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

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

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

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

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 Mechanisms Gioria 2007

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

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: years HM: years

Methods Multi-site comparative approach 3 sites for each invader Comparable invaded and uninvaded areas 4 – 4m 2 plots 5 soil cores 3 depths (0-5, 5-10, cm) May and October Seedling emergence approach (Thomspon & Grime 1979) Unheated greenhouses 240 samples per site

Seed bank of GT and HM 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) 9,762 ± 390 SD seedlings m 2 in October 0-5 cm Synchronous germination Requirement for chilling period Transient seed bank Gunnera tinctoria Heracleum mantegazzianum

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

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

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

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

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

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

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)

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)

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)

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

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

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 m 2 Eradication non realistic HM formed a transient bank - 10,000 seedlings m 2 October Eradication feasible FJ did not set any viable seed

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 years for GT and HM higher invasive potential

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

Acknowledgments EPA Ireland (ERDTI) – NDP Ecophysiology Group at UCD Dr Joe Caffrey Dr Declan Doogue