Long-term implications of Long-term implications of plant invasions: the plant invasions: the

significance of the soil significance of the soil seed bankseed bank

Margherita.gioria@ucd.ieSchool of Biology and Environmental Science

University College Dublin

Margherita Gioria & Bruce Osborne

The Invasive Species Ireland Forum 2009

Predicting invasions by Predicting invasions by IASIAS 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 banksSoil 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 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

MechanismsMechanisms

Gioria 2007

ObjectivesObjectives 3 large herbaceous plant invaders:

Fallopia japonica var. japonica FJGunnera tinctoria GTHeracleum 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

CharacteristicsCharacteristicsLarge stature, biomass, and litter

Reproduction: FJ: exclusively by vegetative means GT: sexual and asexual HM: exclusively by seedsReproductive 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

Methods 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

Seed bank of GT and HMSeed 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 GTImpacts of GT

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

May October

Impacts of HMImpacts of HM

May

Fig. 2. nMDS configurations representing SSB communities invaded by HM at three sites

October

Impacts of FJImpacts of FJ

Fig. 3. nMDS configurations representing SSB communities invaded by FJ at three sites

OctoberMay

Dominance GT Dominance GT

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

May October

Dominance HMDominance HM

May October

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

Dominance FJDominance FJ

May October

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

SSB invaded by GTSSB 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 HMSSB 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 FJSSB 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 Effect of invasive species identityidentity

Source of variation df SS MS F P SS MS F P

Sp 2 67.71 33.86 1.49 0.176 75.50 37.75 2.09 0.062D 2 20.20 10.10 3.79 0.003 16.98 8.49 4.50 0.002S(Sp) 6 136.68 22.78 29.32 0.001 108.27 18.05 13.77 0.001Sp x D 4 9.76 2.44 0.92 0.592 12.99 3.25 1.72 0.024P(S(Sp)) 27 20.98 0.78 1.15 0.024 35.38 1.31 1.77 0.001S(Sp) x D 12 31.98 2.66 5.15 0.001 22.66 1.89 2.65 0.001P(S(Sp)) x D 54 27.95 0.52 0.77 1 38.53 0.71 0.97 0.723Residual 432 290.66 0.67 319.45 0.74 Total 539 605.91 629.76

OctoberMay

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

Invaded seed bank 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 1Conclusion 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

Conclusions 2 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

Implications 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

AcknowledgmentsAcknowledgments

EPA Ireland (ERDTI) – NDP 2000-2006Ecophysiology Group at UCDDr Joe Caffrey Dr Declan Doogue