 • Major effects on the seed bank of invaded areas • Invaded SSB less diverse, abundant • More persistent component • Dominated by seeds of agricultural weeds and Juncus species • GT formed a large persistent seed bank - 30,000 seedlings m2 • Eradication non realistic • HM formed a transient bank - 10,000 seedlings m2 October • Eradication feasible • FJ did not set any viable seed

22. Conclusions 2 • No effect of Species •  similar SSBs • Independent of the reproductive strategy of the invader • Independent of the initial SSB diversity • FJ: despite not setting any viable seed  significant effects on invaded communities • In a short period of time (3-5 years), compared to 40-50 years for GT and HM •  higher invasive potential

23. Implications • Alterations of SSBs could be an important determinant of the invasive success of large invasive plants • Long-term implications • Improving our understanding of such effects •  Important for the development of control and conservation programmes • Disturbance in an attempt to eradicate invasive species •  Promote the germination of seeds of undesirable species • Need for seeds of desirable species

24. Acknowledgments EPA Ireland (ERDTI) – NDP 2000-2006 Ecophysiology Group at UCD Dr Joe Caffrey Dr Declan Doogue