Examining the role of competition in ectomycorrhizal interactions
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Examining the role of
competition
in
ectomycorrhizal interactions Peter KennedyNPER post-doctoral fellow - UC
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Competition:
• Typically highly asymmetric.
• Competitive hierarchies are common.
• Order of arrival can significantly effect outcome.
A major factor structuring natural assemblages
Generalizations
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Research questions
1. How is ECM competition structured in the field?
• What are main mechanisms by which it occurs?
2. Are there competitive hierarchies among ECM fungi?
• If so, are competitive dominants also better symbionts?
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Study SystemPoint Reyes National Seashore, CA
Bishop Pine (Pinus muricata)
Rhizopogon spp.
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Mycorrhizal abundances at Pt. Reyes
Forest type
Young Mature
Rhizopogon occidentalis
Rhizopogon vulgaris
Rhizopogon salebrosus
Rhizopogon evadens
(0-10 yr) (40-60 yr)
ECM Species
Rhizopogon is a dominant colonizer of seedlings in post-fire and primary
successional settings.
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Lab Study Results
•Timing of colonization differed considerably between species
•Strong asymmetry and priority effects wereobserved
•Inoculation curves very similar between species
Kennedy and Bruns (2005), New Phyt. 166: 631-638.
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Field competition experiment
•Two species: R. salebrosus (RS) and R. occidentalis (RO)
•Treatments: no inoculum, single species, two species
•20 replicates/treatment at three sites
•Harvested seedlings after 5 and 10 months
•Analyzed ECM root tips with real-time PCR
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Results: Fungi
0
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D ranch PPP 1B PPP 2B
log (x+1) DNA yield
RS single-speciesRS two-species
R.
a
a
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a
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D ranch PPP 1B PPP 2B
log (x+1) DNA yield
RO single-speciesRO two-species
aa
a
a a
a
R. occidentalis
R. salebrosus
•Highly asymmetric(i.e. mostly all or nothing)
•Priority effect again observed
•lab and field results = good correspondence
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What about the few co-colonized seedlings?
R. salebrosus is not always the
competitive inferior!
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RO RS
Species
DNA Yield
a
a
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Results: PlantsTable 3. Performance of Pinus muricata seedlings in the coastal scrub/grassland at Point Reyes National Seashore, CA, USA.Treatments are non-inoculated (NM), single-species Rhizopogon occidentalis (RO), single-species R. salebrosus (RS), andtwo-species (RO/RS). Values are means with one standard error in parentheses. Different letters indicate significantdifferences at P < 0.05.
Parameter Treatment
NM RO RS RO/RS
Survival 0.72(0.14) a 0.86(0.04) a 0.86(0.10) a 0.84(0.11) a(alive/total)
Total weight 0.14(0.03) a 0.254(0.03) b 0.212(0.02) b 0.206(0.02) ab(g)
Leaf Nitrogen 0.88(0.18) a 1.87(0.09) b 1.87(0.09) b 1.90(0.08) b(%)
Being mycorrhizal is very important in terms of growth, but ECM competition has little effect
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What about the natural pattern?
Forest type
Young Burned Mature Unburned
Rhizopogon occidentalis
Rhizopogon vulgaris
Rhizopogon salebrosus
Rhizopogon evadens
(0-10 yr) (40-60 yr)
ECM Species
Abundances based on Gardes and Bruns (1996), Horton et al. (1998), Taylor and Bruns (1999), Baar et al.(1999),
and personal observation (T. Bruns).
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Types of ECM Competition
Exploitation Interference
Time
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Competitive hierarchy experiment
R. salebrosus
Competitive intransivity
•Four species: Rhizopogon vulgaris (RV), R. salebrosus (RS), R. evadens (RE), R. occidentalis (RO)
•All pair-wise and one three-way combination (RO/RV/RS)
•Spores of competitors added at the same time (106 spores/species)
•10 replicates/treatment grown for 8 months (growth chamber)
•Harvested all seedlings and root tips analyzed with real-time PCR
R. occidentalis
R. evadens
R. vulgaris
R. salebrosus
Competitive hierarchy
R. occidentalis
R. evadens
R. vulgaris
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Results: A Competitive Hierarchy!
R. occidentalis(RO)
R. vulgaris(RV )
R. salebrosus(RS)
R. evadens(RE)
RV RS RO RE
+ + +
(0/10)
(3/10)
_
_
_
_ _
_
+ +
+
(10/10) (7/10) (10/10)
(9/10) (10/10)
(10/10)(1/10)
(0/10) (0/10)(0/10)
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What about the plants?
Seedling biomass Shoot Nitrogen
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RV RS RO RE
Species
Total Seedling Biomass (g)
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bc
c
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RV RS RO RE
Species
Total Shoot Nitrogen
aa
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The best competitors may be the best symbionts
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Applications for Forestry
• Mixed species ECM inoculations may not be effective - competitive interactions are strong and highly asymmetical.
• Differences in timing of spore germination may affect seedling ECM composition - priority effects observed in both lab and field studies.
• More studies are necessary to determine competitive ability - for Rhizopogon, competitive dominants appear to be the best
symbionts.
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Acknowledgements
• Sarah Bergemann, Sara Hortal, Tom Bruns
• Bruns lab members, UC Berkeley
• Point Reyes National Seashore
• National Parks Ecological Research Fellowship
• National Science Foundation
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Ongoing experiments
• Do spore- vs. mycelial-based ECM competition have different outcomes?
• Can priority effects be reversed by altering the timing of colonization?
• Does ECM competition occur mainly through direct or indirect interactions?
• Does spore density and soil heating affect the outcome of ECM competition?