Transcript Homogenization of soil seed bank communities associated

The Invasive Species Ireland Forum 2009
Long-term implications of plant
invasions: the significance of the
soil seed bank
Margherita Gioria & Bruce Osborne
[email protected]
School of Biology and Environmental Science
University College Dublin
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
Mechanisms
Species displacement from
the vegetation
 Reduce seed input
Formation of a large aboveand below-ground biomass
Limitation mechanisms
Changes in conditions for
germination
Reproductive strategies of
resident species
 Saturation
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: 30-50 years
 HM: 30-40 years
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 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
Impacts of GT
May
October
Fig. 1. nMDS configurations representing SSB communities invaded by GT at three sites
Impacts of HM
May
October
Fig. 2. nMDS configurations representing SSB communities invaded by HM at three sites
Impacts of FJ
May
October
Fig. 3. nMDS configurations representing SSB communities invaded by FJ at three sites
Dominance GT
May
October
Fig. 4. Dominance-diversity curves based on SSB data collected in May and October
at three sites
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
Stellaria uliginosa
Spergula arvensis
Ranunculus acris
Juncus bufonius
May
Urtica dioica
Juncus effusus
0
10
20
30
40
50
% Contribution to similarities
60
Cardamine pratensis
Stellaria uliginosa
Spergula arvensis
Urtica dioica
October
Ranunculus acris
Juncus bufonius
Juncus effusus
0
10
20
30
40
% contribution to similarities
50
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
May
Juncus effusus
Urtica dioica
0
20
40
60
80
% Contribution to similarities
100
October
Urtica dioica
0
20
40
60
80
% Contribution to similarities
100
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
Cirsium arvense
Juncus bufonius
May
Ranunculus acris
Urtica dioica
0
10
20
30
40
50
60
70
% Contribution to similarities
80
90
Ranunculus repens
Ranunculus acris
Epilobium hirsutum
October
Juncus bufonius
Juncus effusus
Urtica dioica
0
10
20
30
40
50
% Contribution to similarities
60
70
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
Source of
variation
df
SS
MS
F
P
SS
May
Sp
D
S(Sp)
Sp x D
P(S(Sp))
S(Sp) x D
P(S(Sp)) x D
Residual
Total
2
2
6
4
27
12
54
432
539
67.71
20.20
136.68
9.76
20.98
31.98
27.95
290.66
605.91
33.86
10.10
22.78
2.44
0.78
2.66
0.52
0.67
MS
F
P
2.09
4.50
13.77
1.72
1.77
2.65
0.97
0.062
0.002
0.001
0.024
0.001
0.001
0.723
October
1.49
3.79
29.32
0.92
1.15
5.15
0.77
0.176
0.003
0.001
0.592
0.024
0.001
1
75.50
16.98
108.27
12.99
35.38
22.66
38.53
319.45
629.76
37.75
8.49
18.05
3.25
1.31
1.89
0.71
0.74
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
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
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

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 2000-2006
Ecophysiology Group at UCD
Dr Joe Caffrey
Dr Declan Doogue