long-term implications of plant invasions: the significance of the soil seed bank...
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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
School of Biology and Environmental ScienceUniversity 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 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
CharacteristicsCharacteristicsLarge 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
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